Nitrogen-Fixing Corn?
Endophyte Microbial Partners for Corn
Dr. James White and his team at Rutgers University have worked out many of the relationships between plants and their microbiomes. The endophytic bacteria coming from the seed and from the soil are fed and multiplied in plant cells. In roots, this process is called rhizophagy (root eating), which helps the plant to obtain minerals. Rhizophagy also strongly stimulates root hair production, root branching, the production of root tips within which bacteria multiply, and also nitrogen fixation from the air.
A similar situation can occur in the plant tops where endophytic bacteria are often fostered and fed by plants in different tissues, especially in their epidermis and in the area around plant hairs (trichomes). These microbes are often engaged in nitrogen fixation to feed the plant. The quantity of N2 that can be fixed by such endophytes is presently unknown, but it appears that the bacteria are especially active in young growing tissues where the need for nitrogen and protein is the highest.
At the Mandaamin Institute, we are breeding corn to take advantage of all of these microbial nitrogen-fixing pathways—both above and below the ground. Our research and breeding has yielded some really encouraging results.
Young Root Cells
Mandaamin inbreds have shown the presence of large numbers of seed-borne bacteria that were multiplying and living inside root cells. These bacteria were also visibly excreted from root hairs into the rhizosphere. The bacteria were surrounded by oxidative substances produced by the plants to degrade the bacteria. The Mandaamin inbred seeds and their embryos are highly infected with endophytes and their primal roots and embryo shoots in the embryo are highly colonized. As the small plant grows and cells multiply and expand, the microbes reproduce in tandem. We suspect that the bacteria are being fed with sugary exudates from the chloroplasts. Research has shown that the presence of bacteria strongly supports protein formation in chloroplasts, therefore we suspect that the interaction between bacteria and chloroplast is mutualistically beneficial. We have not observed the colonization of bundle sheath cells in any conventional corn inbreds, but it is present in Mandaamin’s most nitrogen-efficient inbreds.
Epidermal Cells
It is not uncommon to observe microbes in our inbreds repeatedly enter and exit the nuclear membranes during plant growth. Bacterial streaming and bacterial immersion and movement are dynamically occurring in and out of the nuclear membrane.
Root Morphology
Mandaamin’s nitrogen-efficient plants form strongly branching rooting systems with little root rot. Conventional inbreds tend to be sparser on the surface and to be more vertically oriented. The growing root apices slough off colonized, mucoid producing, root cap cells. These cells are filled with both mucoid producing organelles and bacteria whose role is to release colonized mucosa that lubricates the root as it slips through the soil and eventually binds soil with roots into rhizosheaths. Many of the bacteria that were isolated from the root tips of Mandaamin nitrogen efficient inbreds were shown to have the ability to grow in culture without any nitrogen, so they may have the ability to be nitrogen-fixing. Rhizo-sheath formation is fostered by the mucilage and these sheaths help the plants to obtain nutrients by bonding the mineral component of the soil to the roots. The microbiome of our best plants may create a bio-protective sheath which likely helps prevent root rot.
Root Hairs, Trichomes, and Epidermal Cells
Trichomes are hairs that are embedded in the epidermis and tissues of leaves and leaf sheaths. Compared to conventional inbreds, germinating Mandaamin inbreds generally had substantially longer and denser root hairs which were highly colonized by microbes that circulate rapidly and excrete nitrate. Epidermal cells in leaves and leaf sheaths were observed to be the scene for intense microbial activity in Mandaamin inbreds but not in conventional inbreds.
Though domestication and breeding has altered the microbiome in many of our domesticated plants, their health and stress tolerance is still intimately connected with their microbes. These microbes help the plant to thrive by stimulating photosynthesis, germination, growth, and rooting. They can produce growth hormones, alter plant metabolism, antagonize pathogens, and induce systemic resistance to diseases and biotic stress. They can also improve plant resistance to heat, drought, and salt, and assist nutrient uptake from the soil. While new endophytic microorganisms are constantly recruited from the soil in the tips of the roots, a core set of bacterial genera can be passed from generation to generation through the seed. This set dominates the microbiome of juvenile plants.
Many of the observed bacteria stained positive for nitrate. Furthermore, our inbreds show unusually long epidermal cells, likely due to nitrate and ethylene produced by its bacterial partners.
Mature Leaves and Reproductive Tissues
Large differences were seen in the colonization of chloroplasts in mesophyll and bundle sheath cells between our inbreds and conventional types. Ours have a dynamic, interacting system where microbes appear to be associated with both mesophyll and bundle sheath cells. The bacteria also are actively colonizing reproductive tissues including cobs, silks, pollen, and embryos. We have observed impressive flows of bacteria in the silks of Mandaamin inbreds or old cultivars of maize, but not in the newer genetics.
The tassels of the Mandaamin inbreds have more primary and secondary branches than do most conventional inbreds. They also produce more pollen and show more variation than do the conventional inbreds. We suspect that microbial colonization of the tassels may influence the morphology and physiology of branching and pollen production. The microbes are also apparent in the pitch of the cob as well as in the convoluted walls in the bracts and glumes beneath the developing seed of our plants.
Summary
The Mandaamin nitrogen-efficient maize inbreds show incredible levels of plant-microbial cooperation and mutualistic development every year they are grown!
Based on these observations and other field-based trials, our working hypotheses are that:
- In general, the plant fosters bacteria in plant meristem cells, and locates bacteria in its pollen, embryos, and developing tissues. Bacteria are fostered in nuclear membranes and periodically released into cells as they develop.
- The bacteria improve resilience and plant mineral nutrition. They enhance nitrogen nutrition, probably through effects on chloroplasts and nitrogen fixation. They protect roots through the secretion of bacteria rich mucigel.
- These bacteria probably increase root and tassel branching, stimulate the water holding capacity of the xylem vascular system, and increase brace root, root hair, trichome, pollen formation and resistance to fungal diseases. They are integrated into substance mobilization to seed and take part in reproduction.
- These phenomena suggest that to obtain the benefits of partnership breeding, it is important for humans to reconsider what crop plants and plant breeding can become.
