Imbibition of water into dry maize seed occurs within a few hours after exposure, initiating cellular activity, initially in the mitochondria. Examination of mitochondria structure using electron microscopy show poor developed membrane structures in the dry seed. Mitochondria in dry seed have very low amount of oxygen uptake and low activation of enzymes. These characters change with imbibition.
Following 24 hours of imbibition, the mitochondrial membranes show more normal structures. Among the mitochondria, however, there are some that appear to not recover normal structures and function. It is easy to conjecture that these not recovering were either inadequately formed or were damaged during drying process. This may be related to either the nuclear genetics or mitochondrial genetics of the seed parent. Enzymes needed for membrane synthesis in the mitochondria is synthesized in ribosomes and thus dependent upon nuclear DNA and thus indicative of the significance of both hybrid parent genetics, but the mitochondrial DNA is only from the female parent.
These numerous organelles in each cell activated after imbibition become drivers of cell elongation to push the first root tissue to emerge from the seed. More info on this initial activity can be found at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC64868/
Imbibition of water into the seed leads to activation of the cytoplasm within cells. Most of those processes occur along membranous components of mitochondria, ribosomes, plastids and the endoplasmic reticulum. Hydrated proteins now acting as enzymes in break down starch molecules stored in the endosperm and glucose and sucrose molecules are moved thru the scutellum to embryo cells. Diffusion of these sugars through pores of these cells, with cooperation of the cellular membrane and endoplasmic reticulum, these complex molecules composed of carbon, hydrogen and oxygen atoms are transported to mitochondria where they are further metabolized to create the ATP energy needed for other cellular activity. This cellular respiration process allows further cell construction as cells divide in the root and shoot meristems. Elongation of hypocotyl cells, as well as meristem cell division pushes the tissues from the kernel.
Heat energy helps in driving these metabolic activities, including repairing damage to membranes occurring during seed production and storage, as well as those enhanced by hydration of membranes after planting.
ATP (adenosine triphosphate) results from the energy transfer from electrons holding the glucose atoms together to form ATP, releasing CO2 and H2O. This process occurs in the mitochondria. These membrane-intense organelles apparently vary in number and efficiency among corn varieties. Mitochondria, having their own DNA, and yet is dependent upon the rest of the cell for its structural components, are transmitted to the next generation only through the egg cell. This is probably why different female parents of corn hybrids vary in time for seedling emergence and vulnerability to imbibition chilling damage.
It is amazing to see a near-perfect emergence of corn seedlings in a field, given all that had to happen at the cellular level.
Corn seed is vulnerable to damage from soon after pollination in seed field until planting in the field. Each cell in the embryo has membranous tissue that could be damaged from insufficient moisture during embryo formation. Fungal pathogens can infect the seed as it develops. Delaying harvest, perhaps because of weather problems, can result in initiating an aging process of cytoplasm of cells. Too slow a drying process, perhaps because of inadequate dry air movement within the seed facilities can also contribute to cell aging. Excess heat during that drying also affect the membranes within the cells. Rough handling of the seed can result in breakage of the pericarp, allowing faster imbibition of water when in the field. Genetics of the female parent affects vulnerability to each of these factors.
These damages rarely affect all the seed within a lot. There is a tendency for the damage to be greatest at both ends of the ear, with the flat sizes generally having the least damage. It is not clear of the cause but perhaps the embryos in rounds have less physical damage protection. It appears that the damage is not evenly distributed in all cells of the embryo. Individual plants that show malformation during germination often show major injury in shoot meristem, resulting in radical growth but no stem. Injury to cells in the hypocotyl area is believed to be the cause of shoot finally emerging but twisted and clearly behind in growth compared with adjacent seedlings.
Imbibition, in which water allow the dehydrated membranes to swell and activate will allow some damage to repair. Adequate heat is important to generate the energy generated by undamaged cytoplasm to promote repair. Generally, temperatures below 50°F (10°C), inhibit membrane damage in the embryo. Imbibition chilling can result in lowering emergence in the field.
Corn breeders select for multiple performance characteristics, including tolerance of potentially damaging seed production stresses. Combining all the favorable characteristics is never perfect. Seed production methods aim to reduce stresses but nature does not always cooperate.
There are more membranes beyond those of endoplasmic reticulum in all living corn cells. The nuclear membrane surrounds the nucleus. Like other membranes function, it not only contains the contents but also regulates the movement of materials to and from the outside. Messenger RNA is coded by the DNA in the nucleus and moves through the membrane to the ribosome with code for next protein. Ribosomes, mitochondria and plastids (including chloroplasts) are largely composed of membranes. A cell membrane surrounds all cytoplasm of the cell between the cell wall and the cytoplasm. It also functions to regulate transport of materials in and out of the cell.
