Fungi, bacteria and viruses have constant genetic mutations and evolution favor those that adapt to potential food sources. This pathogen diversity must be matched with diversity in corn to avoid crop losses. The list of new, unexpected occurrences in USA and internationally seem to occur every few years. Probably we should not be surprised. As we attempt to improve the hybrids for yield, standability, performance under changing environments we can also inadvertently and unknowingly include genes for susceptibility. Especially vulnerable are genes that allow the recognition of the invader and therefore quick defense response. Furthermore, a corn hybrid that may work well, and not express susceptibility to local pathogens, but when moved to another environment, a pathogen reacts differently.
Microbes also have genes that vary from mutations and sexual recombination. Rapid production of huge numbers of spores, ability to infect multiple hosts often near corn field, and broad, widespread populations of these pathogens is their strength. New variants of the pathogen, adapted to at least a few current hybrids with higher level of pathogenicity, must initially show only in isolated spots in corn fields and easily overlooked. History nearly all ‘new’ races in the USA were not noted until they were widespread. My guess is the genetic variant allowing for the new race (pathotype) was infrequent but present in the pathogen population for some time but was not recognized until damage was common. The more recent occurrence of the bacterial streak of corn, caused by Xanthomonas varicola pv. vasculorum, previously known only in South Africa but within only a few years it was been identified in several US states and Argentina. Was it here for a long time, or distributed by seed or even grain debris? Perhaps it has been a long-time pathogen of other grasses. Or perhaps is a mutant of a related bacterium and we inadvertently selected for susceptibility in corn.
It is doubtful that the battle between new pathogen variants and corn will end. Our best protection must come from careful observations in corn fields and submitting suspicious samples to appropriate specialists for identification. Corn genetic diversity has always allowed selection of resistance, but it does take a few years to implement the hybrid seed production before serious damage to the crop.
The fun never stops!
Constant interactions with the corn plant biology, affected by its genetics, and pathogen biology, affected by its genetics, requires us to carefully observe potential changes. Each participant in these interactions has potential for reducing our final product from the corn crop.
Bacterial pathogenesis of corn differs from fungal pathogenesis mostly because of biological differences in the two types of organisms. Bacteria are single-celled organisms with a single chromosome without a nucleus in its cytoplasm. Cell division in bacteria results in a new, distinct individual cell, whereas fungal cells divide to form a filament, frequently maintaining cell to cell communication and specialization. Bacteria cell single chromosome simply replicates, then separates as a cell wall divides the cell into two halves, each with its own single chromosome. Fungal chromosomes are enclosed in a nuclear membrane, divide by mitosis during cell division but also can undergo meiosis forming single strands of the chromosome and thus exist as haploids until the nuclei fuse to form diploids. These structural and functional differences between bacteria and fungi results in different ‘strategies’ concerning adaptations of these organisms to changing environments. Mutations occur in both, but those in the single bacterium chromosome immediately results in a changed new individual bacterium. Mutation in fungal DNA may not take effect until recombination to form the homozygous recessive. Bacterial biology allowing rapid reproduction of the new forms allows adjustment to new hosts.
Bacteria are ubiquitous, distributed by wind, water, insects and animals. Rapid cell reproduction allows quick spread when in a nutritional environment. Despite their widespread presence, most species cannot infect living corn leaves. Leaves have evolved epidermal cells tightly held together and with a waxy covering, repelling water and prohibiting bacterial infection. Pathogenic fungi establish a multicellular mat (appresorium) from which it produces a penetration peg that enzymatically drills through the wax and epidermal walls to enter the leaf. Bacteria cells can enter through open stomata-some even have flagella to help swim to the stomata. Plant resistance systems can produce antibacterial fumigants to inhibit the bacteria from replicating and there is some evidence stomata close when faced with bacteria invasion.
Small size and few visible structural features has made it difficult to identify a bacterium causing a plant disease. Although some disease symptoms do become associated with specific species, confirmation usually requires culturing and chemical tests.
It often takes a simple mutation in a potential pathogen, or in the corn host, for a relatively minor disease to become major. Good for everyone in corn production to be aware as the crop grows.
