Fungi, like the rest of us non-photosynthesizing organisms, are dependent on plants for existence. Species of the fungal genus Fusarium are ubiquitous, often feeding on dead and living plant materials. They are identified by microscopic observation of their asexually produced spores (conidia) abundantly produced from the filaments (mycelium) growing in and on plant tissue. When these fungi are stimulated to sexually reproduce, they develop spores in microscopic ‘sacs’ called asci. This means of sexual reproduction places Fusarium in a class of fungi called ascomycetes. The genus of ascomycetes that have Fusarium as a conidial form is Gibberella. Consequently, a fungus frequently found in corn stalks, leaves and ears may be identified by its conidia shape as Fusarium verticilloides but if stimulated to form sexual bodies it would be called Gibberella fujikuroi. Another species of Fusarium (Fusarium graminearum) more frequently is found on corn stalks and ears as the sexual stage, Gibberella zeae. This dual naming system, tolerated by mycologists and plant pathologists, occurred because the fungus was initially only known and named by the asexually produced conidia.
These fungi on corn are not aggressive pathogens but mostly invade dead or weakened cell tissue. Infection of corn ears by Fusarium species is often through old silk tissue. Fusarium or Gibberella stalk rot occurs after the stalk tissue has been weakened by desiccation due to roots rot. Fusarium mycelium is easily found in leaf tissue, as if it is an occupant, perhaps feeding on weakened or dead cells within living leaves. Its widespread presence in corn seeds, seedlings, stalks, leaves and ears often leads to difficulty in determining its significance. Was it an aggressive pathogen killing the tissue or was it an invader of weakened tissue? Is seedling blight due to a Fusarium species attacking a vigorous, corn seedling or was it simply infecting a corn seedling weakened by environment? Gibberella stalk rot occurs when the plant-environment- genetics interaction results in roots dying from insufficient carbohydrate to sustain metabolism. Stalk cells die because of consequential wilting and shortage of carbohydrates as well. Fusarium graminearum feeds on the weakened and dead tissue, eventually producing the sexual reproduction bodies of Gibberella zeae and thus allowing us mere humans to call it Gibberella Stalk Rot.
Carbohydrates stored in the endosperm of a corn kernel is a potential source of nutrition for fungi and insects. The pericarp can be a major barrier to attack to these potential invaders. Studies concerning the tropical corn storage insect, Maize Weevil (Sitophilus zeamais), showed that the cross-linked structural components of the pericarp cell walls were highly correlated with resistance to this insect. Other factors included phenols (Afr. Crop Sci. J. 9:431–440) produced by the pericarp cell metabolism and even endosperm hardness (flintiness) contributed to reduced susceptibility to this storage insect (Crop Sci. 44:1546–1552 (2004)).
Pericarp tissue also is a barrier to entrance into the seed by multiple kernel rotting fungi. Most enter the ovary through the silk channel immediately before pollination. This becomes most evident when silks are left exposed for several days in an environment favoring the pathogen. After invasion, the fungus can spread cell-to-cell within the pericarp through small holes (pits) in the cell walls that allows movement of metabolites between cells. Integrity of the pericarp is a significant factor in avoiding invasion by many potential fungal species.
The phenomenon known as silk-cut can expose the seed to fungal infection. After the pollen tube grows down the silk channel and dumps the pollen nuclei into the ovule, silk tissue deteriorates and detaches from the ovary. Not all silks are pollenated even under ideal conditions, leaving some attached to their ovary while adjacent pollenated ovaries grow. These remaining silks interfere with normal contiguous growth of the pericarp cells in the adjacent ovary wall (Plant Disease 81 (5):439-444). This can result in a break in pericarp as the kernels enlarge and thus an opening for invasion by fungi. Genetics and environments influence the occurrence of silk-cut. Stresses that delay silking beyond pollen availability can be an important factor but genotypes vary in vulnerability both to reaction to the stress and probably the tendency of this phenomenon.
The corn kernel is a fruit. Grains are fruits with a single seed enclosed. The ovary wall, part of the female plant, grows after pollination results in enlargement of the single embryo it encloses. The ovary wall thus becomes the pericarp. Genetics of the pericarp cells are those of the female plant and therefore the genetics of the female inbred parent in hybrid seed production or both hybrid parents in the grain field.
These genetics influence the important cell wall components that give both strength and resistance against insect and fungal invaders. Cell walls get their strength from various polymers that are cross-linked. Whereas non-grass species tend to have more lignin chemistry in cell walls than those in grasses and especially the pericarp features less lignin and more of a class of complex compounds called xylan. A major component of these xylan compounds is feruloyic acid. Feruloyic acid is associated with resistance to Maize weevil damage to corn kernels (Crop Sci. 44:1546–1552 (2004). Varieties differ in chemical components of pericarp cell walls that probably influence many aspects of corn grain storage. https://www.frontiersin.org/files/Articles/219955/fpls-07-01476-HTML/image_m
The female parent of a corn seed is the sole genetic source for the pericarp, mitochondria and chloroplasts as well as half of each diploid chromosome.
