Functional and phylogenetic diversity of cultivable rhizobacterial endophytes of sorghum [Sorghum bicolor (L.) Moench]

Abstract

A diverse group of bacteria colonize the exo- and endo-rhizospheres of sorghum and play a critical role in its tolerance to drought and other abiotic stresses. Two hundred and eighty endophytic bacteria were isolated from the surface-sterilized roots of four sorghum cultivars that were grown on three soil types at three different phenological stages of growth. The isolates were subjected to in vitro screening for their plant growth promoting traits. Out of 280 isolates, 70 could produce Indole 3-Acetic Acid (IAA), 28 showed N-fixation, 28 could solubilize phosphate, 24 had ACC deaminase activity and 13 isolates were able to produce siderophores. Functional diversity grouping of the isolates indicated one isolate having five PGP traits and two isolates having four PGP traits; two and 29 isolates having three and two PGP traits, respectively. Among the thirty-four isolates that possessed multiple PGP traits, 19 and 17 isolates were able to produce significant quantities of IAA in the presence and absence of L-tryptophan, an inducer. Eight isolates possessed high levels of ACC deaminase activity. PCR–RFLP of the 16Sr RNA gene revealed a distinct clustering and considerable genetic diversity among these functionally characterized isolates. The 16S rRNA gene based identification of the isolates of single and multiple PGP traits revealed phylogenetic dominance of Firmicutes; Acinetobacter, Bacillus, Enterobacter, Geobacillus, Lysinibacillus, Microbacterium, Ochrobactrum, Paenibacillus and Pseudomonas were the major genera present in the endo-rhizosphere of sorghum. Results of this study are constructive in selection of effective rhizobacterial endophytes or consortia for drought stress alleviation in sorghum.