Virus genetics are probably the simplest of pathogens, some not even bothering with DNA but using mRNA to directly cause the host cell to provide the more complex process of DNA code as well as the ribosomes to produce essential proteins for the pathogen. Fungal pathogens of corn have more complex genetic systems than viruses and corn.
Corn reproduction is relatively easy to understand when compared to most corn pathogens. Most corn cells have a single nucleus with 10 pairs of chromosomes with only the exceptions of the triploid cells of the endosperm and the haploid cells in the pollen and egg cells. The hyphae of most corn leaf pathogens are filaments composed of multiple cells separated by cell walls called septa that have pores allowing exchange of cytoplasm and even nuclei. The nuclei are composed of a single set of chromosomes. Furthermore, a single cell of the fungus causing northern leaf blight may have up to 30 nuclei, all haploid.
Genetics of diploid cells are affected by both sets of the pair, generally with the dominant form of a gene on one set of the chromosomes resulting in the amino acid components of the protein and therefor the trait expressed. Recessive genes get expressed only when both sets have the same recessive i.e., homozygous recessive for that gene. Mutations resulting in new recessive genes are often ineffective because of the dominant form in the other member of the paired chromosome.
In a haploid nucleus, there is only one form of a gene, allowing a recessive gene to be fully expressed. Resistance, especially those with single gene inheritance, in corn to a pathogen such as Exserohilum turcicum, cause of northern leaf blight, is initiated when the host cells recognize the presence a pathogens product (effector). A mutation in the pathogen’s gene for this effector, negates the plants ability to recognize the pathogen and therefore this single gene for resistance is not initiated. This is the weakness of the single gene system for resistance to any pathogen.
Fungal pathogens have the statistical advantage of producing new races because of the haploid nuclei in most hyphae. For example, if a homozygous recessive form is crossed with a homozygous dominant, the first generation of a diploid organism would only produce the dominant gene product. In the haploid hyphae, half of the resulting cells would produce the recessively inherited product. This explains why some pathogens can successfully overcome the single gene type of resistance. Being able to asexually reproduce with thousands of spores with haploid nuclei from the successful pathogen allows quick spread of a new genetic type.
Plant and animal genetics involve a complex involvement of DNA (deoxyribonucleic acid) with long chains of long chains of nucleic acids arranged in sets of 4. These chains are arranged as chromosomes and enclosed in a membrane. The DNA also codes for a related string of nucleic acids called RNA (ribonucleic acid). When a segment of the DNA is signaled to be ‘read’, that string of nucleic acids is transferred to messenger RNA. This mRNA moves through the nuclear membrane to a ribosome in the cell. Each set of 4 nucleic acids link specific amino acids to produce a specific protein. Thus, the DNA-RNA codes result in specific proteins produced in animal and plant cells. Many of these proteins function as enzymes for basic metabolism, ultimately resulting in structure and functions of plants and animals.
Viruses have simplified the process. Some have DNA and some only have RNA. They use the hosts cell’s ribosome for the protein production needed for the virus membrane. Current human epidemic caused by a coronavirus has its genetic code in mRNA. This sufficient to cause the host cell ribosomes to produce the few proteins needed for replication and further infection by the virus. One of those protein forms the ‘spike’ allowing the virus to penetrate the cell membrane. Viruses, such as this one, have self replicating RNA, simplifying the duplication process.
Viruses causing corn diseases have either RNA or DNA codes for duplication within host cells. Most appear to be RNA only genetic codes but a few, such as Maize Streak virus, has its genetic code in the form of DNA. Most corn virus require a vector to penetrate the plant cell walls, but after entering, they use the host cell’s cytoplasm to replicate. Host resistance depends upon successfully producing ‘antibodies’ to inhibit this replication process.
