Mannose regulates water balance, leaf senescence, and genes related to stress tolerance in white clover under osmotic stress

Mannose (MAN), an important monosaccharide, contributes to coping with abiotic stresses in plants. Objectives of this study were to examine whether exogenous MAN (30 mM) could significantly increase drought tolerance and further to reveal MAN-regulated tolerance mechanism in white clover under osmotic stress induced by 18 % (m/v) polyethylene glycol 6000 for 10 d in controlled growth chambers. Results show that the application of MAN significanity alleviated stress damage and the inhibition of growth and photosynthesis in white clover under osmotic stress. The MAN-induced increase in endogenous MAN content and the accumulation of organic osmolytes (proline and water soluble sugars) could be responsible for a lower osmotic potential (OP) in white clover. The exogenous application of MAN also enhanced antioxidant enzyme (superoxide dismutase, peroxidase, ascorbate peroxidase, dehydroascorbate reductase, and glutathionc reductase) activities and maintained ascorbic acid content in white clover during osmotic stress. As concern chlorophyll (Chl) metabolism, the MAN-mated plants showed significantly higher transcription of genes involved in Chl synthesis Mg-chelatase and protochlorophyllide reductase and lower transcription of pheophorbide a oxygenase and chlomphyllase related to Chl degradation and also a senescence associated gene 101 than untreated plants. In addition, the MAN application increased transcription of SK2-, Y2K-, and Y2SK-type dehydrin genes, and dehydrin b in leaves of white clover under osmotic stress. These results indicate that MAN plays important roles in drought tolerance not only acting as a compatible solute for OP but also delaying leaf senescence through enhancing antioxidant metabolism, decreasing CM degradation, and increasing transcription of dehydrin genes contributing to enhanced drought tolerance in white clover.