Mitochondria are not the only membrane source in corn seed cells. Nearly all cell functions are carried out on membranes. Endoplasmic reticulum (ER) is a major component of the cell which acts as transporter of the enzymes and proteins being produced in the cells. It is also the transporter of the messages from the DNA in the nucleus of the cell. Virtually all cell functions are dependent upon the structure of cell membranes.
Development of membranes in seed is dependent upon a combination of the genetics of the variety and the environmental stresses during seed development. Drying the seed also significantly affects the membranes, as the membranes collapse with drying. Corn seed producers are very aware of this potential problem and develop methods to assure that seed drying is carefully monitored so handling is gentle. Seed membrane deterioration can occur in the field, especially if seed is allowed to dry slowly after fully developed at about 35% moisture when the abscission layer (black layer) cuts off nutrition from the plant. Rain preventing harvest at this critical time is one cause of loss of seed viability because it begins an aging process in corn seed.
A combination of drying temperatures below 100°F and quick drying by high air movement is critical to maintaining membrane integrity in corn seed. Although all cellular membranes are probably affected by drying conditions, the fact that germination deterioration is mostly linked to the female parent of a hybrid, it is likely that the mitochondrial membranes are affected the greatest. Each corn genotype varies somewhat in tolerance to these factors, but the principles of drying temperatures and speed of drying appear to mostly involve membrane deterioration. There is also some effect on the pericarp of the grain if it causes breakage. Seed producers apply this knowledge and a lot of art to balance all the variables involved in producing seed with high field emergence rates the following spring.
Respiration of dried seed is needed to maintain membrane integrity while being stored but low temperature and dry atmosphere is needed to minimize the activation of excessive seed metabolism. The combination of seed moisture no greater than 13% and the temperature of 50°F and 50% relative humidity is usually considered adequate for maintaining seed germination quality for several months. Genotypes vary in maintenance of seed quality even in this environment. Stresses during the growing season also can cause some seed to deteriorate even with careful storage monitoring.
Production and maintaining high quality corn seed requires corn genetics, non-stressful growing environment, careful handling at harvest, drying and shelling and proper storage before the next planting season. There is an art to being a good seed producer.
Most new crop seed is tested after drying and shelling and before bagging. Systematic sampling is done after sizing. These samples are tested for genetic purity, warm germination and cold germinations. These results are used for bagging decisions. After sizing and seed treatments are made seed is moved to bagging. Again, systematic sampling is made. Those samples are then tested at least in a warm and cold test. The warm test result is used for the reported germination printed on the bag tag. Germination percentage is within a statistically acceptable range of the published percent germination on the tag and within the month as published.
Not all individual seed within the bag are at exactly the same state of quality- some deteriorating faster than others. This could be related to position on the ear, or within the seed field or handling of seed after harvest. If 5% of the seed did not germinate in a December warm test, a few more percentage may (or may not) be deteriorating in the next few months. Seed companies attempt to estimate this rate by considering the cold test results or perhaps special tests. Suspect seed lots may be retested a few months later.
Each corn seed is a living organism, vulnerable to ageing from membrane deterioration including within the mitochondria, that transfer the energy from stored carbohydrates into usable forms for metabolism needed for cell growth and multiplication. Damage to membranes because of partial metabolic activation prior to artificial drying can affect mitochondrial activity when germination is encouraged. Physical injury to the pericarp can allow imbibition to be too rapid for cell membranes of the dormant cellular components, resulting in breakage. These membranes can self-repair but do this best at warmer temperatures. Leakage from injured kernels attracts micro-organisms that can further inhibit the seed metabolism.
It is a challenge to all involved to correctly predict the percentage of seed that will emerge in the field. Warm tests are often done at 70°F (21°C) as a measure of viability for a week after moistened. Successfully germinated seedlings have clear development of the primary root and shoot. Percentage of seeds showing these structures become the warm test result. Some seed will be slow to push both structures within that time. Those showing only a root are generally called non-germinated and not included in the percent germinated. There is some judgement needed for classification of those that are slow to push out both structures. The cold germination test in which the moistened seeds are kept at 50°F (10°C) for one week before moving to the warm test environment further amplifies the effect of the seed deterioration on germination. Some seed lot samples will express high germination percentages in both warm and cold tests. Usually those showing marginally acceptable warm test results usually have much lower cold test results, assumedly because the individual seeds with membrane damage could not self-repair after imbibition at low temperatures.
Professional Seed Research, Inc. plants corn seed under about ½ inch of artificial soil mix for warm and cold tests. Seedlings are counted when most plants show the third leaf. Individual seedlings that only show the ‘spike’ emerging are counted as not germinated on the assumption that these individuals, if they do emerge in the field, will be non-competitive plants. Uniformity of emergence of the 400 seed sample is also scored in the warm germination report.
The ultimate objective for all involved in corn seed is to allow full expression of the genetics on grain yield and not allow seed germination to be a detraction from that potential.
