Corn seed dried properly to less than 15% moisture allows the mitochondria in the embryo cells to remain intact. Respiration in these mitochondria continues at a very slow rate, releasing sufficient energy to maintain membrane integrity of cell organelles. Genetics, especially of the female parent that supplied the mitochondria from its egg, affected this success as well as the physical conditions during seed development and harvest. Success gets revealed when the seed is planted. That process was described in Corn Journal blog 4/25/2017.
Very soon after the corn seed is planted, imbibition begins. The H2O activates the membrane-bound mitochondria to respire, providing energy for protein production. The enzymatic proteins include those that digest the starch stored in the endosperm into more sugar molecules to be transported through the scutellum to other cells in the embryo, resulting in more energy available to produce structures for cell elongation. Heat energy provides a regulatory function affecting the speed of this germination process. Imbibition occurs at any temperature but metabolic activity in corn is generally thought to be very low if seed environment is below 50°F. Speed of germination increases as the temperature increases.
Membrane integrity within the seed also affects the net speed of this process. Those individual seed with more damage are slower to sufficiently activate the system and thus slower to activate the metabolism needed for cell elongation in root (radicle) and the shoot sections of the embryo. Cool environments, delaying membrane repair, may result in death of the imbibed seed before the shoot can emerge from the soil. Some of these seed, even after warmed manage only to extend the root through the outer wall for the kernel, the shoot never emerging. Other weakened seed may finally get enough momentum to push through the soil surface but days after the healthier seed have emerged, resulting in a season-long competitive disadvantage. Heat energy during germination affects the severity of the effect of membrane damaged seed.
Microbes in the soil are generally warded off by products of seed metabolism in healthy seed. Those individual seeds that are slow to generate sufficient energy for growth are also more easily attacked by microbes, further slowing the germination process. Seed treatments are useful in giving the damaged seed more time to successfully germinate. Healthy seeds can successfully produce normal seedlings despite surrounding common soil microbes but those weaker individuals need the extra protection.
Imbibition is the movement of water through a membrane. This occurs in a corn seed when placed in a moist environment.
Corn seed, dead or alive, will allow water to enter through the pericarp, causing the kernel to swell. Dry cells in the embryo retain many membrane-bound structures including mitochondria, plastids including chloroplasts, nucleus and endoplasmic reticulum. Cellular membranes are composed of phospholipids and proteins organized in a manner that regulates the biological function of the cell organelles including regulation of movement of products in and out of the organelle.
Membranes in a dry corn seed cell are only slightly active, oxygen to pass through, for example but have more of a gel like structure. Within a few hours of imbibition, the structure changes as the phospholipids become moist and swollen. Resulting metabolism with activation of respiration in mitochondria, fueling gene translation in the nucleus, movement of RNA on the endoplasmic reticulum and production of protein in the ribosomes. The water plus metabolism causes the radical part of the embryo to elongate and the germination process has begun.
Two potential problems can stop this process. The seed may no longer have sufficient structural integrity, possibly because the aging process while dry no longer maintained the metabolism needed for maintenance. A second problem can be that the imbibitional process caused breaks in the membranes that were not adequately repaired during those first few hours of swelling as water moved into the cells. Membranes do have the capacity to self-repair and often do when metabolism is active. However, this process is temperature related, and in corn this repair process is very slow when temperatures are at about 50°F. Imbibitional chilling injury is the term used to reflect poor germination of some seed when planted in cold soils.
Every seed within a lot, although genetically identical, has had a slightly different environment experience. Location on the ear, exposure to insect or fungus, location in the drier, handling in the sheller or bagging processor all could affect its tendency to cellular injury. This usually if most profoundly expressed when it imbibes water under cold conditions. Some seed may reflect this by only delaying the germination as it repairs sufficient membrane for metabolism to germinate although later than the other seeds.
From Corn Journal 03/08/2018
It is sad that we, and corn seed, age. The mechanisms between us and them may be similar in that mitochondria are probably involved in all deteriorating living cells. These organelles which can number a few hundred in a cell, are the main sites for transformation of stored carbohydrates into useable energy for other cell functions. Mitochondria have their own DNA and are composed largely of membranes. Dehydrated seed results in mitochondria functioning at a very low level resulting in being unable to repair deteriorating membrane structures. While at very low kernel moisture levels (6-14%?) and cool temperatures (less than 50°F (?) further damage is limited. Precise moisture percentage and temperature for best storage of maize seed probably varies for genotype and seed condition but the general concept remains.
