PvRabA2, a monomeric GTPase, is involved in infection thread formation during early infection events in legume-rhizobia symbiosis

VIRGINIA DALLA VIA, VIRGINIA SAVY, MARÍA EUGENIA ZANETTI, O. MARIO AGUILAR AND FLAVIO BLANCO

Mar de Plata

Congreso; XXIX Reunión Argentina de Fisiología Vegetal, del 17 al 20 de septiembre; 2012

Sociedad Argentina de Fisiología Vegetal

The symbiotic relationship between legumes and nitrogen-fixing bacteria accounts for almost half of the nitrogen incorporated into agricultural soils. Unraveling the molecular mechanisms involved in the establishment of the interaction is crucial to improve this biological process. We have previously shown that PvRabA2, a monomeric Rab GTPase from common bean (Phaseolus vulgaris), participates in polar growth events such as root hair formation, growing and redirectioning in response to rhizobia. Moreover, RNAi silenced plants did not form infection threads (ITs), the tubular infection structures that lead bacteria to the nodule, resulting in the absence of root nodules. Considering the complex post-translational regulation of the monomeric GTPases, we obtained constitutively active (CA) and dominant negative (DN) forms of PvRabA2 by site-directed mutagenesis to further evaluate the function of this protein at early infection stages of the interaction. Overexpression of DN and CA forms did not have a pronounced effect on polar growth of root hairs or in the number of ITs as compared with control plants. However, the presence of any mutated form produced aberrant infection events, which were characterized by the formation of infection pockets, the ramification of ITs within RHs and the premature release of bacteria into the cytoplasm of trichoblasts. Concomitantly, nodule formation and development was also affected in both the CA and DN overexpressing roots when they were inoculated with the high efficient strain of Rhizobium etli SC15, but not with the less efficient strain 55N1. Localization experiments showed that the CA form of PvRABA2 associates to the growing IT. Our results suggest that PvRabA2 participates in the vesicle trafficking associated to deposition of phragmoplast-like material required for IT progression, and might be a key component of  the strain-specific response previously described in the P. vulgaris-rhizobia interaction.