Role Of Myo-inositol In Response To Salinity,Drought And Leaf Senescence In Apple

China is the largest apple(Malus×domestica)producing country in the world.In China,the Loess Plateau region is the largest and best apple production area,but stresses such as drought,salt,and diseases limit the apple cultivation in this area.Myo-inositol is a kind of polyol that widely exists throughout of lives.In higher plants,myo-inositol and its derivatives could not only act as key metabolites to regulate the osmotic balance and eliminate reactive oxygen species(ROS),but also as signaling molecules to mediate multiple metabolic pathways for improving stress tolerance.However,it is unclear whether myo-inositol has beneficial roles under stress conditions in apple.To investigate the function of myo-inositol in plant stress responses,we firstly confirmed that exogenous myo-inositol could improve salt and drought tolerances in apple.Also,these roles were further investigated using the myo-inositol-1-phosphate synthase gene(MdMIPSs)and myo-inositol transporter gene(MdINT1)transgenic apple lines(including over-expressing and silencing).The main results are follows.1. Exogenous application of 50?M myo-inositol significantly alleviated salt-induced damage of Malus hupehensis grown hydroponically.Further analysis found that exogenous myo-inositol significantly slowed the declines in the photosynthesis rate and chlorophyll concentration,and alleviated the growth inhibition under salt stress.This application reduced ROS accumulation by improving the antioxidant system,thereby relieving salt-induced oxidative damage.Application of myo-inositol significantly reduced Na~+concentration and enhanced the accumulation of soluble sugars and sugar alcohols,then maintained ion and osmotic homeostasis.2. MdMIPS1/2-mediated myo-inositol biosynthesis regulates plant senescence in apple.Decreasing myo-inositol biosynthesis in MdMIPS1/2-RNAi lines caused extensive programmed cell death(PCD),manifested by necrosis in both the leaves and roots,and ultimately plant death.Excessive ROS accumulation contributed to necrosis,which was closely associated with the WS pectin-mediated increase of SA and the compromised antioxidant system.In addition,ethylene outbursts in RNAi lines accelerated ROS-mediated PCD.Increasing myo-inositol biosynthesis in MdMIPS1-OE lines delayed leaf senescence.3. MdMIPS1-mediated myo-inositol biosynthesis positively regulates salt tolerance in plants.The myo-inositol level,and tolerance to salt and osmotic stress was significantly enhanced in MdMIPS1-OE Arabidopsis when compared with wild type.Compared with GL-3,the levels of myo-inositol and other osmopretectants were significantly increased in MdMIPS1-OE apple lines under salt stress.Moreover,overexpression of MdMIPS1 in apple enhanced salt tolerance by improving the antioxidant system,scavenging ROS,as well as reducing the Na~+concentration and Na:K ratio.4. MdINT1 was localized in the tonoplast.Transcriptional level of MdINT1 was induced by salt stress.Although the level of myo-inositol in leaves was reduced,the salt tolerance of MdMIPS1-OE lines was enhanced by improving the antioxidant system,concentrations of osmopretectants,as well as maintaining ion balance.RNA silencing of MdINT1 increased the levels of myo-inositol in leaves and roots,but the sensitivity to salinity was increased.MdINT1 may be responsible for myo-inositol utilization by regulating its accumulation in vacuoles,and positively regulates salt tolerance in apple.5. MdMIPS1-mediated myo-inositol biosynthesis positively regulates drought tolerance in apple.Under long-term drought treatment,exogenous application of 50?M myo-inositol significantly promoted growth of Malus hupehensis.Under short-term and long-term drought treatment,the relative water content,chlorophyll content,Fv/Fm and the photosynthesis rate in MdMIPS1-OE apple lines were significantly higher than that in GL-3.Also,the improved antioxidant system benefited the MdMIPS1-OE apple lines under short-term drought treatment.Under long-term drought treatment,the water use efficiency(WUE)and relative growth ratio were significantly increased in MdMIPS1-OE apple lines.