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.
When a poor stand is apparent as the seedlings emerge, we search for a cause. Seed treatments include control of Pythium. After eliminating soil compaction, freezing and insect damage, seedlings can be submitted to a pathology lab for microscopic examination. It is probably that the fungus Fusarium will be found. But that is not a simple answer to the problem.
The fungal species of the genus Fusarium have a complicated relationship with germinating corn seedlings. The most studied species, Fusarium verticilloides (formerly known as Fusarium moniiforme and its sexual stage as Gibberella fujikuroi commonly is found in germinating corn seeds. It often is found in corn plants without symptoms of damage and therefore is characterized as an endophyte because it appears to live within the plant tissue but does not always cause symptoms. It is not uncommon to see growth of this fungus from germinating seed in paper germination test. A study published in 1997 (Plant Dis. 81:723-728) compared seedling growth from seed artificially infected with this fungal species with those that were not infected. There was no difference in germination percentage between infected vs uninfected seed. There was a slight size difference favoring the uninfected seedlings at 7 days after planting but at 28 days those growing from the infected seedlings were slightly bigger and with more lignin in cell walls. Is this because of a hormone (gibberellin?) produced by the fungus or because of some defense compound produced by the plant? The fungus was easily recovered from the seedlings but less from the older leaves. Infected plants showed no symptoms of disease.
It is clear that Fusarium verticilloides can be damaging to germinating seed sometimes but I don’t think all the factors are clearly understood. Is the difference caused by the strain of the fungus, the host plant or the environment? I know from experience that it is so common to find Fusarium growing from a dead leaf sample that one tends to ignore it. It seems to live in much of corn plant’s tissue. It often leads to confusion with diagnosis of problems including stalk rot, almost as if one cannot find other fungi usually associated with rotting stalks, there is always Fusarium.
At about the V3 stage of development, the primary root function begins to be replaced by the nodal, secondary roots. Energy provided by photosynthesis in young leaves, and heat, drive the production of the metabolites for cell division and cell elongation in these young root tissues. Whereas auxin hormone causes increased cell elongation in stem and leaf cells, auxin reduce this activity in the root cells. Consequently, although the nodal roots initially emerge horizontally from the stem nodes beneath the soil surface, gravity causes more auxin to accumulate on the lower root epidermal cells. This results in longer epidermal cells on the upper side than on the lower side, effectively turning the root growth downwards.
Root tip meristem cells rapidly divide, producing the root cap cells below to protect the dividing cells as it pushes through the soil and functioning root cells above the dividing cells. Outer layer root cells composing the epidermis are thin-walled and porous to water via osmosis. A short distance from the meristem of the root tip, epidermal cells form protrusions (root hairs), effectively expanding the surface area exposed to water and minerals of the soil.
Cells in the core of the new root differentiate to form vascular tissue that connects to the stem vascular tissue through the nodes. This vascular tissue allows transport of water and minerals upwards through the xylem and carbs downwards through the phloem. A few cells in this vascular portion of the young root maintain cell division capability, becoming stimulated by another group of hormones (cytokinins) to increase cells laterally, pushing through the epidermal cell layer becoming lateral roots with their own root meristems.
As with all aspects of corn growth a combination of genetics and environment influences the growth of the root system. Total volume of roots and depth of root growth tendencies will vary among genotypes. The fibrous nature of corn roots not only increase absorption from the soil but also provide support for the stalk as it elongates.
Most corn seed planted in the USA has a seed treatment that includes at least one fungicide intended to reduce damage from Pythium. But effectiveness is complicated by differences among Pythium species and environments. A publication in Plant Disease (http://dx.doi.org/10.1094/PDIS-04-15-0487-RE) indicated that 3 of the 4 species isolated from Iowa fields in 2014 were favored by lower temperatures (55-62°F) but one species favored a little warmer temperature of 73°F. Although the fungicides were generally effective there were situations in which the pathogen still effectively infected the seed or seedling root.
It has to be complicated down there. There are the dynamics of the biology of a germinating seed, with some less vigorous than others, soils with varying water holding capacities and organic matter, and competing microorganisms. The latter generally produce chemicals to ward off others as well. Cell contents are leaked into the environment surrounding the seed as the seed swells and begins germination, attracting not only the zoospores of Pythium species but also numerous fungi. The plant responds to invaders by producing phenols that can stop or slow down further invasion. The fact that the germinating seed environment has many complicated interactions makes any attempt to give exact characterizations is difficult and contradictions to conclusions are often seen.
