Part of the glucose use in the corn leaf cell is respiration, providing energy for the many physiology processes in the cell. However, much of the glucose becomes integrated into building cell walls. Polysaccharides, such as cellulose, hemicellulose and pectin are major components, several other molecules that include proteins, phenols and acids join in forming the cell walls.
As a newly formed cell develops a thin, flexible layer forms outside of the plasma membrane of the cell. This first layer formed is nearly gelatinous, composed of carbohydrates, magnesium and calcium pectates. Next the cell forms a layer called the primary cell wall. This is composed of carbohydrates with pectin holding it together. The primary cell wall is flexible, allowing cell expansion. As the cell matures, expanding to its maximum, a thicker layer, called the secondary cell wall forms outside the primary wall. This has the effect of limiting the swelling of the cell as water is imbibed into the cell.
The middle lamella allows for gluing cells together, the primary wall allows some expansion of the cell and the secondary cell wall provides a stronger boundary, limiting cell expansion and offering resistance of invasion by potential pathogens and insects.
Cell walls in each part of the corn plant has its own specific arrangement of these cell walls. The Primary cell walls of the leaf epidermis cells produces a waxy outer layer limiting water absorption and further protection against invasion by pathogens. Cells of the xylem, that essentially form tubes for the transport of water from the roots to the upper plant parts have exceptionally thick secondary layers forming ridged tubes with little obstruction for the movement of water and solutes.
Cell walls have small holes that allow movement of molecules between cells. Regulation of movement into the individual cell is dependent on the living functions of the cellular membrane and often this requires a complex interaction with specific proteins within the cell.
There is a lot going on in these smallest structural components of a corn plant that ultimately contribute to the final product of our interest.
Chloroplasts in cells within mesophyll and vascular bundle cells convert the light energy into chemical energy locked up in sugar molecules. These cell walls, although ridged giving strength, have pores (plasmadesmata) through which the proteins and sugars can be moved from the chloroplast laden cells. Most of the sugars move to the phloem portion of the vascular system. Phloem cells are living cells that are surrounded by companion cells that are the first site of sugar-protein molecules entrance into the phloem tissue. The remaining phloem is called a sieve-tube in which they have reduced cytoplasmic contents and sieve tube openings that connect them with sieve-tubes on above and below. This openings allow the transport of sugars in either direction.
As the concentration of sugars in these sieve tube portions of the phloem increases, water from the xylem moves, from the high concentration of water to the lower concentration because of the high concentration of the sugars. This adds pressure for for movement of the solution from that cell. The diffusion principle continues to guide the movement of the sugars within the phloem. This is basically a source-sink model. Pre-flowering, sugar movement to the growing points at the apical meristem and the root tips is caused by tying up by cellular respiration as well as tying up the sugars into more complex molecules such as cellulose for new cell walls. These sites become the ‘sink’ for diffusion of sugars from the ‘source’, the leaves.
This translocation process from leaves to vascular tissue continues as a source-sink phenomenon after flowering, as the growing point in the kernels become new sinks. These new sinks now compete with the root sinks. Genetics affect the strength of the sugar demand of each kernel and the number of kernels affect the total draw of sugars. Strength of the pull of sugars per kernel also involves the efficiency of the kernel removing sugars to more complex molecules such as starch. The movement of the sugars in phloem is directed by the pressure-driven bulk flow principle of moving from high concentration to lower concentration.
Corn vascular system not only transports leaf products to other parts of the plant, and water and minerals from the roots but has specialized cells that contribute to corn’s advantage as in photosynthesis. In most plant species, chloroplasts in the mesophyll cells capture CO2 and using light energy and a series of enzymes to produce for C3 molecules that are fused to make sugar (C6H12O6.). The potential weakness comes when the stomates are closed at night or during moisture stress causing a process called photorespiration in which instead of utilizing CO2, the system starts consuming oxygen. This not only needlessly uses energy it limits the capacity of the chloroplasts to maximize the light energy capture. Most plant species belong to this C3 photosynthesis system.
