Induced growth promotion and higher salt tolerance in the halophyte grass Puccinellia tenuiflora by beneficial rhizobacteria

Soil salinization limits conventional agriculture since most food-based plant cultivars require low soil-sodium (Na+) levels for robust growth. Moreover, modern agricultural practices, especially in arid environments, can exacerbate soil salinization as belowground water sources utilized in irrigation are frequently tainted with salt. While salt tolerance has previously been shown to be augmented in several glycophyte species by the soil bacterium Bacillus subtilis (GB03), here we reported that this beneficial rhizobacterium promotes growth and augments higher salt-tolerance in halophyte grass Puccinellia tenuiflora. The optimal Bacillus subtilis strain for P. tenuiflora was screened. P. tenuiflora was grown from seeds with NaCl (0, 100, 200 and 300 mM) for salt treatments with or without inoculation of B. subtilis GB03. Growth parameters, chlorophyll content and endogenous Na+ and K+ contents were determined at the time of harvest. Seedlings were grown in medium with 0 or 200 mM NaCl, then were harvested to extract total RNA after 48 h of exposure to GB03 VOCs. Semi-quantitative RT-PCR was used to investigate the relative amount of PtHKT1;5, PtHKT2;1 and PtSOS1 in P. tenuiflora regulated by GB03. The optimal Bacillus subtilis strain for P. tenuiflora was GB03. GB03 significantly improved shoot and root growth at two, three, four and five weeks after inoculation. Under various salinity stresses, GB03 significantly promoted growth of P. tenuiflora seedlings. Na+ accumulation was reduced with K+ accumulation unaffected by GB03 exposure. Therefore, GB03 enhanced selective absorption capacity of P. tenuiflora for K+ over Na+ (SA) from media. Gene expression analysis demonstrated that GB03 up-regulated PtHKT1;5 and PtSOS1, but down-regulated PtHKT2;1 expression, specifically in roots when plants are grown under greatly-elevated salt conditions (200 mM NaCl). Our results presented here established that B. subtilis GB03 promoted the growth and improved the salt tolerance and the selective absorption capacity for K+ over Na+ in the monocotyledonous halophyte P. tenuiflora to a higher level. Interestingly, GB03-triggered up-regulation of PtHKT1;5 and PtSOS1 and down-regulation of PtHKT2;1 in roots reduced Na+ transport from root to shoot as well as Na+ uptake in roots. This study provides the physiological and molecular evidence that application of selected bacteria to salt-tolerant Monocots can ameliorate deleterious effects of high soil saline toxicity.