Overexpression of the Cytokinin Oxidase/dehydrogenase (CKX) from Medicago sativa Enhanced Salt Stress Tolerance of Arabidopsis

Cytokinin oxidase/dehydrogenase (CKXs) are involved in various physiological processes, including cytokinins (CKs) catabolism, root system architecture and response to abiotic stresses in plants. Alfalfa (Medicago sativa) is a widely cultivated forage which is frequently threatened by high salinity, and the potential role of CKXs in alleviating the salt stress in alfalfa lacked attention. In this study, we isolated a CKX gene from alfalfa, MsCKX (MK177192), and identified its biological functions by overexpressing it in Arabidopsis. MsCKX shares high sequence identity with CKX from other legume plants, especially Medicago truncatula (98%). MsCKX was clearly tissue-specific, and it was mainly expressed in roots. In addition, the expression of MsCKX increased under salt stress and abscisic acid (ABA) treatment. Overexpression of MsCKX gene increased the activity of CKX, which led to an enlarged root system in transgenic Arabidopsis plants. Overexpression of MsCKX gene enhanced salt tolerance of transgenic plants by maintaining a higher K+/Na+ ratio, enhancing the activities of antioxidant enzymes to scavenge ROS and increasing the expression levels of stress-related genes (P5CS1, DREB2, ion transporters and H+ pumps). Taken together, these results shed light on the roles of MsCKX involved in salt tolerance and may have applications in salt-resistant breeding of alfalfa.