Cellular membranes consist of a few layers of lipids and proteins. They do degenerate (age?) and require maintenance to replace or repair damage to the membranes. Each of the living cells in a corn seed embryo includes multiples of membranes. Drying of corn seed does cause some shrinkage of membranes and apparently causes membrane damage. Sudden swelling of the cells with hydration can further damage the membranes. Self-repair occurs but does require adequate heat for metabolism to supply the materials for repair. There is some evidence that temperatures below 50°F for the first 24-48 hours after hydration results in permanent damage to its function because membrane damage from imbibition is not repaired. If those first hours are warmer, adequate membrane repair occurs to allow normal embryo growth.
Cold germination tests are intended to detect the percent of seed within a seed sample with vulnerability to imbibitional chilling. Some of those damaged may eventually germinate but later than others within the sample. This could cause uneven emergence in the field if soil temperatures suddenly drop immediately after planting.
Most of us interested in corn concentrate on appearance and performance of the whole plant. Some of us look a little more closely at a few features, such as seedlings or kernels. Only a very few corn people look at individual corn cells. Even with a light microscope magnifying at X1000 one can only see a faint image of the cell nucleus and a somewhat granular cytoplasm. Only with magnification by tens of thousands via the electron microscope do the structures within the cytoplasm become clear. Most of the living cell is dominated by long strings of a membranous structure called endoplasmic reticulum. This connects the import organelles of the cells. Multiple ribosomes, the sites in which, guided by RNA, imprinted with codes from the chromosomal DNA, attach amino acids to each other to form proteins.
Endoplasmic reticulum provides the pathway for these proteins to travel to important metabolic sites such as the mitochondria where glucose is processed into chemical energy in form of ATP. That process is dependent on the specific enzymatic activity linked to the arrangement of the amino acids in the protein. Endoplasmic reticulum structures also connect to chloroplasts where specific proteins assist in photosynthesis providing the glucose.
Endoplasmic reticulum also has a unique function in plants, unlike in animals, in that it can allow transport of proteins between cells through small pores in cell walls called plasmodesma. This allows plant cells to communicate despite presence of cell walls, that are absent in animals.
Endoplasmic reticulum is composed of lipids and proteins arranged as membranes, as with other membranes in cells, the precise arrangement of the specific lipids and proteins affects transport and movement across them. As with the membranes within mitochondria and chloroplasts, endoplasmic reticulum integrity is an essential part of the plant’s life. While we are concentrating on more visual characters of the corn plant, the real activity is happening in the cell cytoplasm at a microscopic and sub-microscopic level.
Seed producers can have the genetics for seed quality, harvest at proper seed moisture, use good seed drying processes and still be disappointed with germination test results the following spring. The most prevalent variable is weather during the seed production season. Drought stress after pollination often the primary stress on seed quality, although rain during harvest time can delay harvest, allowing for deterioration in the field.
Seed produced under stressful environments can lead to near normal germinations for a few months after harvest but faster deterioration than normal before planting in the next spring. Standard warm and cold tests done before preparing seed for packaging that is usually adequate to predict the field emergence the next spring may not be correctly identifying seed that is deteriorating this quickly.
Special tests have been devised to predict these types of potential problems but often have some limitations in establishing standards for every genotype. Balancing the demand for early delivery of hybrid seed to growers with need to detect potential late seed quality problems is not easy.
Seed do age but predicting the rate of aging is not easy. Seed production history and conditions are not identical for each individual seed within a seed lot as well. The goal of establishing a uniform plant stand in the corn field is the goal of everyone involved with corn but the realities of multiple environments and biology does produce obstacles to 100% success.
Successful seed production, like most of agriculture, is the result of managing multiple variables with a combination of science, experience and at least a little bit of luck.
Obtaining an expected plant stand in the field has become increasingly significant to final grain production with modern corn hybrids. These hybrids were selected to tolerate high densities partly by producing more, but smaller, ears than those common 20-30 years ago. Highest grain yields are associated with more ears and, thus, more productive plants. Achieving and maintaining high germination quality in corn requires genetics, field techniques, cooperative weather and carefully monitored handling of the seed after harvest.
Genetics of the female parent is a major factor. Kernel pericarp genetics is totally inherited by the female plant. Pericarp vulnerability to cracking in the field, during handling at and after harvest, and from the drying process is largely affected by those genetics. Cytoplasmic genetics for cellular organelles such as mitochondria and ribosomes come from the egg cell of the female plant. Function of these organelles is linked to integrity of their membranes during the stresses of drying, seed imbibition and aging. Deciding which hybrid parent inbred becomes the female in the seed field is an important part of successful seed production.
