Corn endosperm emphasis
Interest in the domesticated teosinte after the discovery of the mutants that allowed kernels not to be immediately released from the ‘cob’ and absence of the hard encasement around the kernel was selection for endosperm types. This part of the kernel is where the majority of carbohydrates are stored. The C4 photosynthesis process of teosinte was a major contribution of this ancestor of corn.
The endosperm of the corn kernel is composed of two major cellular structures. The outer layer of the endosperm cells, the aleurone, includes a concentration of proteins activated as enzymes to digest the starch into sugars utilized as energy driving the germination activity of the seed. The aleurone is also the site of synthesis of anthocyanins, often functioning as antitoxins that can ward off pathogens. The bulk of the endosperm are cells storing carbohydrates usually in the form of starch. These inner endosperm cells are also the site of carotene biosynthesis.
Ease of transport and potential food use of this new species thousands of years ago led to multiple selections of genetics coinciding with environments from the Andes to lowland tropics in South America. Corn had spread to much of North America before Europeans arrived 7000 years after the initial domestication of teosinte. Human emphasis on endosperm development included not only larger deposits of starch but also specific characteristics.
Carotene synthesis includes multiple steps but if the recessive mutation of the Y1 gene is present this process isn’t completed resulting in no yellow pigment and white endosperm. Aleurone anthocyanin colors are dependent upon three genes affecting that process. A dominant gene, labeled as C1 allows color to be developed, the recessive c1 form of the gene prevents colored aleurone layer. Another dominant gene R1 also allows a colored aleurone pigment. If dominant gene Pr1 is combined with R1 or C1, a purple or blue color is developed in the aleurone layer of cells. Recessive form of the gene (pr1) results in red corn if the C1 or R1 is present. White corn has the recessive y1 activity in the center part of the endosperm and recessive c1 and r1 in the outer aleurone layers. Yellow corn has the dominant Y1 combined with c1 and r1.
As corn was utilized by ancient and modern corn breeders, other endosperm modifications became emphasized. While the attention remains on endosperm characteristics, selections for the multiple genetics involved in plant development appropriate to the environments also occurred. This has led to the large diversity available to current corn breeders.
Corn endosperm significance
All flowering plants utilize the endosperm as storage of starch as an energy source for seed germination. Grains used for human food have been chosen for having larger endosperms than simply needed for germination energy. The variety of teosinte (Zea mayssubspeciesparviglumis) known as Balsas teosinte, believed to be the source of human domestication of maize (Zea mays subspecies mays). Mutations in the Balsa teosinte resulted in absence of the hard encasement around the kernel and absence of abscission layers leading to early release of the kernels from the central rachis (cob). Occurrence and acknowledgment of these mutations occurred to humans about 9000 years ago in the Tehuacan Valley, south of the current Mexico City. Fortunately, this occurrence coincided with the migration of people from northern Asia across the Bering Strait into North America, spreading south through Mexico to South America.
The attraction of these primitive Zea maysseeds as a food source and ease of transportation must have allowed for distribution from that original source to Peru by 6500 years ago and to the eastern base of the Amazon River by 4030 years ago. Recently published research indicates that much development into current corn occurred near the Andes (Kistler et al., Science 362, 1309–1313 (2018).
Human’s interested centered on the endosperm of the corn seed. The fact that this species has at least one generation per year, and that it was cultivated by multiple growers who essentially were corn breeders, selecting for endosperm size and characters. These people did not need to study genetics to realize that if they chose kernels with the most desirable characters in the endosperm to plant, they could increase kernels with those characters. Essentially, there were thousands of corn breeders in a huge number of environments selecting for corn genetics favoring their desired corn endosperm characters. Some preferred certain starch components that could be ground into flour for culture desirable foods. Others chose strong carotene (yellow) colors, still desired because it results in dark yellow egg yolks when fed to chickens. Others preferred the blue or red anthocyanin colors in the aleurone layer of the endosperm.
Selection for desirable endosperm in multiple environments of South and North Americas also allowed for diversity of genetics for other plant characteristics including root growth, time to flowering, leaf size and structure. Multiple internal characteristics affecting photosynthesis, disease resistance and transportation of sugars to the endosperm were also affected by the selection for desirable endosperm by these many corn breeders, across many environments for thousands of years.
An interesting read on the internet on the teosinte and maize relationship can be found at
Corn endosperm origin
The meristem in at least one of the lateral buds of a corn plant develops into an ear. This meristem includes 500-1000 lateral meristems with mother cells with diploid sets of chromosomes, 10 chromosomes from each of that plant’s parents. Meiosis occurs in this diploid cell, resulting in 4 haploid cells, each cell having only a single set of 10 chromosomes consisting of a random mix of the two parent’s chromosomes. Three of the 4 haploid cells degenerate, leaving a single megaspore. This megaspore nucleus undergoes mitosis three times, resulting in 8 cells within the megaspore structure now called the embryo sac. One cell at the bottom of the embryo sac becomes the egg cell while two of the haploid cells fuse in the center of the embryo sac.
