Occurrence, identification and characterization of diazotrophic bacteria from aerial roots of Rhynchostylis retusa (L.) Blume for plant growth-promoting activity.

In this study, the diversity of diazotrophic bacteria of orchid Rhynchostylis retusa (L.) Blume and its potential application in plant growth promotion were evaluated. About 183 nitrogen-fixing bacteria were isolated to screen various plant growth-promoting traits viz. phosphate solubilization,IAA, siderophore, HCN, biofilm and ammonia production. Based on 16S rRNA gene sequencing analysis Achromobacter, Arthrobacter, Acinetobacter, Bacillus, Brevibacterium, Curtobacterium, Erwinia, Kosakonia, Lysinibacillus, Klebseilla, Microbacterium, Mixta, Pantoea, Pseudomonas and Stenotrophomonas isolates were selected and showed positive results for PGP traits. Overall, genus Pantoea, Brevibacterium, Achromobacter, Arthrobacter, Klebsiella, Mixta, Bacillus, and Pseudomonas had the most pronounced PGP characteristics and acetylene reduction among the screened isolates. BOX PCR fingerprinting analysis showed variation in polymorphic banding patterns among diazotrophic strains. PCR amplification of nifH gene and the presence of 37 kDa nitrogenase reductase enzyme band in western blot indicated presence of nitrogenase activity. Our study showed that orchid R. retusa diazotroph interaction helps orchid plant to fix nitrogen, essential nutrients, and control pathogen entry. To the best of our knowledge, this is the first report on characterization of diazotrophic bacterial community from aerial roots of R. retusa.