Production field technique influence seed quality as well. Good timing of pollen supply from male inbred with silking exposure in female plants result in more completely pollinated ears. This includes more seed in the center of ear that tend to have better germination quality. Irrigation timing is important to promote good timing of pollination, as well as maximum silking. Seed of most dent corn genetics begin aging soon after black layer and timing of harvest is critical.
Weather during the growing season can influence pollination by drought stress delaying silking or rain during pollen shed inhibiting dehiscing of anthers resulting in bad pollinations. Rain during silking is often associated with fungal infection of the seed. Drought stress after pollination can be associated with early plant death and poor seed maturation. Weather can also affect meeting the critical harvest timing. I recall witnessing high germinations with a seed field harvested on time but, interrupted by a week of wet weather, the other half of the seed field had very poor germinations.
Dent corn seed must be dried quickly after harvest but without high temperatures to reduced damage to the dehydrating cellular structures in the embryo. Shelling and movement of seed within the seed production facility requires care to minimize damage to corn seed.
The multiple factors involved in obtaining and maintaining corn seed that will result in the expected plant density in the grower’s fields requires experienced management. One of the surprises for people entering the seed corn business is the complexity and significance of seed production on its success. Seed production, like much of agriculture, involves a mix of technology and art.
Genetic diversity in a corn breeders nursery allows for many characters to choose with each season. Each generation of selection, whether nearly instant by dihaploid system, Rapid Inbreeding® system, or traditional selfing has the intent of selecting the plants with a set of genes that will work with another inbred to produce a superior hybrid. A new mix of genes are created with each source used as a starting population. Thirty to 40,000 genes arranged on 10 pairs of chromosomes with a small occurrence of mutations with each generation, affect multiple structural and functional characters of the plant. Much of the DNA codes for synthesis of specific proteins composed of a specific string of amino acids. Arrangement of those specific amino acids affects the enzymatic performance in cellular products, ultimately resulting in the final corn plant structure.
Even with modern methods, the plant breeder mostly needs to depend upon expression of those final characters that can be seen or measured to decide which seed to save. Each method of selfing the plants results in at least a few genes becoming fixed with a DNA arrangement resulting in an undesirable cellular function. Reduction in size of nearly all aspects of corn plants occur as the genetics approach homozygosity because of the accumulation of poorly functional genes. These changes always occur during inbreeding, but the corn breeder must choose the plants with favorable characters also occurring with the new combinations of DNA available from the population.
The ultimate evaluation of inbreeding selections needs to be made by combining the inbred with another inbred that has DNA arrangements compensating for the DNA arrangement deficiency of the other inbred. Hybrid vigor is the genetic expression of the new combination of DNA composition of individual genes in one parent allowing production of a functioning enzyme that was not happening in the other parent.
Despite increasing lab methods for evaluating DNA structure, perhaps the large number of genes, each composed of a string of nucleic acids whose arrangement affects resulting protein structure and function, will not allow much gain in efficiency of making choices in the breeding nursery. Every corn breeding program strives to improve the efficiency of selecting hybrid parents, but the final selection will be determined by hybrid performance in the field.
Probably everyone is at least somewhat driven to try to understand the dynamics of something. It is part of living. Discerning aspects of human behavior is done by everyone but digging deeper into the dynamics involved is attractive to some. Mechanically minded individuals are driven to tear apart a machine to understand how it works. Astrophysicists attempt to understand the dynamics of galaxies within our universe. Biologists are interested in the interactions of factors involved in living things. Most have varying drive to dig deeper within one of these topics but only can afford time to survey the surface of the other topics.
Those of us involved in agriculture certainly fit that description. While each of us have a specialty, the corn grower is managing mechanics, weather, soil structure, biology, human behavior and economics. Most have a deeper interest in one of these aspects of farming but also must have some understanding of each subject to gain success in their occupation.
That person, or those persons, that several thousand years ago discovered the mutant in Teosinte with seed (fruit) that remained attached to the plant instead of scattering to the ground was interested enough to gather those seed for planting the next season. Others driven by curiosity and practical economics carried future mutants from that original source in Central America to its eventual distribution into all continents. Later specialists, partially inspired by their own interests, developed machinery to make the plant more efficient. Those attracted to the diversity within the corn genome emphasized selecting varieties to meet various economic uses.
Genetic diversity in Zea mays has resulted in specialists studying basic aspects of biology, environment, human nutrition, engineering and economics. It also has promoted generalists who attempt to coordinate the knowledge gained by the specialists into efficient farm operation. This species has made multiple contributions to humans beyond the food value.
