Rhizobia in sustainable ag: specificity is key
Nitrogen – vital for plant growth
Nitrogen is essential for plant growth and chlorophyll synthesis. It is one of the most abundant elements on earth, but nitrogen deficiency is probably the most common nutritional problem affecting plants worldwide. A very high percentage of the total nitrogen found in soil is in organic form, this nitrogen form is not directly available to plants. Plants can take up nitrogen from the soil in the form of nitrate (NO3-) and ammonium (NH4+). Nitrogen is also the most abundant element in the earth's atmosphere, but plants are not able to get their nitrogen directly from the air.
These gram-negative soil bacteria are able to fix nitrogen
The role of Rhizobium in nitrogen fixation
This is where rhizobia (Rhizobium, Bradyrhizobium, Sinorhizobium, Mesorhizobium and others) can be of help. These gram-negative soil bacteria are able to fix nitrogen. Rhizobia can convert atmospheric nitrogen (N2) and convert that into ammonia (NH3).
Rhizobium establishment and nodulation
The host plants, in this case legumes, secrete a chemical signal molecule flavonoid to attract the Rhizobium species it can interact with. The Rhizobium responds to this is cry for help by colonizing the root hairs. The bacteria anchor themselves to the root hair surface and both the plant and the bacteria produce compounds that result in a firmer attachment. Then the rhizobia produce compounds called Nod factors that cause the colonized root hairs to curl. (Followed by many more processes to complete nodulation). After this, the Rhizobium bacteria will penetrate the root hairs. Once the bacteria reach the root itself, they stimulate cell divisions that lead to the formation of the nodule.
Rhizobium-crop specificity – a precise interaction
The interaction between Rhizobia and crops is very specific. Rhizobia only form symbiotic relationships with leguminous plants and even that interaction is very specific. The Nod factors that cause the colonized root to curl have been identified as lipochito-oligosaccharides. Variations in these structures determine the compatibility between the Rhizobium species and a specific host plant.
Successful examples of combinations are soybean with Bradyrhizobium japonicum, alfalfa with Sinorhizobium melilotii, peas with Rhizobium leguminosarum bv. Viceae, lotus with Mesorhizobium loti, or beans with Rhizobium legumninosarum bv. Phaseoli.
Some rhizobia species are relatively good nitrogen fixers, while others are not, and colonization by poor nitrogen fixers can suppress the level of nodulation of effective nitrogen fixing strains.
(The above-mentioned microbial genera all consist of a wide variety of species and strains. The precise beneficial effects of these microbes depend on the specific species and strain that is used)
Formulating Biologicals for Agriculture
