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.
From nodal bud to corn ear.
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.
Corn Silk, water and history
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.
Water movement in corn plant
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.
Corn and C4
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.
Variability in pathogens
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.
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.