A few weeks after pollination, dynamics affecting resistance to leaf pathogens changes. Cytokinins are increasingly concentrated in the developing grain embryos, causing more translocation of sugars from leaf tissue to the ear, reducing availability for cellular metabolism in the leaf tissue. Leaves lower in the canopy, in the shadow of upper leaves have reduced photosynthetic rates due to receiving less than 5% of the light intensity as those exposed to full sunlight. Not having sufficient energy to maintain its cells, senescence of these leaves begins. Among those cell functions is the production of anti-pathogen biochemical that limit leaf pathogens.
Disease pressure increases in lower leaves with the higher humidity and longer dew periods that favor leaf pathogens. Cool, cloudy and wet weather in those 50 days of grain fill after pollination further favors the fungal leaf pathogens. This increased disease pressure on as leaf tissues occurs at the time in which they are losing the ability to react to invading organisms. Lowest leaves senesce first as the lower photosynthetic rate and increased disease kills tissue. Even weak pathogens, such as Fusarium species, invade the vascular tissue of such leaves causing the leaf to wilt, while the upper canopy leaves remain green and fully functional.
The senescence pattern progresses up the plant as it gets closer to meeting the active translocation period of 50 days after pollination. If there was exposure to pathogens such as Exserohilum turcicum, the cause of northern corn leaf blight, earlier in the season, the disease appears to move up the plant. This can cause difficulty in comparing resistance levels among hybrids varying in maturity such as in research plots. Those with earlier pollination dates may appear to be more susceptible, especially if the environment favored the disease, simply because the leaf senescence was more advanced than the later hybrids. This can be misleading not only in determining differences in innate resistance levels but also in predicting potential grain yield or increased stalk damage from the disease.
One of the obstacles to evaluating and presenting precise resistance ratings for a disease is relative leaf senescence among corn hybrids. Add this to environmental factors and pathogen intensity pressures and races and one should only expect that disease resistance ratings are not precise predictors of damage from a leaf disease. Corn Journal (8/17/2017)
Corn plant growth is greatly affected by a broad class of complex chemicals called hormones. Two of the kinds of hormones related to grain fill are the cytokinins and abscisic acid (ABA). These two hormones have opposing functions in plants, including the development of corn kernels. Cytokinins function is to increase cell division and delay senescense of tissue. They are produced in roots and transported via the xylem to meristems such as in each kernel. They also may be produced in seed embryos also but evidence for that is elusive. Regardless, cytokinins accumulate in developing seeds where they are responsible for stimulating cell division. Cytokinins are also linked with the transportation or at least the attraction of sugar to the developing kernels.
Abscisic acid, on the other hand, is associated with cutting off of translocation to tissue basically by causing a layer of thick-walled cells impervious to movement of materials. Abscisic acid production increases when the plant is stressed. The black layer at the base of mature corn kernels and at the base of husk leaves in a mature corn ear are stimulated by abscisic acid.
Freshly pollinated ovules have a balance of these two hormones. A non-stressed corn plant normally has a balance favoring the cytokinins stimulating more cell division and, consequently, flow of sugars to the individual kernels. However, if the plant is under heat or drought stress the balance tends to favor abscisic acid. The affect can be abortion of those kernels. Corn kernels within the first 10 days after pollination are most vulnerable, perhaps because the accumulation of cytokinin is too great to be overcome by a short-term increase in abscisic acid.
Genetics and environments influence the production of these two critical hormones affecting grain yield in a corn field.
Pollination was successful and hormones in the new growing points of pollinated ovules begin the process of moving sugars to each new embryo. Shortage of energy movement during the first 10 days is critical. Plant stresses that cut off this movement, having a change in balance of hormones for each kernel.
Two major hormones in corn are cytokinins and auxins. Cytokinins affect cell division and auxins affect cell elongation. Cytokinins are produced primarily in root tip meristems and transported via xylem to other meristems such as those developing in each pollinated ovule. As these embryo meristem cells divide, the attraction of cytokinins increases. Concentration of cytokinins in these meristems also affects translocation of glucose molecules to each developing embryo, as this carbohydrate moves through the phloem from leaves and stem pith tissue to the new cells. Excessive stress affecting water for xylem transport, or, reducing sugar production can reduce the constant flow of cytokinins to developing kernels. If this occurs during the first 10 days after pollination, another hormone, abscisic acid (ABA) accumulates at the base of the ovule. This hormone causes development of thick-walled cells, blocking transport of cytokinins and sugar into the kernel. As a result, the kernel does not develop further.
Genetics and environment have a great effect on the balance of hormones during corn grain development.
