Abstract
Previously, we showed that Bacillus amyloliquefaciens FZB42 can confer salt tolerance in plants by root inoculation under salt stress condition, and the FZB42 volatile organic compounds (VOCs) promoted plant growth and development under non-salt stress condition. In the present study, we investigated the mechanism that allows FZB42 VOCs to confer salt tolerance in Arabidopsis without colonization of plant roots. We found that FZB42 VOCs significantly increased the biomass of Arabidopsis and also maintained the leaf chlorophyll content under salt stress condition. Physiological tests showed that the plant anti-oxidation system was activated by FZB42 VOCs, where higher peroxidase (POD), catalase (CAT), and superoxide dismutase (SOD) activities were detected in plants exposed to FZB42 VOCs compared with non-exposed plants. In addition, FZB42 VOCs increased the leaf total soluble sugars (TSS) content but decreased the proline content compared with the non-exposed plants. Moreover, FZB42 VOCs significantly decreased the Na+ contents of the whole plants and induced the expression of genes (NHX1; Na+/H+ exchanger 1 and HKT1; high-affinity K+ transporter 1) that function to alleviate Na+ toxicity. Furthermore, analysis of mutants with defects in specific hormone pathways showed that FZB42 VOCs induced salt tolerance in plants by modulating jasmonic acid (JA) signaling, which was confirmed by the up-regulation of JA synthesis, defense-related genes, and JA biosynthesis inhibitor tests. The results of this study provide new insights into the molecular mechanism related to the interactions between plant growth-promoting rhizobacteria and plants under salt stress condition.
Highlights
Plant growth-promoting rhizobacteria (PGPR) are naturally free-living soil microorganisms that colonize plant roots and facilitate plant growth (Jastrow and Miller, 1991; Mayak et al, 1999)
Under non-salt and salt stress conditions, after exposure to volatile organic compounds (VOCs) from FZB42 for 15 and 20 days, Arabidopsis (Col-0) plants exhibited robust growth compared with the non-exposed controls (Figures 1A,B) as demonstrated by their enhanced shoot and root biomass (Figures 1C–E)
The VOCs of E. coli strain DH5α could not promote plant growth (Ryu et al, 2003), DH5α has been set as a control to exclude the possibility that the growth promotion and tolerance induction come from the increasing carbon dioxide which released when the bacteria growing (Supplementary Figure S2)
Summary
Plant growth-promoting rhizobacteria (PGPR) are naturally free-living soil microorganisms that colonize plant roots and facilitate plant growth (Jastrow and Miller, 1991; Mayak et al, 1999). Many PGPR have been widely studied and applied to a wide range of agricultural crops for the purpose of growth enhancement, including increased crop yields, plant weight, and seed emergence, due to their ability to form adverse environmentresistant spores (Francis et al, 2010) In addition to their potential effects on plant growth promotion, PGPR play important roles in induced systemic resistance to protect plants against biotic stresses (Yan et al, 2002; Choudhary and Johri, 2009; Rashid et al, 2017; Tahir et al, 2017) and induced systemic tolerance to many abiotic stresses, especially salinity and drought stresses (Kaymak et al, 2009; Yang et al, 2009; Paul and Lade, 2014; Lu et al, 2018; Brilli et al, 2019). The application of PGPR is a simple and economic option for reducing agricultural losses in saline land due to the capacity of PGPR to improve the tolerance of salt by crops (Yildirim et al, 2008), but this effect requires the colonization of PGPR on plants
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