Last few posts have described only a few of the disease surprises in the 1970s and 1980s. Several others have occurred, some showing up for a year or two and then becoming less noticeable. The trend to new occurrences has continued. Bacterial leaf streak, caused by Xanthomonas vasicola, pv vasculorumwas initially found in Nebraska and then in several other midwestern states in USA in 2016. It was only known to occur in South Africa previously. It also was found in Argentina in 2017 but perhaps was there since 2010.
Physodermabrown spot showed up in the scattered areas of the US corn belt in 2017. Although it was known to occur sporadically in southern USA, it showed up in our small nursery in northern Illinois on a few plants.
Tar spot of corn, caused by an obligate parasite (Phyllochora maydis) and usually accompanied by another fungus, Monographella maydis, showed up in Northern Illinois and Southern Wisconsin in 2015/2016 and with more intensity in 2018. It had been known previously in highland areas of South America. There is more to learn about these pathogens, including how they live through the winter.
Were these pathogens in an area long before being identified? Was it only a matter of time before the disease was noticed? Did they spread by wind or seed? We know that spores of Puccinia sorghi, cause of common rust, spread the disease from South Texas of Mexico by wind annually. Corn kernels can easily carry fungal spores whether used as seed or as grain.
Perhaps these corn pathogens were infecting another grass species but a mutation in the pathogen allowed infection of corn. The Physodermain in my nursery was in an outside row near other grasses. Xanthomonas vasicolahas related variants that infect sugar cane and perhaps other grasses. Tar spot spread to northern Illinois is very hard to explain, except the similarity with the environments of highlands in South America. Or were these present here but insignificant and thus unnoticed until susceptible corn genotypes became widespread. Or perhaps susceptible hybrids, lack of crop rotation and minimal tilling allowed increase of the pathogens. It may be a combination of multiple factors.
The Compendium of Corn Diseases, 4th edition, published in 2016 lists more than 70 corn diseases. Many are minor causes of significant damage, at least currently. Unexpected environmental changes, including those related to climate change or inadvertent mutations in corn breeding may result in changes in significance of a corn disease.
We should expect seeing ‘new’ occurrences of diseases to occur probably everywhere corn is grown. Inspecting and reporting to corn disease specialists observation of unusual symptoms in corn fields each season is important to avoiding significant damage by ‘new’ corn diseases.
Gray leaf spot, caused by the fungus Cercospora zeae-maydisin USA, was identified on corn in 1925, but was notable in the 1970’s. The fungus is favored by humid environments and susceptible hosts. Backgrounds that featured B73 was commonly associated with susceptibility that was intensified if the other parent of a hybrid was also susceptible. A few very successful hybrids in terms of other desirable features like grain yield and stalk quality were driven from the commercial market by this disease as it spread through much of the central corn belt. Emergence, spread and significance of this disease was a surprise to most in the corn industry.
Maize chlorotic mottle virus (MCMV) was first identified as a corn pathogen in Peru in 1974. In 1976 it was associated with severe damage to corn in Kansas and Nebraska when plants were also infected with another virus such as MDMV or wheat streak mosaic virus (WSMV). Since then it has also been identified in corn South America, Asia and Africa. Disease caused by MCMV and either of these other virus is called maize lethal necrosis or corn lethal necrosis. Most commonly used genotypes are susceptible, but resistance can be found with effort. MCMV is transmitted by insects such as beetles and thrips. MDMV is spread by aphids and WSMV by the wheat leaf curl mite. MCMV can remain in beetle larvae overwinter and transmitted to young corn seedlings by rootworm feeding. Most damaging affect on corn happens when the other viruses are also infecting corn at very young development stage.
Head smut of corn, caused by the fungus Sphacelotheca reiliana, has caused damage to corn erratically in North, Central and South America, Australia, China, Europe and South Africa. The fungus teliospores commonly are spread to the soil, where they germinate and infect seedlings. The mycelium grows within the plant towards the floral tissue, ultimately replacing the ovules and pollen with fungal tissue including more teliospores. It is commonly associated with susceptible genotypes, continuous corn, light and dry soils.
We get surprised continually with outbreaks of corn diseases. There is no reason to think that this pattern will change.
Race T of Helminthosporium maydis, cause of the 1969-1971 southern corn leaf blight epidemic got the attention of everyone involve in corn. This was not the first ‘new’ corn disease that seemingly suddenly emerged. Probably occurring in isolated areas of isolated fields these diseases may have been present for some time before gaining enough attention for specialists to make note and study. Several gained notoriety in the 1960’s.
Eyespot, caused by Kabatiella zeae, was first identified in Japan in 1956 and later in northern corn areas of USA, and in Argentina, Brazil, Europe, China, India, and New Zealand. The fungus is only known to infect corn, overwinters on corn debris. It is not known to infect seed but apparently it is associated with distribution via seed at least one case (https://www.cabi.org/isc/datasheet/29297). The disease severity is related to use of susceptible hosts. In the USA, the wide use of inbreds W64A was associated with the damage from this disease in areas with high humidity and cool summers. Leaf lesions are found on other genotypes but spread and damage is limited on more resistant genotypes.