The meristem in at least one of the lateral buds of a corn plant develops into an ear. This meristem includes 500-1000 lateral meristems with mother cells with diploid sets of chromosomes, 10 chromosomes from each of that plant’s parents. Meiosis occurs in this diploid cell, resulting in 4 haploid cells, each cell having only a single set of 10 chromosomes consisting of a random mix of the two parent’s chromosomes. Three of the 4 haploid cells degenerate, leaving a single megaspore. This megaspore nucleus undergoes mitosis three times, resulting in 8 cells within the megaspore structure now called the embryo sac. One cell at the bottom of the embryo sac becomes the egg cell while two of the haploid cells fuse in the center of the embryo sac.
The embryo sac (ovule) is enclosed in an ovary, at part of the female part of the parent plant. Part of this female flower is the silk., extending from a single ovary and attached to its ovule. The male flower also produces pollen via meiosis followed by a single mitosis, resulting in two haploid nuclei. A pollen grain adhering to the silk, germinates and extends down the silk to the ovule. Upon entrance of the ovule, one nucleus fuses with the haploid egg cell forming a diploid nucleus to become the seed embryo. The other pollen haploid nucleus fuses with the two ovule nuclei in the center of the ovule resulting in a triploid nucleus, having two sets of chromosomes from the female parent and one from the male. This triploid nucleus undergoes mitosis to become the endosperm of the seed.
Whereas the inheritance of the embryo, and its resulting mature plant, is determined equally by the genetics of the male and female parents, characteristics of the endosperm is slanted towards the genetics of the female parent. If the female parent has the recessive Y1 gene, and thus a white endosperm, but the pollen is from a parent with dominant gene and thus has yellow endosperm pigment, the resulting endosperm will be lemon white in color. The female genetics has the major affect on endosperm function in the maize seed because it contributes two of the three sets of chromosomes in endosperm cells.
Corn apical meristem switches to producing male and female flowering parts, but quickly changes to male development only. Each glume in the tassel is an individual floret containing three anthers. Within these immature anthers are hundreds of microspore mother cells in which meiosis occurs. As a result, each of these cells with 2 sets of the 10 chromosomes (diploid) before meiosis now contain 4 microspores, each with only 1 set of the 10 chromosomes (monoploid). Whereas the diploid stage in hybrid corn, included 1 set from the parent male parent and 1 from the female, after meiosis, each microspore includes a random mix of two parents. There are a minimum of 1024 different combinations of the two parental genetics among the microspores. The 4 microspores separate over a 4-day period and begin to become separate pollen grain with thicker walls. Nutrients are absorbed from the liquid contents of the anther during the microspore and pollen grain stages over about 10 days, at least in one study. During this period, the anther dehydrates as it is filled with pollen grain. By the end of this period, the pollen grain has many starch granules, two haploid nuclei, a thick outer wall and a thin inner one. Total time from beginning of microspore production to mature pollen is 14-17 days. Each pollen grain remains viable for only about two days after maturity and less when under high temperatures.
A pore at the end of the anther opens to release the pollen. This process involves dehydration and is affected by drops in surrounding relative humidity. There is no release during rain and pollen release is common in mornings as relative humidity drops with rising daytime temperatures.
Each floret of the tassel has slightly different time of development as the apical meristem expanded. Consequently, each finishes the process of pollen development at different times, frequently over 10 days. A typical hybrid tassel has about 6000 anthers, although hybrids and environments vary. It is common for a single tassel to produce millions of pollen grains.
Millions of pollen grains in a hybrid field, and yet only a small percentage land on the stigma of the female flower. The sticky hairs on the silk assist in holding the pollen in place. Attached pollen grain rehydrates from the moisture in the silk. A germ tube emerges through the pollen grain pore, invading the silk usually through the silk hair. It continues to be dependent upon silk moisture as the germ tube grows down the center of the silk, probably with the assistance of hormones to guide it. A nucleus at the tip of the tube is active in providing the DNA codes for the growth, while two other nuclei are carried along the tube. Although a few pollen grains may simultaneously be germinating on an individual silk, usually only one succeeds in progressing all the way. The silk begins to collapse as the winning pollen tube grows, shutting off water to later pollen grains- and to potential fungal invaders. The winning pollen tube reaches the ovule in up to 24 hours, depending upon the distance from where it penetrates the silk. Nutrition for pollen tube growth and metabolism is supplied by the silk.
After reaching the ovule, the other two haploid nuclei in the pollen tube are released into the cytoplasm of the ovule. One nucleus migrates towards and joins the egg cell nucleus to form a diploid embryo. The other nucleus migrates to the cell that already has two haploid nuclei to form the triploid nucleus of the endosperm.
The genetics of the next generation is set for this seed. Hopefully, the winning pollen grain was the one intended by the humans involved.
