It is amazing how humans have adapted a tropical annual plant like Teosinte to produce large starch-storing food for people and their livestock. Along those few thousand years of selection as corn was moved to temperate zones it became adapted to shorter growing seasons and cooler environments. Among the remaining challenges is adaptation to cool spring environments and invaders. Pathogens and other feeders of the nutrients in the corn seed and seedling have followed the adaptation to these conditions as well. Resistance to such invaders is affected by the host plants biology and environment of the young corn plant. Sometimes it is not easy for us to sort out the significance of the outside invader of corn, the host interactions and environment.
The fungal species of the genus Fusarium have a complicated relationship with germinating corn seedlings. The most studied species, Fusarium verticilloides (formerly known as Fusarium moniiforme and its sexual stage as Gibberella fujikuroi commonly is found in germinating corn seeds. It often is found in corn plants without symptoms of damage and therefore is characterized as an endophyte because it appears to live within the plant tissue but does not always cause symptoms. It is not uncommon to see growth of this fungus from germinating seed in paper germination test. A study published in 1997 (Plant Dis. 81:723-728) compared seedling growth from seed artificially infected with this fungal species with those that were not infected. There was no difference in germination percentage between infected vs uninfected seed. There was a slight size difference favoring the uninfected seedlings at 7 days after planting but at 28 days those growing from the infected seedlings were slightly bigger and with more lignin in cell walls. Is this because of a hormone (gibberellin?) produced by the fungus or because of some defense compound produced by the plant? The fungus was easily recovered from the seedlings but less from the older leaves. Infected plants showed no symptoms of disease.
It is clear that Fusarium verticilloides can be damaging to germinating seed sometimes but I don’t think all the factors are clearly understood. Is the difference caused by the strain of the fungus, the host plant or the environment? I know from experience that it is so common to find Fusarium growing from a dead leaf sample that one tends to ignore it. It seems to live in much of corn plant’s tissue. It often leads to confusion with diagnosis of problems including stalk rot, almost as if one cannot find other fungi usually associated with rotting stalks, there is always Fusarium. It’s complicated!
Corn seedlings are frequently attacked by a fungal-like organism of the Pythium genus when the soil is extremely cool and wet. Pythium belongs in a group of organisms called Oomycetes. These were once considered fungi, but more recent research has shown that they are more related to algae. Other pathogenic oomycetes include those causing diseases such as downy mildew of several grasses (including crazy top of corn) and Phytopthora root rot of many crops. Oomycetes were considered fungi because they usually produce filaments and spores plus they absorb nutrients from plants and animals. They differ from fungi by mostly lacking individual cells within the filaments, resulting in many nuclei within the long filaments. Most fungal cell walls are composed of chitin whereas the oomycete filament walls are cellulose, a difference that relates to the host resistance system. Often the first signal of fungal invasion is of the chitin, triggering the production of defense systems. Detection of an oomycete invasion requires a difference in plant chemistry.
Oomycetes frequently produce swimming spores that are released from overwintering spores (oospores) in the soil. Pythium spores, with flagella, swim to host root cells on which they grow hyphae to invade the host. Consequently, they cause the biggest problems in fields that are temporarily flooded with heavy rains and cool conditions. Studies have shown that 55°F is optimum for most Pythium species infecting corn in Midwest USA. The low temps probably slow down the host plant’s growth rate and inhibit potential competition from soil fungi.
As many as 18 species of Pythium have been shown to be pathogenic on corn. Often, they destroy the outer layer of the new root tissue. If this occurs before secondary roots become the main source of water uptake, the seedling will wilt. Although seed treatments can offer some protection, genetic diversity within a species often includes resistance to most treatment chemicals. Pythium genetic diversity also includes ability to attack rotation crops such as soybeans. Pythium species are easily isolated from soil but completely accounting for all the species or diverse genetics in regard to effective seed treatments or genetic resistance is not easy.
Control of the disease is best done with controlling water holding capacity of the soil.
Corn seedling vigor is affected by their environment including soil water-holding capacity, temperature and pathogens.
