Abstract
Salicylic acid is an important signalling molecule in plant-microbe defence and symbiosis. We analysed the transcriptional responses of the nitrogen fixing plant symbiont, Rhizobium leguminosarum bv viciae 3841 to salicylic acid. Two MFS-type multicomponent efflux systems were induced in response to salicylic acid, rmrAB and the hitherto undescribed system salRAB. Based on sequence similarity salA and salB encode a membrane fusion and inner membrane protein respectively. salAB are positively regulated by the LysR regulator SalR. Disruption of salA significantly increased the sensitivity of the mutant to salicylic acid, while disruption of rmrA did not. A salA/rmrA double mutation did not have increased sensitivity relative to the salA mutant. Pea plants nodulated by salA or rmrA strains did not have altered nodule number or nitrogen fixation rates, consistent with weak expression of salA in the rhizosphere and in nodule bacteria. However, BLAST analysis revealed seventeen putative efflux systems in Rlv3841 and several of these were highly differentially expressed during rhizosphere colonisation, host infection and bacteroid differentiation. This suggests they have an integral role in symbiosis with host plants.
Highlights
Plants produce and secrete a diverse number of compounds into the rhizosphere
In the case of symbioses with rhizobia plants do not usually elicit a defence response [6], salicylic acid may be important in controlling host range and regulating nodule formation
Exogenous application of salicylic acid has been shown to decrease or inhibit nodule formation when Bradyrhizobium japonicum or Rhizobium leguminosarum are grown on Soybean and Vetch respectively [7,8]
Summary
Plants produce and secrete a diverse number of compounds into the rhizosphere These include a myriad of phytoalexins and signalling molecules which mediate plant defences but orchestrate plant microbial interactions including symbiosis with nitrogen fixing symbiotic rhizobia [1,2]. One key molecule in the response of plants to microbes is salicylic acid, which is a phenolic hormone with varied roles in plant metabolism and physiology including plant defence [3]. In the case of symbioses with rhizobia plants do not usually elicit a defence response [6], salicylic acid may be important in controlling host range and regulating nodule formation. A number of studies have investigated salicylic acid levels and nodule formation. Decreasing endogenous levels of salicylic acid in Lotus japonicus led to increased nodule numbers when inoculated with Mesorhizobium loti [6]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Free) 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
