Bacillus subtilis QM3, a Plant Growth-Promoting Rhizobacteria, can Promote Wheat Seed Germination by Gibberellin Pathway

The mechanism underlying seed germination promotion by plant growth-promoting rhizobacteria (PGPR) remains unclear. This study investigated the effects of Bacillus subtilis QM3 (a PGPR) and gibberellic acid (GA(3)) on germination indexes, seedling growth, and amylase activity in wheat seeds (Triticum aestivum L.) (caryopses) at different germination times. The results showed that B. subtilis QM3 exhibited a significantly greater increase in most measured indicators compared to GA(3) (p < 0.05), and the coordinated effect of bacteria QM3 and GA(3) was superior. GA(3) showed no promotion of radicle length (LR) from day 3 to day 7. Conversely, the addition of uniconazole (U), a gibberellin synthesis inhibitor, significantly reduced all indexes except for fresh weight (FW) and root-shoot ratio (RSR). However, both B. subtilis QM3 and GA(3) significantly alleviated the inhibition (p < 0.05), especially in the germination rate and amylase activity. The gibberellin (GA) content and expression of two key biosynthetic enzyme genes were also characterized. Further investigations revealed that the GA content in the B. subtilis QM3 group was significantly higher than in the control group at 12 h, 24 h, and 48 h during seed germination (p < 0.05). Moreover, B. subtilis QM3 significantly increased TaGA3ox-B2 expression (p < 0.05), with 1.44 times that of the control group, while there was no significant effect on TaGA20ox-D2 expression. Furthermore, amylase activity was positively correlated with the germination rate (GR) and GA content was positively correlated with amylase activity according to Pearson's correlation analysis, with R-2 values of 0.8102 and 0.8365, respectively. Our results demonstrate that one of the mechanisms by which B. subtilis QM3 promotes wheat seed germination may be the induction of GA biosynthesis.