Maize dwarf mosaic disease, caused by several strains of a virus named MDMV, gained it earliest attention in the 1960’s as one of its overwintering hosts, Johnson Grass, gained prominence in south central US corn belt. An aphid vector feeding on this alternate host carried the virus to young corn plants, causing significant damage. The aphids also would spread the virus to sweet corn planted later in northern corn areas. Most dent and sweet corn hybrids were (and are) susceptible. Damage is increased when the plants are infected with another virus as well. Maize Chlorotic Dwarf Virus (MCDV) was identified in the late 1960’s to also infect Johnson Grass. MCDV transmitted by a leafhopper (Graminella nigrifrons) to corn also infected with MDMV in young plants can result in 100% yield loss in susceptible, infected plants. Better control of Johnson Grass, insect control and use of resistant hybrids has led to less damage from this disease.
Goss’s bacterial wilt (caused by Clavibactermichiganensissubspecies nebraskense) was first identified in Nebraska in 1969, it was especially associated with physical damage to corn leaves such as from hail. It was most notable in the early 1970’s on hybrids using susceptible inbreds such as A632. Switching to more resistance hybrids greatly reduced the incidence of the disease. It emerged with minor epidemics outside of Nebraska in recent years again associated with susceptible inbreds.
These fungal, virus and bacterial diseases emerged with significance in the USA during the 60’s. Continuance of this pattern after that period will be the focus of the next blog.
Race t of Helminthosporiummaydis(Bipolaris maydis) (Cochliobolus heterostrophus) spread across most corn growing areas in USA and elsewhere in 1970. The traditional version, race 0, of this pathogen was common in the Southeastern USA where temperatures and humidity favored the biology of the fungus. A related fungus Helminthosporium carbonum(Bipolariszeicola) (Cochliobolus carbonum) was a common pathogen of corn but tended to be more frequent in the northern part of the US corn belt. The summer of 1970 not only featured epidemics of race T of H. maydisbut spread of this pathogen to much of northern corn belt. This allowed the co-mingling of the two species.
The two species were distinguished by microscopic examination of their conidia, the asexually produced spores associated with spread of these fungi. H. maydisspores were consistently curved and appeared to be gray when viewed with a light microscope. H. carbonumconidia were darker in pigment and mostly straight. Both species had shown to have similar sexual reproduction structures and to have distinct sexual mating types.
Seed companies, including the one that I had just joined, were checking their inbreds and hybrids in the summer of 1972 to make sure there was no remnant susceptibility left among their materials. I was surprised to find a wide range of shapes and sizes of lesions naturally occurring among materials that looked like southern corn leaf blight in our central Illinois nursery. Examining the spores under microscope showed a range of spore shapes intermediate to H. maydisand H. carbonum. Other pathologists found the same thing. It had been shown previously that these two species could cross in lab experiments and now it appeared that the wide spread distribution of H. maydisinto regions where H. carbonumwas common allowed multiple opportunities for sexual crosses between the two. This apparently accounted for the range of conidia morphology seen in the summer of 1972.
The resulting population of these multiple crosses further sorted in virulence on corn. H. carbonumalready had been found, with one race (race 1) to produce a toxin affecting a few inbreds homozygous recessive to the toxin. Another group of this pathogen appeared to mildly pathogenic on corn leaves was defined as Race 2. After 1972, some inbreds and hybrids were found to be susceptible to Race 3, resulting in distinctive long, narrow lesions. In 1980, inbreds with B73 backgrounds, commonly used as female parents in seed production fields, were infected with a distinct race 4 of H. carbonum. Apparently, the crosses of the two related fungal species resulted in new genetic combinations.
This was the second experience for this novice corn pathologist of the interactive dynamics of pathogen and corn genetics. These dynamics continue.
Several plant species feature mitochondrial genetic mutations that result in lack of pollen production, allowing hybrid production even in normally self-pollinated species such as sorghum and rice. In each case, production of viable hybrid seed requires a method to produce hybrids that do produce viable pollen. In all cases of cytoplasmic male sterility (CMS), that restoration of pollen viability has come from the chromosomal genetics in which dominant, fertility restorer (Rf) genes are involved.
Corn is known to have three main types of CMS, designated as CMS-T, CMS-C and CMS-S. Each involve different mutations in the mitochondrial DNA but all leading to inability to adequately process the transformation of carbohydrates into usable form of energy (ATP) needed for pollen production. These mutations create the wrong protein to function as an enzyme in this pathway within the mitochondria. These genes are referred to as ORF genes.