Some species, including corn, have evolved a system to avoid this wasteful system. Chloroplasts in the corn leaves make normal photosynthesis process but then break down the C3 molecules, have them transfer them to the specialized, vascular bundle cells surrounding the vascular system that are loaded with special chloroplast for C4 molecules. These molecules are then enzymatically combined to make sugar which is moved elsewhere in the leaves and other parts of the plant. This system occurs in species that are native to dry, hot environments such as that of corn’s central America origin. The ultimate advantage is that corn can continue to produce carbohydrates despite environments that cause stomates to close. Whereas most C3 plants such as soybeans, wheat and rice do not utilize light intensity greater than 3000 foot-candles, corn photosynthesis rate keeps increasing with light intensities to our maximum sun brightness of 10000 foot-candles. Those few cells surrounding the xylem and phloem of a corn leaf vein have a special role in allowing the photosynthetic efficiency of maize.
Those folks in Mexico that selections from Teosinte did not know about C4 photosynthesis but we have benefited from 10000 years of selections leading to what we call corn (or maize).
Corn leaves, as well with as other monocots, have multiple veins located parallel to each other within the leaves and ultimately connecting with vascular system of the stalk. They are critical to the distribution of the cell products and ultimately the grain. Structure and function of this system is summarized in Corn Journal blog of 5/17/2016.
Corn leaf veins run parallel to each other to the length of the leaf. There are primary veins easily seen on each side of the leaf midrib and smaller secondary ones as seen in the photo. Basic function is to move water and minerals from the roots to the leaves and distribute photosynthesis products to other parts of the plant. It is complicated, of course. A single row of thin-walled cells surround the vascular bundle. These cells function to regulate and transform products into and from the other parts of the vein. In C4 plants like corn, special chloroplasts in the bundle cells perform the final stages of photosynthesis making sucrose and finally starch. At night the starch is broken down into smaller molecules, allowing it to move into the phloem components of the vascular bundle.
Phloem tissue includes two cell types. Companion cells are the immediate recipients of materials from the bundle sheath cells. Chemical processes change the carbohydrates from the molecules as they arrive into forms that allows continual input. More changes allow the movement of sugars and proteins to be moved to the second cell component of the phloem tissue, the sieve cells. These living cells share cytoplasm with adjacent sieve cells above and below through pores, and thus allow the movement of carbohydrates to sink as dictated by plant hormones. The phloem thus becomes the means of moving carbohydrates from the leaf to meristem for more growth, to roots for growth and metabolism and to the grain. Phloem tissue is living, requiring energy to function. Death of phloem tissue stops translocation of carbohydrates. Phloem tissue also can be an avenue for viruses to spread through the sieve cell pores.
The xylem portion of vascular bundles is composed of dead cells that function as a tube allowing water and minerals to move by capillary action from the roots to leaves and other parts of the plant. Water molecules consumed by photosynthesis or passing through stomata by evaporation (transpiration) are replaced because of the cohesion character of water. Fungal spores and bacteria can be moved through xylem cells as well, although xylem arrangements at the stem nodes can foil movements of larger particles such as these.
A significant cellular component in corn leaves are the chloroplasts. Cells in the mesophyll receive the light and CO2 and thru multiple steps captures the light energy, changing it to a form of chemical energy as it locks carbon, hydrogen and oxygen molecules together. Cells in the bundle sheath surrounding the leaf vascular tissue finish the job, converting this energy into sugars. Blog from 4/26/2016 summarizes the role of these organelles.
Plant cells have plastids, distinguishing them from animal cells. They are believed to have been derived from cyanobacteria, such as single cell blue green alga, when a symbiotic relationship with a single celled organism merged with it, perhaps a billion years ago. Like any symbiosis between organisms, each one benefits, and they often become interdependent. Plastids, like bacteria, have two surrounding membranes and DNA organized in a circular manner as opposed to the chromosomal arrangement in all other organisms with a membrane-bound nucleus. Plastids multiply by division independent of host cell division but are carried along with new cells. Consequently, in corn, they are present in the female egg cell. After pollination, as the fertilized egg cell divides and ultimately forms meristems, each cell includes the plastids. These are called protoplastids because they are not fully developed. Those in the cells reaching the light quickly are transformed into chloroplasts. Although plastids have their own DNA and capability to produce the many enzymes and other components of chlorophyll, as in other cases of symbiosis, they are also dependent upon the host cell to provide some proteins and plant hormones such as cytokinins needed for proper development.
