Combined herbicide and saline stress differentially modulates hormonal regulation and antioxidant defense system in Oryza sativa cultivars

Plants are simultaneously exposed to a combination of biotic and abiotic stresses in field conditions. Crops respond to the combined stress in a unique way which cannot be understood by extrapolating the results of individual stress. In the present study, effects of individual and combined stress of herbicide (2,4-dichlorophenoxyacetic acid) and salinity (NaCl) on two Oryza sativa cultivars (ZJ 88 and XS 134) were investigated. Both herbicide and saline stress affected the plant growth differentially and produced oxidative stress in rice cultivars. Interestingly, the combination of herbicide and salinity showed a significant protection to both rice cultivars by reducing ROS (H2O2, O2−) and lipid peroxidation through modulation of enzymatic (SOD, POD, CAT and APX) and non-enzymatic (TSP, sugars, phenolic and proline) antioxidants. In addition, active regulation of transcript levels of genes encoding Na+ and K+ (OsHKT1;5, OsLti6a,b, OsHKT2;1, OsSOS1, OsCNGC1, OsNHX1 and OsAKT1) transporter proteins reduced sodium and enhanced potassium accumulation under combined stress, resulted a better growth and ionic homeostasis in both rice cultivars. The production of ABA and IAA was significantly higher in cultivar XS 134 compared to cultivar ZJ 88 under control conditions. However, combined herbicide and saline stress enhanced the accumulation of phytohormones (IAA and ABA) and transcription of ethylene in cultivar ZJ 88, which might be one of the factors responsible for poor salt tolerance in sensitive cultivar. These findings indicated that herbicide application under saline stress confers tolerance to salinity in rice cultivars, likely by reducing oxidative damage, modulating mineral absorption, upgradation of antioxidant defense and by dynamic regulation of key genes involved in Na+ and K+ homeostasis in plants.