Use of homozygous inbreds as corn hybrid parents is important to producing new hybrid seed with identical parents’ genes successful in past seasons. Traditional method of obtaining homozygosity has been done by self-pollinating selected plants from a segregating population for 5-8 generations. Studies from the 1950’s showed that basic heterosis with another parent could be expressed after 3 selfing generations, but the remaining selfing generations are needed to assure repeatability of minor traits. Corn breeders have tried various methods to try to reduce the time and testing expense to this process.
A quicker method of obtaining homozygous inbreds by inducing haploids to be doubled was known since 1959, but the method became more common after the year 2000. Pollen from haploid inducer is crossed to a prospective heterozygous genetic source. From 2-10 percent of the resulting seed will only have the genes from the female ovule although only one member of each pair of chromosomes of the female plant. These seed can be identified visually with pigments if the inducer differs from the female plant in these pigments or perhaps by molecular methods. Haploid chromosomes are doubled by either submersion or by injection with chemicals such as colchicine or specific herbicides. It is estimated that, on average, to obtain 100 new inbreds by DH system, requires initial pollination of 100 - 200 plants by the inducer to obtain 1000 haploid plants to be grown in field, after doubling chemical treatment. It is intended to obtain 100 homozygous, diploid inbreds as a result.
Professional Seed Research, Inc. (PSR) began experimenting with another method of quickly reaching near homozygosity by utilizing their Seedling Morphology Fingerprint (SMF™) technology used in their other services. This utilizes the fact that because corn has only 10 pairs of chromosomes, there is are a notable percentage of near-homozygous individuals among a segregating population. That percentage increases if the F2 population is between related parents. PSR selects these near-homozygous plants based upon seedling characters. These plants are transplanted to the field and selfed. Resulting plants range in homozygous levels equivalent to 4-8 selfing generations. This is sufficient to evaluate heterosis with prospective parents and yet may have sufficient heterozygosity to select for simple inherited characters such as ear height, pollen production or disease resistance. PSR refers to this system as Rapid Inbreeding®.
The objective of all three systems is to reach a level of homozygosity in productive hybrid parents that can be repeated with each increase. Traditional method of selfing each generation allows for selection of desirable characters in each generation but costs time. Dihaploid breeding is quicker to reach complete homozygosity than traditional methods but requires specialized efforts and genetic backgrounds affects the success rate. Rapid Inbreeding® (RI) offers the speed of dihaploid breeding to reach level of sufficient homozygosity for hybrid selection, works with any segregating population and allows further selection of minor characters in final inbred. PSR Global Genetics uses RI technology to develop thousands of inbred lines each year.
The nucleus of every living cell of the corn plant has 30-40 thousand genes on each chromosome. Each gene is composed of a string of four nucleic bases (adenine, cytosine, guanine and thymine) the order of which is the DNA code for that gene. The DNA structure is a double strand of these nucleic bases wound around a sugar molecule (deoxyribose) and a phosphate molecule. The strands are held together by the attraction of bonds of adenine to thymine and cytosine to guanine. A single gene may include a string of hundreds to thousands of these 4 nucleic bases.
When a gene is transcribed, the double helix strand is separated, and the single strand copied onto a different sugar (ribose) and phosphate, moved from the cell nucleus to the ribosome in the cytoplasm. Amino acids from the cytoplasm are attached to each other according to the sequence of the nucleic acids. Three consecutive nucleic bases code for a single amino acid. The string of amino acids become a protein once the genetic code for a single gene is read in the ribosome.
Amino acids are compounds of nitrogen, hydrogen carbon and oxygen atoms attached in 20 distinct patterns, including differing side branches. A single protein molecule often includes hundreds of amino acids combined in complex shapes because of the different charges of those side branches in each amino acid. Many of the proteins function in cells as enzymes affecting all cell metabolism.
The complexity of thousands of genes each composed of strings of 4 bases translated into strings of amino acids to form a single protein that may affect the process of producing a single cell compound is overwhelming. The stability of the system is responsible for corn plants looking like corn plants. Probably 95-99% of the genes in all corn plants are similar.
Genetic variability allowing the differing expression of favored traits affecting timing of flowering, silk extension, water and mineral uptake, leaf size and shape, disease resistance may involve only 1-5% of the DNA code. Selection of random assortments of the DNA codes has led to the variation among varieties within the corn species. Combining the inbreds to maximize the expression of favored traits is the job of the corn breeder. Providing a dependable system of genetics is the responsibility of the cell nucleus and translation of the gene is up to the cell cytoplasm.
The vast majority of genes in corn must relate the basic structures and functions of all corn plants. Mutations of these genes tend to be recessive and therefore are usually not expressed if the dominant form of the gene is present in the matching area of its paired chromosome. On the other hand, recessive genes with potentially beneficial (to us) traits need to have both members of the paired genes be in the recessive form. The enigma of attempting to benefit from hybrid heterosis and yet obtaining repeatable genetics by making homozygous parents is that the process of reaching homozygosity allows expression of the recessive traits.
