Secondary metabolism and antioxidants are involved in the tolerance to drought and salinity, separately and combined, in Tibetan wild barley

Drought and salinity are the two most common and frequently co-occurring abiotic stresses constraining crop productivity worldwide. Secondary metabolites play a major role in the adaptation of plants to the environment and in overcoming stress conditions. To reveal the physiological and molecular differences between Tibetan wild (XZ5, drought-tolerant; XZ16, salinity/aluminum-tolerant) and cultivated barley (salinity-tolerant cv CM72), secondary metabolites and their related genes were studied in response to the individual and combined stresses (D + S) of drought (4% soil moisture) (D) and salinity (S) and the subsequent recovery. Callose content and activities of chitinase, sucrose synthase (SuSy), sucrose phosphate synthase (SPS) and acid invertase (AI) increased more in XZ5 and XZ16 under drought and salinity, both alone and combined, compared with control than in CM72. Elevated phenol and flavonoid content were also observed under single and combined stresses in the two wild genotypes relative to control. The induced expression of genes related to secondary metabolism (GST1, PPO, SKDH, PAL, CAD and chi2) was demonstrated under all stress conditions in wild barley and accompanied an increase in activities of the respective enzymes, with the greatest increase observed in XZ5. During rehydration and recovery, activities of all enzymes increased except for phenylalanine ammonialyase (PAL) and cinnamyl alcohol dehydrogenase (CAD), which only increased in XZ5. Moreover, microscopic imaging of leaves revealed DNA damage with increasing tail moment under all stress treatments, but XZ5 and XZ16 were less affected than CM72. Our findings suggest that high tolerance to D + S stress in Tibetan wild barley is closely related to enhanced callose, chitinase and carbohydrate metabolism as well as ROS level control through modulation of antioxidant enzymes, their secondary metabolism and their translation level.