ACC-Deaminase-Producing Endophyte Brachybacterium paraconglomeratum Strain SMR20 Ameliorates Chlorophytum Salinity Stress via Altering Phytohormone Generation

The higher content of salt in soil is the key constraint to crop productivity. Induction of stress ethylene production from 1-aminocyclopropane-1-carboxylate (ACC) in the plant system is one of the consequences of salt stress responsible for toxicity to the plant. Several transgenic plants have been developed by expressing the ACC deaminase bacterial gene as a stress management strategy which apart from being a costly proposition lacks public acceptability. Our main objective was to investigate if a plant inoculated with ACC-deaminase-producing bacterial endophyte will effectively mitigate salt-induced damage and act as an alternative to transgenics. An ACC deaminase-producing salt-tolerant endophyte SMR20 (Brachybacterium paraconglomeratum) was isolated from the roots of Chlorophytum borivilianum, a medicinal plant sensitive to salt stress. Initially we tested ACC-deaminase-producing SMR20 for amelioration of salt stress-induced damage at three different concentrations of salt, that is, 0, 75, 100 and 150 mM NaCl. 150 mM salt stress was selected for further study because wider differences between inoculated and uninoculated plants were noticed under this concentration of NaCl stress. Inoculated with SMR20 successfully protected C. borivilianum from salt stress-induced crop losses. SMR20 significantly alleviated the negative impact of salinity by potential deamination of ACC (precursor for ethylene generation) in the host Chlorophytum roots leading to decreased production of stress ethylene, delayed chlorosis and senescence that resulted in improved yield of plants. Apart from reducing stress ethylene, the presence of the endophyte also affected other phytohormone quantities like indole-3-acetic acid and abscisic acid in plants. It also altered the amount of total leaf pigments and biochemical compounds like proline, malondialdehyde and enhanced foliar nutrient uptake. Due to these outcomes, it appears that an ACC-deaminase-producing endophyte can enhance crop tolerance against salinity stress and can be an important area to explore.