The stems of most vascular plant species have specialized cells at the base of where leaves are attached. These nodes often also have a bud with its own meristem. Genetics and environment determine the development of stem tissue from those nodal buds. Thousands of years of human selection towards specialized expansion of buds to producing only one or two buds into special branch convenient to produce grain and harvest.
Those specific meristems are activated as a short stem with several nodes are attached to short leaves. Then the enclosed stem produces several hundred flowers. Each flower in modern hybrid grain corn, has only the female parts as corn in these flowers developing from these nodal branches. Each flower consists of an ovule and attached stigmata. The activated meristem at this node produces hormones causing flow of sugars to this activated stem and flower tissue. One corn stigmata (silk) grows from one ovule. The cells within each silk are metabolically active and cell expansion is fueled by energy supplied by photosynthesis in leaves and water pressure supplied through vascular system in the short stem and ultimately leading to the roots. Cellular growth extends the silk about 1-1.5 inches per day for about 10 days as it emerges from the surrounding leaves composing the husk. Exposed tip of each silk cell grows side extensions that promote the capture of pollen.
Silk life is relatively short, the tissue degrading after pollination and extension of the pollen tube to the attached ovule. If unpollinated after several day after exposure the cells die. The nodal bud of a corn plant has been selected to produce very specific function in corn. Genetics and environment influence it ability to carry out that function.
Silk, the stigmata of the female flowers of corn, grow from each flower in the lateral branches of the corn plant that we call a corn ear. Extension of the the silk growth is powered by the water pressure as those molecules migrate via xylem cells through this stem tissue of the ear shank. Water availability during this time is critical to this exposure of all the silk, each leading to a single flower in the ear. We can thank corn’s history for this interaction.
Teosinte, the ancestor of corn, has lateral meristems at each node, producing about 10-20 female flowers in two rows attached to a narrow rachis and each flower with a long stigma protruding beyond the modified leaves that surround the flowers. In most Teosinte species the male flowers develop from the apical meristem, much like a corn tassel. As people selected for mutants that had more grain like corn, they also maintained the leaves (husk) that helped protect the ovules and, later, the developing grain from invasion by insects and pathogens. It was also convenient as a wrap for cooking corn (tamales anyone?)
The ear of corn is composed of parent plant tissue and DNA surrounding a group of flowers attached to a central stem (rachis) we know as the cob. Each flower has a single stigmata that we know as silk that extends beyond the outer leaves (husk). The silk tissue, husk, cob and the outer layer of cells of the ovules are parent plant material and therefore controlled by genetics of the hybrid plant. Benefiting from the vast genetic diversity, breeders over the millennia selected for variants with different husk characters that met their specific needs. Heavy insect pressure environments favored those with longer and perhaps thicker husk leaves. Short season environments requiring quicker field drying the mature grain favored those with thinner and shorter husks.
Silk growth also had to accommodate the husk length of husks in order to get exposure to pollen. Silk growth is largely a cell elongation process. Like all cell growth, water pressure is needed to extend the silks, thus it is dependent upon the plant environment. Genetics also is a significant factor that requires breeders to select for good silk extension even with drought pressure. Timing of the silk emergence from the husks is also important because of the limited time in which viable pollen is available. Although the first silks to enlarge are the oldest at the bottom of the ear, those with a shorter distance, perhaps an inch from the bottom reach there first.
Although the movement of sugars to the various growing points of a corn plant requires energy from photosynthesis to move through the living cells of phloem tissue, water movement from roots to leaves through the non-living xylem tissue of the vascular bundle is mostly due to the physical character of water molecules.
Sugar solubility of water favors the initial movement of water into root hairs. This process of movement of water across root hair cell membranes is a physical phenomenon of osmosis, as water moves from a higher concentration outside the root hair to a lower concentration in the sugar (and other molecules) dissolved in water within the cells. This osmotic pressure also promotes water movement into the xylem tubes within the vascular bundles of the roots.
Water cohesiveness keeping the water molecules together along with the removal of water molecules from transpiration through leaf stomata, essentially pulls the water up the plant, carrying with it the minerals dissolved in the water.
Corn stems and leaves with multiple vascular bundles in the stem contribute to stability of water uptake and distribution throughout the leaves. The veins are parallel to each other in the corn leaves. At the base of the leaf, where it connects to the stem at the node, the system becomes much more complex. The vascular tissue goes horizontal with fusions between the individual veins. Also, the xylem ‘tubes’, (vessels) have end walls, forcing the water moving up from roots and stem through small pores that act as filters. The pores are sufficiently small to filter out any particles being carried upward with the water. Many bacteria and even some viruses are too large to pass through the pores. Each node of corn, even in the small seedling, has this complexity of the vascular tissue. Root vascular tissue connects with the stem vascular system at the first leaf node. Whereas an individual leaf may have up to 20 main veins, the node may have 100 horizontal vascular bundles and with fusion of vessels at the nodes. This redundancy protects the plants from a problem in single vascular bundle or one root branch from blocking transport of water and minerals to the leaves. Likewise, the movement of carbohydrates from leaves to roots gets distributed to all roots. Water soluble substances such as minerals and toxins can move freely up the plant with water through the xylem, but most fungal spores and bacteria are filtered out by the pores.