Some of the expression of homozygous recessive traits may be drastic, such as albino plants. In some cases, it is expressed as susceptibility to a pathogen, such as race 1 of Bipolaris zeicola (Helminthosporium carbonum), resulting in an occasional inbred homozygous recessive for susceptibility. The majority of negative recessive genes affect the chain of biochemical pathways resulting in a reduction in effectiveness of those products. Highly inbred plants are less vigorous than hybrid plants because of this overall reduction in the physiological processes contributing to cell and plant structures. It is assumed that this is caused by the expression of recessive genes that are not expressed when paired with dominant forms of these genes.
Corn breeders attempt to overcome the negative effects of inbreeding by mating inbreds with compatible inbreds to cover the deficiencies of each parent inbred. Considering the large number of genes in corn, and the diversity of its history, identifying the best combination for maximum desired performance is not easy. It is known that inbreds developed from certain ‘families’ are most likely to combine well to make productive hybrids in most environments. A broad group of varieties generally grown in the western part of central USA were intercrossed for several generations to create a breeding population known as Iowa Stiff Stalk. Inbreds developed from this population tend to create vigorous hybrids when crossed with certain other populations originating from the Eastern USA. This principle of crossing inbreds originating from distinct populations applies to hybrid corn breeding in all regions where corn is grown.
The challenge for corn breeders is to not only select the appropriate breeding source based upon desired history of agronomic characters, but also identify prospective hybrid parents with desired characters and identify the appropriate other parent for maximum hybrid performance. Despite our gains in ability to analyze DNA, physiology and environments, the variables are multiple. There has never been the perfect corn hybrid for multiple years and locations- and the complexity of plant and environments will maintain this history for many years as well.
Corn’s history and biology has resulted in diversity beyond what most of us see in any single season. Advantages of hybrid plant uniformity for yield, harvestability, disease and pest resistance and genetic repeatability requires development of homozygous inbred parents. Each of many seed companies produce multiple hybrids each year and there are about 40,000 genes in each corn plant that are available to influence something, whether needed or not.
Corn researchers in 1920’s became aware of a need to collect and share many of the genetic sources in corn, forming a Maize Genetics Cooperation Stock Center- it’s history is summarized at http://maizecoop.cropsci.uiuc.edu/mgc-info.php. This collection started and continues to emphasize mutants affecting some identified trait, such as those involved in sweet corn, waxy corn or amylose corn and many that may not have a specific economic advantage but are useful in understanding some biochemical pathway in the corn metabolism. Study of these mutants contributed to location of genes on each chromosome and add to growing knowledge of corn DNA codes for many traits.
Despite these efforts, corn’s genetic diversity is large due to selection by humans over diverse environments. Our experiences with ‘new’ diseases as a pathogen such as the bacterium causing Goss’ wilt suddenly appears, with a previously unknown susceptibility gene in corn became part of popular corn hybrids, or susceptibility of race T of southern corn leaf blight associated with mitochondrial gene in t-cytoplasm male sterile corn. Resistance to Maize Chlorotic Mottle Virus was found in corn genetics in USA after it occurred in Kansas in the 1970’s and in Africa in 2016.
Often the strong resistance to these diseases are associated with single genes already present in corn apparently without intended human selection and without known selection pressure in absence of the disease. Perhaps there was exposure somewhere in its history where the gene was favored but also it is likely that randomness of mutations, segregation of genes during miosis, cross pollination and historic diversity of corn’s environments have provided many genes for characters that we have yet to identify. These genes must be influencing multiple internal aspects of absorption of light wavelengths, translocation of carbohydrates, absorption and movement of minerals, water uptake and conservation, and structures of leaves. Among this diversity is the future adaptation needed for changing environments.
Breeders witness diversity within their nursery as they see differences in plant structures and growers see differences among hybrids in performance each year. At Professional Seed Research Inc., we see differences among hybrids in structures of seedlings (Seedling Growout®). Genetic diversity will continue to be an important contributor to this crop as it interacts with changing environments.
The kernel used as grain is the product of the hybrid plant between a male and female parent. It is bred for maximum production of carbohydrate from conversion of solar energy. Genetic contribution of both of the inbred parents is utilized by the grower to maximize the grain yield. Use of the kernel as a seed has different genetics and handling requirements. Maximizing seed yields requires experience to match timing of silking of female inbred with pollen distribution of male inbred. Amount of male pollen can be limiting with some male parents. Membrane deterioration, critical to mitochondria function in the seed embryo, can begin if the moisture levels remain high for prolonged time, a character that varies among seed parent inbreds. Consequently, seed is usually harvested before normal black layer and dried quickly to at least 13.5% moisture. Whereas high germination quality is not of much significance with kernels used as grain they are highly significant with use as seeds. Not only is seed dried with minimal heat addition, and lots of air movement, the harvest at high moisture requires harvests by ears, gentle handling within the facility becomes essential.