A major structural feature of chloroplasts is formation of multiple layers of membranes (thylakoids) with the chlorophyll molecule and thereby enhancing the capacity for photosynthesis. The plant hormones classified as cytokinins, perhaps produced more by the host cell but some from the chloroplast itself, apparently affect the size and quantity of these layers. Host cell genetics, those inherited from both parents of a corn hybrid, thus influence the chloroplast development and function despite the fact that the protoplastids are carried along in only the female parent egg cells.
All protoplastids do not develop into chloroplasts. Those remaining below soil surface and some others do not become green and become sites for starch storage. Some others accumulate other pigments, contributing to other colors expressed in plants. Some chloroplasts located near the veins in plants develop slightly different carbon-fixing methods that allows corn’s photosynthesis to be among the most efficient of plants to convert light energy into carbohydrates.
Those corn seedlings, soon to be emerging in US fields, will have protoplastids in coleoptile and other new leaves with newly-formed chloroplasts converting of light energy into the chemical energy needed for growth of the plants.
Corn cells are the location of corn plant activity of its growth and its ultimate grain production. The cells include a nucleus with chromosomes with the DNA genetics. Leaf cells contain chloroplast for converting light into chemical energy, including the carbohydrates that eventually end up in grain. Mitochondria, another type of organelle in all living cells convert the carbohydrates into a form of energy (ATP) that can drive the multiple other biological processes for cell and plant growth. These organelles can barely be seen with 1000X power of a light microscope. The rest of the cytoplasm within the cell is even more difficult to distinguish at that magnification but the rest of the cell cytoplasm can only be distinguished with the power of electron microscopy.
The endoplasmic reticulum (ER) is a thin, tubular membrane that has multiple folds as it connects many of the other operating particles within the cell cytoplasm. Part of this membrane has a rough appearance because it is dominated with multiple ribosomes. These organelles are the sight of translating to mRNA code into proteins. The ER assists in movement of the mRNA to the ribosome and the proteins to other function particles in the cytoplasm. Part of the ER appears as smooth because it lacks the ribosomes but remain as the sites for the multiple products of the cells. Chemical products within the smooth ER are essential to most plant functions producing the lipids such as those needed for cell wall construction and anti -pathogen toxins. Folds in the smooth ER also commonly separate toxins from other potential detrimental organelles of the cell.
Endoplasmic reticulum also assists in the movement of products of chloroplasts to other cells such as carbohydrates moved through the phloem to the root cells. Endoplasmic reticulum is considered an organelle although it is not as easily distinguished as chloroplasts and mitochondria.
A lot is going on that living corn plant.
Chromosomal DNA in corn cells is located in the nucleus of the cell. Within the 10 chromosomes of corn are a total of about 40000 genes. Each gene consists of a specific string of nucleotides that ultimately gets translated into a string of specific amino acids resulting in specific functionality of the protein. Translation of a small portion of DNA in the cell nucleus results in specific coded RNA designated as mRNA. This RNA molecule must travel to a ribosome in the cytoplasm outside of the nucleus to produce the protein that it codes.
Transport of the mRNA through the nuclear membrane requires a special transport protein that attaches to the mRNA. This interaction allows movement through pores in the two layers of nuclear membrane into the cytoplasm of the cell. Multiple ribosomes are located on the strings of endoplasmic reticulum. When the mRNA is attached to the ribosome, another form of RNA called translation RNA (tRNA) attaches to one of the 20 amino acids as called for by the nucleoside code. This process occurring in the ribosome results in specific amino acids ordered by the nucleoside code.
While we appreciate the performance of a corn hybrid plant as we observe them in the field, it is amazing to think that in all those cells, individually only visible by microscope, that the real work is going continuously in the thousands of cells of the plant.
Chromosomal DNA located in the nucleus codes of about 40000 genes but it is not the only DNA in cells. Chloroplasts have their own DNA that codes for about 100 genes and mitochondria have DNA as well. DNA strands in these two organelles is single strand, unlike the two-stranded DNA in the plant nucleus. This feature, as well as the double-layered membrane of these two organelles, have contributed to the concept that they originated early in the evolutionary change to advanced forms of life. Chloroplasts are believed to happen 3-400 million years ago when a cyanobacterium (a blue green algae) became engulfed in a non-green single celled organism. The chloroplast self-duplicates in plant today, dividing much like bacteria. Its DNA gets translated into RNA, that moves to ribosomes within the chloroplast for production of the proteins needed as enzymes to convert light into chemical energy. Despite producing some of its own proteins, about 90% of the proteins in chloroplasts come from the cells’s nuclear DNA.