Let’s hope that the recently revealed vaccines for current human coronavirus that apparently involve a segment of the mRNA will successfully halt the ability of the virus to replicate.
tEach seed to be planted next year will have a full set of the hybrid genes located in each cell of the embryo. Natural drying of seed in the field allows a temporary delay in cellular activity in corn seed about 40 days after pollination. Respiration and other metabolism in the embryo cells slow, preventing germination. Preservation of seed viability for the next season requires continuation of the seed drying to 14-7% to prevent faster metabolism. Corn seed producers control this drying process, carefully avoiding higher temperatures that could destroy the cellular physiology but yet quickly dry the harvested seed. This is mostly done by using fans to remove the released moisture from environment surrounding the seed.
Drying speed is significant because moisture levels between 32% and 15% allow some degradation of cellular membranes but without the adequate replacement that occurs in actively growing plants. This ageing process shortens the eventual viability of the seed.
Multiple studies have shown that seed respiration rates are affected by storage conditions and that these eventually affect germination percentage and seedling growth rate. Genetics also enter into these factors. Pericarp wall density affects the ease of moisture movement into the embryo. Some corn genotypes are more vulnerable to uptake of atmospheric moisture than others. Pericarp tissue is part of the female parent of the corn kernel and therefore vulnerability to seed germination deterioration due to moisture during storage is often associated with the hybrid female parent. Two inbreds may combine to give identical hybrid parent characteristics, but one of the inbreds may be superior for germination preservation when used as the female parent.
Successful preservation of high germination rates in the spring are dependent on multiple factors including stresses during seed development, drying conditions and genetics of parent seed.
As the Corn harvest season draws towards a close, once again a corn grower witnesses the variability within and among individual fields. Differences in soil types, crop history and perhaps even rainfall distribution affected corn hybrids’ grain yield and stalk performance. Genetic diversity among corn hybrids provide opportunity to adjust with choices among next year’s hybrids although weather for the next season is not easily predicted.
Zea mays features tremendous genetic diversity due to its cross-pollination biology and humans involvement of exposing it to multiple environments throughout its relatively short history. The large number of genes on only 10 chromosomes allows fairly rapid expression of mutations. Hybrids with deeper roots allow water absorption in sandy soils while those with more branching of roots near the surface are better adapted to soils with lots of organic matter from the previous year crop. If next year’s weather tends to be dry, especially during flowering time, the deeper root hybrid may be favored. If it is wet during that critical time period, the shallow root type may do better in grain yield and stalk quality. Mid season wind pressure may favor the hybrid with roots with more branching.
Previous crop environments may influence need for resistance to some leaf diseases. Corn breeders attempt to select away from extreme susceptibility to pathogens such as Exserohilum (Helminthosporium) turcicum, cause of northern leaf blight, and Cercospora zeae-maydis, cause of gray leafspot, but genetic variability among pathogens and weather patterns influence the ultimate threat to the crop. Genetic variability among corn hybrids and within potential pathogens will always present changes in the affect of corn diseases on performance. We are dependent upon researcher in both genetics of both host and pathogen to monitor these possibilities.
We are also dependent upon growers to access all agronomy knowledge, make the most economic and technical decisions in choosing hybrids for each field for next season-and then roll the dice!
Seed producers attempt to make genetically pure hybrid seed by using homozygous parents. This is not easy. Inbreeding theoretically makes genes with the identical code on the other member of the chromosome pair and thus be homozygous dominant or homozygous recessive. Small mutations can occur along the way of developing these inbreds that can interfere with the process. A larger potential source of problems can come as the inbred in increased when unintended pollen invades the seed increase field.
Corn pollen viability can last only a few hours if the weather permits. Most corn pollen falls within a few feet to a few hundred feet, depending upon wind, although there is evidence of viable corn pollen found ½ mile from the source if wind and humidity is favorable. Fields for increasing parent seed inbreds are consequently isolated from other corn fields at long distances to minimize the contamination.
