Corn leaf epidermal cells are protected from the many fungi that would benefit from the cells’ nutrients. Major protection comes from the hydrophobic wax deposited on the exposed surface of the epidermis. A few fungal species have evolved methods to overcome the waxy protection. The spore of such a fungus germinates when moist, growing a hypha strand on leaf surface. Adhering with some mucilage released from the germ tube tip. It chemically detects and links to molecules in the wax. At that point, the cytoplasm of the spore flows into the swelling, special hyphae now called an appresorium. Now a barrier (septum) develops cutting off the spore from the growing appresorium.
Turgor pressure in the appresorium hyphae increases as more water moves by osmosis into the structure. The region of the appresorium adjacent to the leaf cell wax produces a specialized hypha called a penetration peg through the wax and into the host epidermal cell. From there haustoria develop, penetrating cells and absorbing more nutrients. The northern leaf blight corn pathogen Exserohilum turcicum uses this basic method to infect leaves as the hyphae branch to spread elsewhere in the leaf. Most corn leaf pathogens use this basic method to enter the plant, although the rust fungi enter the stomata instead of drilling through the wax.
This first line of the corn plant’s defense is effective against the many saprophytic fungi of the corn plant’s environment but more interactive defense systems are stimulated into action after the pathogens have penetrated the leaf epidermal wax.
Corn leaf blades, like leaves of most plant species, have small ‘hairs’ extending from some epidermal cells. These are called trichomes. The functions of trichomes are not fully understood although in some species they are associated with insect resistance. Nearly all corn varieties have these structures, at least after the 6th leaf stage, an indication of some significant importance to the corn plant.
Trichomes originate from the epidermal cells and vary with in shape within and among maize varieties. There are mutants that extend the smooth epidermis of the younger plants to the older leaves. Although single gene mutants can nearly eliminate all macrohair trichomes, the actual production would still be under the control of several genes and apparently influenced by a plant hormone such as gibberellic acid.
Although corn trichomes probably lead to less insect injury to leaves, they have been associated with infection by a few pathogens. Three species of the fungal genus Fusarium (F. graminearum, F. proliferatum and F. verticilloides) have been shown to infect the epidermal cells via the trichomes (http://www.sciencedirect.com/science/article/pii/S1878614616300654 ).
The bacterium causing Goss Wilt (Clavibacter michiganensis subsp. nebraskense) has been shown to occasionally enter the leaf tissue through the trichome at the point where it attaches to the epidermal cell. This does not occur with the frequency of leaf injury being associated with entrance of this pathogen. It does explain the occasional Goss-diseased plant that has no visible physical injury similar to what we have seen a few rows away from inoculated plants in our disease nursery.
A review of corn trichome genetics can be found at http://www.genetics.org/content/166/3/1451
Corn leaf whorl
As the corn plant approaches the 10-leaf stage, before extensive stem cell elongation, cells in the new leaves rapidly elongate. Newest leaves are wrapped around the older ones forming a whorl of leaf blades until the leaf blade elongate enough to separate the leaves. If the growth is too rapid the whorl tends to get very tight or twisted. This is usually associated with extra warm temperatures, especially when following days of cooler than normal temperatures. Growth hormone herbicides can have the same effect. If twisted too tightly for more than a few days, sections of the leaves do not get sufficient light for chloroplasts to produce pigments, resulting in a temporary yellow leaf band when those sections finally emerge. Twisted whorls has been associated with drought conditions apparently because of the effect on cell elongation.
The corn leaf whorl also provides a nice warm, moist environment for pests and pathogens to infect the plant. European corn borer (Ostrinia nubilalis) larvae and corn leaf aphids (Rhopalosiphum maidis) are favored in this cozy environment. Uredospores of the rust fungi Puccinia sorghi and Puccinia polysora often being their initial infection in the humid, moist environment of the leaf whorl. Not only does this allow the spores the perfect germination and infection environment but the whorls provide a pocket to catch the spores arriving with rain. These fungi do not overwinter in the central USA corn belt but the spores are carried north by winds from Mexico and Caribbean corn plants. Spores deposited by the rain into the Midwestern leaf whorls find a good home to become established in the field, and to spread to the rest of the plants if the season weather favors them.
Spores from many corn fungal pathogens overwintering on the previous season’s corn debris often make the initial infection in the whorls, again because of this favorable environment. This is usually revealed with a band of a disease symptoms in the leaf a few weeks later, as the leaves emerge. The smut fungus Ustilago maydis, is believed to enter corn plants only through injured tissue and the leaf smut phase is associated with injury to the whorl when it is twisted.
