Researchers say bacteria that colonize root cells stimulate root hair growth and increase water and mineral absorption
Plant roots draw water and nutrients from the soil but much more goes on underground than a simple process of harvesting food.
A new study has shown that the bacteria that colonize root cells stimulate root hair growth in a series of complex interactions. Root cells depend on bacteria to grow hairs, which greatly increase the surface area of the root and increase absorption of water and minerals.
Scientists at Rutgers University in New Jersey have been studying the relationship between roots and the internalization of soil bacteria in a process called the rhizophagy cycle.
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The end game is to use the microbes not only to grow crops and reduce dependence on inorganic fertilizers but also produce crops resistant to the stresses of climate change and resilient in the face of extreme weather.
“What has been perplexing is that root cells do not enlarge and form root hairs unless bacteria are present within the root cells,” said James White, a professor in the department of plant biology and pathology at Rutgers University.
“In order for root hairs to elongate, root cells must maintain bacteria within them for an extended period of time. This sustained (symbiotic relationship) appears to be somehow necessary for root hair development.”
While root cells are dependent on the bacteria to grow hairs, the bacteria must come directly from the soil or be bacteria on or within seeds. It is the reason why soils should be managed to preserve microbial diversity and why seeds should be managed to preserve natural bacteria in them.
“Many different bacteria are internalized into root cells and may be oxidized or degraded in rhizophagy,” said White. “However, some of the bacteria function optimally in root cells, stimulate elongation (of root hairs) and may be ejected back into the soil.”
He said that plants appear to prefer to internalize bacteria that optimally function within their root cells and exclude those that do not work. But how plants actually select these optimal microbes is still unknown.
Soil microbes are also known to release ethylene, a plant growth hormone, and they begin secreting the ethylene immediately upon entering the root cells. As the cells grow, they release some nutrients in the form of sugars back to the bacteria in a sort of co-dependent relationship. But even this, White says, comes with conditions.
“Root hairs actually grow from specialized root cells called trichoblasts,” he said. “They have a little pocket, or sock, on them that will accumulate bacteria en masse within them. If bacteria are present within the trichoblast pocket, bacteria produce ethylene, and this causes the root hair to elongate. If there are no bacteria within the trichoblast, the root hair does not elongate. We do not really know what triggers the bacteria to begin secreting ethylene. If there is something that comes before ethylene secretion, it would be a signal from the plant root cell.”
He said that the signal from the root cell could be a secretion of the amino acid arginine. In another level of co-dependence, roots are known to release small amounts of arginine and other amino acids. Bacteria produce ethylene using an enzyme called microbial ethylene synthase, which, in turn, uses arginine as the substrate. Therefore, said White, it would be logical that arginine could be both the signal and the substrate for ethylene production.
Root cells also secrete superoxide onto bacteria. Plants use this compound to control the bacteria that has been absorbed into root cells. Bacteria respond protectively with secretion of antioxidant forms of nitrogen, which combine with superoxide to form nitrate that is absorbed into root cells.
“The nutrient exchange of sugars (from plants) and nitrogenous antioxidants (from bacteria) are like a trap for the bacteria in root cells in that they cannot exit from the nutrient exchanges once engaged,” said White. “If bacteria stop secretion of ethylene, they do not receive sugars from root cells; if bacteria stop secretion of nitrogenous antioxidants, they will be degraded by superoxide. We have termed this the nutrient exchange trap.”
He said it is possible that the trap is the mechanism whereby plants force some bacteria to fix atmospheric nitrogen and transfer it to plants.
“We are still working on this,” he said. “What is clear is that plants are using soil bacteria to acquire and perhaps generate nutrients within their growing cells. We think the trap is fundamental for nutrient exchange between intracellular bacteria and plant cells.”
This underground organization has yet more complexities driven by the process of root hair growth happening in spurts as plants absorb bacteria from the soil and give some bacteria back.
“In this process of root hair growth in spurts, plants are doing several things,” he said. “They are extracting nutrients from bacteria and repopulating the soil with bacteria that function well in the rhizophagy cycle. This way, they continue to acquire nutrients optimally using their most adapted bacteria, which get soil nutrients and carry them back to the plant to be internalized into root cells again.”
A lot of the nutrients acquired by plant roots come from soluble soil nutrients while other nutrients are acquired from mycorrhizal fungi. Plants not only take advantage of multiple ways to acquire nutrients but have developed an organized process that puts them in control of the cultivation of bacteria both in their roots and the soil solely for nutrient gain.
“Plants are not merely passive recipients of fertilizers. They are active in using soil bacteria to carry nutrients from soils, which they extract by internalizing the bacterial into root cells and oxidizing them. There is a kind of ‘intelligence’ that is exhibited by plants in how they manage microbes. I am astonished by how plants manage them and interact with them within their cells. There is a beauty in it, and it is astonishing.”
The study was published recently in the journal Microorganisms.