Movement of water into stem and leaf cells also is physical, water moving from higher water concentration in the xylem tubes through membranes of the living, metabolically- active cells, allowing direct utilization of water in photosynthesis and other activities. Cohesiveness allows more water to follow.
We are apprehensive that corn plants lose water through transpiration but also should appreciate that because of the loss of water through the stomata, not only is CO2 allowed in the plant, and oxygen escapes, the process allows uptake of water and transport of minerals also is occurring.
Cells between the upper and lower epidermis of corn leaves make up the mesophyll and the vascular bundles (veins). These cells carry out many normal cell functions of producing proteins and anti-pathogen substances but most notable is the photosynthesis performed in the chloroplasts. Most (95%) plant species have mesophyll cells located immediately adjacent to the epidermis. These species have a C3 system of photosynthesis. This system obviously works ok even with some inefficiency in the final conversion to sugar. However, this inefficiency results in excess water consumption for that final step and even higher use of oxygen instead of releasing it to the environment. The problem is accelerated at higher temperatures in which more oxygen is consumed and less sugar produced.
Some plant species of tropical origin, such as the Teosinte species from which corn was developed, separated the photosynthesis two steps making the process more efficient even when under tropical conditions. With a change in the enzymes involved, the first step finishes with a 4 carbon ring instead of a 3 carbon ring. This compound is then moved to another cell’s chloroplasts for the final combination with carbon dioxide molecules and production of sugar. In C4 plants like corn, the mesophyll cells are not lined up close to the epidermis but dispersed closer to the vascular bundles where that second stage of sugar production takes place. Bundle sheath cells, in C4 plants, have the specialized chloroplasts that make the final product. Being adjacent to the mesophyll cells is essential to the efficiency of the process.
The effect of this cell arrangement and the slight change in enzymes in the chloroplasts of C4 plants and participation of the bundle sheath cells allows corn to become a greater user of light intensity than most crops. Whereas photosynthesis in soybeans and wheat peaks out at about 3000 ft candles (32000 lux) of light intensity, photosynthesis rate in corn increases to the brightest of sunlight (10000 ft candles or 107000 lux). It also explains why corn photosynthetic rates decreases with slight changes in light intensity such as shading within the canopy as well with clouds. On the other hand, this cell arrangement and unique photosynthesis process makes corn one of our best crops at removing CO2 from the atmosphere and for storage of chemical energy captured from the sun.
We benefit from the photosynthesis in plants, as it provides us with the energy for our existence and growth. Capturing light energy and converting it to chemical energy useful to us and other animals allows existence. Corn has an additional photosynthesis tool that makes it more productive in capturing light energy than most plants. This is addressed in several Corn Journal blogs including this one of 12/8/15.
Corn is planted in all continents and certainly is productive in many temperate zone areas of the world. But one important contributor to its success as a major supplier of carbohydrates to people and animals that they feed evolved in its original tropical past. Most plants have a photosynthesis system with an inefficiency that limits its productivity. This system, labeled as C3 photosynthesis, peaks in its ability to fully use total light intensity to about 3000 foot candles where-as unclouded sunlight has 10000 foot candles. In corn, with it C4 photosynthesis, it continues to produce carbs in direct relation intensity of the light with maximum photosynthesis in bright sunlight.
Carbon dioxide enters plants through holes in leaves called stomata. These structures also allow oxygen to escape from leaves to the benefit of all of us. Water vapors also go through the same stomata. Stomata open and close. At night they close with the benefit of avoiding unnecessary loss of water when photosynthesis cannot occur. But when plant tissue is stressed from lack of water, these stomata also close, limiting the water loss but also interfering with uptake of carbon dioxide for photosynthesis. C3 photosynthesis doesn’t make carbohydrates out of all the CO2 it absorbs, using some of it in other molecules. No problem when environment provides plenty of moisture, is generally cool and have long summer days, but some plant species that evolved under hot dry conditions evolved systems to overcome that limitation.