As with most parts of the corn plant’s biology, a lot is going on that is not obvious even during the first few months after planting.
Adhesion of pathogens to corn cuticle
Among the functions of leaf cuticle is providing protection from potential invasion by microbes. This waxy layer does ward off many microbes as the rain washes them off the leaf surface. However, successful pathogens develop mechanisms to adhere to the hydrophobic cuticle until the germinating spore produces hyphae to successfully invade the leaf. Many pathogenic species such as Exserohilum turcicum, cause of Northern Leave Blight of corn, has a thin gelatinous outer layer that sticks to the leaf cuticle long enough for the germinating spore to set up ‘drilling station’ called an appresorium to enzymatically forced entrance with a penetration peg into a leaf epidermal cell.
Another approach is to develop an adhesive pad that includes enzymes that partially dissolve the cuticle wax. Spores (uredospores) of corn rust the fungi, Puccinia sorghi and Puccinia polysora, produce such a pad when moistened on a corn leaf. After securing itself the cuticle, it germinates to grow thin hyphae across the leaf surface until it finds a stomatal pore, apparently by touch, to enter the leaf between cells. After reaching a living mesophyll cell, it produces an appresorium to attach to the cell and then a peg into the cell wall. It does not penetrate the cell membrane but instead produces a membrane adjacent to the host cell membrane called an haustorium. This allows this obligate parasite to keep the host cell alive as it provides nutrients to the rust fungus. Eventually the rust fungus produces more uredospores to spread the disease. These spores are very easily picked up in winds for long distance distribution.
The gray leaf spot fungus Cercospora zeae-maydis has a slightly different tactic to overcoming the potential defense mechanism of the leaf cuticle. Whereas most pathogens must enter to leaf tissue within a few hours or the spores will dehydrate after exposure to the dry air, C. zeae-maydis progresses slowly on the leaf surface as it requires nearly 100 hours of 90-100% humidity to slowly grow on the leaf surface until it finds a stomatal opening from which it enters the mesophyll.
Leaf cuticle layers offer a system to prevent many fungi from entering living plant tissue. Relatively few of the many fungal species exposed to the plant have mechanisms to overcome this first line of defense. After entry into the plant, other corn plant methods must be employed to limit the disease.
One protection from excessive water loss in corn is a waxy layer deposited on the outside of the leaf and stem epidermis cells. This cuticle is composed of waxy polymers that are hydrophobic. Leaves younger plants up to V4 stage tend to have more crystalline forms but older leaves form smother, more flat, smooth formats. Cuticular waxes are manufactured in the cytoplasm of the epidermal cells as a solute in solvents such as alcohols, ethers and fatty acids. The solution move through the cell walls to the surface, allowing the solvent to evaporate and deposit the wax layers.
At least 18 genes, referred to glossy genes have been identified affecting cuticle formation and composition of corn leaves. Various combinations of these genes influence the thickness of wax as well as the nature of the cuticle. Most corn varieties have a crystalline form in the younger leaves although some gene mutations will extend crystalline type of wax beyond these development stages. The change from crystalline wax to the smoother wax apparently is related to the tendency for herbicides (plus surfactants) to be absorbed more in mature leaves than in immature leaves.
Cuticle waxes provide several important functions to the corn plant. Loss of water via evaporation through epidermal cells is greatly reduced. Waxes become the initial barrier for potential pathogens both because of becoming a structure for the pathogen must overcome but also, being hydrophobic, encouraging water runoff. Only a few fungi and bacteria species can manage to overcome this protection. Cuticle waxes also offer protection against UV radiation with its potential mutagenic effects.
Corn plants respond to dry atmosphere by producing more wax on leaf surfaces. Leaves of corn grown in the drier air of Nebraska develop a thicker waxy surface than the same hybrid grown in Ohio.
Push and pull of water in corn
Maize, like all plants, has special structural adaptations to the chemical characteristics of water. Water molecules share hydrogen bonds with each other resulting in a cohesiveness between them. This plus a tendency for adhesiveness to certain xylem wall components allows for capillary action in the small xylem tubes.
Water moves from soil through the cell walls of the root by process of osmosis. Higher concentration of water outside the cells because of solutes such as minerals or sugar in the cell cytoplasm draws water into the root tissue mostly through the root hair extensions of the epidermal cells. The same physical phenomenon causes the water to move to the dead xylem tissue. Meanwhile, water on the surface of the mesophyll cells in corn leaves, exposed to open stomata, evaporates into the atmosphere. This also is due to the cohesive nature of water molecules as the moisture in the air has a lower concentration of water molecules than that between the cells beneath the open stomata. A small percentage of the water is tied up by photosynthesis and some is utilized as a medium of cell metabolism but most escapes through the stomata.