With favorable temperatures, moisture and oxygen levels, we know high quality corn seed generally overcome the potential problems with fast root and shoot growth. We also know that every seed can be slightly different in cell membrane status because of factors that includes genetics, maturity, drying, handling, and storage conditions. Field conditions vary in soil type, temperatures and moisture levels. Pathogen intensity and seed treatment effectiveness may vary with all of the above conditions. It is a wonder that we actually usually get 90+% stands in the fields. It is to the credit to everyone from the corn breeder, seed producer, seed quality workers, public and private researchers and the grower that this happens. (Corn Journal Blog 3/7/2016)
Corn is often planted in temperate zones as early as possible to take full advantage of the warmth of spring and summer. The spring weather is not always predictable, and temperatures affect the growth rate and metabolism of the seedlings. A battle begins between potential pathogens and corn plants.
The field environment of corn germination includes many organisms. One group active in early spring are the Oomycetes. These organisms were once classified as fungi but now their distinctiveness has most specialists agreeing that they are more closely related to brown algae. Fungi have chitin cell walls whereas Oomycetes have cellulose walls. Oomycetes have swimming spores, zoospores, whereas this is not a feature of most true fungi. This is the feature that makes Oomycetes genera such as Pythium so significant to corn seedling survival.
Pythium species reproduce with swimming sperm cells fertilizing egg cells, while in infected live or dead plant tissue. These then form a thick-walled oogonium that persists during stress, including winter temperatures. When in water, and spring temperatures in the 50’s, sporangia growing from the oogonia release the swimming zoospores. Attracted to sugars released by primary roots and the mesocotyl of corn seedlings. In some cases, the oogonia produce filaments (hyphae) that infect the roots also. Infection of these tissues can cause the seedlings to die, cutting off water to the emerging leaves. If the seedlings survive this early infection of the primary root and mesocotyl, secondary roots emerging from the crown area bypass the infection and outgrow the damage. Low temperature and oxygen deficiency because of water-soaked heavy soil contribute to the seedling vulnerability to damage. Seeds with previous membrane damage resulting in slow early seedling growth are often the most vulnerable, perhaps because they are slower to produce the more resistant secondary roots.
There is evidence that the same Pythium species infecting corn also infect soybeans and several grasses as well. Pythium species do exist in a competitive environment with other microorganisms capable of inhibiting Pythium success. Apparently low oxygen, cool environment of water-soaked heavy soils favor the Pythium species. Seed treatments on corn (and soybeans) are often aimed at not allowing the seedling infection. Races of Pythium are known to overcome some of the treatments. It is unfortunate that genetic variability works for all organisms!
Primary roots supply water to the mesocotyl and energy from the endosperm via the scutellum stimulates the elongation of shoot cells in the embryo. Outer layer of embryo leaves is a modified one called the coleoptile, essentially enclosing the other leaves in the emerging shoot.
Elongation of cells in the mesocotyl pushes the corn seedling coleoptile towards the soil surface. Cells in the coleoptile are also elongating as it grows upward but cell function changes drastically when light strikes the emerging coleoptile. Meanwhile the immature leaves encased by the coleoptile are also slowly enlarging. Almost immediately after the coleoptile is exposed to light, hormones are produced that essentially shut down the mesocotyl growth. Other plant hormones, auxins, are produced in the shoot tips and transported to the node at the bottom of the coleoptile, stimulating the growth from root primordial cells to produce the secondary root system. Movement of this auxin in the opposite direction of the flow of water from the soil requires energy, as it must go from individual cell to cell. That energy is now being supplied by photosynthesis occurring initially in the emerged coleoptile and then by the new leaves that pushed out of the coleoptile enclosure. Previous to emergence, energy for growth was supplied by the seed endosperm and influenced by heat. Water supplied by osmotic pressure in the primary root tissue allowed for cell elongation in the mesocotyl. Now with exposure to light, a new source of energy moves the seedling to new phases of development above and below the soil surface. Mesocotyl significance to the seedling reduces as the above soil structures take over the physiology of the young corn plant. This transition does allow the plant to be vulnerable to negative temperature, moisture and pathogen affects but if everything goes right, the mesocotyl remains intact until the secondary roots function as the main supplier of water and nutrients for above ground growth of the seedling.
The corn embryo is alive but dormant until water is imbibed. The water causes membranes in the cells to activate. Mitochondria absorb the surrounding carbohydrates and those coming from the endosperm via the scutellum. RNA, some of which is newly transported from being coded by the DNA in the nucleus is translated in the proteins needed for processing the sugars into the chemical energy ATP. This energy, and that provided by heat, is utilized for cell division and elongation in the root and shoot meristems.
