Actin Cytoskeleton Organization in Symbiotic Nodules of Pisum sativum L. and Medicago truncatula Gaertn

Using immunolocalization and confocal microscopy the actin cytoskeleton organization in pea and M. truncatula nodules was studied. The pea wild-type line SGE and the M. truncatula wild type line A17 and their corresponding mutant lines with mutations in the orthologous genes efd-1 and sym40 forming hypertrophied infection droplets, ipd3 and sym33 characterized by the absence of bacterial release were used.

Both legume species were characterized by a similar organization of the actin cytoskeleton in nodules. It was also shown that the organization of actin cytoskeleton was not affected by mutations. Nevertheless, the use of these mutants with increased infection structures made it possible to study the organization of actin microfilaments around infection threads and droplets more clearly. Dense network of actin microfilaments surrounded the nucleus and connected it with the cell periphery. Similar network of microfilaments was observed around the vacuole. Thickened endoplasmic actin microfilaments passed along infection threads and surrounded infection droplets. After bacterial release into the host cell cytoplasm, a network of thin and short actin microfilaments was developed among bacteroids in the infected cells.

Quantitative analysis of actin microfilament orientation revealed several patterns in infected and uninfected cells. In pea and M. truncatula the angle patterns of uninfected cells were similar, but they were different in infected cells. The latter can be explained by the differences in bacteroid morphology of two legume species.

This work was supported by Russian Science Foundation 16-16-10035.