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.
Visit us at the ASTA in Chicago, Dec 9-12 (booth G207)
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.