Multi-omics Analysis of the Regulatory Effects of Plant Growth-Promoting Rhizobacteria on the Growth and Development of Different Lettuce Cultivars

Lettuce (Lactuca sativa L.) is an important leafy vegetable widely cultivated worldwide. To mitigate the environmental problems caused by the irrational application of chemical fertilizers, developing environment-friendly approaches to improve soil fertility is of great significance for increasing lettuce yield per unit area. As a novel green growth promoter, plant growth-promoting rhizobacteria (PGPR) have become a research and application hotspot in the agricultural field. To screen PGPR inoculants suitable for new lettuce cultivars, this study used Bacillus velezensis strains JB0319 and SAAS-6-3 as the test strains to investigate their regulatory effects on the growth and development of two new lettuce cultivars, Huqian and Binfen-3. The results showed that inoculation with both strains exerted a significant growth-promoting effect on Huqian, with remarkable increases in agronomic traits including fresh weight, plant spread, leaf width and plant height, and SAAS-6-3 exhibited a superior growth-promoting effect to JB0319. In contrast, the growth-promoting effect on Binfen-3 was not significant, with no obvious changes in fresh weight, plant height, leaf length and other indicators compared with the control group. Detection of physiological indices revealed that the soluble protein content of lettuce increased significantly after inoculation treatment, while the chlorophyll and soluble sugar contents showed no obvious changes. Determination of plant hormones indicated that the contents of gibberellins (especially GA3) and auxins (especially IAA) were significantly up-regulated. To elucidate the molecular regulatory mechanism, an integrated transcriptomic and metabolomic analysis was conducted on the inoculant-treated groups, which identified a total of 672 differentially expressed genes (DEGs). These DEGs were mainly enriched in pathways such as plant hormone signal transduction, zeatin biosynthesis, anthocyanin biosynthesis and phenylpropanoid biosynthesis. Untargeted metabolomic analysis showed that differential metabolites were significantly enriched in ABC transporters, alanine-aspartate-glutamate metabolism, galactose metabolism, tricarboxylic acid cycle, carbon metabolism, arginine biosynthesis and other pathways. The results of this study provide key evidence for clarifying the molecular and metabolic mechanisms underlying PGPR-mediated regulation of lettuce growth, and also lay a theoretical foundation for the precise application of such inoculants in the green cultivation of lettuce