Some new aspects of the pea (Pisum sativum L.) root nodule ultrastructure

Unequal cell divisions were observed in the meristem of pea root nodule. Since after such divisions only the bigger cells become infected then those divisions play a significant role in the formation of the three-dimensional structure of the bacteroidal tissue. In the infected cells of the young ineffective bacteroidal tissue the first host reaction to the incompatibility of the symbiotic system is the RER membranes aggregation. In effective symbiosis RER membranes form permanent sites of contact with the peribacteroidal membranes thus connecting all the symbiosoms in the cell. Possibly that ensures the synchronisation of the differentiation processes of the bacteroids and/or their simultaneous degeneration. The presence of membraneous structures in the form of rings is a characteristic feature of effective bacteroids. It is postulated that the structures are directly connected with nitrogen assimilation. Structures X and Y which are present in the bacteroids of the effective and ineffective symbiosis may be connected with the adaptation of bacterial cells to lowered oxygen pressure in bacteroidal tissue and their transformation (structures X) into bacteroids. The presence of the cytoplasm (or cytoplasmatic remnants) of the infected cells was observed in the intercellular spaces. It is sugested that it is a way, so far unknown, of the gas diffusion regulation in bacteroidal tissue.