Mitochondria, the organelles that convert sugars into the chemical energy of ATP, also have single stranded DNA, double-layered outer membrane and divide like bacteria. Thus, the theory that they originated as bacterial. Like chloroplasts, they are also dependent upon the host cell’s DNA for part of their existence. Just as with chloroplasts, mitochondria have a symbiotic relationship with the host cells.
While the whole corn plant gets our attention, the real work is happening in its smallest components.
Most of us are interested in the corn plant as a whole but it is clearly an expression of its parts and those parts are an extension of the smallest parts, the cells. Cell function is dictated ultimately by components of the cell nucleus especially the DNA. The process of transforming a string of chemical compounds, the nucleic acids into ultimate structure and function of any living organism is amazing- or should we say: ‘a-maize-ing’.
DNA in each of the 10 chromosomes of each living cell of corn is composed 2 strands of DNA wound around each other. Each string is composed of nucleotides. Each nucleotide is composed of a sugar molecule of deoxyribose , a phosphoric acid molecule and a nitrogen molecule. There are 4 nucleotide molecule: Thymine, Cytosine, Adenine and Guanine. These are abbreviated as T,C,A and G. The sequence of these nucleotides in the DNA ultimately determine a gene and its product.
An enzyme causes the two strands of DNA to separate briefly to begin the RNA replica of a group of the DNA nucleotides. Some codes in the DNA called starter codes become the beginning of the RNA. The replication continues until it reaches another code called the stop code. This new RNA, strand migrates from the nucleus into the cell cytoplasm. It is called messenger RNA or mRNA as it is conveying genetic information to the ribosome in the cell.
The ribosome imports amino acids that are attached to each other according to the RNA nucleotide sequence. The string of different amino acids become a protein. The proteins enzymatic potential is determined by the sequence of the amino acids. This enzymatic power affects all other chemical processes needed for the living function.
We need to think both small and large about the corn plant.
Diversity among humans is obvious to us as our tendency is to look for physical features that are easily seen. But real diversity is hidden by those obvious features as internal differences and culture are the real diversity. Corn diversity affected by mutations in DNA and RNA for multiple differences in adaptation to environments and the balance we demand between for grain production, quality and harvestability. Some are obvious but much goes unseen.
Not only are small changes in ‘error’ in duplication of chromosomal DNA significant but RNA, the chain of nucleic acids transferring the codes from the chromosomes to the ribosomes for protein construction, can have their own errors. In both cases, proteins essential for some physiological process can be affected. Transportation of glucose to roots, production of new cells or number of stomates can be affected, causing drastic affects on final performance of the corn plant.
Production of the components that allow the recognition of microbe-associated molecular patterns is an example of an essential physiological component to the plant being able to respond to a pathogen attack. Critical mutations in production of this system are an import component to resistance systems.
Corn’s exposure to multiple environments allows us to discard those with detrimental mutants, accounting for the relatively short life of any commercial hybrids. Fortunately, the long, varied history of this annual crop has allowed for a vast genetic base to draw upon for new genetic combinations, and mutations, to draw upon for final performance in expected environments of the next season.
Just as in humans, some of those obvious, visible trait difference do not predict the inner differences. It is performance that is importance.
How diverse is corn? That issue is often expressed with a concern that it is becoming too narrow. Certainly, the selection pressure for performance under today’s USA agriculture environment does move the genetics towards performance in environments that have changed during the past 40 years. Higher plant density and more minimum tillage have increased needs for more tolerance of stresses on plants. Other plant characteristics have also been chosen in today’s commercial needs for grain quality.
But is more diversity, if needed, available? We tend to only recall the diversity in the characteristics that we see or receives our attention. If corn is viewed from the road as we pass by fields, it looks the same in nearly every field. If one is a student of corn, one sees a range of leaf structures, kernel depths, kernel quality, root structure, flowering timing, and tassel branches. Measuring grain and standability differences at the end of the season shows diversity at the end of the season. These observations of outward characteristics are not a complete analysis of the unseen diversity that may or may not be expressed- at least to us.