Restoration of pollen production appears to be related to the proteins produced in the cells associated with specific Rf genes. These proteins are believed to be close in structure to the missing protein in the CMS mitochondria, allowing the carbohydrate transformation to ATP to continue. Each of the CMS versions ORF gene products are different and thus appropriate restorer genes are different. Restoration of CMS-T requires two Rf genes, designated as Rf1 and Rf2. Restoration of CMS-C is associated with Rf4. CMS-S is restored by the gene Rf3.
Corn breeders cross inbred designated to be female parents by crossing into related inbreds of the desired CMS after the inbred has been shown to not have the appropriate Rf genetics. Some inbreds are not possible to get complete sterility, probably because they include some Rf genetics. If the male of a hybrids does not already restore fertility to a specific CMS, the appropriate Rf gene is crossed into the inbred. CMS-T was desirable because it always gave complete male sterility. CMS-C and CMS-S occasionally, in some environments and some genetics, produce a small amount of pollen and therefore could result in some selfing in seed production fields. Seed producers usually remove tassels to prevent this problem.
Corn, having male flower parts easily removed from the plants designated as the seed parent, can produce hybrids without CMS. It is done mostly for cost and labor efficiently. For mass production of hybrid Rice or Sorghum seed, CMS and Rf genes are essential.
We moved from Tennessee back to the Midwest in August of 1970 with a fresh PhD in Botany with a specialty in Mycology to take a temporary Post Doc position at Illinois State University to study fungi. I recall seeing corn fields turning brown, thinking that is not consistent with my younger Iowa experience. It was quickly realized by many others that the early corn death was associated with a race of a relatively moderate corn fungus, Bipolaris(Helminthosporium) maydis, producing a toxin attacking corn leaves. Susceptible hybrids all had t-cytoplasm.
Most genetics in a corn plant are located on chromosomes in the cell nucleus. A few of the organelles, such as plastids and mitochondria, in cell cytoplasm have their own genetics, consistent with the theory of their ancient derivation from bacteria. Mitochondria are the site in which carbohydrates are processed to provide energy for all metabolic pathways in living cells. Mitochondria duplicate and multiply within cells and are generally only passed onto the next generation of an organism through the female ovule, but not the sperm cells. They accumulate in areas of most cell activity, such as cell division leading to pollen production. The base cells that undergo meiosis and cell multiplication resulting in corn pollen contain 40 times as many mitochondria as other corn cells.
After it was realized that some cytoplasmic genetics would interrupt production of viable pollen, the cytoplasm was crossed into female parents of hybrids to reduce effort and cost of producing hybrid seed. It is believed that, by 1970, 85% of hybrid corn planted in USA had the T-cytoplasm. Use of this sterility system was also used extensively in many other countries.
The mutant mitochondrial gene (T-urf13) encodes for a protein in the mitochondrial membrane, ultimately disrupting normal mitochondrial function, and thus interfering with production of normal pollen. It was discovered after the corn disease outbreak that the same membrane defect allows the fungal toxins produced by B. maydis and a much less significant fungus (Mycosphaerella zeae-maydis), cause of yellow leaf blight, to reduce the natural genetic resistance to these pathogens.
Full realization of the linkage of disaster associated with t-cytoplasm resulted in urgent winter seed production by all corn seed companies in an attempt to avoid repetition of the problems. In the 1971 season, I was asked to assist in monitoring the 1971 progress of the disease by Funks G Hybrid company and thus got involved in corn pathology. I can now claim that a mutation in a corn mitochondrion got me lost in the corn field and that I have yet to find my way out!
There are many studies on race t and t-cytoplasm. One is: https://lib.dr.iastate.edu/cgi/viewcontent.cgi?article=1060&context=bot_pubs
Variety parent seed is identical to the final seed product. Parents of hybrid seed are not identical to the hybrid. Single cross corn hybrids are inbreds selected primarily for favorable performance when combined as a hybrid but each with homozygosity for repeatable genetics. This inbreeding process does result in some genetic expression of negative characters in one parent that are covered up by the other when combined as hybrids. After identifying such a combination of inbreds, selection of which parent will become the source of the seed and which will become the pollinator becomes significant to the commercial production of hybrid seed.
The corn kernel is a fruit. The outer layer, the pericarp, is a structure of the female plant. It does not include any genetics of the inbred chosen to be the pollen source. The bulk of the corn seed within the kernel is the endosperm where storage of starch is made available as energy utilized for germination of the seed. Cells within the endosperm include 2 copies of female plant chromosomes and one of the pollen parent. Only the embryo has equal genetics from both parents of hybrid seed.