Damage to the kernels whether in the field prior to harvest or within the facility after harvest, genetic factors also become significant. Inbreds differ in vulnerability to these factors. Some of this vulnerability involves pericarp structure features of the female parent. Disease resistance of the female parent can be a factor also. Inbreds are intended to be homozygous to assure repeatability but this also allows unexpected homozygous recessive genes for susceptibility to a race of a pathogen such as that of Races of Helminthosporium carbonum(Bipolaris zeicola) that can cause premature death of the female plants as well as infection of the kernels. Use of T-cytoplasm corn for sterile female plants was associated with defective mitochondria membranes, allowing a toxin produced by race t of Helminthosporium maydis(Bipolaris maydis). Although the hybrid seed embryo is a product of the genetics of both parents, some cellular organelles such as mitochondria originate only from the female inbred.
Dual use of the corn kernel as a grain and as a seed requires effort by different specialists, the grain farmer and the team of seed experts.
The dual function of a corn kernel, from human’s perspective, adds to the difficulties of improving grain production with this crop. In most ‘wild’ plants the fruit functions in protection and distribution of the seed. Fruits originate from the ovary of the female plant and have evolved specific structures for distribution by wind or animals. Grass species have reduced ovary contribution of the fruit as it functions as protection of the single seed within it. The seed endosperm, originating from genetic contribution of both male and female parents, is the bulk of the grass fruit. Human’s effort to increase grain production has led to increase the endosperm production of a corn plant while maintaining its function of plant reproduction.
Most growers prefer high grain production per area of land- bushels per acre of corn at kernel moisture adequate for storage. It is not only the size of the kernel but also the female plant characteristics that allow rapid drying of the grain after completion of expansion of the endosperm but also maximum number of plants per acre or hector. Successful hybrids have DNA contributions from both parents that lead to structures favoring high production, efficient movement and storage of carbohydrates in grain and fruit structures that protect the grain from pathogens while allowing loss of moisture. It is not surprising that it takes DNA contributions of two parents to accomplish improvement of corn grain production.
Both hybrid parents contribute genetics leading to plant structures such as plant size, and structure growth rate, disease resistance, flowering time, root growth and mineral uptake. Diversity of origin of the two parents adds to the probability of favorable genes being found and combined in a desirable hybrid.
The female parent has the additional ‘responsibility’ of contributing to the germination efficiency of the seed. Mitochondria DNA of the hybrid seed is solely inherited from the female parent. The fruit outer wall, the pericarp, is a structure of the female parent of a corn seed. Successful gemination and uniform emergence, two major contributors to grain production per acre, is mostly dependent upon the female parent of the hybrid seed.
Among the contradictions in corn culture is the need to have corn stalks maintaining upright plants through harvest but rapid deterioration in soil between seasons and /or efficient decomposition for fermentation to recover the carbon in ethanol or energy for cattle. Primary strength during the growing season is derived from a combination of the tight connection of the pith cells to the outer rind cells, fibers and near the outer rind and thick cell walls of the outer rind cells.
Stalk components after harvest range among hybrids. About 50% of the solid weight is composed of carbon but most of it is involved in complex molecules such as cellulose, hemicellulose and lignin. Although lignin composition is only about 7% of the stalk, it is the most difficult to digest and often is wrapped around the more easily decomposed cellulose molecules.
Multiple fungal species in the soil produce enzymes capable of breaking and modifying the lignin molecules. Tree wood, mostly composed of lignin, is slowly destroyed by fungi specializing in production of lignocellulolytic enzymes. These initial wood rotting species are succeeded by other fungal species that enzymatically degrade the cellulose into its components. Genetic variation among fungi and competitive pressure for obtaining the energy locked up in corn stalks provides multiple sources to break down the complex carbon compounds that provided strength for the corn stalk previous to harvest.
Among the challenges for all interested in corn is to identify hybrids that produce stalks that remain upright through harvest but can be efficiently digested by cows, fermentation and soil organisms.
Nearly all plant species shed plant parts with the function of allowing continuation of the species. Temperate zone deciduous tree leaves reduce the supply of nutrients to the chloroplasts as summer ends. Nitrogen and other nutrients are transported from the chloroplasts and other leaf cell contents to plant parts that will survive the winter. This physiological change reduces movement of auxin hormones to the cells at the base of the leaf where it is connected to the stem. This stimulates the production of lignin and suberin in cells immediately below a layer of cells called the abscission layer. This thickening of cell walls essentially cuts off all translocation of water and nutrients from the stem to the leaves. It also causes the leaves to break from the stem at the point of the abscission layer while preserving the moisture in the stem.
The same process occurs in all plants, including in the tropics, although not as remarkably simultaneously colorful as with trees in the temperate zone at this season in north America. Similar physiological phenomena occur as fruit are released from the base plant. This was also true for the Teosinte species selected 10,000 years ago by someone because, contrary to most Teosinte plants, this genotype formed abscission layer at the base of the fruit (kernel) but did not quickly allow the kernel away from the parent plant. Maintaining the attachment of the kernel to the rachis (cob) even after abscission layer (black layer) formed allowed more efficient harvest for those folks 10,000 years ago, as it does for the modern farmer. Selection of this character by corn breeders since that time have attempted to maintain appropriate attachment of the kernel (fruit) to the cob thru harvest.