Seed imbibition of water is a physical phenomenon with little inhibition from the pericarp or seed coat. Seed treatments added to seed can slow down the imbibition, apparently giving the renewed mitochondrial function more time to repair damaged membranes. On the other hand, only increasing kernel moisture slightly can cause more membrane damage to occur, but not repaired. Increasing moisture more while at low temperatures (50°F) has the same effect. Corn seed planted in cold soils will imbibe water but the low temperature inhibits normal cell function, including repairing mitochondria. Those individual seeds with the most mitochondrial damage are likely the ones that struggle to germinate when the soil temperatures do heat up.
Seed producers are aware of the significance of inadvertently adding a small amount of moisture, such as from a seed treater before bagging by designing their process to limit the water and allowing for drying after application. I recall a case in the Thailand in which a new fungicide seed treatment was applied to control downy mildews but the humid environment did not allow the seed to dry after application. Seed germination quality quickly deteriorated as a result. Accelerated aging test of corn seed is based upon placing seed in an environment of 100% humidity and 113°F for 3 days, then planting in germination test to record the reduced germination. It is intended to predict the viability of the seed after storage. It is notable that even under this condition, all seed within the sample are not equally affected. Some germinated normally, some eventually and some not at all. This is typical of normal, well treated aging seed lots. Each individual seed is in a slightly different condition. We expect maximum performance when emergence is uniform. Seed quality is a factor influencing this trait.
From Corn Journal 3/16/2017
We all want things to be simple but it is amazing how rarely this happens
After overcoming the leaf surface protection against fungal invasion, the battle inside the leaf begins. This corn journal blog in 2018 illustrates the next phase of the battle between host and pathogen.
A recent report in Science (Vol.359, 1399-1403) describes how a single fungal gene controls plant cell-to-cell invasion by the rice blast fungus. It is an interesting description of how a plant pathogen manages to invade a plant cell and manages to travel cell-to-cell, evading the plant immunity system, eventually killing significant sufficient tissue to sporulate and spread to other plant tissue. This study involved a lot of biochemistry and microscopy and uncovered interactions that are probably common to other fungal-corn interactions.
Living plant cell walls have microscopic holes, called plasmodesmata, that allow passage of sugars and proteins to adjacent cells. These holes are smaller than normal fungal filaments (hyphae). In this study in which the pathogen Magnaporthe orzae, cause of rice blast disease, initially invades the plant by forcing the outer leaf cells with an appresorium, to occupy a cell but maintaining the cytoplasm in the cell. The hyphae of this fungus reduce the size of the hyphae to about 1/10th to squeeze through the plasmadesmata into the next cell. The plant cell resistance includes reacting to the presence of the invader by depositing callose to close the plasmodesmata, and therefore restricting the fungus to the initially invaded cell. The researchers found that a single fungal gene delays the resistance reaction until the pathogen has passed on the next cell. Mutants of this gene in the fungus are not able to pass onto the next cell. Resistance is related to a quicker reaction in closing the plasmodesmata as well as repression of the fungus within the infected cells.
This study involving the interaction between a fungal pathogen and host is probably common to many leaf diseases. Relatives of this fungal species attack other grass crops such as wheat and barley but apparently not corn. The study illustrates the evolution of methods of attack by pathogens and competing defense systems by plants. It is also interesting that the study was done by several specialists employing multiple techniques and understandings of their science.
The continual battle between the carbon producer, like corn, and the organism seeking carbon nutrition involves biology of both organisms. Each corn pathogen has evolved different biological features for success. Exserohilum turcicum, the fungus causing northern leaf blight of corn produces spores (conidia) with thick wall that do not immediately desiccate, germinate within a few hours of exposure to moisture and set up an appresorium and penetrate the leaf surface within a single day. The relatively heavy spores tend to not travel far within a field from the original infection site.
Cercospora zeae-maydis, cause of gray leaf spot, by contrast has relatively thin-walled conidia that germinates on leaves with only high humidity (80-90% RH). After germination the mycelium remains on the leaf surface until being exposed to 95% humidity for a total of nearly 100 hours. The mycelium can tolerate as low as 60% humidity in a somewhat dormant and then continue to grow when higher humidity returns. After meeting the minimum, an appresorium forms and the fungus enters to leaf surface. The thinner walled spores of Cercospora zeae-maydis is more easily moved in the winds and therefore spreads easier within a field.
