Seedling disease dynamics
Upon planting seed is exposed to an environment with many factors affecting the success of germinating and successfully producing a growing plant. When corn seed, with its own biological potential problems, is placed in soil populated by multiple micro-organisms ready to feed on organic matter, multiple battles begin. As the seed imbibes water, internal membranes swell, causing some leakage of stored sugars and proteins into the soil. Many soil organisms grow towards the damaged seedlings, attracted by the leaked substances. First tissue exposed to the potential invaders are the primary root and mesocotyl, two tissues that are later disposed when the secondary roots take over for the main root functions. Invaders are detected chemically by the corn cells near the invasion, turning on local production of resistance antibiotics. Success in suppression of the invaders is affected by aggression and intensity of the potential pathogens, environmental effect on pathogens and seedling and the biological vigor of the seedling.
Several fungal species are associated with seedling root rots. Fusarium, Penicillium, Pythium and Rhizoctonia species are among the most prominent fungi found in diseased corn seedlings. Fusarium species are common in nearly all soils with any organic matter. Every corn seedling is exposed to this fungus and yet only a few show symptoms. The actual cause of the seedling disease is usually more complex than simply identifying the fungus present in the corn tissue. Analysis of the cause should include consideration of environmental factors such as water concentration and temperature, and seed germination quality. Vigorous seedling growth can quickly dispose of primary root and mesocotyl dependency as secondary roots develop and new leaf tissue produces the quantity of resistance factors needed to control potential invaders such as these fungi.
Seed treatments can give the seedling some temporary relief from fungal invasion pressures. Genetic variability within the pathogen population can weaken its affect in some circumstances. Vigorous early growth due to corn hybrid genetics and seed quality favor escape from seedling disease pathogens even when environments are not optimum for early corn growth. Corn genetics and seed viability tests are strong deterrents to seedling diseases. Favorable environments also contribute.
Uniform seedling emergence
45 years ago, when pursuing the question of why one plant died early with stalk rot and the adjacent plant did not, I hypothesized that the dead plant was that one emerged late as a seedling. When most of the plants in the test plots showed their 5th leaf, a tag was put near those that had only 3 leaves and another marking those with only a spike. Notes were taken of these plants and their adjacent plant during the season. At pollination, it was clear that even those tagged at three leaves were not silking in time with adjacent plants and tended to have more slender stalks. Many of those tagged as spiking no longer were present, but those that survived were far off in pollination timing, had very small, narrow stalks and, eventually, small tassels. Ears were harvested at end of season and kernel numbers were counted. Those tagged with three leaves had only 20% of kernels of adjacent plants and those tagged as spike only were barren. Delayed emerging plants did not develop stalk rot but clearly the delay affected yield.
To eliminate the possibility that these delayed plants were not ‘selfed’ inbreds instead of hybrid plants, an experiment was performed the next season to confirm that emergence delay was the main factor. Seed was planted with twice the normal plant-to-plant spacing. When those seedlings spiked, the same hybrid seed was planted between the seedlings. This would be an unusual delay but the effect was the same as the first observation. Barren plants, skinny stalks, small tassels were characteristic of the delayed emergence. Apparently plant competition for late emerging plants has a drastic affect.
Others have done similar experiments, before and after these, done by a young guy beginning to learn about corn. My conclusion was that individual plants developing stalk rot were not the late emerging ones and that uniform emergence was an important factor in corn yields. Also, it was interesting that those late emergers could be confused with selfs, as confirmed by the fellow who normally evaluated hybrid purity in winter growouts. That eventually led to developing a different purity test method (Seedling Growout® test).
My experiments were done in the 70’s with hybrids and plant densities common at that time. It would be useful if similar experiments were done with more recent hybrids selected for consistent ear development at the higher plant densities used today. It is notable that experiments done by others have shown that among germination test methods, the cold test is the best predictor of field emergence and that it accounts for about 70% of differences among seed lots in field emergence.
Corn plants in the field compete with each other for light, nutrition and water. Although genetically identical, this small disadvantage of the late emerging plant can detract from the final yield in the field.
Corn seed aging
I am amazed when I see our grandchildren move with seemingly little effort. Did I ever move that way? Sadly, we do age and and it is not only in our minds but in all cells of our body. The speed in which seed and us age is influenced by our genetics, and environments after our 'birth'.