Teosinte, the species of origin for corn in Central America, has a C4 photosynthesis system. Plants with this character have additional structures in their leaves surrounding cells that perform photosynthesis. These cells function to reduce the loss of CO2 by causing these molecules to be recycled into more carbohydrates. The combination of extra enzymes and structures comes at some energy cost but the net gain is both more net carbohydrate and better utilization of CO2, even if stomata are closed.
Fortunately, corn that was moved out of the original dry hot environment, kept that C4 photosynthesis system. Along with that came the C4 photosynthesis advantages and its superior production of carbohydrates. Sorghum and sugar cane also are C4 plants but wheat, rice and soybeans are C3 and will not be able to match corn in carbohydrates per acre because of this trait. Although only about 3% of all plants species are C4, it does occur in a few plants in many plant families, suggesting that it can evolve independently. This would seem to raise hope that the efforts of the International Rice Research Institute and others trying to convert that C3 species to a C4.
Plants like corn are not the only organisms in the corn field that gain genetic variability through mutations and recombination from fusion of chromosomes from haploid nuclei. Although the pathogenic growth of most fungi is with the haploid form, eventually chromosomes from two mating types fuse, meiosis occurs, and new combinations of the two parent’s genetics are formed. This is usually realized when a single gene form of resistance in corn is no longer effective.
The range of sexual of reproduction methodology among fungi is wide. Cellular features of fungi are not completely different than plants and animals. Their DNA is in a distinct nucleus, they have mitochondria for transformation of energy to forms used for growth and function and ribosomes for protein production. While most fungal growth is separated into distinct cells, there are some such as in the Pythium genus in which the hyphae do not have distinct cells and have multiple haploid nuclei. Most of the life in most fungi, the nucleus has only one set of chromosomes, instead of the two sets found in most plants and animals. The process of sex has slightly different definition with fungi. Joining of two cells, and then of two nuclei followed by meiosis in which new combinations of chromosomes are distributed into haploid nuclei can occur without sexual distinctions between the two individuals, or, in some species, only with distinct mating types. It can result in creation of distinct (to us) morphological structures or within the normal appearing hyphae. The requirement for having two, or more, mating types certainly causes difficulty for us humans to completely understand the sex life of many fungi but genetic variability for characters that we follow in corn pathogens seems strong. Because of the ability to asexually reproduce quickly the unique genotype that has found a susceptible host, a new variant has been witnessed often in agriculture, whether produced by mutation or sexual recombination.
A lot of complex biological things are happening in the corn field beyond the biology of the corn plants.
Pathogens of corn have genetics too. A discussion of the fungal pathogen causing Northern Leaf Blight was discussed in this Corn Journal blog from 11/16/2017.
Northern leaf blight is the name usually given to the corn disease caused by the fungus Setosphaeria turcica. The original name for this fungus was Helminthosporium turcicum was given in 1876 and was based upon the pigment and shape of the conidia spores. Taxonomy researchers attempt to group closely related organism into a common genus name. Fungi have limited morphological characters to use with spores being the most commonly as a stable feature. It is indicative of the prevalence of asexual reproduction in this fungus that the genus name changed to different names based upon asexual characters from Helminthosporium to Bipolaris, Drechslera, Luttrellia and then Exserohilum in 1974. The sexual stage of this fungus was not confirmed until the 1950’s, initially named Trichosphaeria turcica and later in 1974 to Setosphaeria turcica. The time lapse between the recognition of this pathogen based upon it asexual conidial stage and identification of its sexual stage indicates the significance of asexual reproduction of this pathogen.
Setosphaeria turcica sexual reproduction occurs when hyphae of two mating types (MAT1 and MAT2) fuse, followed by the combining of the nuclei chromosomes, recombination and eventual segregation into new haploid nuclei. These form 4-6 individual spores within a sack called an ascus. Sexual reproduction does assure new genetic diversity but the rarity of finding the two mating types perhaps indicates that this is not the most significant source of genetic diversity of this species.
Those of us that have grown isolates of this fungus in artificial culture media frequently see differences in growth patterns, pigments and sporulation among the isolates. This pathogen is widely distributed on corn. Although it usually prefers cooler environments of 15°-25°C (59°F-77°F) for infection it is adapted to most temperate and semitropical environments. The higher frequency of both mating types in more tropical environments, especially in Mexico, suggests that it originated along with the early development of corn and perhaps was distributed with the crop.
Distribution to many geographic locations, exposure to multiple corn genotypes, haploid hyphae producing huge numbers of conidia has resulted in diversity within the species, whether we call it Setosphaeria turcica, Exserohilum turcicum or even Helminthosporium turcicum.
Current news shows concerns that the Covid-19 virus is mutating. The strand of RNA coding for the protein affecting infection has a mutation in one of its 1300 nucleic acid codes, resulting in the substitution of the amino acid glycine for the amino acid aspartic acid. This substitution apparently increases the infectivity by the virus.
