Bacillus velezensis SX13 promoted cucumber growth and production by accelerating the absorption of nutrients and increasing plant photosynthetic metabolism

Rhizosphere colonization by plant growth-promoting rhizobacteria (PGPR) facilitates the ability of PGPR to promotes plant growth and health. Among the rhizosphere microbial communities, the dominant PGPR strains should be tapped to analyze their growth-promoting mechanism, and then promote the development and utilization of related agricultural products. In the current study, Bacillus velezensis SX13, Bacillus paralicheniformis SX21, and Bacillus tequilensis SX31, which can dissolve inorganic and organic phosphorus and produce auxin (IAA; except B. tequilensis SX31), were isolated from cucumber rhizosphere. Three weeks after the strains were separately inoculated into the rhizosphere of cucumber plants, the plants treated with the three strains showed improved root structures, photosynthetic parameters, increased growth rate, and high degree of biomass accumulation, compared with the control plants. Plants inoculated with B. velezensis SX13 had the highest seedling growth rate. Experiment using different inoculation doses was conducted. Results showed that B. velezensis SX13 promoted the absorption of elements by the roots and the transport of these elements to the shoots by improving the root structures and up-regulating the expression of CsNRT1 subfamily genes in the roots. Increases in element absorption and transport rates promoted photosynthesis and the activities of enzymes related to carbon metabolism (including ribulose-1,5-bisphosphate carboxylase/oxygenase, sucrose phosphate synthase, phosphoenolpyruvate carboxylase, ribulose-1,5-bisphosphate carboxylase/oxygenase activase, and fructose-l.6-bisphosphate aldolase) and nitrogen metabolism (including nitrate reductase, glutamine synthase, glutamine-2-oxoglutarate aminotransferase, and glutamate dehydrogenase) in the leaves. In addition, the levels of glucose, sucrose, fructose, soluble proteins, and amino acids increased in the leaves. Induced by B. velezensis SX13, increase in photosynthetic metabolism promoted biomass and element accumulation in the roots and shoots and significantly improved fruit quality and yield. However, a dose effect was observed in the case of B. velenzensis SX13 at the doses assayed in this work.