Can interaction between silicon and non–rhizobial bacteria benefit in improving nodulation and nitrogen fixation in salinity–stressed legumes? A review

Salinity stress adversely affects root nodulation symbiotic relationships, and ultimately the nitrogen fixation capacity and the growth and yield of leguminous plants. Improving growth and biological nitrogen fixation of leguminous plants grown on salt–affected soils are considered to be a striking challenge. Most of the findings reveal that co–inoculation of legumes with salt–tolerant non–rhizobial endophytic/rhizosphere bacteria and rhizobial bacteria is a sustainable solution for boosting nitrogen fixation and the productivity of leguminous plants grown on salt–affected soils compared to those formed by a single rhizobial bacterium. In separate studies silicon was also found to boost growth, nodulation and nitrogen fixation in leguminous plants by various mechanisms under non–saline and saline conditions. In this paper we first review the implication of endophytic/rhizosphere non–rhizobial nodule bacteria in the plant's resistance to salinity stress. Next we review the implication of silicon biogeochemistry in plant's response to salinity stress. Last, we propose that non–rhizobial bacteria mediate the response to salinity together with silicon in mitigating salinity stress. This review mostly focuses on the important role of silicon and beneficial non–rhizobial endophytic bacteria during the nodule formation process, accentuating their importance to legume growth promotion under salinity stress. Based on our literature review, non–rhizobial bacteria in combination with silicon were often more effective at improving nodulation and nitrogen fixation in salinity–stressed legumes than solely inoculated non–rhizobial bacteria. We propose the combined use of non–rhizobial helper nodule bacteria and silicon together can be a powerful and sustainable strategy to nodulation and nitrogen fixation in legumes under salinity–stress conditions, and hence research on the dual use of silicon and non–rhizobial bacteria in legumes under salinity stress can be an effective strategy. A better grasp of the interactions between plant silicon and non–rhizobial nodule bacteria and plant responses to salinity stress will help implement more effective fertilizer practices under saline conditions.