Exploring Growth Promoting and Drought Related Functional Traits of Pink Pigmented Facultative Methylotrophs from Okra (Abelmoschus esculentus L.)

Abstract

Pink-pigmented facultative methylotrophs (PPFMs) are common plant associated bacteria with diverse biochemical traits that may contribute to plant interactions. This study characterised functional variation among 36 PPFM isolates recovered from okra phyllosphere and rhizosphere samples collected in Kerala from March to July 2024 and assessed their in vitro plant growth-promoting (PGP) associated traits and growth under (Polyethylene Glycol) PEG 6000 induced osmotic stress. The isolates were examined for morphology, Gram reaction, oxidase and catalase activity, indole-3-acetic acid (IAA), gibberellic acid (GA), extracellular ammonia and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. Twelve PPFM isolates were selected based on their overall PGP-associated profiles across the measured traits and were tested at 0, 10, 20, 30 and 40% (w/v) PEG 6000. IAA ranged from 2.43 to 53.80 µg mL⁻¹ with tryptophan supplementation in the medium and from 0.16 to 42.02 µg mL⁻¹ without tryptophan supplementation. GA ranged from 0.05 to 98.82 µg mL⁻¹, while extracellular ammonia was detected in 22 isolates.   ACC deaminase activity varied from 0.77 to 16.60 µmol mL⁻¹. PPFM R5 and PPFM P10 displayed broad PGP profiles, whereas PPFM P25 recorded the highest mean optical density at 600 nm (OD600) at 20 and 30% PEG, with values of 0.97 and 0.95, respectively. Partial 16S rRNA gene sequence analysis identified PPFM R5 as closely related to Methylobacterium brachiatum (99.51% sequence identity) and PPFM P10 as closely related to Methylobacterium radiotolerans (98.33% sequence identity). The findings demonstrate considerable functional heterogeneity among PPFM isolates from okra and indicate that  PGP-associated traits and tolerance to PEG 6000induced osmotic stress can vary independently among isolates. These results provide a basis for selecting functionally distinct PPFM isolates for further evaluation as plant associated microbial inoculants.