ACC deaminase containing diazotrophic endophytic bacteria ameliorate salt stress in Catharanthus roseus through reduced ethylene levels and induction of antioxidative defense systems

Abstract

Among a total of 27 cultivable salt tolerant endophytic bacteria isolated from Catharanthus roseus grown in highly salt affected coastal region of cuddalore district, Tamilnadu, India four isolates were found to be positive for nitrogenase activity. The isolates were evaluated for their stress tolerance efficiency and screened for different PGP traits. Based on the above studied parameters, and ability to produce 1-aminocyclopropane-1-carboxylate (ACC) deaminase (4.24 μmol α-ketobutyrate mg_1 protein h_1) the salt tolerant diazotrophic isolate AUM54 was selected for further investigation and identified as Achromobacter xylosoxidans by 16S rRNA gene sequencing. The ability of this isolate to ameliorate salt stress in C. roseus was evaluated under gnotobiotic and pot culture conditions. At 150 mM NaCl level A. xylosoxidans AUM54 treated plants recorded ethylene level of 394.1 p mol ethylene g−1 FW h−1 compared to the ethylene level of 516.0 p mol ethylene g−1 FW h−1 recorded in the un inoculated control. A. xylosoxidans AUM54 inoculated plants recorded the maximum germination percentage of 98.3, vigor index of 2231.4, plant height of 120.4 cm, root dry weight of 53.24 g Plant_1 and ajmalicine content of 1.60 mg g−1, compared to the germination percentage of 91.6%, vigour index of 1511.5, plant height of 105.8, root weight of 47.2 g Plant−1, and ajmalicine content of 1.23 mg g−1 in uninoculated plants grown without NaCl treatment. This isolate also decreased plant ethylene levels by 11–23% and increased the antioxidative enzyme content of inoculated C. roseus plants to the tune of 19–32% for ascorbate peroxidase (APX) activity, 20–30% for superoxide dismutase (SOD) activity and 4–16% for catalase (CAT) under normal and salt affected conditions.