Seeds are planted in environments that vary every few inches for water holding capacity, organic content and microbes. Furthermore, each individual seed varies slightly in its cellular membrane status. With imbibition causing swelling of the membrane bound cell contents, some seed can have problems getting effective metabolism for early cell growth to push out the root and stem structures.
Cell metabolism includes producing the response to attacks by potential pathogens in the soil. These anti-pathogen chemicals (phytoalexins) are usually produced with a complex system of detecting the microbe and concentrating the phytoalexin into the area of the attack. Weakened seed not only are likely to release more carbohydrates and proteins into soil because of membrane injury, but also be less capable of responding to the microbes invading root and mesocotyl tissue.
Diagnosis of seedling disease becomes complicated also. Pathologists can isolate a fungus such as a Fusarium species or an oomycete like a Pythium species, but the actual cause probably involves some interaction between the microbes, metabolic quality for the ‘diseased’ seedling, and a complex environment not only providing potential pathogens but also affecting the seedlings metabolic rate. Soil organisms are affected by the environments as well. Leakage of carbohydrates directs their growth toward the seedling roots but temperatures favor some over others. Pythium’s swimming spores do well in cool wet environments but can be inhibited by certain seed treatments that have very little effect on fungi such as Fusarium species. Other seed treatments can inhibit the latter group of microbes but are less effective against Pythium. Corn seed genetics and seed quality can be greater factors than either group of chemicals. Cold wet heavy soils for a prolonged time can overcome all methods of defense.
After the stress on the seedlings is reduced, remaining plants that emerge can give normal production especially if they are uniform in growth with adjacent corn plants. The metabolism of these plants will promote the recovery and normal root growth. Those plants that survive but emerge later than adjacent plants will have difficulty competing for light and mineral uptake which will be reflected in grain productivity.
It is remarkable that we usually get a high percentage of ‘normal’ corn plants with all the potential of problems surrounding that small seedling.
Corn seedlings have recently emerged in fields of North Central USA. It is exciting to see the new plants arrive. Most of the activity is occurring out of site, however. Beneath the soil surface, close to the seed, the primary root is growing downwards, and the apical meristem is producing new cells via cell division. Heat energy is contributing to driving the cellular activity at both ends of the young plant.
Imbibition of water into the seed leads to activation of the cytoplasm within cells. Most of those processes occur along membranous components of mitochondria, ribosomes, plastids and the endoplasmic reticulum. Hydrated proteins now acting as enzymes in breaking down starch molecules stored in the endosperm and glucose and sucrose molecules are moved thru the scutellum to embryo cells. Diffusion of these sugars through pores of these cells, with cooperation of the cellular membrane and endoplasmic reticulum, these complex molecules composed of carbon, hydrogen and oxygen atoms are transported to mitochondria where they are further metabolized to create the ATP energy needed for other cellular activity. This cellular respiration process allows further cell construction as cells divide in the root and shoot meristems. Elongation of hypocotyl cells, as well as meristem cell division pushes the tissues from the kernel.
ATP (adenosine triphosphate) results from the energy transfer from electrons holding the glucose atoms together to form ATP, releasing CO2 and H2O. This process occurs in the mitochondria. These membrane-intense organelles apparently vary in number and efficiency among corn varieties. Mitochondria, having their own DNA, and yet is dependent upon the rest of the cell for its structural components, are transmitted to the next generation only through the egg cell. This is probably why different female parents of corn hybrids vary in time for seedling emergence and vulnerability to imbibition chilling damage.
The remarkable growth from a corn seed during a few months all coming from the cellular activity as coded in the genetics of the seed and its environment as assisted by the corn grower.
Emerging corn seedlings initially utilize the primary root for absorption of water and nutrients as this root tissue is powered initially by the endosperm and then the photosynthesis from the first seedling leaves. Initial stem nodes remain under the soil surface. Soil temperatures affect the growth rate of the seedling but by time the third leaf is visible inside the whorl of the seedling, lateral secondary roots emerge from the nodes below the soil surface. Photosynthate are moved to these root tips stimulating more cell in new root tips as hormones direct the growth down. Cells outside the dividing root tip cells develop a strong epidermis allowing the root to push through the soil.
