Question

What are lectins, and what is its role in nitrogen fixation?

Answer 1

The ability of the nitrogen-fixing bacteria Rhizobium to form a symbiotic relationship with legumes is characterized by a high degree of selectivity. For example, Rhizobium leguminosarum forms a symbiotic relationship only with peas and not with other legume species such as soybeans and jack beans. Understanding the molecular basis of this specificity currently is a goal of scientists studying nitrogen fixation.

Complete answer:
After recent research, scientists have come to a conclusion that after formation of a complex called Rhicadhesin-lectin complex, nodule formation in the roots of leguminous plants is initiated by Rhizobium. It is just like pollen-pistil interaction, where if the species (the chemical matching proteins) of the pollen and pistil do not match, pollen germination stops. Thus, the lectin secreted by the roots of leguminous plants attract and induce division in the Rhizobium after the formation of the said complex, and by this, to confer specificity.
In general, the formation of nodules in leguminous plants follows the following steps:
i. Rhizobium secrete rhicadhesin.
ii. Roots of leguminous plants secrete lectins.
iii. If the lectin is recognized and accepted, then the rhicadhesin-lectin complex is formed. How the formation of this complex is triggered is still a matter of research.
iv. Threads of Rhizobium go up into the cortex areas where the pectose-cellulose layer gets soluble.
v. There Rhizobium secrete the Nod gene.
Growth factors are secreted by the Nod gene.
vi. Growth factors proliferate cortex cells (mitotic factor) to form nodules.
vii. Within nodules, iron is collected from the plant body, protein produced by Rhizobium. This leads to formation of leghemoglobin.

Note:
It is to be noted that the role of lectins in nitrogen fixation is still under research. It possibly helps in the nodule formation which may involve genetic aspects too. Once the nodule formation has occurred, lectins have no role after that. Thus, its role is confined to only allowing specificity to the plant species which Rhizobium recognizes and invades the roots.