The mechanisms between us and seed may be similar in that mitochondria are probably involved in all deteriorating living cells. These organelles which can number a few hundred in a cell, are the main sites for transformation of stored carbohydrates into useable energy for other cell functions. Mitochondria have their own DNA and are composed largely of membranes. Dehydrated seed results in mitochondria functioning at a very low level resulting in being unable to repair deteriorating membrane structures. While at very low kernel moisture levels (6-14%?) and cool temperatures (less than 50°F (?) further damage is limited. Precise moisture percentage and temperature for best storage of maize seed probably varies for genotype and seed condition but the general concept remains.
Seed imbibition of water is a physical phenomenon with little inhibition from the pericarp or seed coat. Seed treatments added to seed can slow down the imbibition, apparently giving the renewed mitochondrial function more time to repair damaged membranes. On the other hand, only increasing kernel moisture slightly can cause more membrane damage to occur, but not repaired. Increasing moisture more while at low temperatures (50°F) has the same effect. Corn seed planted in cold soils will imbibe water, but the low temperature inhibits normal cell function, including repairing mitochondria. Those individual seeds with the most mitochondrial damage are likely the ones that struggle to germinate when the soil temperatures do heat up.
Seed producers are aware of the significance of inadvertently adding a small amount of moisture, such as from a seed treater before bagging by designing their process to limit the water and allowing for drying after application. I recall a case in the Thailand in which a new fungicide seed treatment was applied to control downy mildew, but the humid environment did not allow the seed to dry after application. Seed germination quality quickly deteriorated as a result. Accelerated aging test of corn seed is based upon placing seed in an environment of 100% humidity and 113°F for 3 days, then planting in germination test to record the reduced germination. It is intended to predict the viability of the seed after storage. It is notable that even under this condition, all seed within the sample are not equally affected. Some germinated normally, some eventually and some not at all. This is typical of normal, well treated aging seed lots. Each individual seed is in a slightly different condition. We expect maximum performance when emergence is uniform.
Energy for germination of a corn seed not only comes from stored carbohydrates but also from surrounding environment. Heat energy is a major contributor. Although imbibition is not inhibited by cool temperatures, heat energy allows the orderly repair of the membranes damaged during the dehydration of drying seed and then the hydration after planting.
Very soon after the corn seed is planted, imbibition begins. The H2O activates the membrane-bound mitochondria to respire, providing energy for protein production. The enzymatic proteins include those that digest the starch stored in the endosperm into more sugar molecules to be transported through the scutellum to other cells in the embryo, resulting in more energy available to produce structures for cell elongation. Heat energy provides a regulatory function affecting the speed of this germination process. Imbibition occurs at any temperature but metabolic activity in corn is generally thought to be very low if seed environment is below 50°F. Speed of germination increases as the temperature increases.
Membrane integrity within the seed also affects the net speed of this process. Those individual seed with more damage are slower to sufficiently activate the system and thus slower to activate the metabolism needed for cell elongation in root (radicle) and the shoot sections of the embryo. Cool environments, delaying membrane repair, may result in death of the imbibed seed before the shoot can emerge from the soil. Some of these seed, even after warmed manage only to extend the root through the outer wall for the kernel, the shoot never emerging. Other weakened seed may finally get enough momentum to push through the soil surface but days after the healthier seed have emerged, resulting in a season-long competitive disadvantage. Heat energy during germination affects the severity of the effect of membrane damaged seed.
Microbes in the soil are generally warded off by products of seed metabolism in healthy seed. Those individual seeds that are slow to generate sufficient energy for growth are also more easily attacked by microbes, further slowing the germination process. Seed treatments are useful in giving the damaged seed more time to successfully germinate. Healthy seeds can successfully produce normal seedlings despite surrounding common soil microbes but those weaker individuals need the extra protection.
Corn seed planting time is approaching in the northern hemisphere. The processes of successful producing and storage of the seed will soon come to fruition. The selection of seed parents, care in the seed field of the last season and then the drying, shelling and storage all have an affect on reaching the goal of getting a uniform emergence in this next stage of growing corn.