Newly formed cells of the elongating roots, near the root tip includes epidermal cells with thin-walled protrusions called root hairs. These protrusions into the soil affectively expand the net root surface area of the root allowing flow of water and nutrients into the root by osmosis.
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 more lateral root branches form, along with their root hairs the water and nutrients shipped to the developing seedling leaves the upper leaves are formed and photosynthesis increased.
This early coordination of shoot and radical root emergence from the seed. Initially with energy from the seed endosperm and then from photosynthesis, allows the new plant to develop for its annual lifetime.
The initial roots growing from the embryo radical supply the emerging seedling with water and nutrients. Other changes then occur.
After the first corn leaves emerge, the hormonal message to the mesocotyl tissue is to stop pushing upwards. Apical meristem, at the tip of the mesocotyl is now below the soil surface where the first leaf is attached. Photosynthesis now drives the metabolism of the young seedling as it switches from dependence upon the seed endosperm for carbohydrates. Cell division in the meristem produces new leaves, each attached to the young stem under the soil surface and attached in distinct clusters of newly dividing cells called nodes. These nodes are then stimulated to produce roots at about the time the 4th leaf appears in the young seedling. Because the roots are being produced from stem tissue, they are called adventitious roots. As the primary root, that had grown initial seed, loses its energy source, adventitious roots become the main roots for the plant.
The first 4-5 nodes of the young stem remain underground, each producing the roots for the plant, even as the first leaves remain attached at the same locations. What appears to be stem in a 4-5 leaf seedling is a compilation of leaf sheaths tightly wrapped together while the actual stem remains beneath the surface. The underground stem portion, formerly attached to the mesocotyl, with adventitious roots becomes known as the crown. Eventually the mesocotyl deteriorates as it is deprived on nutrition and loses resistance to the many soil organisms.
As the stem growing point eventually emerges above the soil surface a few exposed nodes will often form the brace roots to further support the adult plant. A lot of changes in a relatively short time after seed is planted.
As we look for those first seedlings poke up to reach the light it is easy to miss the amazing feat that has been accomplished. Breeder efforts is selecting genetics, seed producers care during production, and growers efforts with soil preparation and care in seed planting results in the biology of each seedling pushing the shoot up and root down. A cooperative effort by all results in a uniform emergence.
The first appearance of corn seedling tissue emerging from the imbibed seed is the new root. This young seedling primary root can be considered as three regions: meristem, elongation and mature. The cell elongation area provides the main initial force for pushing through the kernel pericarp. Cell elongation and maturation involves production of many new molecules composing the cell walls as they grow. The pectins, hemicelluloses and celluloses that compose the new cell walls are composed of several sugar-related molecules joined together through specific reactions, assisted by enzymes, heat energy and chemical energy such as from ATP.
Energy and components for the biosynthesis of these cell wall components comes mostly from the endosperm. Starch in the endosperm is broken down with enzymes into sucrose molecules, moved to root cell cytoplasm where other enzymes break down the sucrose into glucose and fructose. With other enzymatic action, the fructose is made into more glucose. Modification of these sugars allows other new carbohydrate based molecules that become linked to form the more complex polysaccharides such as pectin, hemicellulose and cellulose for the new cell walls.
We see what looks like a rather simple process- seed swells, root protrude and a few days later the stem emerges from the seed. What we don’t see is a complex utilization of stored energy, production of complex proteins some of which act as enzymes assisting in linking molecules together and thus giving outer strength to cells. Also, unseen is production of anti-microbial compounds to ward off the many organisms attracted to the very molecules stored and manufactured in the seed. We don’t witness the genetics that programs for these processes. Humans successfully selected for these features from a wild plant species, adapting it to worldwide growing environments. The complexity of corn seed germination still can be challenged in a cold, wet spring such as being experienced in the central U.S.A. spring, but it is amazing that these processes work even under tough environments
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.