Mutations that occur with every reproduction often do not affect physiological processes that we observe. Some may affect some process that has no affect in current environment but may be significant later. Maize chlorotic mottle virus became significant in Nebraska in 1976. Although most common hybrids were susceptible, a few older inbreds were found to be resistance. When the disease broke out in Africa, within a few years breeding programs identified genotypes with resistance. Goss wilt, caused by a bacterium that apparently came from grasses, caused severe damage to a few popular corn genotypes, but resistance was found in other adapted corn hybrids. Unhidden diversity within corn has continually contributed to undesirable characteristics, such as susceptibility to a ‘new’ disease and also to resistance to a potential pathogen.
Corn’s history of movement to multiple environments, its annual reproduction and large number of genes have contributed to an immense diversity that is available for future versions of the crop.
Many mutations occur during cellular replication but those occurring in haploid cells can have extreme expression because often these are in recessive genes. Dominant versions of the mutated, recessive gene are covered up in most diploid genotypes. Inbreeding objective is to make all genes homozygous as the breeder attempts to obtain consistent, repeatable genetics but with the potential cost of making homozygous some recessive genes with negative effects on the plant. Not only does this result in smaller corn plants as the inbreeding progresses, but also carries risk for a few diseases.
One example is susceptibility to Race 1 of Bipolaris zeicola (Helminthosporium carbonum). This variant of the fungus apparently is among other grass leaf pathogens of this species. It has genetics resulting in production of a toxin that is controlled by a dominant gene in corn. During the inbreeding process, however, and recessive version of this gene is made homozygous. Consequently, these inbreds are frequently heavy infected in many seed fields exposed to the pathogen. More information on this pathogen race can be found in 7/11/19 blog of Corn Journal.
Much of the increase in corn grain production, adaptation to multiple environments, disease resistance (and susceptibility), and specialty traits are the result of naturally occurring genetic mutations in this annual plant. Humans benefit that mutations occur in corn, as other forms of life, but we should not be surprised with changes from mutations that are often expressed in inbreds- and hope the other parent of a commercial hybrid covers up the defects.
Corn, being an annual plant, has opportunity for quick expression of mutations. Simple substitution for one of the nucleotide bases within the DNA for a gene, can result in an amino acid change in a protein critical to some physiological process in the plant. Many of these types of ‘errors’ made during meiosis do not cause meaningful or notable changes to the expressed corn phenotype, but a few have huge effects.
Opportunity for mutations to occur among the 40,000 corn genes, each gene having about 50-150 sets of 3 nucleotide bases makes genetic variability inevitable. Fortunately for us, this variability has been mostly beneficial, allowing for adaptation of this crop to multiple environments. Humans have been able to select those appropriate genotypes. Some rare mutants resulting in waxy and high amylose endosperm have special uses. White endosperm is another recessive gene that was due to a mutation in gene responsible for carotenoids in yellow kernels. Pure white kernels also require genes for colorless pericarp. Other obvious mutations involve different anthocyanin genes resulting in blue and red kernels.
Most mutations have resulted in less obvious differences affecting plant height, leaf uprightness, leaf width, root growth and direction, mineral uptake, general and specific disease resistance and even more subtle differences in photosynthesis and cellular respiration.
Mutations, although random, have given us a crop adapted to multiple human needs.
A review of the relationship of nucleotides to DNA and RNA can be found at:
A maize data base of corn mutants is maintained at maizegdb.org. This online site has an interesting summary of notable mutants at:
Not only is nuclear DNA vulnerable to mutations but also that in mitochondria. One of the dramatic examples was the occurrence of sterile cytoplasm in corn. And the unexpected variant in a pathogen that attacked the variant mitochondria. Below is a post from 7/27/2017.
MITOCHONDRIAL DNA, CORN MALE STERILITY AND RACE T
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.
Corn’s past and its future is driven by mutations, allowing for humans to select desirable characteristics. Because most meaningful mutations generational result in changes is recessive genes that only become expressed when, in diploids, homozygous for that recessive gene, the mutations may not become evident in hybrids but only after inbreeding.
Bacteria and fungi also have mutations but most of their life cycle is controlled by haploid versions of DNA. Consequently, a mutation can have immediate effect on a potential pathogen. Most of these organisms have high rates of reproduction and spread. Mutations in a potential pathogen resulting in a new protein that allows blocking the detection of a pathogen by the host plant can result in success of the pathogen to further invade the plant. Races of Exserohilum turcicum have specific mutations that block the turning on of specific lesion size restriction in corn of Ht1, Ht2, Ht3 and Ht4. Clavibacter michiganensis is a bacterium species with multiple subspecies that are essentially mutants adapted to specific hosts including wheat and tomatoes. A mutant was identified on corn in Nebraska in 1970 as the cause of Goss’s wilt of corn.
