Phosphate deficiency reduces nodule formation through a phosphate starvation response-like protein in Phaseolus vulgaris

Phosphate deficiency reduces nodule formation in various legumes, which hinders nitrogen fixation and crop yield. We previously showed that phosphate deficiency reduces nodule formation by activating the autoregulation of nodulation (AON) pathway. We also observed that some genetic components of the AON pathway contain Phosphate Starvation Response 1 binding site cis-regulatory elements in their promoter regions, which are recognized by the Phosphate Starvation Response 1 transcription factor. This evidence led us to hypothesize that host plant phosphate levels regulate the expression of genes essential for forming nodules through a PHR-Like protein. In the present study, we provide evidence supporting the participation of PvPHR-Like 7 (PvPHR-L7) in regulating nodule formation in Phaseolus vulgaris. Modulation of PvPHR-L7's expression by RNA interference and overexpression suggested that this transcription factor may control the expression of crucial symbiotic genes involved in nodule development in P. vulgaris. An RT-qPCR analysis revealed that the expression of PvPHR-L7, PvNIN, and PvTML is regulated in accordingly to the plant host Pi levels. Transactivation assays in Nicotiana benthamiana and P. vulgaris transgenic roots indicate that PvPHR-L7 can upregulate the expression of PvNIN and PvTML in the absence of rhizobia. In contrast, PvPHR-L7 downregulates the expression of PvNIN under symbiotic conditions with rhizobia. The data presented shed light on the potential role that PvPHR-L7 plays in the root nodule symbiosis.