Because of the osmosis, water is pushed into the root cells and by cohesion-adhesion, every molecule of water lost through stomata is replaced by a molecule traveling to the leaf. Breakage of the capillary via insects or pathogens can cause a wilt in a small area of the plant but xylem lateral connections usually avoid complete wilting. For example, the northern leaf blight fungus Exserhilum turcicum plugs the xylem for a small area of the leaf making a wilted lesion of about 2-3 inches but is eventually stopped by the plant resistance system. Water moves around the wilted area and pulled up through the xylem tissue, some going via osmosis to living cells and most moving out into the atmosphere through stomata.
It is the molecular nature of H2O resulting in cohesiveness that leads to the push of water into the roots and the pull of water up and out of the corn plant.
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.
Corn root expansion
At about the V3 stage of development, the primary root function begins to be replaced by the nodal, secondary roots. Energy provided by photosynthesis in young leaves, and heat, drive the production of the metabolites for cell division and cell elongation in these young root tissues. Whereas auxin hormone causes increased cell elongation in stem and leaf cells, auxin reduce this activity in the root cells. Consequently, although the nodal roots initially emerge horizontally from the stem nodes beneath the soil surface, gravity causes more auxin to accumulate on the lower root epidermal cells. This results in longer epidermal cells on the upper side than on the lower side, effectively turning the root growth downwards.
Root tip meristem cells rapidly divide, producing the root cap cells below to protect the dividing cells as it pushes through the soil and functioning root cells above the dividing cells. Outer layer root cells composing the epidermis are thin-walled and porous to water via osmosis. A short distance from the meristem of the root tip, epidermal cells form protrusions (root hairs), effectively expanding the surface area exposed to water and minerals of the soil.
Cells in the core of the new root differentiate to form vascular tissue that connects to the stem vascular tissue through the nodes. This vascular tissue allows transport of water and minerals upwards through the xylem and carbs downwards through the phloem. A few cells in this vascular portion of the young root maintain cell division capability, becoming stimulated by another group of hormones (cytokinins) to increase cells laterally, pushing through the epidermal cell layer becoming lateral roots with their own root meristems.
As with all aspects of corn growth a combination of genetics and environment influences the growth of the root system. Total volume of roots and depth of root growth tendencies will vary among genotypes. The fibrous nature of corn roots not only increase absorption from the soil but also provide support for the stalk as it elongates.
Corn Seedlings structure
The stem of a young corn plant, up to emergence of 4th or 5th leaf is below ground. The plant is supported by the tightly-wrapped leaf sheaths. Corn leaf sheath structure further supports this function as they are composed of vascular bundles with xylem tubes surrounded by lignin deposits. Vascular bundles carry out the important functions of xylem tubes for transporting water to the leaf blades and phloem tissue to move sugars to the roots and that growing point of the stem near the soil surface.
Leaf blades of these young plants develop more structures adapted to their photosynthesis. Genetics largely affects the shape of the leaf blade but each part is differentiated to carry out the functions. Epidermal cells include stomata that provide openings for absorption of carbon dioxide and release of oxygen molecules. Epidermal cells are often covered with a waxy cuticle functioning to reduce excessive loss of water. On the other hand, the movement of water through stomata and other small pores (hydathodes) on the edges of the corn leaf blade through evaporation and the cohesiveness of water molecules results in the pulling of water from the roots to the leaves through the xylem.
Majority of photosynthesis is carried out in the central part (mesophyll) of the leaf blade in the chloroplasts. These organelles, with their own genetics controlling much of their function and division, are the actual sites of photosynthesis. Other cell genetics control the movement of essential components of the photosynthetic process as part of the symbiotic relationship between chloroplasts and rest of the plant. Mitochondria, another symbiont with all living plant cells, are the sites for transferring the energy captured by the chloroplasts to useable forms of chemical energy allowing for more growth of young corn plant tissues.
Mitochondria in the meristematic tissue of the stem, still below the soil surface, also provide the energy for production of the nodal root system, greatly increasing the absorption of water and minerals, thus increasing the momentum for growth of the young corn plant.
Genes of the nucleus greatly influence the coordination of these cellular functions and structures within the constraints of the environment. Genetic diversity within the species is visible with close observation of these young corn plants but much more is happening out of our sight
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.