Mutations occur frequently in plants, animals, bacteria, fungi and viruses. The random slight change in a single nucleic acid in RNA or DNA generally is of no consequence and not noted. But these are the changes that allows for evolution of new species, including the development of Zea mays from a Teosinte species about 8000 years ago. Mutations contribute to the adaptation of variation in pathogens to overcome resistance in their hosts. Races of Exserohilum turcicum, the pathogen causing northern leaf blight, that overcomes single gene resistance in corn is such an example. We benefit from some mutations and we fight others.
Mutations not only occur in nuclear chromosome DNA but also in of cellular organelles such as mitochondria. Such a mutation in mitochondria DNA inhibited some corn varieties to not produce pollen. Because mitochondria are carried into hybrids only from the female parent, hybrid seed production was made easier by reducing need for detasseling corn. Male fertility was overcome in growers field by using males with a mutation that overcame to mitochondria mutation and thus produced normal pollen in the hybrid. Unfortunately, the pathogen fungus Bipolaris (Helminthosporium) maydis had mutants producing a toxin that destroyed these mutant mitochondria resulting in the race t of the pathogen destroying a large portion of the 1970 corn crop in the USA.
All aspects of life interact with natural occurrence of mutations. Further discussions of corn mutations in Corn Journal can be found by searching mutations in this issue.
We generally characterize corn varieties by physical characters that we easily see. Plant height, grain color and hardness, ear height, standability and disease resistance are probably the characters we assume are mostly genetic expression. Each of these characters are mostly directly affected by 1-4 genes. The remaining 30000 genes in every corn plant are producing products not so easily observed but are really the ones affecting final hybrids performance.
The real action influencing everything the corn plant does is occurring in the cells. Not only are the 30000 genes in the cell nucleus on the 10 chromosomes being turned on at appropriate times to produce proteins active in cell metabolism but the DNA and RNA in cell organelles such as chloroplasts, ribosomes and mitochondria are active as well.
The breeding process eventually leading to developing inbred parents for hybrids offers many new combinations of the the genetics. Mutations naturally occurring along the way also contribute to genetic differences not easily detected visually. We may characterize hybrids by obvious features but inside the plant there are differences among hybrids. Although each plant is a single cross hybrid should be genetically identical, Individual plants within a breeding population is not.
It is much similar to the ease with which we classify individual humans by simple visible characters such as skin color or hair color without acknowledging that each individual is genetically different from the next person with same skin or hair color. We have between 20000 and 25000 genes in our chromosomes, and a couple hundred thousand years of genetic mutations within our species and apparently some crossing with related species along the way.
Just as with humans, corn genetics were affected by selection in specific environments. The resulting genetic diversity contributes to new combinations that will drive the future with important adaptations.
Corn Journal has discussed genetics that can be found in the search on Corn Journal page under genetics. One of those blogs from Corn Journal 9/14/2017 follows:
At least 32000 genes in the ten chromosomes plus the independent DNA of mitochondria and chloroplasts in corn plants. We know the function of relatively few of these genes. We have selected genetics based upon field performance for the traits that we desire for the most part but we don’t know the actual genes involved in establishing grain yield and standability. Certain physiological processes such as photosynthesis can be studies, discerning the enzymes that can be traced back to a genetic code. Based on mutations we can determine the genes involved in endosperm starch formation. Resistance to some diseases can be linked to specific genes.
But how about the genetics that determines number of stomata, allowing for passage of CO2 into the leaves, or loss of water. Do genetics influence the photosynthesis in stomata guard cells determining when they open or close? Chloroplast and mitochondria DNA influence the membrane structure of these organelles. Replication of chloroplasts and mitochondria must involve the interactions of genetics of these organelles with that of the host cells. Movement of minerals into cells and photosynthetic products out is partially determined by cell wall structures as influenced by genetics. Size and number of vascular bundles must be important to movement of water from roots to leaves and ears as well as carbohydrates from leaves to roots and ears.
Genetics influence corn stalk rind thickness, duration of life in pith cells and carbohydrate storage capacity. Root branching, formation of root hairs and ability to absorb water and minerals from the soil are affected by products of the corn plant’s DNA. Kernel number and size also limited by genetics. It is no wonder that corn has a lot of genes.
Many of these genes had to have been established in those Teosinte plants that humans tapped several thousand years ago. Natural occurrence of mutations and human selection of traits expressing adaptation to their environments and desires provide us with large genetic variability. Despite modern molecular techniques to study corn DNA, the complexity of interactions within the corn plants, we are still stuck with our somewhat crude method of field testing in several environments for the best hybrids. We do this with the knowledge that many unknown genes are influencing the final performance and the hope that there remain new genetic combinations that will lead to better performance in the future.