Root tip cells are surrounded by a special layer of cells (coleorhiza) that act as a protective covering when the root tissue, also called the radical, pushes through the pericarp of the seed. Root tips include special cells with organelles (statoliths) that are heavier than other parts of the cell. Consequently, they accumulate on downside of the outer layer cells of root tissue. These cells lead to production of hormone-like chemicals (auxins) that inhibit root cell elongation on the lower side of the emerging root. With greater cell length on the upper side, the root grows downwards, regardless of the orientation of the seed when planted.
This initial root is called the primary root. It is usually unbranched and relatively short lived as secondary roots grow from the lower nodes of the stem portion of the embryo. Between the two major parts of the corn embryo between these two is the mesocotyl.
The shoot portion of the embryo already has several nodes, each with undeveloped leaves. Energy and water stimulate cell growth and division in the meristem causing the shoot to push through the pericarp usually after the primary root has emerged and begins absorbing soil moisture and minerals from the soil to be transported to the shoot.
Shoot tips cells also produce similar organelles also affected by gravity. They also produce auxins, but these hormones have the opposite affect on shoot cell elongation. Those cells on the gravity side with more auxin become longer than those on the upper side. Consequently, the shoot grows upwards.
Affect of gravity on plant growth direction is called geotropism. After shoots emerge, phototropism becomes dominant, causing the plant to grow towards light because cells on the shaded side produce more auxin and consequently longer cells.
When all works as planned healthy seedlings begins the season.
Corn, like other grasses, is a monocot- the seed has a single cotyledon as part of the embryo. Unlike the dicot species, such as soybeans, in which the two cotyledons emerge and photosynthesize, the corn cotyledon remains underground. The shape of this thin structure led to the name scutellum, which is Latin for ‘small shield’. It attaches to the rest of the embryo by a small channel with vascular tissue and is positioned between the shoot-root portions and the endosperm. The scutellum has its own enzymes that are activated with imbibition to digest the starch and oils stored in the scutellum as well as assist in the movement of sugars from the endosperm.
The scutellum is a storage location although much smaller than the endosperm. Efforts to select for high oil corn, carried on for many years at the University of Illinois, resulted in larger scutellum for more storage of oils, and smaller endosperm. Other parts of the embryo also were larger in high oil corn seeds as compared to the original ‘normal’ varieties before selections. Having a carbohydrate storage capacity and having a vascular connection with the shoot and root parts of the embryo makes it a target for infection by pathogens. This connection has been utilized by scientists to transmit trait DNA via infection by the symbiotic bacterium Agrobacterium tumefaciens carrying the DNA into the scutellum when excised from the embryo. Activity in the scutellum is turned on with imbibition as germination begins. (Corn Journal Blog 4/5/2017)
The corn ‘seed’ appears as a single entity, but its parts have distinct origins and functions. The outer layer, the pericarp, is completely derived from the female parent and does not include genetics from the male parent. Immediately inside the pericarp is a product of the union of both parent. Aleurone cells are biologically active and include anthocyanin and carotenoid pigments affecting the color of the corn kernel. Pericarp and aleurone cell layers surround the the embryo and and endosperm of the corn kernel.
Most of the grain’s carbohydrate is stored in the endosperm. The embryo includes tissue adjacent to the endosperm called the scutellum that is rich in mitochondria and therefore ready to produce the energy needed to make enzymes such as amylases that break down the starch into it’s glucose components that will be moved to the other embryo cells.
Mitochondria show only slight activity in the dry seeds. Many apparently are only partially formed but a little respiration is occurring. However, once exposed to water, and the seed imbibes, the cells and its components, including mitochondria, swell. Partially formed mitochondria are not only activated but gain the more membranes needed to get the glucose transformed into the chemical energy needed for germination.
Studies have shown that temperatures affect this transition. Not surprising to anyone experienced with growing corn, the mitochondrial activity is higher at 77°F than at 57°F. Some of that activity is responsible for the membrane reproduction and repair not only in mitochondria but other membranes in the cells of scutellum and other embryo cells.
Another site of activity in cells of the embryo are the ribosomes, also inherited from the female parent. Ribosomes are the site of protein manufacturing. RNA molecules, as coded by the nucleus DNA, migrate through the nuclear membrane to ribosomes in the cell. Chemically energy from the mitochondria provide the power for import and combination of amino acids in the ribosomes for production of proteins to become the enzymes and structure of cell replication and growth in the germinating seed.
A lot of things are going on in that seed after it begins germination.
Corn grain production is dependent on maximum percent of plants for its field environment and plant to plant growth uniformity. Seed quality is a major factor in determining these characteristics.
Crop agriculture is dominated by multiple environmental and biological factors. Everyone participating with corn seed attempts to define and control these interactions. Breeding procedures can mostly assure that a hybrid is genetically uniform, production methods are intended to maintain this purity and testing methods can evaluate for level of genetic purity.