Common rust fungus, Puccinia sorghi, is even more easily spread in the wind. Spores (urediniospores) have thick walls but are round and about a third the size of the E. turcicum spores. Thick walls prevent desiccation allowing long distance travel and light weight encourages it. The spores cannot withstand the absence of susceptible hosts between live corn presence during the temperate winters. Consequently, production of a new urediniospores occurs in more tropical areas with continuous corn growth. High altitude winds carry the spores to the temperate zones where new corn plants are growing. These spores quickly germinate in moist corn leaf surface, germinate, then an appresorium above a stomata, penetrating the leaf. This requires about 6 hours of moisture. The leaf whorl, always moist, fits this process. New spores are quickly produced and spread within the field can happen quickly.
These are three examples of corn fungal leaf pathogens with different infection and biology.
The disease is caused by the fungus Exserohilum turcicum (Setosphaeria turcica). The fungus lives in infected, dead or live leaves. It asexually produces spores (conidia) when the diseased tissue is moist and temperatures are proper for corn growth. The conidia have 4-6 cells arranged in a row and are light enough to be distributed by air currents within a corn field. After landing on a corn leaf, with a little moisture, in 3-6 hours the cells on both end of the conidia, begin dividing, emerging as germination tubes. These new hyphae quickly form a base on the surface called an appresorium, from which the fungus grows into the leaf epidermis. Within 12-18 hours after the conidia have landed on the leaf, it has successfully penetrated the leaf. There appears to be no difference in time to leaf penetration between the susceptible and resistant corn hybrids.
Chloroplasts near the infection point soon lose pigments as nearly 100 cells die, perhaps because of enzymatic activity of the fungus. This can be observed when small (0.5-1cm) circular, yellow spots show in the leaves in 24-48 hours after infection. From this initial location, the hyphae grow between cells towards the vascular bundles. Penetration of the vascular bundles is followed by plugging the xylem causing further death of surrounding cells dependent upon the water supplied through these tubes.
Resistance systems appear to begin after initial infection and perhaps mostly once the fungus has reached the vascular system. There does seem to be differences in that initial yellow spot in a couple days after infection and I wonder if the brighter color is not associated with greater quantitative resistance. Could be part of quantitative resistance or be self-destruction of cells near the infection point, depriving the fungus of nutrition? It does seem that initial spot is less obvious and possibly smaller in the more susceptible corn genotypes.
The wilted areas surrounding the plugged xylems eventually are depleted of living host cells. The fungus responds by producing new conidia within 14 days of the initial infection, ready to spread to more live leaves. Exserohilum turcicum remains viable in dry leaves for a number of years in dry environment, ready to produce conidia within 24 hours after moistened. Tillage, crop rotation and hybrid resistance become major factors in crop damage from this disease.
Much is written about this disease. An interesting summary of the morphology aspects is included in this report: Journal of Applied Biosciences (2008), Vol. 10(2): 532 - 537. ISSN 1997 – 5902: www.biosciences.elewa.org (From CornJournal 11/21/2017).
Corn leaves are constantly surrounded by fungal spores during the whole season. The waxy leaf cuticle and tight compaction of the epidermal cell walls inhibit penetration by the vast majority of fungi. Even entrance into the leaf through open stomates is restricted by antimicrobial fumes from the cells inside the stomata.
A relative few fungi have methods to overcome these defense systems. Spores of Exserohilum turcicum, the pathogen causing northern leaf blight, germinated within a few hours of exposure to moisture on the leaf. The hyphae growing from the spore responds to the surface hardness producing mucilage promoting adhesion to the leaf surface. Soon the tip of the hypha forms a special cell called an appresorium. Exposed cell wall of the appresorium cell thickens but the wall adjacent to the leaf surface remains thin. The cell gains turgor pressure from moisture as cytoplasm in appresorium cell absorbs glycerol from the fungal spore hyphae. Turgor pressure results in formation of new cell that forces through the leaf cuticle and through the epidermal cell walls.
Penetration into the corn leaf by the pathogen causing northern corn leaf blight occurs about 10-12 hours after the spore and moisture on the leaf surface. Favorable environment for spore germination and appresorium development usually is present in the whorl of pre-flowering plants, with fog, high humidity and dew formation.
The battle between plant defense systems and pathogen offense continues.
Resistance to leaf diseases in corn that limits the number and size of lesions is called horizontal or quantitative resistance and usually involves 3-5 genes. Ratings for this type of resistance involve a scale developed after considering disease pressure from different environments but does provide a stable type of resistance. Resistance to some leaf pathogens can have another system, usually involving a single gene, that stops some races of a pathogen more completely than horizontal resistance. This latter type of resistance is called vertical or qualitative resistance.