Mutations in potential pathogens will continue, as will mutations in corn. Diversity is good, most of the time, and necessary for all of us into the future.
We can be thankful for 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.
Differences among all living things is driven by mutations. A simple change in the nucleic acid position within the DNA or RNA code can affect the structure of the protein being produced by a gene. That protein’s function among the several affecting production of some cellular process ultimately can be significant to the organism’s appearance or function. Every cell division is vulnerable to such slight changes in these types of slight changes in DNA. Those occurring during cell division of cells not involved in reproduction result in a group of cells differing from adjacent cells is called a chimera. It could result in a distinct streak in a leaf of one corn plant. Most chimeras do not continue to the next generation if the mutation was not present in the nuclei of the reproduction cells.
Mutations during meiosis are the main source of genetic diversity in organisms with two sets of chromosomes (diploids). Multiple mutations accumulate over generations resulting in distinctive characteristics among genotypes. Organisms with shorter life cycles are likely to gain diversity quicker. Selection of those best fitting an environment can include variations utilizing different physiological processes to accomplish this success. The randomness involved in mutations also includes differences that we humans do not know are of necessary function but simply are present.
These genetic differences among corn varieties can be detected by comparing plant structures, proteins or DNA. Professional Seed Research, Inc. compares leaf structures of three leaf plants when growing in uniform environments, allowing the distinction between expected phenotype versus those produced by the wrong parent, such as selfing or pollen from wrong male parent. Careful observation of mature plants also makes this distinction. In all of these methods, we detect diversity but not necessarily significant differences in desired function of the plant.
Although diversity is frequently random or at least without recognized useful function, we use it as a means to detect distinct varieties. Taxonomists specialize in observations of these differences to define species. Humans use phenotype expression of facial characters to distinguish among individuals. It is best, at this point, not to comment about those human characteristics most desirable to me, but just as with corn, some diversity due to genetic diversity would seem less functional than others.
Among the 30000-40000 genes in corn, some mutations have occurred over time to be useful to humans and some are distinctive but not obviously useful in today’s environments. In general, we should celebrate diversity in all organisms. Mutations are great!
Our exposure to diversity, in environments, plants, pathogens and in our species shows that diversity is basically good even if it causes some temporary stress. And we have the ability to adjust to changes and move on. This blog from Corn Journal on 7/5/2016 illustrates how it works in corn.
One study of a single corn inbred (B73) indicated that it had 30,000 genes. We benefit from a species with a huge genetic potential and a pollination system that encourages new mixes of genetics. This has allowed the species to be used for food in a wide range of environments. Some of those genes are turned on in response to the many microbes searching for the products of photosynthesis for their nutrition. Microbes have genetics too! Some, like rust and smut fungi, survive by attacking living corn cells, drawing carbohydrates to the cell, and then moving on via spores before the host cells respond to the fungus. Many fungal pathogens of corn simply kill a limited area of the leaf tissue, feed on the dead tissue, produce spores and infect new areas. Corn varieties differ in how quickly and strongly they respond to the invasions.
One fungus that I find interesting is Bipolaris zeicola. It was formally known as Helminthosporium carbonum. There are genetic variants of this species that apparently feeds only on dead leaf tissue, often caused by insect damage or simply physical injury. These variants apparently lack the genetics for either penetrating the live corn leaf tissue or overcoming the resistance system of most corn varieties. At least one variant of this fungus produces a toxin that kills corn plant cells but most corn varieties have dominant gene that effectively blocks this toxin. However, very occasionally, a mutation of that dominant corn gene does occur while developing new inbreds. If this mutation, now a recessive gene, becomes homozygous during the inbreeding process, the inbred is vulnerable to the toxin. The result is practically no defense to this variant (race 1) of B. zeicola. The pathogen kills small leaf area of leaf, produces spores and spreads to new leaf tissue and eventually causes the whole corn plant to die early. Because susceptibility is recessive, and the dominant toxin-resistant gene is present in most corn inbreds, this creates a problem for seed producers but not for hybrid growers. Good that corn has genetic diversity.