Seed viability is affected by environments during seed production in the field and after harvest. Each individual seed within a seed lot has a distinct experience with this process, ultimately affecting the ability to germinate with viable shoot and root tissue. Individual seed may have some damage to membranes within cells that require metabolic repair before being able to elongate the root (radicle) part and shoot part of the seed embryo after imbibition. This may be affected by genetics, often of the female seed plant and perhaps of the mitochondria in the female parent.
Germination tests to identify seed viability, usually defined as a seeds ability to produce a shoot and root when placed in a controlled environment can be done with reasonable repeatability. Results are determined after a specific time with specific definition of a root and shoot. Defining and characterizing differences among the seed’s vigor, or the time it takes for that individual seed to produce a root and shoot is more difficult. The seed analyst may see differences in vigor among germinating seed but communicating these differences becomes a major problem.
How to characterize a seed lot that has a high percentage of seed that meet the definition of viability but does not germinate uniformly in test conditions? Generally, those seed lots with delayed germination in warm conditions have lower germination percentages when tested under cold conditions (50°F) but there are exceptions to that as well.
The ultimate goal is to reduce the possibility that seed viability and vigor affect hybrid performance in the grower’s fields where environments present their own variables. It is understood that late emerging seedlings, regardless of cause, have difficulty in competing with adjacent corn plants. They often remain less vigorous because competitors reduce light on leaves and outcompete the late-emerging plant’s roots for minerals and water. Often late emerging plants produce ear shoots later than most adjacent plants resulting in poorly pollinated ears. Genetics of hybrids probably differ in ability for late emerging plants to remain nearly fully pollinated and thus the detriments of lack of uniformity is not exactly the same for all hybrids.
Everyone in corn agriculture wants maximum performance from the seed. We attempt to remove known variables by measuring viability and vigor and by preparing planting conditions. There remain uncontrollable environments and difficulties in defining and communicating seed vigor. Late emerging corn plants detract from maximum yield potential of a hybrid, but how late is the emergence and how much is the yield reduction? Like most of life’s experiences, we wish for clear definition but often the variables make that difficult. Our germination tests at PSR report uniformity of emergence with a 1-5 rating as the seed germinates in the uniform artificial soil, heat and moisture environment of our greenhouse (www.psrcorn.com/seed-testing.html).
I recall a phrase used by the plant physiology professor in a lecture that he used to make a point. While traveling through mountains, he was arguing with his wife as to whether they were going up hill or down, confused by surrounding terrain. He claimed he got out of the car and poured water on the road to see which way the water flowed. Then, he made the point to the students “water runs downhill!”.
Moisture from soil moves into the drier tissue of the planted dry corn seed. This imbibition causes the membranes of cells and their contents to expand, sometimes damaging the membranes. Cellular membranes have the ability to self-repair, but this process occurs more quickly with heat energy. Fast movement into the seed under cold soil conditions can cause significant damage to cell function ultimately resulting in slow or no germination.
Movement of water into the seed is slowed by the outer layer of cells in the pericarp. Breakage of this layer, results in more rapid uptake of water by the seed, potentially inhibiting seed germination. Seed treatments include a polymer coating to slow the uptake speed of water into the seed, allowing for membrane repair even when planted in cooler conditions.
Small breaks to the outer layer of cells in corn seed is practically unavoidable during the movement of harvested seed, despite extreme care of the seed producer. Ultimate uniform and nearly complete germination of the seed is enhanced by polymers applied to the seed outer layer. The warm germination test distinguishes seed sufficiently injured that cannot recover but the cold test identifies seed that cannot recover under the usual cooler soil conditions common for temperate zone corn planting. Pericarp cells, derived from the female plant, have some influence affecting the speed of water imbibition. Coating of the pericarp with a polymer can slow the imbibition process and resulting in less membrane damage.
Imbibition does not require living cells but simply the physical act of water moving down hill.
Storage of seed requires very slow metabolic activity, enough to keep membranes, but not enough to cause premature death of the embryo. Low seed moisture is mostly responsible for maintaining life in this semi-dormant condition. After planting, however, we want the seed to imbibe enough water to stimulate more activity. This process of imbibition was addressed in Corn Journal blog of 4/4/2017.
As the corn kernel is developing after pollination, embryo shoot and root cells are formed in a way that could quickly germinate with the moisture present in the tissue. A temporary dormancy prevents this from happening. Removal of the milky endosperm from the embryo as early as 10 days after pollination will allow the growth of root and shoots. Abscisic acid (ABA) in the endosperm appears to be the hormone involved in avoiding germination in seed before the full development. At least 10 single gene mutants are known to overcome dormancy in developing maize seeds, resulting in germination while on the ear (vipipary). Drying of the kernels initially by displacement with starch formation and then by air inactivates the dormancy.
