Salicylic acid improves drought-stress tolerance by regulating the redox status and proline metabolism in Brassica rapa

Salicylic acid (SA) has been regarded as the important phytohormone in improving abiotic stress tolerance. However, the physiological mechanisms on drought stress tolerance remain largely unknown. To investigate the role of SA in drought-stress tolerance in Chinese cabbage (Brassica rapa subsp. pekinensis) with regard to cellular redox control and proline metabolism, Chinese cabbages were pretreated or untreated with SA for 7days, then grown under well-watered or drought-stressed conditions for 14days. The osmotic potential and chlorophyll and carotenoid contents were decreased in the drought-stressed plants; however, the negative effects of drought on these parameters was substantially ameliorated in the SA-pretreated plants. Drought stress resulted in increased O-2(-), H2O2, and malondialdehyde (MDA) contents in the non-SA pretreated plants relative to the SA-pretreated plants. Superoxide dismutase, catalase, guaiacol peroxidase, and ascorbate peroxidase were highly activated in drought-stressed plants, whereas these substances were further activated in the SA-pretreated plants under drought stress. Drought stress resulted in a significant decrease of the reduced/oxidized glutathione (GSH/GSSG) and reduced/oxidized nicotinamide adenine dinucleotide phosphate (NADPH/NADP) ratios, whereas plants pretreated with SA had levels similar to those in the well-watered plants. Under drought conditions, SA pretreatment significantly increased proline content by up-regulating the expression of genes encoding pyrroline-5-carboxylate synthase (P5CSA and P5CSB) and down-regulating the expression of the gene encoding proline dehydrogenase (PDH) compared to non-SA pretreated plants. These results indicate that a pretreatment with SA improves drought-stress tolerance by maintaining redox homeostasis and activating proline biosynthesis.