CYTOSKELETAL ARRAYS IN THE CELLS OF SOYBEAN ROOT-NODULES - THE ROLE OF ACTIN MICROFILAMENTS IN THE ORGANIZATION OF SYMBIOSOMES

Within the infected cells of root nodules there is evidence of stratif ication and organisation of symbiosomes and other organelles. This org anisation is likely to be important for the efficient exchange of nutr ients and metabolites during functioning of the nodules. Using immunoc ytochemical labelling and confocal microscopy we have determined the o rganisation of cytoskeletal elements, microtubules and actin microfila ments in soybean nodule cells, with a view to assessing their possible role in organelle distribution. Most microtubule arrays occurred in t he cell cortex where they formed disorganised arrays in both uninfecte d and infected cells from mature nodules. In infected cells from devel oping nodules, parallel arrays of microtubules, transverse to the long axis of the cell, were observed. In incipient nodules, before release of rhizobia into the plant cells, the cells also had an array of micr otubules which radiated from the nucleus into the cytoplasm. Three act in arrays were identified in the infected cells of mature nodules: an aster-like array which emanated from the surface of the nucleus,a cort ical array which had an arrangement similar to that of the cortical mi crotubules, and, throughout the cytoplasm, an array of fine filaments which had a honeycomb arrangement consistent with a distribution betwe en adjacent symbiosomes. Uninfected cells from mature nodules had only a random cortical array of actin filaments: In incipient nodules, the density of actin microfilaments associated with the nucleus and radia ting through the cytoplasm was much less than that seen in mature infe cted cells. The cortical array of actin also differed, being composed of swirling configurations of filaments. After invasion of nodule cell s by the rhizobia, the number of actin filaments emanating from the nu cleus increased markedly and formed a network through the cytoplasm. C onversely, the cytoplasmic array in uninfected cells of developing nod ules was identical to that in the cells of incipient nodules. The cyto plasmic network in infected cells of developing nodules is likely to b e the precursor of the honeycomb array seen in mature nodule cells. We propose that this actin array plays a role in the spatial organisatio n of symbiosomes and that the microtubules are involved in the localis ation of mitochondria and plastids at the cell periphery in the infect ed cells of root nodules.