Imbibition is the movement of water into the seed. It is a physical phenomenon, independent of the germination quality of the seed and of temperature. Movement of water into the seed is relatively fast, most occurring within a few hours. There is some evidence that slowing the imbibition by some seed treatments reduces the harm to membranes by giving them more time to repair. Rehydration of cellular tissue and chemicals cause swelling. Membranes, shrunken by drying, strengthen and activate but some solutes escape before damage from the drying is repaired. As the mitochondria are activated, ribosomes begin producing proteins needed for more cell growth, starches from endosperm are digested to form glucose molecules to be transported to the mitochondria in embryo cells. Oxygen uptake into the seed increases rapidly during imbibition - an indication of the active respiration occurring in cells. Heat, moisture and oxygen availability influence the speed in which imbibition initiates the germination of the corn seed.
Humans, without knowing cellular physiology, selected for these traits for Zea mays. And we get the benefit.
Dry corn seed are alive and breathing. Respiration in the seed, like in the rest of us, occurs in the mitochondria of the cell. The process of breaking down sugar provides the energy for creation of enzymes needed to maintain membrane structures in the seed that will be needed when germination begins.
Membrane deterioration increases as the temperature and moisture increase. These interactions of storage temperature and moistures have been shown to have a drastic affect on eventual germination percent of stored seed. Even size of root and shoot length of the seedlings is reduced if the seed was stored under conditions of higher humidity and temperatures.
Membranes not only surround individual cells but also the main sites of activities in cell organelles. Enzyme activity along these are the sites of protein production in ribosomes, transfer of proteins and other products are often done via cellular membranes. Much of this activity is guided by DNA within the cell nucleus, its integrity and activity affected by the nuclear membrane. Because natural breakdown of membranes increases with temperature and moisture, the need for higher respiration rate increases when seed is stored under poor environments. Seed stored at 9% moisture content and 10°C (50°F) retained nearly 100% germination for 4 years whereas the same lot of seed stored at 15% moisture content and uncontrolled temperatures as high as 38°C (100°F) had o% germinations. Reducing the seed moisture percent to 11% even under the warm conditions increased the percent germination to 90% (Plant Physiol. (1967) 42, 1071-1076). Many seed studies and experiences since then have verified the principle that drying seed and storing under low temperatures are essential to maintaining eventual high percentage germination of corn seed.
The pericarp surrounds the whole corn kernel, affecting the insect and pathogen invasion of the kernel and water penetration in the kernel. It may be 2-20 cells thick and is, genetically, female tissue. It is also without pigments.
Surrounding the starchy endosperm portion of the corn kernel, but within the pericarp is a single (usually) layer of cells known as the aleurone layer. These cells are part of the seed, the result of fusion of one nucleus from the pollen grain with two nuclei from the egg cell in the ovule. Whereas the cells in the rest of the endosperm function mostly to synthesize and accumulate starch, aleurone cells maintain more metabolic activity. Although only a single layer of cells, it can include 30% of the total proteins of the endosperm.
Anthocyanin production occurs within the aleurone cells, resulting in red and purple or blue corn kernels. Genes for lack of anthocyanin in the aleurone, allows the yellow color of starch endosperm cells carotenoid production to show in the common yellow corn kernels. Corn genetics for lack of carotenoid production in starchy endosperm, along with genes for no anthocyanin in aleurone, results in white corn kernels.
Aleurone metabolic activity contributes to much of the seed activity. Phytosterols infuse into the pericarp, contributing to insect and pathogen resistance. Although 80% of the oil in corn kernels is located in the embryo, 12% of the oil located in that thin layer of aleurone cells. Fibers from the aleurone cells and pericarp are processed together as corn bran, the aleurone contributing the oil to the bran animal feed.
The scutellum and aleurone cells are stimulated to produce amylase when moisture and temperatures are appropriate for germination. This enzyme assists in breaking down the starch of the endosperm, and thus making energy available for the growth of the embryo.
This layer of cells is an important component of both the use of corn grain and the growth of the next generation.
This thin outer layer of the kernel, originating from the female plant becomes an important contributor to ability to fend off pathogens and insects searching for the carbohydrates stored in the endosperm of the enclosed seed.
Maize Weevil (Sitophilus zeamais), showed that the cross-linked structural components of the pericarp cell walls were highly correlated with resistance to this insect. Other factors included phenols (Afr. Crop Sci. J. 9:431–440) produced by the pericarp cell metabolism and even endosperm hardness (flintiness) contributed to reduced susceptibility to this storage insect (Crop Sci. 44:1546–1552 (2004)).
