Among the specialized cells in corn leaves are the stomata. Each stomate consists of two guard cells and two companion cells. Corn, being a monocot, has these specialized cells organized parallel to the length of the leaf at a rate of about 36000 stomata per square inch on the top of the leaf and about 50000 per square inch on the bottom to the leaf.
Blue light wavelengths detected by a carotenoid activates a process in which potassium ions flow into the guard cells. Thus, the water concentration in the cell drops, resulting in osmotic pressure for water to enter the guard cells. The shape of the guard cells allows uneven swelling and a pore opening up between the two guard cells. Photosynthesis produces sucrose that then contributes to the osmotic pressure in the cells. At the end of the daylight, starch is synthesized from the sucrose and potassium ion concentration is reduced and the opening between the two guard cells closes. Other compounds within these cells also contribute to this phenomenon. https://academic.oup.com/jxb/article/57/2/381/489968.
This is essential, of course, to allow diffusion of CO2 into leaves for photosynthesis in other leaf cells. Open stomates allows O2 to be released to the atmosphere but also water loss. Water evaporation through stomates (transpiration) is affected by the relative humidity in surrounding atmosphere as the water concentration within the leaf spaces is nearly 100%. Cohesiveness of water molecules ‘pulls’ water up to the leaves so that essentially every molecule of water that goes out the stomata is replaced by one from the root tissue.
During the day, stomata are open, carbon dioxide moves into the leaf, oxygen moves out and so does water. At night, photosynthesis in the guard cells stops, water moves out of the guard cells causing the swelling to be reduced and the pore is closed. We will discuss more about this phenomenon later as the plants grow.
Stomata are essential to corn and have a dramatic affect on productivity as import of carbon dioxide and export of water occurs through the stomata. This topic is covered in Corn Journal 5/10/16.
When the plant apical meristem cells interpret the combination of daylight length and accumulated heat, as determined by genetics, the terminal shoot meristem and at least one of the meristems located at a stem nodes, begins to produce cells for the tassel and shoot. Resulting modified stems and leaves now produce the male and female flowers of the corn plant.
Maize male and female flowers are on separate branches of the corn plant, thus the species is called monoecious, as opposed to the dioecious flowers of soybeans. Both the ear-forming branch and the terminal tassel is composed of multiple flowers. Each kernel that forms in the ear traces to a single flower with a single ovule within the fruit wall, the ovary. Both male and female flowers of corn begin as dioecious but the male portion in the ear and the female flower in the tassel are aborted very early in the development of each. A mutation or an environmental factor can overcome the abortion, resulting in tassel seed or terminal tassel on an ear.
A corn tassel may include up to 1000 spikelets, each one including 2 florets. These individual flowers are enclosed in the modified leaves called glumes. Each of the florets have three stamens, consisting of filaments and anthers. Each anther includes multiple cells called microspore mother cells or microsporangia. Meiosis occurs in these diploid cells resulting in 4 haploid microspores per mother cell. This occurs over a period of 3 days. Microspores become free of each other as they grow for a few more days. The individual haploid nucleus in each microspore undergoes mitosis, resulting in two haploid cells within the individual pollen grain. The pollen grain secretes a pollen wall within another 7 days. Starch crystals accumulate within the pollen grain during that wall formation period as the cytoplasm of the pollen grain dehydrates. A small pore is formed in the pollen wall.
Pressure from the growing pollen grains and dehydration of anther walls causes the split that allows the release of pollen grains. One thousand spikelets each with 2 florets with three anthers each with hundreds of pollen grains easily produces a cloud pollen. Production of the spikelets over a period of days results in daily release. Pollen longevity may only be a few hours in high heat but the release over consecutive days in a field of corn usually assures viable pollen reaching most viable female stigma.
The remarkable human selection and development of maize adapted to multiple environments because of available genetic diversity is largely due to the separation of male and female flowers. (Corn Journal 7/17/18)
New above-ground cells are produced with cell division in the apical meristem. Genetics and environment control the eventual size and function of those new cells. This was addressed in the Corn Journal Blog on 8/8/2019:
Corn shoot apical meristem is genetically controlled to switch from producing new leaf and cells to the terminal male flowers of the tassel. The main environmental factor influencing this switch in temperate zone corn is heat energy. Earlier maturing corn requires less heat to trigger this change in apical meristem products, allowing corn to mature in short seasons far from the tropical environments of corn’s origin.
