Transcriptional and biochemical profiling of Bacillus strains regulating the growth of tomato via altering morpho-physiological and hormonal traits

Endophytic Bacillus species can increase plant biomass and serve as biofertilizers. Five Bacillus strains were evaluated for their potential to generate extracellular enzymes. All strains demonstrated proteolytic and cellulolytic activity in an in vitro experiment and were able to solubilize insoluble starch except Bacillus altitudinis TM22A. All strains failed to exhibit phosphate solubilization but able to produce IAA. B. altitudinis TM22A and B. cereus CP1 were lacking the ability to hydrolyze lipids and produce siderophores. Transcriptional analysis revealed that B. altitudinis TM22 possessed all tested growth-related genes viz, acdS deaminase, PvD, AsbA, PqqE, EpsO, SatA, and XerC. TM22 produced IAA in the highest concentration. In the greenhouse experiment, TM22treated plants showed higher growth promotion than TM22A and enhanced fresh weight shoot, shoot length, fresh weight root, dry weight shoot, and dry weight root. TM22-treated plants had greater leaf area, photosynthetic rate, and stomatal conductance. Root scanning revealed that TM22-treated plants showed higher total length, surface area, diameter, and volume relative to TM22A treated plants. The expression of growth-related genes viz., expansin (EXP-1 and EXP-2), cytokinin (CKX), auxin (IAA-1 and IAA-6), and gibberellin (GA20OX1 and GA20OX-2) was upregulated in TM22 treated plants while the expression of ethylene-responsive genes ACO-1 and ACO-2 was downregulated in both TM22 and TM22A treated plants relative to control.