Leaves surrounding the corn ear, the husks, undergo similar abscission layers after the black layer is formed in the kernels. This cuts off the water movement into the husk leaves, usually causing them to withdraw from being tightly wrapped around the ear. This is associated with faster evaporation of water from the kernels. Likewise, abscission occurs in cells at the base of the cob. This further removes water movement into the cob, allowing for further drying of cob and kernels. Corn breeders challenge is to select genetics that allow rapid kernel drying while maintaining attachment of the ear to the corn plant until harvest. Stress conditions may cause premature formation of abscission layers in kernels and husk leaves. This has been associated with ears dislodging from the plant. Pests such as European Corn Borer have been associated with premature dropping of ears in the field.
The balance of timely formation of abscission layers to allow drying of grain, maintaining adequate attachment of kernels to cob and ears to the plant when grown in multiple environments continues to be a complex genetic feat. Combining these characters with maximum translocation of sugar to the grain before formation of the abscission layer at the base of the kernel adds to the corn breeders’ accomplishments for the past 10,000 years.
Corn stalk rot involves nearly all biology of the corn plant. Consequently, the genetics of resistance is complex. The common fungi associated with stalk rot such as Gibberella zeae, Fusarium verticilloides and Diplodia zeae are often identified as the main fungi associated with destruction of stalk tissue because of their obvious symptoms but no specific genetic resistance to these fungi has been identified. Anthracnose stalk rot, caused by Colletotrichum graminicola, is associated with a few specific resistance genes that can be identified but this has minimal effect on the overall occurrence of the corn stalk rot.
Resistance to premature deterioration of the corn stalk involves nearly all aspects of the plant, at least post-flowering. Identity of the fungi most obvious in the dying stalk tissue is mostly irrelevant. Genetics of resistance involves those affecting root growth and function in that season’s environment. This could involve resistance to feeding damage from soil insects, nematodes and pathogens. Genes affecting the structures for water translocation to the stem or the metabolic efficiency of the living root cells influence the plant’s ability to function when under stress.
Photosynthesis rates during the season are affected by genetics of leaf shape and size. Leaf disease resistance is affected by both the general physiology and genes specific to each potential pathogen. Translocation of sugars to the roots and to the grain after pollination involves many physiological and structural aspects of the corn plant, each influenced by different gene interactions.
Movement of sugars to the grain is influenced by the number of ovules pollinated. Genes for establishing that number are multiple. Rate of movement to each kernel is affected by genetics not only within the embryo but also those affecting the vascular structures to carry the sugars. Multiple genes must be involved in the timely extension of the silks allowing successful pollination and fertilization of the ovule.
Successful maintenance of living pith cells in corn stalks until completion of translocation of sugars to kernels involves multiple genes affecting most parts of the corn plant in its environment. It is unlikely that a single gene for stalk rot resistance combined with acceptable grain yield will ever be identified. We will continue to realize that stalk rot resistance involves multiple genes influencing most of the corn plant’s biology just as does grain yield.
Ten thousand years ago, individuals in central Mexico harvested this new version of grain, derived from a wild plant, Teosinte that remained attached to the central rachis of the flower instead of shattering. They realized the increased carbohydrate increased the flour available for cooking. The annual plant spread by humans throughout the Americas for the next eight thousand years as others moved the seed to their environment, annually selecting genetics that performed best in their environment and their culture. This history of annual selection for adaptation in multiple places, separation of male and female flowers and unique C4 photosynthesis has attracted people to this species each harvest season since that initial discovery in Mexico.
All corn people from the first to those in 2018 have to be impressed with what they saw with corn. Kernel color or hardness, or amount of grain variation allowed selecting and saving desired genetics. Superior harvest was supported by the variety having resistance to local disease pressure, root structure adapted to local soils, timing of pollination matching weather conditions and maturity adapted to time between freezing.
It became apparent over a hundred years ago that crossing varieties adapted to different environments could result in new corn varieties superior to either of the two parents. This ultimately led to concentrated efforts to match breeding populations that contribute genes resulting in superior hybrid performance. With nearly 40,000 genes to work with corn breeders have realized the wealth of genetic diversity that this history has wrought.
We humans have pushed this energy converting crop to increasing yields in multiple environments. Every corn grower will be evaluating the current varieties to choose hybrids for the next season. Every corn breeder share in the excitement at harvest time as they witness the progress within the genetics that they select. Corn gains each year in some aspect of production due to the tremendous genetic diversity due to people’s selection for their preference over the past 10,000 years. It is historic!