Plant height is determined by the number of cells produced by cell division at the apical meristem before switching to producing the cells that becomes the tassel and the elongation of the cells. Elongation of the stem cells is enhanced by water pressure applied to the young cells before maturing with less flexible cell walls. Thus, water availability to the roots, root volume and transport of water to the expanding cells in upper plant also affects the eventual plant height.
Corn planted later than normal in temperate zones, accumulating heat units quicker than usual, produce fewer stalk cells because apical meristem is induced to produce tassel cells quicker. If water availability for cell expansion is less than optimum, the result of these two factors will be shorter plants than usual for a hybrid.
Corn stalk cells of the rind have thick walls with lignin, hemicellulose and cellulose, all carbon-based compounds formed after carbohydrates were shipped to the stem locations. Rind cells are major barriers to pathogens and insects and contribute to the withstanding of lodging pressures. Hybrids vary in the thickness of the rind with strength measured with special penetration equipment.
Stalk pith tissue is composed of parenchyma cells with thinner walls allowing import of sugar molecules. Cytoplasmic activity in these cell plasmids converts the glucose into starch molecules. This functions as an energy storage for future use in roots and grain, as hormones directs the movement. As grain begins to form, sugars are moved at a steady daily pace. When stresses, such as leaf disease, or hail damage to leaves or cloudy weather reduce photosynthesis, the reserve from the stalk cells is pulled to the grain.
Vascular tissue in the stalk becomes the vehicle for the movement to the grain and root, while the xylem supplies moisture to the stalk cells from the root.
Stalk pith cells connect to the rind cells, essentially a solid rod of the stalk and thereby adding to the stalk total strength. Some have estimated that this is about 33% of the total stalk strength.
Movement of sugars to the grain can result in deprivation of sugar needs for root tissue, resulting in early death of root tissue. This reduces the uptake of water by roots eventually causing the leaves to wilt and the stalk parenchyma cells to collapse. The latter results in pith cells to pull away from the rind, essentially changing the rod structure to a tube. Death of these cells allows the advance of fungi as active cell metabolic resistance is no longer effective. Consequently, the stalk easily lodges. We often refer to these as stalk rot as fungi present are identified. The real problem, however, was the starving of roots.
Photosynthetic stresses combined with the draw of sugars to the grain reduced the available of chemical energy for root cells. Death of the root resulted in wilting of plant and redaction of stalk pith cells. Withdrawal of the pith cells away from the rind cells weakened the strength of the stalk.
Cells of the corn plant are the source of the plant structure and function. Converting light energy into chemical energy allows for growth and production of cell structures and chemical compounds allowing for all of living cell function. The totality of construction and function of the plant is determined by its DNA and interaction with environment. Ultimately, however, the movement within the plant of different cell products requires communications among the cells.
Movement of water within the plant is mostly made by simple physical principle of diffusion and osmosis. I recall a plant physiologist professor many, many years ago illustrating by sharing a story of being on a mountain road in a discussion with wife whether they were going uphill or not. He claimed he got out of car, poured water on the road and declared that water runs downhill! Water moves from a high concentration to a low concentration, such when diluted by sugar. That principle applies to movement of minerals and compounds as well, if they can make it through barriers such as cell membranes.
Hormones are actively involved with movement of cell products. Four notable groups of hormones are auxins, gibberellins, cytokinins and abscisic acid. Each is associated with different functions in the plant.
Auxins, such as indole acetic acid (IAA), affect cell elongation. Produced in the apical meristem it is associated with initial elongation of the stem tip. Absence in the lateral buds at base of each corn leaf, prevents branching. Removal of the apical meristem allows production of more auxin in lateral buds and consequential branching of the corn plant.
Gibberellins, such as gibberellic acid, promotes elongation of the cells in the stem but not in the apical meristem.
Cytokinins promote cell division in the meristems such as in the newly pollinated embryos. Zeatin is a cytokinin in corn embryos.
Abscisic acid inhibits cell growth. It is most active in developing the restriction of flow of more carbohydrates into the mature grain, causing the cells at base of kernel to form a ‘black layer’.
