Biofilm-forming Bacteria from Farm Pond Water Show Plant Growth-promoting Traits and Enhance Seedling Growth of Green Gram (Vigna radiata L.)

Biofilm formation can improve the persistence and root colonisation of plant growth-promoting bacteria (PGPB), yet farm-pond water has been little explored as a source of such strains. Fifteen bacterial isolates (R1–R15) were obtained from two ponds on the SDAU campus and screened for biofilm formation by tube, microtitre plate and Congo red agar assays. The five strong biofilm formers (recoded RBF1–RBF5) were tested for indole-3-acetic acid (IAA), siderophore, ammonia, HCN, zinc and phosphate solubilisation, growth on nitrogen-free medium and antagonism against Fusarium incarnatum; identified by 16S rRNA gene sequencing; and evaluated on green gram (cv. GM 4) in a net-house pot experiment. Five of 15 isolates were strong biofilm formers (OD₅₇₀ 0.659–0.789; control 0.0376). IAA ranged from 10.25 to 16.05 µg mL⁻¹, tricalcium phosphate solubilisation from 9.61 to 18.95 µg mL⁻¹, zinc solubilisation index from 1.22 to 2.66, and inhibition of F. incarnatum from 48.5 to 74.2%. No isolate produced HCN, and four of five grew on N-free semi-solid medium. The isolates were identified as Bacillus cereus group (RBF1), Paenibacillus illinoisensis (RBF2), P. polymyxa (RBF3), Niallia nealsonii (RBF4) and Providencia rettgeri (RBF5). P. polymyxa RBF3 gave the best seedling growth: root length 10.37 cm (control 6.67 cm), shoot length 21.76 cm (14.15 cm) and seed vigour index 3213 (2082). Farm-pond water thus yielded biofilm-forming bacteria with multiple PGP traits. P. polymyxa RBF3 is a promising candidate for further evaluation, although its performance was not explained by the in vitro traits measured and needs field validation.