Hybrid production using pure inbred parents also must struggle with the same problem but with a few more dynamics. Both parents are inspected visibly looking for off type plants in the male and female rows, removing those that are obvious. Most off types will show some hybrid vigor over the inbreds and thus can be easily identified and removed before pollination. Hybrid seed is produced by preventing pollen from the designated female plants by either removing the tassels or use of male sterile female inbreds. The intent of the seed producer is to have adequate pollen from the male parent to cover all exposed female silk as they emerge. The small presence of foreign pollen in the air at the same time always causes the potential for contamination, making the timing of male parent pollen production essential. Environments greatly affect the success of this endeavor. Dry field conditions tend to delay silk elongation whereas it has little effect on pollen production. This can cause most male parent pollen to be released before silks are exposed. Wet field conditions allow more elongation of silks causing the risk of silks exposed before male inbred pollen is released. Exposure of silks when little intended pollen is present increases the probability of the wrong pollen landing the on the silk and thus fertilizing the egg cell in the ovule at the end of the silk. The first viable pollen grain to arrive ‘wins’!
Unintended genetics in production of hybrid seed production because of potential contamination is nearly impossible to avoid. If foreign pollen comes from a commercial hybrid field, all of the off types will not be identical. Genetics of that hybrid will include a strand of DNA from that hybrid’s male parent and a strand from the female parent. With meiosis occurring in the production of the hybrids pollen resulting in a minimum of 1024 new combinations of the 10 chromosomes in the pollen, contaminating pollen leads to many new genetics in new seed. Consequently, the outcrosses among intended hybrid seed vary greatly in appearance, some being taller than the canopy and some being shorter, along with a wide range of other morphological features. Usually contamination is associated with timing and production of intended male parent pollen thus the outcrosses often are more common within a seed size with the seed lot. First silks to emerge are from the base of the ear and thus the larger sizes. Poorly pollinated ears, perhaps from those in which silks emerged after most intended male pollen was exhausted and thus more likely to be pollinated by outside pollen results in more large, round seed. Best purity usually is in those from the middle of the ear of well pollinated ears (the medium flats).
Although a few contaminants in hybrids are common and have practically no effect on hybrid yield, they gain significance in special trait hybrids. It does take special effort to produce pure hybrid seed.
Observations of corn hybrids and inbreds at flowering stage and later allows distinction of genetic differences, especially if several varieties are planted in unform conditions. Usually plant height differences are clear. Close observation of ear characters also allows distinguishing between varieties especially when grown near each other with same micro-environment. Closer observation of the plants for other morphological differences such as tassel and leaf shape and size also can allow distinguishing genetic differences, although can these can be vulnerable to confusion with the differences caused by some virus infections and poor seedling emergence.
Inbreds and hybrids differ from each other in many other genes than only those observed in mature plants. Close observation of plants at all stages show other leaf character differences that becomes obvious when plants are in identical environments. Multiple leaf character differences become obvious when observed by experienced researchers. Pure single cross hybrid seed planted in controlled soil and temperature environment will be identical to each other for many leaf characters. Modified single cross hybrids, such as when the female parent is a cross of two related inbreds, will be reflected in seedlings with slight morphological differences. Outcross plants caused by pollen blown into the hybrid seed production field will show multiple character differences among the seedlings. A seed mix accident in which two hybrids in the same sample will show two distinct seedling plant types. This is also true where one of the inbred parents was mixed in the seed production field or if contaminated with the male parent from an adjacent field.
Accident selfing problems in the seed field results in some plants not only with distinct seedling morphology, and usually smaller than the hybrid seedlings. These ‘selfs’ are also morphologically identical to each other. If compared to a sample of the female parent, these plants can be confirmed as ‘selfs’.
Professional Seed Research, Inc. has used this information and experience to evaluate purity of hybrid and inbred seed for seed companies for more than 30 years (Seedling Growout® test). 400 seed are planted adjacent to each other in uniform environment with natural light. Experienced researchers carefully observe each plant for morphological differences to identify those that are unlike the others. If all the offtypes differ from each other, it is assumed that they are from contamination from outside hybrid fields. If identical to each other then probably the source is a seed mix or possibly contamination from an adjacent seed field. If the plants are smaller for most leaf characters yet distinct from most plants and identical to each other, they are assumed to be selfs.
This method of evaluation of purity of seed samples allows for results within 2 weeks of planting, and larger samples (400 seed) than most other grow out methods. It has been used with temperate and tropical hybrids. Corn’s diverse genetics are expressed at all stages of the plant’s development.
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.