Effect Of Exogenous Strigolactones On Drought Resistance Of Apple Seedlings And Transcriptome Analysis

Apple（Malus domestica Borkh.）is an important fruit cash crop in China,which is widely cultivated with high economic benefits.The largest main producing areas of Chinese apple industry are distributed in loess Plateau area,but drought stress due to low precipitation and high temperature are the most important reasons limiting the sustainable development of apple industry in this area.Therefore,how to mitigate the damage of drought stress efficiently and without pollution is an important issue facing the apple industry.Strigolactones（SLs）are a class of carotenoid-derived phytohormones that regulate plant growth and development and have been shown in recent years to help a variety of plants withstand abiotic stresses,including drought stress.However,there are no reports on whether SL can alleviate the growth inhibition of apple seedlings by drought stress and the mechanism of action.In this study,we used apple rootstock variety‘Malus hupehensis var.Mengshanensis’as the experimental material,and the physiological mechanisms of drought tolerance in apple seedlings induced by GR24,a homologous analogue of strigolactones,were investigated by measuring relevant physiological indicators and transcriptome sequencing analysis.The main findings are as follows:（1）Different concentrations of GR24 were applied externally to apple seedlings under drought stress,and it was found that the growth and development of apple seedlings were improved,and 1μmol/L GR24 had the best alleviation effect.（2）Exogenous GR24 increased the photosynthetic capacity of apple seedlings under drought stress.Stomatal width and length were significantly increased by 188.7%and 34.0%,respectively.Total chlorophyll content and carotenoid content were significantly increased by32.5%and 52.2%.（3）Exogenous GR24 increased the antioxidant capacity of apple seedlings.Drought stress led to charge imbalance and excessive accumulation of reactive oxygen species in apple seedlings.Exogenous GR24 increased the antioxidant enzyme activity and non-enzymatic antioxidant content of apple seedlings under drought stress.H₂O₂and O₂^·-content decreased significantly by 15.7%and 66.3%.The degree of membrane lipid peroxidation damage was reduced,and MDA content and REC decreased by 25.4%and 12.4%.（4）Transcriptome data analysis showed that there were 6490 differential genes in drought-treated plants after exogenous GR24 treatment compared to drought plants.GO analysis showed that DEGs are mainly enriched in the“defense response”process.KEGG analysis revealed that DEGs were enriched in the synthetic pathways of various secondary metabolites.（5）Exogenous GR24 activated the expression of related genes in the Ca²⁺signaling pathway and induced the expression of related transcription factors such as WRKY and NAC.（6）Exogenous GR24 reduced the synthesis of endogenous strigolactones but enhanced the signal transduction of strigolactones and other hormones.In conclusion,exogenous GR24 could effectively enhance the drought resistance of apple seedlings under drought stress.On the one hand,exogenous GR24 increased the expression of drought-related genes,and on the other hand,exogenous GR24 affected the activity of enzymes in various metabolic pathways,enhancing the antioxidant capacity and photosynthetic capacity of plants.In addition,exogenous GR24 could synergize with other hormones to promote plant growth and development.The results of this study provide a theoretical basis for the future use of strigolactones to improve the drought resistance of crops,which will help to improve the sustainable development of apple industry under drought stress.