The embryo sac (ovule) is enclosed in an ovary, at part of the female part of the parent plant. Part of this female flower is the silk., extending from a single ovary and attached to its ovule. The male flower also produces pollen via meiosis followed by a single mitosis, resulting in two haploid nuclei. A pollen grain adhering to the silk, germinates and extends down the silk to the ovule. Upon entrance of the ovule, one nucleus fuses with the haploid egg cell forming a diploid nucleus to become the seed embryo. The other pollen haploid nucleus fuses with the two ovule nuclei in the center of the ovule resulting in a triploid nucleus, having two sets of chromosomes from the female parent and one from the male. This triploid nucleus undergoes mitosis to become the endosperm of the seed.
Whereas the inheritance of the embryo, and its resulting mature plant, is determined equally by the genetics of the male and female parents, characteristics of the endosperm is slanted towards the genetics of the female parent. If the female parent has the recessive Y1 gene, and thus a white endosperm, but the pollen is from a parent with dominant gene and thus has yellow endosperm pigment, the resulting endosperm will be lemon white in color. The female genetics has the major affect on endosperm function in the maize seed because of contributes two of the three sets of chromosomes in endosperm cells.
Aleurone layer of corn kernel
A single layer of cells immediately inside the pericarp is the aleurone. Contrary to the pericarp’s origin from the female plant’s ovary, the aleurone is derived from the pollinated ovule. It develops from the endosperm portion and thus from the contribution of one sperm nucleus combining with two haploid egg nuclei. Cells of the endosperm, including the aleurone cells is triploid, having three sets of chromosomes in their nuclei. It originates with other endosperm cells as they multiply after pollination. These cells surrounding the inner, starch filled endosperm cells, have an important role in seed germination, providing the enzymes to digest the starch in the rest of the endosperm into the glucose molecules.
Although only a single layer of cells, it can include 30% of the total proteins of the endosperm. It is also the location of pigment molecules influencing the color of the corn kernel. Pigment expression in corn kernels is mostly influenced by the carotenoid and anthocyanin pathways. A dominant gene (Y1) codes for synthesis of a protein in the carotenoid pathway for production of yellow pigment all of the endosperm, including the aleurone layer. If the female and male parents of a hybrid have the recessive version of this gene, and lacks recessives for the anthocyanin genes, the hybrid kernels will be white. However, if the male parent is yellow and the female parent white, the result will be the intermediate color between white and yellow. If both parents have the recessive gene of Pr1 the kernel will have purple anthocyanin pigment in the aleurone cells. The color will show as more blue pigment if the recessive carotenoid gene gives a white endosperm. Red kernels likewise are more intense with the absence of carotenoid pigments and presence of the recessive version of R1 gene for anthocyanin production.
Mutations of genes influencing pigments produced in the aleurone layer of cells was utilized a few thousand years ago by local cultures as corn was moved from that original Teosinte base. We use it today to make those colored corn chips.
Fumonisin in corn
The toxin fumonisin causing severe disease in horses and other mammals is produced in corn by the fungal species Fusarium verticillioides. This species, once known as Fusarium moniliforme, if a commonly associated with corn worldwide. Whereas other Fusarium species associated with corn are frequent in soil, this species is more dependent on existence is corn debris between crop seasons. The fungus does invade corn seedling roots, especially if root growth is slow due to low temperatures and/or injured by insect feeding.
Fumonisin production by the fungus is linked to a few genes in the fungus leading to some variation among the isolates of F. verticilloidesin production of fumonisin. There is evidence that fumonisin production by the fungus assists in overcoming the plant’s resistance system by causing plant cell death. Fumonisin produced in roots has been shown to be transported to the leaves of the corn plant.
Growth of this fungus in seedling roots is linked to slow growth of the seedling, mostly due to low temperatures. After initial infection, the fungus produces conidia small enough to be carried in the xylem through the complex first node separating the mesocotyl from the coleoptile. It has been noted that low light conditions are associated with more rapid spread from the root to the above ground-portions of the corn plant. This implies that plant physiological condition associated with photosynthesis affects the ability of the fungus to spread within the corn plant.
Fusarium verticilloidesalso infects the kernels through the silk (style). There is evidence that corn genetics associated with the size of the small opening at the tip the corn ovary allowing the pollen entrance is associated with successful early invasion of hyphae of this fungus. Environmental stresses during kernel formation, including insect feeding is also associated with invasion by this fungus.
Corn’s association with this fungus is complex and seemingly ubiquitous. The fungus often appears to be an endophyte, causing no visible harm. Not all variants of the fungus produce fumonisin. Seedlings growing in good environments infected with F. verticilloidesmay express limited damage. Not all kernels infected with this fungus produce fumonisin. It is one of the inhabitants of corn’s environment that can produce a toxin.
