Screening and Application of IAA-producing Bacteria for Seed Priming to Enhance Rice Germination and Salinity Stress Tolerance
Abstract
Background: Rice is a staple crop that plays an important role in global food security, but seed germination and early seedling establishment are often limited by salinity stress. Salinity reduces water uptake, disrupts metabolic activity and impairs early seedling growth through osmotic and ionic stress. Seed priming with plant growth-promoting bacteria, particularly indole-3-acetic acid (IAA)-producing bacteria, may improve germination performance and salinity tolerance during early growth. Methods: Bacterial isolates were obtained from saline soils in northeastern Thailand and screened for IAA production and salinity tolerance. Four selected isolates were identified and applied to rice seeds of the cultivar Khao Dawk Mali 105 through seed soaking and seed priming. Seed quality was evaluated under laboratory and greenhouse conditions and the most promising isolate was further tested under salinity stress using NaCl concentrations from 0 to 100 mM. Result: The selected isolates were identified as Bacillus megaterium isolates 1, 2 and 3 and Microbacterium proteolyticum isolate 4. B. megaterium (isolate 1) produced the highest IAA concentration, but M. proteolyticum (isolate 4) showed the best overall seed performance. Under laboratory conditions, seeds soaked with M. proteolyticum (isolate 4) had the highest germination percentage of 97%, the highest speed of germination of 9.41 plants/day, the shortest mean germination time of 2.95 days and the greatest root length of 15.63 mm. Under greenhouse conditions, the same treatment produced the highest germination percentage of 99%, the highest speed of germination of 9.89 plants/day, the shortest mean germination time of 4.76 days and the greatest shoot length of 6.62 cm. Under salinity stress, M. proteolyticum priming improved germination performance under selected NaCl levels, particularly by maintaining higher germination percentage at 40 and 60 mM NaCl and higher germination potential and germination index under severe salinity. These results indicate that effective bacterial seed priming depends not only on IAA production but also on the stress-adaptive capacity of the bacterial isolate.