Investigations were made to identify, screen and characterize rice apoplastic fluid endophytic bacterial strains for osmotic stress tolerance and plant growth promoting activity. These bacterial strains were identified phylogentically as Bacillus subtilis TSAC5, Bacillus subtilis TSAA2, Bacillus sp. R2AA10, Bacillus cereus R2AA7, Bacillus marisflavi TSAC7, Cupriavidus alkaliphilus TSAC1, Delftia sp. TSAC2, Janibacter melonis R2AA1, Microbacterium oleivorans R2AA6, Kocuria rosea R2AA5, Pseudomonas aeruginosa NAAI3 and Sphingobium yanoikuyae R2AI1. Of these 12 strains, eight were obtained from drought tolerant rice varieties such as Anna R4 and IR DRT 64 and four from normal rice variety CO51. These endophytes exhibited varied level of osmotic stress tolerance created using different concentrations of PEG 6000 (-1.5, -3.0, -4.0, -4.5, -5.0 and -5.5 MPa) under in vitro conditions in liquid and solid medium. Among the 12 strains, Cupriavidus alkaliphilus TSAC1, Delftia sp. TSAC2, Bacillus marisflavi TSAC7, Janibacter melonis R2AA1, Kocuria rosea R2AA5, Pseudomonas aeruginosa NAAI3 and Sphingobium yanoikuyae R2AI1 showed moisture stress tolerance up to -5.5 MPa. Further increase in moisture stress of -6.0 MPa inhibited growth of 12 tested strains. These strains also exhibited Indole acetic acid (IAA), gibberellic acid (GA3) and extracellular polysaccharide (EPS) production and also showed ACC deaminase (ACCD) activity. Maximum extracellular proline production was found in Bacillus marisflavi TSAC7 under both normal and water stressed conditions. Higher IAA productivity was registered in Janibacter melonis R2AA1 followed by Cupriavidus alkaliphilus TSAC1 and Sphingobium yanoikuyae R2AI1 in the absence of PEG 6000. In the presence of PEG 6000 Delftia sp. TSAC2 recorded maximum IAA productivity. Bacillus subtilis TSAA2 showed greater ACC deaminase activity. EPS production was in the order of Pseudomonas aeruginosa NAAI3 followed by Sphingobium yanoikuyae R2AI1. Sphingobium yanoikuyae R2AI1 produced maximum quantity of GA3 both under normal and water stressed condition. None of the tested strains indicated insoluble phosphate, silicate and zinc solubilisation.
Key words : Rice apoplast, Moisture stress, PEG 6000, GA3, IAA, ACCD