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.
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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.