Pericarp tissue also is a barrier to entrance into the seed by multiple kernel rotting fungi. Most enter the ovary through the silk channel immediately before pollination. This becomes most evident when silks are left exposed for several days in an environment favoring the pathogen. After invasion, the fungus can spread cell-to-cell within the pericarp through small holes (pits) in the cell walls that allows movement of metabolites between cells. Integrity of the pericarp is a significant factor in avoiding invasion by many potential fungal species.
The phenomenon known as silk-cut can expose the seed to fungal infection. After the pollen tube grows down the silk channel and dumps the pollen nuclei into the ovule, silk tissue deteriorates and detaches from the ovary. Not all silks are pollenated even under ideal conditions, leaving some attached to their ovary while adjacent pollenated ovaries grow. These remaining silks interfere with normal contiguous growth of the pericarp cells in the adjacent ovary wall (Plant Disease 81 (5):439-444). This can result in a break in pericarp as the kernels enlarge and thus an opening for invasion by fungi. Genetics and environments influence the occurrence of silk-cut. Stresses that delay silking beyond pollen availability can be an important factor but genotypes vary in vulnerability both to reaction to the stress and probably the tendency of this phenomenon.
The pericarp, being completely dependent of the female plant genetics, adds to the significance of the hybrid seed producer’s decision of which parent inbred to use as the female. Both parents contribute to the hybrid ‘vigor’ but the female parent determines the pericarp characteristics.
The fact is a corn kernel is a fruit, composed of a part of the female plant, the thin outer wall, and the remaining from fertilization of the female egg by the male sperm nuclei. The cells in the resulting embryo include organelles such as mitochondria that are duplications of those from the female plant as the male’s contribution is only half of the resulting DNA in the seed. Mitochondria genetics, as inherited from the mother plant, may have subtle affects on the hybrid plant but could be major in seed aging and germination.
The powerhouse of almost all living cells in all plants and animals is a very small, bacteria-like organelle called a mitochondrion. It is similar to bacteria in its size, shape of chromosome, organization of its DNA and function. Mitochondria presence in all from the smallest of single cell animals and plants to the largest has led to the hypothesis that it originated as symbiotic relationship with a bacterium 3 billion years ago. The clear advantage of having this organelle that could transform carbohydrates into chemical forms of energy that allowed production of proteins for growth and movement of muscles in animals is overwhelming.
Mitochondria are the size of bacteria and therefore visible only with a strong light microscope magnifying at 1000X but the details require electron microscope power at 30000-50000X. With this extreme level of magnification, mitochondria are shown to be composed of a surrounding double layer of membranes enclosing many folds of membranes. Membranes are significant to function in that these are the sites in which the enzymatic action allowing the energy holding the glucose molecule together is released and combined with nitrogen and phosphorus into another chemical compound, Adenosine triphosphate (ATP). This compound released through the membranes into the rest of the cell for normal cell metabolism. It is also the site in which CO2 is released during respiration.
The fact that mitochondria have their own DNA has had dramatic affects on corn. Individual cells may contain from a few to hundreds of mitochondria and they replicate on their own, independent of nuclear chromosomes. However, when sexual reproduction occurs in plants or animals, and the nucleus from the male donor fuses with the nucleus of the female egg cell, no mitochondria are passed along. Consequently, the mitochondria in the progeny are only those from the female parent. Although the size and genetics of the nuclear DNA is overwhelmingly greater than that of the mitochondrial DNA, and the most profound genetics remains with the nucleus, mitochondria inheritance has had some dramatic affects on plants and animals, including us humans.
Amazing how we are so dependent upon such small things like mitochondria!
The corn kernel is a fruit with one giant seed. We humans mostly bred and selected this grain for its use as a food source, increasing the endosperm size with carbohydrates. Selecting for desirable seed traits has been at least somewhat secondary to the grain production. On the other hand, uniform and reliable field emergence is a major contributor to corn grain production in modern corn hybrids. This is dependent on the science and experience of seed producers.
Much of the propensity for high germination is dependent upon the female seed parent. Pericarp, being totally part of the female plant, affects rate of moisture loss during drying, vulnerability to physical damage from handling of the seed and susceptibility to ear molds. Mitochondria genetics are totally inherited from the female parent. Much of the damage from rapid, cold imbibition of water at the initial stage of germination involves the mitochondrial membranes.
Maize kernels handled as grain need to be stored at 15% moisture to avoid mold. Modern hybrids are usually allowed to dry in the field well beyond the 30-32% moisture level that black layer forms and completion of movement of carbohydrates to the kernel. It is usually most economic to allow drying in the field before finishing with artificially drying.
