What are the Rhizobial Infection, Nodulation, and Nitrogen Fixation Processes?

When the correct Rhizobium species reads the chemical signals from the specific legume roots, they congregate at those roots at the root hairs (Step 1). Infection begins when the root hair “curls” around the Rhizobia and encloses them (Step 2). An “infection thread” then forms (Steps 3 and 4). The infection thread is simply a folding-in of the root hair membrane so that it encloses the replicating Rhizobia while they grow into the root until they reach the root cortical cell layer. The thread branches out into the cortical cell layer (Step 5) and the cortical cells begin to multiply due to the infection (Step 6) forming the young nodule (Step 7). The Rhizobia multiplies further in these nodule cells and then enlarges to become bacteroids, or the form that is capable of fixing nitrogen (Step 8). Nitrogenase, the actual nitrogen-fixing enzyme, is synthesized inside the mature nodule and bacteroids (Step 8). However, nitrogenase is deactivated by oxygen, so another critical compound produced is leghemoglobin which is responsible for capturing oxygen and moving it away from the nitrogenase enzyme. Therefore, when the mature nodule and bacteroids are functioning properly, nitrogen gas seeps into the nodule and is reduced chemically into ammonia which is in turn converted to amino acids that are transported throughout the legume plant for its metabolism and growth as protein. The Nitro-Coat® process places the proper Rhizobium next to the seed so that infection begins as soon as the root hairs emerge.