Commercially acceptable female parents of hybrids need to have reliable and consistent elongation of silks even when under some moisture stress. Silks need to be receptive to fertilization after pollenated. High number of ovules is favored. Pericarp structure must be inclined to withstand stress with minimal cracking. The most important character of the female parent is consistently high percentage of germination. A major factor linking this to the female seed parent is the genetics of the mitochondria within the embryo cells. Mitochondrial genetics originate only from the ovule. These sources of transforming energy stored as carbohydrates into that needed for cell metabolism are full of membranes that can be damaged by rapid swelling when water infuses into dry seed. Maintenance of the integrity of these membranes become essential to the germination process. Tolerance of natural stresses on emergence of silks, of pathogens and stresses on pericarps and of function of mitochondria are all associated with the female parent of a corn hybrid.
Pollen sources for hybrid seed production do have some responsibility as well. Most critical is reliable and timely production of live pollen grains. Release of pollen grains from the anthers is affected by genetics, as the anther chambers must dehisce as the relative humidity drops. Timing with the presence of receptive silks on the female parent is essential. It is probable that part of the pressure for selecting parents that increase grain yield involves shifting the genetics for energy needed to produce pollen to that of more grain results in less pollen.
Commercial hybrid corn breeding programs identify which hybrid parent is best as the female or male based upon quantity and germination of the seed. These are determined by the genetics affecting the biology within the corn seed.
Breeding corn as a variety, the seed selected from open-pollinated plants displaying the traits preferred for grain production is relatively simple compared to breeding parents for hybrids. The advantages of single cross hybrids coming from uniformity of crossing 2 homozygous inbreds that have genetics resulting in heterosis in traits that provide superior performance for grain production, standability and disease resistance drive the incentive for the more difficult process of hybrid breeding.
Experience and experimentation showed that crosses of inbreds developed from different general backgrounds have a high probability of expressing heterosis. The inbreeding process, however, sorts those 30-40000 genes, in each generation of inbreeding from the beginning population with randomness. Although each inbred from a base population may share some characteristics, and many can be immediately discarded by the breeder visually with each generation of selfing, the ultimate test of acceptability comes after evaluation of performance after crossed with potential heterotic partners. Hybrid corn breeding programs devise methods to solve the conundrum of selecting preferable inbred characters while considering heterotic performance efficiently.
Traditional methods of selfing heterozygous seeds for several generations before crossing with potential hybrid partners to evaluate hybrid performance has the frequent disappointment of considerable effort over several generations because of hybrid performance. Making hybrids from potential, but not completely, homozygous inbreds has been used to more quickly and efficiently select desirable inbreds but carries the risk of genetic drift after further selfing.
Dihaploid corn breeding involves crossing heterozygous plants with pollen from a haploid inducer, resulting in up to 10% of resulting embryos only having the one set of chromosomes from the female plant and none from the male. Treatment with specific chemicals can cause up to 50% of these embryos to double this single set of chromosomes, resulting in completely homozygous two sets of chromosomes, a dihaploid. These dihaploid plants can be selfed to produce a small quantity of distinct inbreds in three generations. These dihaploids are still a random set of genes from the parent stock, the value of which must be identified in hybrid performance.
PSR's Rapid Inbreeding® (RI), a system of shortening the time to developing corn inbreds, utilizes the natural homozygosity that occurs in any genetic segregating population. PSR utilizes its skills of evaluating seedling phenotypes to pick out the individual plants that display characters most identifiable with near homozygosity. These seedlings are transplanted, grown to flowering and selfed. This seed is then crossed with potential heterotic inbreds for hybrid evaluation. These near-inbred plants are sufficiently homozygous to assure that future increases from these plants will show similar hybrid performance to that shown in the initial tests.
Breeding for hybrids is more complex than variety breeding but the final reward can be greater as well.
As it became apparent to some corn breeders in the early 1900’s that consistency and repeatability of genetics in corn required creating homozygous inbreds from populations frequently expressing heterosis. The enigma was that inbreeding greatly reduced the volume of hybrid seed to be planted but the production of grain from those hybrid seed was greater than produced by indigenous varieties. A few academic corn breeders pushed the idea of using the hybrids as parent seed to make double cross hybrids to overcome the seed volume problem. They encouraged several farmer seed producers to adapt this concept in the 1930s. The significance of heterosis resulting from crossing specific inbreds became obvious to many during the 30’s, stimulating investigation into the genetics and botany of corn in academia and entrepreneurship among farmer breeders.
As more farmers switched to using hybrid seed, public and private corn breeders increased inbred breeding programs. New synthetic populations were created by breeders by crossing seed from existing varieties, selecting for heterosis by crossing with opposing inbreds, recycling the best, testing again and repeating the cycle to create new improved populations from which new inbreds could be created. Stiff stalk synthetic population created at Iowa State University became and continues to be a powerful source of new inbreds that commonly used as female parents of hybrids. Populations derived from varieties with origin in Eastern USA and grossly identified as Lancaster often became sources of inbreds expressing heterosis with stiff stalk derived inbreds.
