Phosphate concentration alters the effective bacterial quorum in the symbiosis of Medicago truncatula-Sinorhizobium meliloti

The symbiosis of Medicago truncatula-Sinorhizobium meliloti is affected by phosphate (P) deficiency in the environment. Quorum sensing (QS) is a regulatory pathway in S. meliloti that controls various functions of free-living and symbiotic bacteria in response to phosphate availability and regulation is mediated by a periplasmic protein PstS, and also bacterial density. The quorum sensing pathway of S. meliloti, involves three genes named sinI, sinR and expR and also some bacterial auto-inducers such as N-acyl homoserine lactones (AHLs). In the current study, the expression of the different genes of quorum sensing and pstS were evaluated under 0.1, 0.5 and 2 mM P. The qRT-PCR results showed an increased expression of pstS and also the quorum sensing genes sinI and sinR but not expR, following phosphate starvation. Indeed, the enhanced level of sinR induces the expression of sinI that is responsible for the N-acyl homoserine lactones (AHL) production in S. meliloti. The different response of expR may be due to its negative control on sinR expression. In the symbiosis of M. truncatula-S. meliloti, it was shown that the concentration of phosphate in the medium alters the effective inoculating bacterial quorum (density). By increasing the phosphate concentration in the medium from 0.1 to 0.5 and 2 mM, considering the optimal plant growth and pink nodule (nitrogen-fixing) formation, the effective inoculating bacterial densities were 10⁵, 10⁷ and 10⁹ CFU ml^?1, respectively. Therefore, low phosphate concentrations can compensate for a low bacterial density by inducing the quorum sensing pathway and establishing a symbiosis. Conversely, bacterial density plays the main role in the formation of symbiosis at high phosphate concentrations.