Quorum sensing activities and genomic insights of plant growth-promoting rhizobacteria isolated from Assam tea

Secretion of quorum sensing (QS) molecules is important for the effective colonization of host plants by plant growth-promoting rhizobacteria. The current study aims at the isolation and characterization of tea rhizo bacteria, which produce the QS molecules, acyl homoserine lactone (AHLs), along with multiple plant growth-promoting (PGP) activities. Thirty-one isolates were isolated from the tea rhizosphere, and screening for PGP activities resulted in the selection of isolates RTE1 and RTE4 with multiple PGP traits, inhibiting the growth of tea fungal pathogens. Both isolates also showed production of AHL molecules when screened using two biosensor strains, Chromobacterium violaceum CV026 and Escherichia coli MT 102(jb132). The isolates identified as Burkholderia cepacia RTE1 and Pseudomonas aeruginosa RTE4 based on genome-based analysis like phylogeny, dDDH, and fastANI calculation. Detailed characterization of AHLs produced by the isolates using reverse-phase TLC, fluorometry, and LC-MS indicated that the isolate RTE1 produced a short chain, C8, and a long chain C12 AHL, while RTE4 produced short-chain AHLs C4 and C6. Confocal microscopy revealed the formation of thick biofilm by RTE1 and RTE4 (18 and 23 mu m, respectively). Additionally, we found several genes involved in QS, and PGP, inducing systemic resistance (ISR) activities such as lasI/R, qscR, pqq, pvd, aldH, acdS, phz, Sod, rml, and Pch, and biosynthetic gene clusters like N-acyl homoserine lactone synthase, terpenes, pyochelin, and pyocyanin. Based on the functional traits like PGP, biofilm formation and production of AHL molecules, and genetic potential of the isolates B. cepacia RTE1 and P. aeruginosa RTE4 appear promising candidates to improve the health and growth of tea plantations.