There are many reports of ear rot this year in the US corn production. Ear rot is caused by several fungal species, each influenced by the environment at the time of infection. In most cases, infection occurred at pollination time. Environments that were unfavorable to successful pollination when the silks extended beyond the ear husks but were favorable to a fungus with potential for feeding on the silks is linked to infection.
Rainy days influence rapid extension of the silks, exposing them to pollen. However, release of pollen grains from anthers involves desiccation of the anther sac and dry air influences the movement of corn pollen. Consequently, rain and extremely high humidity allows the unpollinated silk to be exposed to fungal spores for a prolonged time. Pollen normally grows rapidly down the silk channel towards the ovule, and the silk collapses behind it, leaving a dry mass of lignified tissue not easily and quickly penetrated by fungi. Delay in pollination results in a more nutritious substrate for fungi such as Fusarium verticilloides, Gibberella zeae and Stenocarpella(Diplodia) maydis.
Aspergillus flavus and the smut fungus Ustilago maydis are favored by an extreme dry pollination period. This stress can cause delay of silk extension. Pollen maturity and release is not as affected by drought and consequently may be mostly spread before the silks are available for pollination. The drier environments favor the spread of these two fungal species.
The fungi grow on the surface of the silk to the ovules. If the infected silk channel does later support pollination or if adjacent ovules are pollenated, the mycelium spreads to adjacent, newly-formed kernels. Some ears may have infection limited to a few kernels, but others may have mycelial growth covering the whole ear.
The smut fungus often grows within the infected kernel, replacing it with a gall of mycelium and spores. These spores (teliospores) overwinter in the soil, germinate the following summer to produce basidiospores that infect plants of the next season. Infection of leaf tissue can result in gall formation on leaves or stems, providing inoculum for infection of ears.
Infection of a few ears in a corn field by any of these fungi is not unusual as each plant’s environment is slightly different. Genetic susceptibility evaluation is difficult because it involves tendencies for silking and pollen production during stressful conditions. This is especially confusing in small plots with multiple hybrids. A hybrid that has silk emergence timing different than most other entries may have more vulnerability to fungal infection.
Ear rots are result of host and pathogen biology interacting with environment variability. Timing is everything concerning corn ear rots.
Harvest of corn for grain depends upon the stalk strength to maintain upright plants. Environment and genetics were major factors influencing the ability of the plants to be upright at harvest time.
Photosynthesis supplied the carbohydrates deposited as the strong lignin, cellulose and hemicellulose components of the outer rind cells. These molecules not only provide physical strength but are difficult for digestion by invading microbes. Factors influencing photosynthesis during the season affect the supply of carbs for formation of these complex carbohydrates. Plant density affects the light intensity per plant. Leaf disease can detract from carb production. Minerals influence affect photosynthesis and construction of these complex carbohydrate molecules in the outer rind of the corn stalk.
Net production and distribution of carbohydrates in the corn plant influences the stalk strength not only by effecting the deposition in the outer rind but also by maintaining the connection of the pith tissue to the rind. Monocotyledon plants like corn differ from dicotyledons in the distribution of the vascular bundles. Xylem cells, with their solid outer walls of lignin are organized in the center of the dicot plant such as the wood of a tree. Xylem cells in monocots are in separate vascular bundles scattered among weak-walled parenchyma cells in the center of the stem. Parenchyma cells in a corn stalk are living cells that become temporary sugar storage cells. Pith contributes to the stalk strength only when it is physically connected to the rind. If the plant wilts before completion of grain fill, dehydration of these cells results in withdrawal from the rind tissue, not only reducing a rod to a tube, but reducing the strength contribution of the vascular tissue.
Genetics effects photosynthesis, formation of complex molecules in the rind tissue and maintenance of pith tissue’s connection to the rind. Environments influence photosynthesis. Harvest time allows us to observe how these factors were in action for this season.
2018 harvest in US Corn belt is showing some lodging problems. Analysis of cause(s) should begin with distinguishing between what is called root lodging and that named as stalk lodging.
Root lodging generally occurs anytime during the growing season when the lateral roots of the plant are insufficient to avoid the plant leaning when faced with winds. This vulnerability can be associated by feeding on roots by insects, soil texture or moisture inhibiting growth of the lateral roots or simply inherited tendencies for deeper ‘taproot’ that may be a favorable character for reaching water during drought conditions but a disadvantage if water soaking in upper soil softens the soil. If root lodging occurred before flowering, geotropism will cause new growth to result in curved plants. These plants and those that root lodged later in the season usually maintain green, lower unbroken stalks. Pre-flowering lodging may cause incomplete pollination and therefore reduced grain yield and later lodging may complicate harvest.
Stalk lodging has different causes and symptoms than root lodging. Symptoms of stalk lodging begins about 15 days before kernel abscission layer (black layer) forms. Causes involve the supply and translocation of carbohydrates to the roots, stalk and grain. Nearly always, root tissue death causes the plant to wilt because insufficient water is absorbed from the soil to meet the water loss from transpiration in leaves. Wilt causes cell death in all tissue including the stalk. Dessication of the stalk pith cells causes withdrawal from the outer rind, reducing the stalk strength. Various fungi invade and further weaken the rind tissue. Late season wind now can cause the stalk to break at the lower internodes. Early wilt results in stoppage of translocation of sugars to the grain and therefore light grain density. Broken stalks maybe difficult to harvest.
