Apoplast-associated Bacillus amyloliquefaciens LAS10 for plant growth promotion and drought stress tolerance in little millet (Panicum sumatransae)

Bacterial endophytes thriving in the apoplast could be tailored to withstand environmental challenges, thereby contributing to plant stress resilience. Developing plant growth-promoting (PGP) bacteria resilient to drought is crucial for sustainable agriculture and improved crop production under stress conditions. Thus, the present study aimed to decode the drought-tolerant apoplast-associated PGP bacteria from little millet (Panicum sumatrense L.) and profile its functional traits including the metabolites for PGP and drought resilience. The drought tolerant PGP bacterial isolates from apoplastic fluids of little millet (Var. ATL1) grown under stressed conditions (-10 bars) on polyethylene glycol (PEG) (PEG 6000) infused agar plates revealed that a total of 12 isolates, were initially screened for their ability to tolerate drought up to -36.6 bars (-3.6 MPa) and of which, 6 isolates (LAS1, LAS2, LAS4, LAS6, LAS9, and LAS10) were selected. Further, screening for their plant growth-promoting (PGP) traits such as 1-aminocyclopropane-1-carboxylate deaminase (ACCd) production, exopolysaccharide production, potassium (K), phosphorous (P), and Zinc (Zn) nutrition, indole acetic acid (IAA) production, siderophore production, ammonia, and hydrogen cyanide (HCN) exhibited higher PGP potentials in LAS10 followed by LAS2 and LAS4. Of three potential isolates (LAS10, LAS2, and LAS4), LAS10 produced the highest ACCd (147 n moles alpha ketobutyrate mg-1h-1), IAA (15.9 mu g mL-1), siderophore (54.9 % units) and P solubilization compared to other isolates. Further, isolate LAS10 was identified as Bacillus amyloliquefaciens based on 16S rRNA sequence analysis. Metabolite profiling of B. amyloliquefaciens LAS10 in GC-MS under induced stress conditions yielded several drought-tolerant metabolites belonging to the class viz., organic acids, fatty acids, amino acid and its derivatives, organoheterocyclic compounds, and benzenoids. The compounds responsible for drought tolerance such as phenol, proline, fumaric acid, ascorbic acid, and gibberellic acid are more pronounced under induced drought stress (PEG 6000) which would aid in drought stress tolerance and facilitate plant health and fitness. These results implied that B. amyloliquefaciens LAS10, an apoplast-associated drought-tolerant endophytic bacteria, would enhance plant growth under drought stress, and can be further extrapolated as a newer bio inoculant for abiotic stress mitigation and sustainable production in little millet.