Classification
Two main types of nodule have been described: determinate and indeterminate.
Determinate nodules are found on tropical (sub)legumes, such as those of the genera Glycine (soybean), Phaseolus (common bean), Lotus, and Vigna. Determinate nodules lose meristematic activity shortly after initiation, thus growth is due to cell expansion resulting in mature nodules which are spherical in shape.
Indeterminate nodules are found on temperate legumes like Pisum (pea), Medicago (alfalfa), Trifolium (clover), and Vicia (vetch). They earned the moniker "indeterminate" because they maintain an active apical meristem that produces new cells for growth over the life of the nodule. This results in the nodule having a generally cylindrical shape. Because they are actively growing, indeterminate nodules manifest zones which demarcate different stages of development/symbiosis:
- Zone I—the active meristem. This is where new nodule tissue is formed which will later differentiate into the other zones of the nodule.
- Zone II—the infection zone. This zone is permeated with infection threads full of bacteria. The plant cells are larger than in the previous zone and cell division is halted.
- Interzone II–III—Here the bacteria have entered the plant cells, which contain amyloplasts. They elongate and begin terminally differentiating into symbiotic, nitrogen-fixing bacteroids.
- Zone III—the nitrogen fixation zone. Each cell in this zone contains a large, central vacuole and the cytoplasm is filled with fully differentiated bacteroids which are actively fixing nitrogen. The plant provides these cells with leghemoglobin, resulting in a distinct pink color.
- Zone IV—the senescent zone. Here plant cells and their bacteroid contents are being degraded. The breakdown of the heme component of leghemoglobin results in a visible greening at the base of the nodule.
Read more about this topic: Root Nodule