Membranes dominate the structures in corn cells, being major components of the endoplasmic reticulum, mitochondria and plastids such as chloroplasts. The nucleus of the cell also includes a double layer of membrane, composed of lipids and proteins. It functions as a gateway for movement of complex molecules and minerals in and out of the nucleus. As a segment of the chromosome DNA for a gene is activated to produce a RNA code for a protein, the RNA moves to a ribosome to hook the amino acids together forming a protein. Although some of the ribosome action occurs within the nucleus, much happens after the RNA moves through the nuclear membranes into other ribosomes in the cytoplasm. Auxins and other plant hormones interact on the activation of the DNA, requiring regulation through the nuclear membranes.
Membranes for each organelle of the cell require very specific proteins, each dictated by the DNA code. Many of those proteins are coded from the nuclear chromosomes but are also affected by the single chromosomes in mitochondria and chloroplasts. Amino acids with differing nitrogen, hydrogen and oxygen ions arranged around carbon chains determine the composition of proteins and the phosphor-lipids that compose membranes are critical to all cellular function. Eventual germination of the seed is dependent on formation during seed development and maintenance of membranes during seed storage.
Poor stands in the field are associated with weather stresses resulting in some seeds not germinating or delayed in germination, resulting in plants much later than adjacent plants. We look at the plant as a whole, but most of the damage is being done at the cellular level. Plant cells are not empty structures but are the vessels where the real action of the plant occurs. Membranes, those long chains of lipids and proteins, not only form barriers to control what enters the cell and its organelles like mitochondria, chloroplasts and ribosomes but also membranes are major components of these structures. These are sites where all of the corn plant’s life must occur. Maintenance of integrity of the membranes within the corn embryo cell membranes is critical to timely germinations.
Cell membranes are especially vulnerable to damage during the drying process of the seed and then the expansion after imbibition. They do self-repair, a process requiring both heat energy and supply of carbohydrate energy. Embryos removed from the endosperm will germinate, producing a shoot and root, but adequate heat energy must be supplies and no invasion of pathogens. But they need the stored energy from the endosperm to push through the soil to emerge and receive fresh supply of carbohydrates from photosynthesis.
Cool wet conditions in the field will result in inadequate repair of cell membranes, delaying the emergence of some seedlings. It may be greater with some individual seeds than others because of some seed production and handling conditions or some specific field situations.
More about membranes can be found in the Corn Journal in the search.
We generally think of corn genetic differences as expressed in grain yield and grain characteristics. However, genetics unique to each hybrid influences its reaction to environmental as well as appearance through out the season. Nuclear genes in each cell plus the independent genetics of some cell organelle such as mitochondria and chloroplasts are being expressed in the reactions to environments as well.
Basic gene function must be effective in all living corn plants except in those few with major mutations. Corn breeders attempt to select individuals that have the characteristics, and thus, the genetics, preferred by the ultimate use of the hybrid. Unseen gene products carry out most physiological functions without our intentional interference. Thousands of genes are regulated and activated for the growth and function of all corn plants without our direct genetic intervention. We do, however, attempt to select those relatively few genes that affect the products most desired by the user of corn. Each of these traits are inherited by relatively few genes.
We can select for flowering timing, relative ear and plant height, grain quality characteristics from the variability present within a breeding population. Resistance to each potential corn disease usually only involves 3-4 genes available in some genetic source within corn. Grain quality characteristics are mostly affected by only a few genes. The challenge is to select for these relatively simple inherited characteristics within the background of those other physiological and morphological functions influence by those thousands of other genes. Furthermore, the expression of those genes must be relative to the varied environments faced by the growing crops.
Added to this breeding difficulty, we must stabilize the genetics by selfing to make inbreds and then match inbreds to make a hybrid combination for repeatable performance in the field. It is no surprise to find appearance differences among hybrids within a variety display plot. Each hybrid exhibited desirable product performance to be commercialized. Each got there by slightly different genetic pathways and because of the necessity of having homozygous parents, each plant within a hybrid will appear identical to each other but different from the other hybrids. Characters such as shape of canopy, length of leaves, and color and shape of tassel are inherited and uniform within a single cross hybrid as the result of uniform homozygosity of the hybrid parents.
Homozygosity of hybrid parents results in uniform and identical genetics for each plant of the hybrid. This applies to each morphological character when the plants are grown in a uniform environment. This applies to corn seedlings as well. PSR has utilized this concept for 33 years assisting seed companies in assuring seed genetic purity of each lot of new seed production. Genetics affect all function and appearance of corn at all development stages.