Breeding efforts to select more productive seed parents, improved seed production methods and economics of corn grain led to the introduction of single cross hybrids in the USA in the late 1960s. A similar pattern developed in the multiple environments on other continents as well. Continual selection by humans from the diverse genetics selected by previous human generations has led to continual improvement of grain productivity of this species. Its biological features of separation of male and female flowers, C4 photosynthesis, easily transported seed and 30-40000 genes has served us well.
A ‘Compilation of North American Maize Breeding Germplasm’ published in 1993 by Crop Science Society of America includes a list of more than 500 distinct, open-pollinated dent and flint varieties that existed in North America. Their locations of origin and use illustrate the varied environments corn occupied in one temperate zone continent.
Adaptation of corn to a wide range of environments included a range of length of growing seasons. This included selections by people for varieties that would flower with minimal accumulation of heat, such as the Gaspe Flint variety used by indigenous people on the Gaspe peninsula in north Eastern Quebec at least as early as the year 1524. This variety, growing in relatively cool environment, manages to mature with minimum heat units. If the same variety is grown in Central United States, it matures in only 40-50 days after planting at a height of only 30 inches (76cm). Tropical hybrids respond to short day lengths rather than heat accumulation to trigger flowering and thus cannot complete a generation in Central USA unless artificially given short days.
Diversity encouraged by the world-wide selection of desirable genetics adapted to local conditions not only affected factors affecting time to maturity in corn, but also multiple metabolic and structural aspects of the species. It is probable that size of vascular systems in stems, affecting movement of minerals and photosynthates, root structures and efficiency of absorption of individual minerals, number and reactivity of stomata, number and efficiency of chloroplasts and mitochondria must have been among the multiple inadvertent selections made by those early corn breeders as they selected for adaptation to their environments. This variability is offered to current corn breeders as sources for continual adaptation to human needs.
A Germplasm Enhancement of Maize (GEM) project, a collaborative effort by USDA-ARS and public and private research scientists cross USA adapted maize germplasm with exotic germplasm in attempt to expand adaptation of traits selected under diverse environments. These new combinations are intended to be sources of new gene combinations for future corn hybrids available to all potential corn growing environments. More information about GEM can be found at https://usda-gem.public.iastate.edu/GEM_Project/GEM_Project.htm
Corn spread from its origin in central Mexico to much of North and South America over an 8000-9000 year period. Its annual life cycle, transportability and large endosperm favored its use as a food source. Cross pollination biology of corn favored genetic variability and thus adaptation to multiple environments as it coincided with movement of humans across these two continents. As people settled in different areas, they tended to select desired genetics favored by that location and their culture. Over time, distinct types, or races, of corn developed. These races varied in time to flowering, size of kernels, types of starch stored in the endosperm and pigments in endosperm. Obtaining these features also required genetics for resistance to local corn pathogens and insects, root structures to fit the soil types and moisture levels, leaf shapes to capture maximum sunlight, and hormone systems to stimulate timely flowering for both male and female flower parts.
This pattern spread to other parts of our earth after the European immigrants arrived, not only getting introduced to this new food source but also bringing livestock that would thrive on corn. This species was then spread to other continents where it became adapted to other environments.
The culture of identifying ‘varieties’ of corn that was perceived as best fit for their livestock and food needs was adapted by American farmers a few hundred years ago. These farmers saved the seed from the plants with the most favorable features, especially emphasizing the amount of clean grain from single plants growing in their fields. As result they continued the isolation of hundreds of individual, genetically-diverse populations. These populations experienced some inbred depression, as some recessive genes becoming homozygous would cause a negative effect on yield performance.
Occasional mixing of populations occurred however with unexpected results. The Reid family in Ohio had a poor field emergence with their soft starch variety (Southern Dent) that they had traditionally grown. They filled in the bare spots with an earlier variety of a hard-starched new England Flint variety. Saved seed from that field produced much larger yields than their original seed. This principle of hybrid vigor after crossing varieties with distinct genetics became evident in the late 1800’s, leading to university and USDA geneticists, and eventually others to develop methods to utilize the historical genetic development of this species to expand the use of its diverse history. That pattern continues as science and experience with corn advances.
The unique history of origin and humans’ interactions with corn are presented in many writings available in journals, books and the internet. This interaction is probably closer than that of any other between humans and a plant species. Humans have been dependent upon some plant species; the history of human migration and unique biology of this plant species has led to a distinct relationship.
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.