Cells in a developing embryo are sufficiently mature to germinate in only 15 days after pollination if separated from the endosperm. Early germination (vipipary) is inhibited by the presence of the plant hormone, abscisic acid, in the endosperm which negates the growth stimulant hormone, gibberellic acid, in the embryo. This allows normal seed desiccation as water is replaced with starch deposits and eventual seed germination inhibition because of low water content. Seed producers know that each genotype differs in the percent moisture to be used to harvest for optimum seed quality. Generally, for most corn dent hybrids, that moisture is higher (+/-36%) than normal black layer moisture of 32%. From there the moisture level must be rapidly decreased to less than 13 % to prevent excessive aging caused by damage to the cell membranes. It is the art and science of the seed producer to bring the moisture down rapidly without using excess heat which also could damage the membranes.
Intactness of the pericarp affects the speed of imbibition of water in the soil. Allowing the membranes surrounding each cell as well as the cell internal membranes to swell back to normal size with minimum injury is essential to their functions. Cell membranes being the main structural component of embryo cells needed for energy conversion from starch in the mitochondria and translation of DNA to proteins, have a major effect on the germination quality of seeds. Genetics of mitochondrial membranes, at least partly affected by the mitochondrial DNA, are probably one of the major reasons that seed quality varies between hybrid parents. Mitochondria of the seed are replications of those in the female parent. This is the major reason why choice of which hybrid parent becomes the seed parent.
Another major factor affecting on seed quality is the growing environment in the seed production field. Stress, from drought or disease during the seed maturation period shows later in seed germination quality. Such seed often becomes evident in germination tests done 4-5 months after seed harvest, sometimes in contrast to tests done only a few months earlier. It is as if these seed had some membrane pre-harvest injury that when added to normal aging during these 4-5 months surpassed the minimum for normal germination. These individual seeds either fail to germinate or are slower to emerge than other seed in the same seedlot.
Diversity is good
Not only is our species dependent upon diversity for survival as our environment, including pathogens, changes. We are depended upon diversity among our crops as well. Corn being naturally cross-pollinated and diploid provides this opportunity for the diversity that comes from naturally occurring mutations. It is basic to providing the eventual variability that has driven and continually drives evolution. It allowed the deviants in Teosinte that was selected by people in Mexico 10000 years ago and the multiple selections in corn as it was moved worldwide since then. Most research has verified that most of these genetic mutations result in recessive genes and thus the presence of the mutation is not often expressed in a diploid present in which the dominant member of the paired gene is expressed. The su genes resulting in sweet corn is only expressed when the recessive gene is expressed in both members of the diploid plant. Same is true of the mutants wx for waxy. This is true for multiple other homozygous recessive traits.
Occurrence of mutations can be an advantage or a disadvantage. In most cases, being recessive, the mutation may not be detected by performance of the hybrid. Selfing to achieve homozygosity during the inbred development reflects the negative affect of making some recessive genes more homozygous. This is reflected in reduction of plant size from the heterozygous parent used for inbred development. The selection process with each generation does allow elimination of some negative homozygous recessives. Double haploid systems do not allow generational selection because the homozygous condition is fixed.
Expression of hybrid vigor when an inbred is crossed with another specific inbred is mostly due to dominant versions of the negative recessive genes of the inbred parents. That is probably why prospective commercial hybrids are from crosses of inbreds with distinct ‘families’, each not likely to share the same negative recessive versions of important genes. Corn has 40000 genes, including some negative recessives, perhaps due to mutations. The seed industry uses hybrid testing, and inbred development to select for hybrid performance. Further selection among those near-inbreds can allow for selection against the few negative traits found among some plants to improve inbred performance in hybrid production. That has been consistent with our experience in our proprietary Rapid Inbreeding® program. Diversity is good!
Mutations that count
Opportunities for mutations occurs with every cell division but those that really count are those that occur during meiosis, in which all cells dividing from the fertilized egg cell will lnclude the new DNA arrangement for that segment of the chromosome.
Corn advanced from that first mutation in Teosinte, allowing and exposed kernel to be easily used as grain. More mutations that occurred with each annual reproduction was utilized by people over the past 9-10000 years. Mutations occur naturally in all organisms. For example, each new human baby, on average, has 100-150 new mutations different from either parent. Majority of the human mutations are unnoticed and insignificant but a few can be drastic. But human generation reproduction is once every 20 years whereas an annual plant such as corn produces new mutants with each seed generation. Although the mutation rate per gene may be low in comparison with some organisms, having 32000 genes allows for a probability of some mutations to occur with each generation. Not all of these mutations will be expressed because they will usually occur in one strand of the paired DNA strands, allowing the other dominant version of the gene to affect the trait associated with the gene.