Most maize seed begin losing germination capacity if left in the field during those final days of slow drying in the field. Studies have shown that greater germination percentages are retained if seed is harvested in the 35-40% moisture level and then is quickly dried with lots of air and less than 100°F. Retaining the higher moisture level for some time initiates metabolism in the embryo, essentially an artificial aging process. Seed dried to about 12% moisture is considered optimum of storage and retention of high germination rates.
Producing and retaining high germination is the result of research of each hybrid parent’s vulnerability as well as experience with weather and facility. Drought damage during grain fill, rain delaying harvest, drying at too high temperature and not enough air, rough handling during processing and adding too much water during seed treatment can contribute to below standard germination. One can write a manual for production of seed corn but ultimately it takes some experience to apply the science. Like most agriculture.
Leaf epidermal cells walls and the waxy leaf surface provide the first line of defense against microbes. Pathogens adapted to overcoming this defense set off the next defense system after penetrating the leaf. This is initiated by the plant detecting the presence of the intruder. Plant cells nearby detect the presence of a protein exuded by the pathogen. Such proteins are called effectors, as they are detected chemically by host cells near the invader. Upon detection, these adjacent host cells produce potential microbe-inhibiting compounds such as reactive oxygen, nitric oxide, specific enzymes, salicylic acid and other hormones to effectively thwart the pathogen growth. Much initial reaction is limited to host cells adjacent to the infection site.
Resistance to corn leaf pathogens such as Exserohilum turcicum, cause of northern leaf blight, Cercospora zeae-maydis (gray leaf spot) and Bipolaris maydis (southern corn leaf blight)
Involve detection of that specific pathogen and production of more general antimicrobial products in the immediate area of the pathogen. These two steps are inherited independently. Perhaps the pathogen detection system is more specific to the pathogen, accounting for a corn variety being more resistant to one pathogen than another. On the other hand, I am suspicious that if two pathogens arrive in the same area of the plant, only one will survive, as if the plant reacts to the first one by producing general resistance compound that inhibit the infection by the second one to arrive in the same area.
The system described above is referred to as general or horizontal resistance. It is controlled by 3-5 genes for products to detect and reduce spread of the pathogen. Horizontal resistance is expressed in corn plants by fewer leaf disease lesions. Evaluation of varieties for this type of lesion has some ambiguity however, because the number of lesions or amount of leaf damage is also affected by the intensity of disease pressure. Heavily diseased leaves from the previous season in fields of low tillage, with frequent early season rain can result in more leaf lesions in a variety of good general resistance to a pathogen than will occur in one of poor resistance with little disease pressure.
Characterization of horizontal resistance level to a pathogen requires a rating scale that has some consideration of disease pressure and relativity to other varieties. It is best done when each variety is exposed to the same pathogen intensity at the same stage of leaf maturity. Differences expressed as lesion numbers, size of lesions and percent of leaf destruction can be used to indicate the level of general resistance to that pathogen. I prefer to make ratings based upon several plants exposed to the pathogen in what I project to be somewhat heavy disease pressure in most USA corn environments. With artificial exposure to the pathogen by placing spores in the plant whorl, each plant receives more-or-less the same pressure. Expression of resistance will show 1-2 weeks later. Those varieties with abundance of larger lesions are deemed more susceptible than those with fewer and often smaller lesions. Consequently, it is assumed that will simulate the reactions in fields with somewhat heavy pressure from that pathogen.
Any evaluation of horizontal resistance includes consideration of disease pressure and relativity to other varieties. From Corn Journal 7/11/2017.
Virus genetics are very simple. They penetrate the genetically complex host cells, utilize the hosts metabolism to duplicate themselves and move on to another host cell. COVID 19 virus has 15 genes in its RNA. Humans have about 20000 genes in its nuclear chromosomes plus independent genes in some cell organelles such as mitochondria. Corn has about 40000 genes in its chromosomal DNA plus genes in chloroplasts and mitochondria. The human selected genes that allowed the development of modern corn from its Teosinte origin only involves 2-4 percent of the total genes in present corn varieties.
One marvels at the complexity of the interactions that are occurring within each cell of a corn plant as it not only absorbs light energy, translates it into metabolic energy for sustain growth and more metabolism. Meanwhile the corn plant is fending off potential invaders of insects and pathogens. Mutations in genetics of those invaders can overcome the simple detection method of the host that triggers the corn plant to produce metabolites to stop the pathogen. Human selection of more stable resistant corn has resulted in resistance inherited by several genes. Usually, 3-5 genes are involved in limiting a pathogen success in a corn variety. Occasionally a single gene in corn is effective but often only for a short time. The Ht1 gene for stopping Helminthosporium (Bipolaris) turcicum was useful in USA in late 1960s for about 15 years but eventually the mutants in the fungus produced metabolites escaping the Ht1 gene’s products, making use of the gene no longer effective.