It is not known completely how these hormones cause these affects. It is mostly assumed that they are acting with cell DNA but some reactions, such as auxin causing roots of a germinating seed to turn to go downwards because cells on one side elongate more than those on the other side, seems too quick to only be reacting with DNA. Regardless mechanism, plant hormones are essential participants in the communication among corn cells.
Although many cells of the corn roots have the basic structures similar to other plant parts, there are significant differences appropriate to the root functions. Root cell production originates from active root tips, producing epidermal cells, intermediate parenchyma cells and vascular bundles complete with xylem and phloem tissue.
The important function of uptake of water and minerals occurs mostly through the youngest and newest cells near the root tips. This is enhanced by extensions from some epidermal cells called root hairs, vastly expanding the exposure to the root surface for absorption that occurs mostly by osmotic pressure. This is enhanced by diluting the water in root hair cells with sugars supplied ultimately by leaves and transferred through the phloem.
Root hair cell walls block large items, such as fungal mycelium, but smaller molecules pass through to the cell membrane that is selective in allowing entrance. Root hair extensions of the epidermal cells live for only a few days but as the root tip produces more new cells that produce new root hairs, the enhanced absorption of water and minerals continues. Auxin hormones are involved in the initiation of the root hairs. The short life is probably useful as the permeability enhances the potential for invasion by pathogens. After the root hairs disintegrate root cells increase the function of transporting to the xylem.
Of course, genetics influences the branching of roots, and production of root hairs. In addition to the multiple genes probably involved, a single root hairless mutant gene rth3 gene has been identified and in trials has shown to be associated with significant losses in grain yield. https://www.ncbi.nlm.nih.gov/pubmed/18298667/.
Lot of things happening in the corn plant that we don’t see.
Xylem cells in the vascular system from the roots to the leaves is the main tube that allows water and minerals to be moved from roots to leaves. A major portion of the xylem, tracheids, are dead cells at maturity that have strong thick walls providing the strength to withstand strong water pressures but have small pores to allow flow into surrounding cells. Also, part of the xylem tissue, are living parenchyma that allow movement of water and minerals to other cells. These cells accumulate potassium ions to be distributed into mesophyll of leaves. Xylem parenchyma cells accumulate starches in the corn stalk pith areas, to be later moved thru the phloem vessels to the developing kernels after pollination.
Tracheid cells in the xylem do not have end walls allowing them to form a tube. Water pressure from water entering the root cells to the movement of water to leaves where a portion of it evaporates through open stomata openings, causing transpiration pull. This push and pull causes a constant flow of water through the tubes of the xylem.
Phloem cells provide the transport of sugar and protein molecules up and down the plants. Xylem cells provide the flow of water and minerals up the plant. Movement in the phloem occurs via living membranes and requires energy. Movement of the water from roots upward does not require energy but simply removal of water through stomata and tendency of water molecules to adhere to other water molecules (cohesion). Thick xylem tracheid walls form the tube to contain the force.
Pores in cell walls allow movement of cell products between cells. These pores are filled with an extension of the endoplasmic reticulum, the membrane-like filaments within a cell. This extension called the plasmodesmata is essential to plants because of the cell walls. Animal cells do not have cell walls. These typically are large to move small molecules such a glucose but often need some energy dependent assistant to move larger ones such as proteins. It is hypothesized that when some corn varieties are chilled the movement of the compounds from the chloroplasts in the leaf cells to the bundle cells adjacent to the veins for the final production of glucose is inhibited (https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2958785).
Larger molecules such as RNA and proteins also move via plasmodesmata between cells with some specific chaperones’ molecules assistance. This same type of assistance allows viruses to move between cells as well, eventually reaching the growing point to cause a systemic disease.
Movement of carbohydrates from leaves to growing points, roots and corn stalks is facilitated with the plasmodesmata between cells of the phloem. Hormones are also moved in the phloem to other plant parts by the same system. Root cells also have plasmodesmata for hormone attraction of carbohydrates and proteins for growth.
Plasmodesmata represent another unique feature of plants, hidden from most of us, that is affecting the performance of the corn hybrid each season in our fields.
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.
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.