We rightfully watch and care about the corn plants as they grow in our fields. We observe the field as a whole for uniformity of stand and expected growth rate. We may note some individual plants that are behind others or perhaps show some differences, perhaps with disease symptoms. We don’t see the individual cells within any of the plants where the real action is occurring. One of those things was described in this Corn Journal blog written in June 2016.
Corn plants now in much of the US corn belt are stretching upwards, for the most part showing little signs of stress. We have little cognizance, however, of the internal battles that are going on in each of those plants. All plant parts are exposed to potential invaders, through injuries, through stomata or other openings and through direct enzymatic attack from pathogens outside the plants. Plants have systems to fight the invaders by responding with anti-microbe chemicals or even initiating cell death to limit the damage.
One of the key components of that mechanism is salicylic acid. This chemical was known by Hippocrates about 2400 years ago in an extract from willow bark that could relieve humans with headaches. Yes, it is the main component of aspirin. Salicylic acid production in plants increases when cells are stressed from pathogens, drought, or toxins. It functions as a signal molecule, triggering the production of a series of proteins to limit the damage. Of course, the response time for salicylic acid production and consequential protein production to stop the potential pathogen is dependent on the plant genetics and nutrition. Pathogens, no slouches in evolution either, often include mutations to slow down the production of salicylic acid by either tying up its component compounds or interfering with the production of the resistance compounds. It’s a battle out there!
It must be human nature to try to make complex things into simple. We see this in politics, economics and probably many aspects of human relations seem to want it simple, even if it isn’t. Those of us that have studied corn and its diseases and certainly anyone growing a corn crop know that the actual environmental interactions with the crop is complex but we still are inclined to try to simplify the interaction between a microbe and the corn plant.
In reality, corn roots are invaded by a variety of fungi and bacteria, some of which simply live off of plant products and don’t cause any visible harm to the plants. Some would call these organisms as endophytes (living with plants but not causing damage).
Presence of these may be detected by the host plant, causing it to produce compounds that restrict the growth of these endophytes into more active plant cells. In some cases, this appears to restrict more active pathogens. Species of the genus Trichoderma have been noted as a type of biological control, but also some studies have noted fungal species of Fusarium, Acremonium, Aspergillus, and Botryodiplodia have similar interactions with corn.
It becomes more difficult to classify organisms that may once be a harmless endophyte but later, perhaps as the plant begins senescence either because of age, stress or simply shortage of adequate products of photosynthesis in some tissues. Cells in these areas perhaps cannot produce the resistance products needed to stop the foreign organism from killing weakened host tissue. Do we now designate the organism as a pathogen?
Often it is easier to name a disease, implying that an aggressive pathogen attacked the plant is appealing. Often, however, looking at the more complex aspects that allowed the organism to attack the plant could help avoid the repeat in the future. With many plant physiology, environmental and micro-organisms dynamics it is difficult for research as well as to adequately and completely describe.
Major growth regulation in corn is done with hormones. There are three major types: cytokinins, auxins, and gibberellins. Each has specific functions in the metabolism and growth of the corn plant.
Cytokinins, originally produced in the corn seed scutellum, migrate to the root tip where they stimulate cell division. Later, cytokinins trigger the cell division in all the growing points of the corn plant. These include the lateral root tips, the stem meristem and each of the lateral stem buds, including the one (or more) that becomes the ear. Cytokinins also are active in delaying senescence of leaf tissue. Zeatin is a common cytokinin in corn and other plants.
Auxins influence cell elongation, stimulating it in stem cells but inhibiting it in root cells. Auxins inhibit elongation of lateral buds countering the cytokinin effect of cell division. It is the balance of the two hormones that affects corn plants tendency to tiller. The most common auxin is indole-3-acetic acid (IAA). Apical dominance in plants is controlled by this auxin. This auxin also influences flowering and inhibition of abscission layers at the base of leaves and maturing kernels. Herbicides such as 2,4-D and dicamba are auxins that disrupt plant growth and development.
Gibberellins include more than 100 compounds that effect shoot elongation, seed germination and maturation of grain. These hormones are produced in root and stem meristems as well as tips of new leaves and seed embryos. Gibberellic acid is the most common compound that can be artificially added to plants. Gibberellins tend to delay kernel maturation and are effective in determining plant height.
Synthesis of these hormones is determined by genes, of course. Plant height of different varieties involves these genes as the hormone synthesis involves several steps, with a few major genes causing dwarfness, and multiple genes affecting slight differences in plant height. Nearly all aspects of corn plant growth is affected by hormones.
Soil microbes also produce auxins and cytokinins that can affect root development and ultimately affect phosphorus uptake by changing the balance of hormones in roots. Potential microbial seed treatments attempt to use these interactions to stimulate early corn growth.
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.