Adaptability of corn to multiple environments is due to the large genetic resource among those 40000 genes. Corn being an annual plant, separation of male and female flowers and abundant genes for selection has allow humans to desirable traits. Mutations and new mixes of genes from different backgrounds has allowed these selections to continue the increase in grain production by this plant. Research in the nature of the corn genetics continues as molecular methods discern more about corn genes. One article summarizing current status of corn genes can be found at https://www.cell.com/plant-communications/pdf/S2590-3462(19)30010-0.pdf.
At the same time that the complexity stimulates the research interest for some to explore with their molecular research, the simple pollination of the corn species, and more complex testing for desirable hybrids by those making selections for current environments has allowed participation of a large number of humans in improving this crop.
The genetic code of all living things exists as a long string of 4 nucleotides adenine, thymine, guanine and cytosine. We abbreviate as A, T, G and C. Each nucleotide is composed of a phosphate, a sugar and a nitrogen base. They slight differences in their composition that affects their chemical behavior. RNA and DNA differ by the sugar, ribose for RNA and deoxyribose for DNA. A gene is composed of a chain of these nucleotides interpreted as sets of three after the start sequence is established. Each set eventually gets translated to produce an amino acid when moved to the cellular ribosome where the amino acids are linked to form proteins. These proteins often become the enzymes needed to carry out the metabolism of the organism. Enzymatic function is often affected by the sequence of the amino acids within the proteins. Exact duplication of the DNA is required for each nucleotide sequence to result exact duplication of the protein and expected function in some metabolic process.
Some of the ‘errors’ made in the RNA or DNA result in meaningless mutations and some allow the natural and human-driven selection of variability for choice corn varieties. Of course, the diversity mechanism is active in all things with RNA and DNA, resulting in changes in some pathogens of corn as well. The opportunity for change gives reasons to appreciate beneficial
variability as well as to be alert for those from which we do not benefit.
Corn Journal blog of 7/27/2017 addressed one of those dramatic events affecting corn.
All living cells of plants and animals have mitochondria, organelles that convert carbohydrates into the useful form of energy that drives synthesis of metabolites in cells. Mitochondria are believed to be descendants of bacteria that became symbiotic with cells in the early evolution of most living forms. They retained their own DNA, are transferred to the next generation only in eggs cell and not sperm. They replicate within cells but the host cells have some control on the rate of replication. Energy conversion in mitochondria occurs on their folded membranes in a series of chemical reactions. Regions of the plant undergoing rapid cell duplication have more mitochondria. This includes the tassel cells of a corn plant. The pollen mother cells in that region undergo meiosis and duplication, driven partly by the energy conversion by concentration of mitochondria in those mother cells.
A small defect in mitochondrial DNA of an inbred caused a defective membrane product in those mitochondria resulting in incomplete development of pollen. This was found in a corn breeding program in Texas. As the inheritance of this condition was known to be only transmitted independent of nuclear DNA, it was called Texas male sterile cytoplasm. It became a useful tool to corn hybrid seed production because it was easily transferred in breeding programs to the female parent of a hybrid, and thus avoiding manual removal of tassels in seed production fields. Use of T male-sterile cytoplasm became common in the worldwide corn in the 1960’s.
It was noted in the Philippines in 1961, that a fungal pathogen, then known as Helminthosporium maydis, was especially aggressive on several hybrids with T cytoplasm. Despite a few scattered reports elsewhere it was not until 1969 that the connection between increased occurrence of this disease and T cytoplasm became alarming. Majority of seed produced for 1970 corn season had T cytoplasm, the main exceptions being new hybrids in which the conversion to sterility of the female parents was incomplete.
Although the pathogen was normally found in the southern half of the corn belt, and adequately controlled by products of nuclear DNA genes, this disease was found highly destructive in northern corn belt areas as well. A race of the fungus (now named Bipolaris maydis and by its sexual stage Cochliobolus heterostrophus) called race T, produces a toxin that causes death to cells with mitochondria having the DNA with the defect associated with T male sterility. All cells of the corn plant with these defective mitochondria were vulnerable to the fungus. This included the cells in developing seed resulting in diseased stored grain as well as overwintering leaves and stalks. Normal resistance mechanisms to the pathogen were ineffective because the toxin destroyed these defective mitochondria.
As the relationship with T cytoplasm was realized, seed companies worked to change, and within a few years, the disease subsided back to its normal distribution. It was a new learning experience of interaction of corn and pathogen biology.
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.