Inspection of the stalks before harvest can discern the difference between root lodging and stalk lodging. Lower internodes of root lodged plants will be green whereas stalk lodged plants will have brown outer rind color in the lower internodes. Genetics involved in these two types of lodging are different. Root lodging involves genetics affecting root type of growth whereas stalk lodging genetics involve factors affecting photosynthesis and sugar translocation factors including multiple plant biology interactions. A deep-rooted hybrid may be favored in drought seasons because it reached water, favoring silk extension and therefor more kernels than the hybrid with more lateral roots towards the soil surface. The latter may suffer with fewer kernels because of silk extension missing pollen. On the other hand, if the late season is unfavorable for photosynthesis, the deep-rooted hybrid could be vulnerable to stalk lodging.
Competition between roots and grain for products of photosynthesis can result in root death followed by premature wilting and death of the corn plant. This is followed by collapse of stalk tissue, followed by invasion by stalk rotting fungi. Increasing grain yield while maintaining stalk quality obviously requires increasing photosynthesis either per plant and /or per acre. We have seen progress in this during the past 30 years by increasing the leaf area per acre, reducing the grain per plant but increasing the grain per land area. Selection for better stalk quality along with grain yield under higher plant density also could be selecting the genetics for improved photosynthetic efficiency.
Some genetic input to leaf structure that affect capturing more light could be effective. Chromosomal genetics and chloroplast genetics affect the number of chloroplasts and their productivity. Mitochondrial genetics influences the efficiency of converting the carbohydrates into useful energy need to drive cellular physiology as well as building of the plant structures. Some beneficial characters such as leaf disease resistance are visible, but much is not obvious. A recent paper (Plant Physiology:10.1104/pp.18.00176) identifies mutants affecting the opening and closing the stomata, which affects the loss of water when open but also allows the CO2 movement into the leaf for carbohydrate synthesis. There must be multiples of other mutants to critical aspects of increasing net photosynthesis per acre that are available but unknown.
We currently mostly select for these mutants by yield testing for improved grain yield and stalk quality. We don’t know the specifics of what genetics are responsible, but we have witnessed that this improvement has occurred and expect that it will continue even if we do not which mutations in corn’s 30000-40000 genes were mostly responsible.
Agronomic practices are also contributing to improved yield and stalk quality affecting total plant physiology. Weather also contributes as it affects the plants directly, nullifying some genetic advantages and favoring others. Fortunately, the breadth of genetic variability available in this species allows general improvement in grain production.
Humans benefited from the diversity of genetics in corn. Completion of a generation within months, easy transportation of the result of reproduction, cross pollination allowed quick selection of genetics fitting the corn growing environments. Corn breeding techniques continue to be developed to further exploit this diversity.
Other organisms affecting cultivation of corn also have genetic diversity. Selection pressure to survive and reproduce encourages favorable genes in pathogens of corn. As the Ht1 gene was widely used for resistance to Helminthosporium turcicum (Exserohilum turcicum), a gene within this fungal species population increased in the USA within about 10 years. The corn gene severely suppressed the reproduction of the more prevalent previous race, allowing quick increase in the new race. Small genetic changes in several pathogens has resulted in appearance of new corn disease problems. Goss’s wilt bacterium may have become a corn pathogen after a gene change that allowed it to move from a minor grass pathogen to causing a major disease on a few corn genotypes. Race t of Helminthosporium maydis(Bipolaris maydis) was a mutant that produced a toxin causing disfunction of the mitochondria of corn that was utilized by seed corn producers to facilitate seed production. Multiple races of Helminthosporium carbonum(Bipolaris zeicola) have evolved adaptation to specific corn inbreds while being maintained on other grasses. Diversity in potential pathogens will continue to challenge corn and diversity in corn will continue to allow selection of genotypes to resist the new race.
Hybrid corn does benefit from the genetic diversity between the two parents. Selection for grain yield when each parent covers the other’s genes that have a negative influence on plant performance often includes resistance factors to a specific race of the pathogen. Often parent inbreds are more susceptible than the hybrids. Pathogens often have the advantage of rapid reproduction, but corn has the advantage of active human’s ability to select for resistance within the diverse genetics available.
Advantages of genetic diversity applies to all species- and we should not forget that it also applies to our species. Cultural diversity among humans is also beneficial.
About Corn Journal
The purpose of this blog is to share perspectives of the biology of corn, its seed and diseases in a mix of technical and not so technical terms with all who are interested in this major crop. With more technical references to any of the topics easily available on the web with a search of key words, the blog will rarely cite references but will attempt to be accurate. Comments are welcome but will be screened before publishing. Comments and questions directed to the author by emails are encouraged.