Down-regulation of a Phaseolus vulgaris annexin impairs rhizobial infection and nodulation

Legumes establish a symbiotic relationship with N2-fixing soil bacteria known as rhizobia. This process is initiated by a molecular dialog between roots and bacteria. Rhizobia secrete lipochitooligosaccharide signals, known as Nod factors, into the rhizosphere that are perceived by root hairs and induce an increase in Ca2+ levels, ROS production, and transcriptional activation of nodulin genes. Plant annexins are a family of calcium- and membrane-binding proteins involved in growth and development and in several abiotic and biotic responses. In this study, we identified thirteen annexin genes in the Phaseolus vulgaris (common bean) genome. We show that PvAnn1 transcript abundance was higher in root hairs than in root apices and stripped roots, and that during symbiosis, PvAnn1 transcript abundance was substantially higher in nodules than in nodule-stripped roots. RNAi knock-down of PvAnn1 resulted in reduced root hair lengths and ROS production. Furthermore, PvAnn1-RNAi transgenic roots that were inoculated with rhizobia showed a decrease in the progression and number of infection threads, possibly due to changes in Ca2+-ROS homeostasis and nodulin signaling. Furthermore, a reduced number of nodules and less nitrogen fixation was observed in the transgenic plants at 21 days post infection. Together, these data suggest that PvAnn1 is involved in rhizobial infection and nodule development and function.