Fusarium and corn
Fungi, like the rest of us non-photosynthesizing organisms, are dependent on plants for existence. Species of the fungal genus Fusarium are ubiquitous, often feeding on dead and living plant materials. They are identified by microscopic observation of their asexually produced spores (conidia) abundantly produced from the filaments (mycelium) growing in and on plant tissue. When these fungi are stimulated to sexually reproduce, they develop spores in microscopic ‘sacs’ called asci. This means of sexual reproduction places Fusarium in a class of fungi called ascomycetes. The genus of ascomycetes that have Fusarium as a conidial form is Gibberella. Consequently, a fungus frequently found in corn stalks, leaves and ears may be identified by its conidia shape as Fusarium verticilloides but if stimulated to form sexual bodies it would be called Gibberella fujikuroi. Another species of Fusarium (Fusarium graminearum) more frequently is found on corn stalks and ears as the sexual stage, Gibberella zeae. This dual naming system, tolerated by mycologists and plant pathologists, occurred because the fungus was initially only known and named by the asexually produced conidia.
These fungi on corn are not aggressive pathogens but mostly invade dead or weakened cell tissue. Infection of corn ears by Fusarium species is often through old silk tissue. Fusarium or Gibberella stalk rot occurs after the stalk tissue has been weakened by desiccation due to roots rot. Fusarium mycelium is easily found in leaf tissue, as if it is an occupant, perhaps feeding on weakened or dead cells within living leaves. Its widespread presence in corn seeds, seedlings, stalks, leaves and ears often leads to difficulty in determining its significance. Was it an aggressive pathogen killing the tissue or was it an invader of weakened tissue? Is seedling blight due to a Fusarium species attacking a vigorous, corn seedling or was it simply infecting a corn seedling weakened by environment? Gibberella stalk rot occurs when the plant-environment- genetics interaction results in roots dying from insufficient carbohydrate to sustain metabolism. Stalk cells die because of consequential wilting and shortage of carbohydrates as well. Fusarium graminearum feeds on the weakened and dead tissue, eventually producing the sexual reproduction bodies of Gibberella zeae and thus allowing us mere humans to call it Gibberella Stalk Rot.
Pericarp invasion resistance
Carbohydrates stored in the endosperm of a corn kernel is a potential source of nutrition for fungi and insects. The pericarp can be a major barrier to attack to these potential invaders. Studies concerning the tropical corn storage insect, Maize Weevil (Sitophilus zeamais), showed that the cross-linked structural components of the pericarp cell walls were highly correlated with resistance to this insect. Other factors included phenols (Afr. Crop Sci. J. 9:431–440) produced by the pericarp cell metabolism and even endosperm hardness (flintiness) contributed to reduced susceptibility to this storage insect (Crop Sci. 44:1546–1552 (2004)).
Pericarp tissue also is a barrier to entrance into the seed by multiple kernel rotting fungi. Most enter the ovary through the silk channel immediately before pollination. This becomes most evident when silks are left exposed for several days in an environment favoring the pathogen. After invasion, the fungus can spread cell-to-cell within the pericarp through small holes (pits) in the cell walls that allows movement of metabolites between cells. Integrity of the pericarp is a significant factor in avoiding invasion by many potential fungal species.
The phenomenon known as silk-cut can expose the seed to fungal infection. After the pollen tube grows down the silk channel and dumps the pollen nuclei into the ovule, silk tissue deteriorates and detaches from the ovary. Not all silks are pollenated even under ideal conditions, leaving some attached to their ovary while adjacent pollenated ovaries grow. These remaining silks interfere with normal contiguous growth of the pericarp cells in the adjacent ovary wall (Plant Disease 81 (5):439-444). This can result in a break in pericarp as the kernels enlarge and thus an opening for invasion by fungi. Genetics and environments influence the occurrence of silk-cut. Stresses that delay silking beyond pollen availability can be an important factor but genotypes vary in vulnerability both to reaction to the stress and probably the tendency of this phenomenon.
The corn kernel is a fruit. Grains are fruits with a single seed enclosed. The ovary wall, part of the female plant, grows after pollination results in enlargement of the single embryo it encloses. The ovary wall thus becomes the pericarp. Genetics of the pericarp cells are those of the female plant and therefore the genetics of the female inbred parent in hybrid seed production or both hybrid parents in the grain field.
These genetics influence the important cell wall components that give both strength and resistance against insect and fungal invaders. Cell walls get their strength from various polymers that are cross-linked. Whereas non-grass species tend to have more lignin chemistry in cell walls than those in grasses and especially the pericarp features less lignin and more of a class of complex compounds called xylan. A major component of these xylan compounds is feruloyic acid. Feruloyic acid is associated with resistance to Maize weevil damage to corn kernels (Crop Sci. 44:1546–1552 (2004). Varieties differ in chemical components of pericarp cell walls that probably influence many aspects of corn grain storage. https://www.frontiersin.org/files/Articles/219955/fpls-07-01476-HTML/image_m
The female parent of a corn seed is the sole genetic source for the pericarp, mitochondria and chloroplasts as well as half of each diploid chromosome.
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.