Abstract
1-aminocyclopropane-1-carboxylate deaminase (ACCD)-producing endophytic Streptomyces sp. GMKU 336 and its ACCD-deficient mutant were inoculated into Thai jasmine rice Khao Dok Mali 105 cultivar (Oryza sativa L. cv. KDML105) under salt stress (150 mM NaCl) conditions. The results clearly indicated that Streptomyces sp. GMKU 336 significantly increased plant growth, chlorophyll, proline, K(+), Ca(+), and water contents; but decreased ethylene, reactive oxygen species (ROS), Na(+), and Na(+)/K(+) ratio when compared to plants not inoculated and those inoculated with the ACCD-deficient mutant. Expression profiles of stress responsive genes in rice in association with strain GMKU 336 were correlated to plant physiological characteristics. Genes involved in the ethylene pathway, ACO1 and EREBP1, were significantly down-regulated; while acdS encoding ACCD in Streptomyces sp. GMKU 336 was up-regulated in vivo. Furthermore, genes involved in osmotic balance (BADH1), Na(+) transporters (NHX1 and SOS1), calmodulin (Cam1-1), and antioxidant enzymes (CuZn-SOD1 and CATb) were up-regulated; whereas, a gene implicated in a signaling cascade, MAPK5, was down-regulated. This work demonstrates the first time that ACCD-producing Streptomyces sp. GMKU 336 enhances growth of rice and increases salt tolerance by reduction of ethylene via the action of ACCD and further assists plants to scavenge ROS, balance ion content and osmotic pressure.