Isolation and Identification of Rhizosphere Growth-Promoting Bacteria of Myroides odoratimimus PJ-3 and Their Salt/Alkali-Tolerance and Growth-Promoting Effects on Maize

Soil salinization is a significant limiting factor for global agricultural production and food security. Based on the mutualistic symbiotic relationships between rhizosphere beneficial bacteria (PGPR) and plants, this study aimed to screen out PGPR strains that possessed multiple functions, such as salt tolerance, alkali tolerance, and promoting growth, with the salt-stressed soil from maize rhizosphere. The study would provide raw materials and theoretical basis for the development of microbial agents for salt-alkali soil improvement and support the comprehensive management of salt-alkali land.

The PGPR strains were isolated by using the dilution plate coating method. The isolates were identified through morphological and microscopic observation, Biolog Gen III microplate analysis, and 16S rRNA gene sequencing. A microcosmic experiment was employed to assess the salt-alkali tolerance and plant growth promotion potential (PGP) of the PGPR strains.

23 PGPR isolates were screened and obtained, and the PJ-3 isolate exhibited the optimal salt-alkali tolerance and PGP potential. It was identified as Myroides odoratimimus with the attributes of Gram-negative. Specifically, the strain PJ-3 possessed a robust fructose-glycolytic pathway and lactic acid-pyruvate and phosphosugar transformation systems, enabling tolerance to extreme saline-alkaline conditions of up to 5% NaCl with a pH of 11. Pot experiments further demonstrated that under moderate saline-alkaline stress, the strain PJ-3 significantly increased maize germination rate of maize by 70% and promoted plant height, leaf length, leaf width, aboveground fresh weight, and belowground fresh weight by over 50%. Further analysis revealed that its superior tolerance and PGP mechanisms included the secretion of proteases, siderophores, and the plant hormone IAA (indole-3-acetic acid), activation of reactive oxygen species (ROS) scavenging by antioxidant enzymes SOD (superoxide dismutase) and CAT (catalase) in maize. Moreover, it could stimulate the conservative growth strategy in roots, with the enchantment in root surface area, root tip number, branching frequency and root vigor. As a result, these positive effects improved the improved chlorophyll a-dominated photosynthesis.

Collectively, it was the first to report that Myroides odoratimimus PJ-3 possesses multiple novel functions, including remarkable salt tolerance, alkali tolerance, and the promotion of both aboveground and belowground development in maize. It demonstrated broad application potential for the integrated management of saline-alkaline lands and agricultural development.