The Ht1 gene was discovered in 1964. It prevented the northern leaf blight pathogen, Exserohilum turcicum, from developing the normal wilted lesion symptom after it reached the vascular tissue. Instead of the normal lesion formation, a small yellow streak developed and, most importantly, the fungus failed to reproduce with spores capable of spreading the disease within the corn field. This seemed ideal in the USA because its presence was easily identified and damage from the disease was eliminated. Consequently, the Ht1 gene was utilized in commercial hybrids during the 1970’s. In 1979, a race of this fungus was found in several locations in the U.S. Corn Belt that overcame the Ht1 gene resistance, resulting in normal lesion and sporulation. It appears that the fungal gene responsible to overcome the Ht1 gene was present in a low frequency within the widespread population of E. turcicum. Its frequency increased as it gained competitive advantage over those individuals without this gene. Similar races of this fungus had already been noted in South Africa and the Philippines.
Other single genes (Ht2, Ht3, Htn) for resistance to E. turcicum have also been identified, as well as races of the fungus that overcome those resistance systems. This is not a new phenomenon. Genetic diversity within pathogens have repeatedly shown an increase of individual genes producing products to overcome single gene resistance. It should be noted that the term race for a pathogen refers to only a single gene difference within the pathogen population. It probably existed as a mutation, allowing a slight structural change in a protein that happened to be attacked by the host plant’s resistance product. Qualitative, vertical resistance to a disease in corn offers quick answers but stable, long-term benefits are best when quantitative, horizontal involving several genes are employed in corn hybrids.
Leaf epidermal cells walls and the waxy leaf surface provide the first line of defense against microbes. Pathogens adapted to overcoming this defense set off the next defense system after penetrating the leaf. This is initiated by the plant detecting the presence of the intruder. Plant cells nearby detect the presence of a protein exuded by the pathogen. Such proteins are called effectors, as they are detected chemically by host cells near the invader. Upon detection, these adjacent host cells produce potential microbe-inhibiting compounds such as reactive oxygen, nitric oxide, specific enzymes, salicylic acid and other hormones to effectively thwart the pathogen growth. Much initial reaction is limited to host cells adjacent to the infection site.
Resistance to corn leaf pathogens such as Exserohilum turcicum, cause of northern leaf blight, Cercospora zeae-maydis (gray leaf spot) and Bipolaris maydis (southern corn leaf blight) Involve detection of that specific pathogen and production of more general antimicrobial products in the immediate area of the pathogen. These two steps are inherited independently. Perhaps the pathogen detection system is more specific to the pathogen, accounting for a corn variety being more resistant to one pathogen than another. On the other hand, I am suspicious that if two pathogens arrive in the same area of the plant, only one will survive, as if the plant reacts to the first one by producing general resistance compound that inhibit the infection by the second one to arrive in the same area.
The system described above is referred to as general or horizontal resistance. It is controlled by 3-5 genes for products to detect and reduce spread of the pathogen. Horizontal resistance is expressed in corn plants by fewer leaf disease lesions. Evaluation of varieties for this type of lesion has some ambiguity however, because the number of lesions or amount of leaf damage is also affected by the intensity of disease pressure. Heavily diseased leaves from the previous season in fields of low tillage, with frequent early season rain can result in more leaf lesions in a variety of good general resistance to a pathogen than will occur in one of poor resistance with little disease pressure.
Characterization of horizontal resistance level to a pathogen requires a rating scale that has some consideration of disease pressure and relativity to other varieties. It is best done when each variety is exposed to the same pathogen intensity at the same stage of leaf maturity. Differences expressed as lesion numbers, size of lesions and percent of leaf destruction can be used to indicate the level of general resistance to that pathogen. I prefer to make ratings based upon several plants exposed to the pathogen in what I project to be somewhat heavy disease pressure in most USA corn environments. With artificial exposure to the pathogen by placing spores in the plant whorl, each plant receives more-or-less the same pressure. Expression of resistance will show 1-2 weeks later. Those varieties with abundance of larger lesions are deemed more susceptible than those with fewer and often smaller lesions. Consequently, it is assumed that will simulate the reactions in fields with somewhat heavy pressure from that pathogen.
Any evaluation of horizontal resistance includes consideration of disease pressure and relativity to other varieties. (Corn Journal 7/11/2017)
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.