Actin monoubiquitylation is induced in plants in response to pathogens andsymbionts

Most dramatic examples of actin reorganization have been described during h ost-microbe interactions. Plasticity of actin is, in part, due to posttrans lational modifications such as phosphorylation or ubiquitylation. Here, we show for the first time that actins found in root nodules of Phaseolus vulg aris are modified transiently during nodule development by monoubiquitylati on. This finding was extended to root nodules of other legumes and to other plants infected with mycorrhiza or plant pathogens such as members of the genera Pseudomonas and Phytophthora. However, neither viral infections nor diverse stressful conditions (heat shock, wounding, or osmotic stress) indu ced this response. Additionally, this phenomenon was mimicked by the additi on of a yeast elicitor or H2O2 to Phaseolus vulgaris suspension culture cel ls. This modification seems to provide increased stability of the microfila ments to proteolytic degradation and seems to be found in fractions in whic h the actin cytoskeleton is associated with membranes. All together, these data suggest that actin monoubiquitylation may be considered an effector me chanism of a general plant response against microbes.