2019 diverse US corn belt environments was a strong reminder of all the significance of diversity among corn hybrids. This principle applies to all areas of earth in which corn is planted. Following is from Corn Journal on 11/13/2018 attempting to discuss corn diversity.
Corn’s history and biology has resulted in diversity beyond what most of us see in any single season. Advantages of hybrid plant uniformity for yield, harvestability, disease and pest resistance and genetic repeatability requires development of homozygous inbred parents. Each of many seed companies produce multiple hybrids each year and there are about 40,000 genes in each corn plant that are available to influence something, whether needed or not.
Corn researchers in 1920’s became aware of a need to collect and share many of the genetic sources in corn, forming a Maize Genetics Cooperation Stock Center- it’s history is summarized at http://maizecoop.cropsci.uiuc.edu/mgc-info.php. This collection started and continues to emphasize mutants affecting some identified trait, such as those involved in sweet corn, waxy corn or amylose corn and many that may not have a specific economic advantage but are useful in understanding some biochemical pathway in the corn metabolism. Study of these mutants contributed to location of genes on each chromosome and add to growing knowledge of corn DNA codes for many traits.
Despite these efforts, corn’s genetic diversity is large due to selection by humans over diverse environments. Our experiences with ‘new’ diseases as a pathogen such as the bacterium causing Goss’ wilt suddenly appears, with a previously unknown susceptibility gene in corn became part of popular corn hybrids, or susceptibility of race T of southern corn leaf blight associated with mitochondrial gene in t-cytoplasm male sterile corn. Resistance to Maize Chlorotic Mottle Virus was found in corn genetics in USA after it occurred in Kansas in the 1970’s and in Africa in 2016.
Often the strong resistance to these diseases are associated with single genes already present in corn apparently without intended human selection and without known selection pressure in absence of the disease. Perhaps there was exposure somewhere in its history where the gene was favored but also it is likely that randomness of mutations, segregation of genes during miosis, cross pollination and historic diversity of corn’s environments have provided many genes for characters that we have yet to identify. These genes must be influencing multiple internal aspects of absorption of light wavelengths, translocation of carbohydrates, absorption and movement of minerals, water uptake and conservation, and structures of leaves. Among this diversity is the future adaptation needed for changing environments.
Breeders witness diversity within their nursery as they see differences in plant structures and growers see differences among hybrids in performance each year. At Professional Seed Research Inc., we see differences among hybrids in structures of seedlings (Seedling Growout®). Genetic diversity will continue to be an important contributor to this crop as it interacts with changing environments.
Breakage of stalks in the 2nd or 3rd internode above the soil is a big concern during harvest. Multiple studies have been done attempting to sort out the dynamics of physical strength of the stalk and fungi associated with lodged stalks. Multiple fungi capable of digesting the cellulose and lignin of corn cells surround the corn plants in the field. Most of these are warded off by the anti-microbe metabolites of living corn cells. Fungi that successfully attack dying or dead cells, producing recognizable fungal structures such as Diplodia (Stenocarpella) maydis, Gibberella zeae, Colletotrichum graminicola and Fusarium sp. as well as several others that are found in the deteriorated stalk.
Methods of Evaluating Stalk Quality in Corn, published in 1970, (https://www.apsnet.org/publications/phytopathology/backissues/Documents/1970Articles/Phyto60n02_295.PDF ) is a summary of the dynamics crushing strength of lower stalk pith and rind versus intensity of Diplodia maydis. Both pith integrity and rind thickness are significant contributors to the crushing strength. Their study and others point out that the Diplodia fungus grows only in dead pith tissue, and, therefore, correlation of this fungus with weakened stalks is mostly related to death of pith tissue.
When individual plants wilt, usually because of root rot, the pith cells dehydrate, pull away from the rind and lose production of the metabolites needed to restrict growth of the fungi of the stalk. This results in weakening the strength of the stalk by changing the dynamics of pith attaching to the rind plus allowing the growth of fungi that can break down the rind cells.
It is interesting to observe (from the roadside) multiple fields in our area with very little stalk lodging. Plants are obviously dead from maturity and low temperatures. In general, lower stalks that make it to black layer without wilting, maintain strength for a long time. Dynamics involving environment, genetics of response to environment and vulnerability to root rot are significant in corn stalk lodging.
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.