Mutation causes are associated with errors that can happen during meiosis and recombination of gametes during reproduction. Point mutations occur when a different nucleic acid is substituted during DNA replication. This small change can code for a different amino acid when placed in the eventual protein produced from the DNA-RNA-protein process. This can lead to a difference in some biochemical process that the original protein, acting as an enzyme, would affect. It may affect drastic and very visible differences in the plant but in most cases, it is insignificant and not noticed by most observers. Point mutations are probably the most common cause of mutations but a few other more drastic causes can be related to major errors in DNA duplication as part of meiosis. It is common, however for some breakage in one of the pair allowing a segment to be exchanged with a portion of the member of that pair. This process, called a crossover, is utilized by corn breeders in backcrossing procedures in which the objective is to cross a specific gene, such as a BT gene, into a desirable inbred without disturbing most of the genetics of the original inbred. Backcrossing in a gene, long used as a breeding procedure before use of GMOs, has been relatively successful in recovering the essential genetics of the original inbred but now with the desired gene such as the Ht gene for resistance to the fungus causing northern leaf blight or wx (waxy corn gene).
Corn growth is done by enlargement of cells, often with the assistance of water pressure before the cell walls have solidified and by cell duplication. Cell division is is a remarkable process that first requires the duplication of the chromosomes within the nucleus by a process called mitosis. This delicate process should result in exact duplication of each chromosome pair, followed by new membranes forming around each set of 10 chromosomes. This process is called mitosis is nicely described and illustrated in https://www.nature.com/scitable/topicpage/mitosis-and-cell-division-205.
This process is not mutation proof. During the chromosome duplication process a segment of a DNA string may not reattach properly, resulting in an important expression of a trait to be changed or missing in the resulting cells. Because this new cell and other cells duplicated from this mutated cell will likely contain the same mutation it may become evident to our eyes like a long non- pigmented streak in a corn leaf. Most such mutations are hidden from our eyes and have minor affect on the plant performance.
Mitosis in a corn plant occurs mostly in the root and shoot tips from the embryo stage completion of shoot and tassel formation. Another remarkable process occurring mostly out of site in the corn field.
Articles about the recent outbreak of the Covid19 often includes mention that this simple virus has its 15 genes tied to it’s mRNA. I am pretty sure that RNA was not mentioned (or I wasn’t paying attention) in my college genetics course in 1960. I recall teaching my secondary class in biology in 1963- and 1964 in Sarawak that the genes in the nucleus of the cell controlled the cell biology, but I could not explain how. I became fascinated to learn of DNA and its interaction with mRNA and protein manufacturing in cells. Lots has been learned since the 60’s and there is a lot more to come. Certainly, most of the younger readers of this blog have been educated on this but, as a review for some of the ‘oldies’ I will try to summarize.
Corn genes, like in all plants and animals, are organized in the nucleus in chromosomes. Corn has 10 pairs and humans have 23 pairs. Each chromosome of the diploid part has one strand of DNA from the male parent and one from the female parent. Corn has 30,000 to 40,000 genes spread across the 10 chromosomes. DNA code are a string of nucleic bases (adenine, cytosine quinine and thymine) attached to a sugar (deoxyribose). The sequence of these four becomes a significant factor in the ultimate expression the gene.
When called upon to transcribe the gene, a molecule called RNA (ribonucleic acid) is constructed by transcribing the DNA of the gene, using its codes for the sequence of nucleic bases. The gene has a start and stop code to stop the transcription. This newly formed RNA molecule, now called mRNA (messenger RNA) migrates through the nuclear membrane into the cytoplasm of the cell.
mRNA enters the cell ribosome, a small organelle of the cell. The mRNA now become translation RNA (tRNA). Each set of 3 consecutive nucleic baes signal for a specific amino acid to be attached within the ribosome creating a long string which becomes a protein. The specific arrangement of the aminos acids dictate the potential action of the protein as an enzyme in cell activity. This enzymatic activity becomes most important determinant of the cells, and therefore the organism’s, behavior.
Many viruses, such as the one causing Covid-19, have relatively few genes, no DNA, that is coded to invade human cells, codes for the ribosome to produce the protein that allows formation of the virus spike to penetrate the cell. The host cell thus produces more of the virus.
The response of the corn plant to its environment is ultimately tied to those small DNA codes in every living cell of the corn plant.
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.