| Plant growth-promoting rhizobacteria(PGPRs)can promote plant growth,improve crop yield,induce plant systemic resistance against biotic and abiotic stresses.Plant rhizosphere in different habitats contains unique bacterial communities that can help plants to adapt to their habitats.Haloxylon ammodendron,a succulent and dominant species in desert area,has formed a series of complex and unique stress-resistance mechanisms during long-term evolution.We proposed that PGPRs from H.ammodendron rhizosphere have the characteristics closely related to their host plants and habitats and can affect the growth and improve resistance of host plants through various mechanisms.Therefore,it is especially important to explore the PGPRs from H.ammodendron rhizosphere and investigate their growth-promoting effects and mechanism in improving plant stress resistance.The results will provide a physiological foundation for further exploring the molecular mechanism of their growth-promotion on plants,and also provide excellent PGPR resources for the development of new compound microbial fertilizers.In this work,four strains(WAW-10,WA30-5,WM30-21 and WM13-24)isolated from H.ammodendron rhizosphere were used as PGPR strains.Their physiological and biochemical characteristics and effects on the promotion and salt tolerance of leguminous forage were investigated;at the same time,the whole genome of WM13-24 was sequenced and analyzed by PacBio RS II sequencing platform and Single Molecule Real Time sequencing technology,and a series of genes relevant to growth-promotion and stress-resistance were found.The main results are as follows:1.The strains WAW-10,WA30-5 and WM30-21 belong to the genus of Mesorhizobium sp.The three strains are saline-alkali resistance Rhizobiu and can grow in the pH range of 5 to 11 and temperature range of 20 to 37°C.The highest survival salt concentration was 4%NaCl(0.684 mol/L)for WA30-5 and WAW-10 and 5%NaCl(0.855 mol/L)for WM30-21.The three strains have good ability to fix nitrogen and solubilize phosphorus.2.Under normal condition,WAW-10,WA30-5 and WM30-21 showed fine growth-promoting effects on Medicago sativa.Under 300 mM NaCl,three strains significantly improved the salt tolerance of Medicago sativa through increasing biomass,chlorophyll content,root activity,soluble sugar content,proline content,catalase activity,K~+content and K~+/Na~+ratio and decreasing leaf osmotic potential,malondialdehyde content,Na~+content.WM30-21 showed the best effect.3.The strain WM13-24 belongs to the genus of Bacillus subtilis.A variety of active enzymes from WM13-24 were identified.The strain can use a variety of carbon sources and can grow in the pH range of 4 to 11 and temperature range of 20 to 45°C.The highest survival salt concentration was 17%NaCl(2.91 mol/L)for WM13-24,indicating it is a saline-alkali resistance and high temperature resistant bacterial strain.The whole genome size of WM13-24 was 4,230,925 bp and GC content was 43.88%.29 genes related to growth promotion and 30 genes related to abiotic stress were preliminarily identified in the genome.4.WM13-24 has significant growth-promoting effects on Medicago truncatula and white clover.WM13-24 significantly increased the plant height,root length,biomass,root volume,root activity,branch numbers and photosynthesis rate of the two legume forage.In summary,the three Rhizobium strains from H.ammodendron rhizosphere showed significant growth-promoting effects and salt tolerance improvement on Medicago sativa.The whole genome of Bacillus strain WM13-24 provided abundant reference data to enrich the whole genome database of Bacillus and supplemented research data of the functional genes of PGPR.This study will provide a theoretical foundation for clarifying the mechanism for PGPR-plant interaction to improve plant growth-promoting and stress-resistance;and also provide excellent PGPR resources for the development of new compound microbial fertilizers,which is significant for green and sustainable agriculture. |
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