Functional Analysis Of MhYTP1 And MhYTP2 In Apple Water Use Efficiency Regulation Under Drought Condition

YTH-domain containing RNA binding proteins(YTPs)participate in many plant stress responses and development processes.MhYTP1 and MhYTP2 cloned from Malus hupehensis participating in responses to abiotic stresses,including short-term water deficits,water-logging,high and low temperature,high salinity and nutrient deficiencies,have been reported previously.Besides,MhYTP1 and MhYTP2 also participate in Marssonina apple blotch disease resistance regulation.China dominates global apple production.Shaanxi Province,on the Loess Plateau,is the largest apple-growing region in China.However,long-term moderate drought is the main limiting factor for orchards here.Therefore,identifying genes involved in water use efficiency(WUE)regulation under drought condition can assist future apple drought resistance breeding with the help of biotechnology.This experiment aims at providing functional analysis of RNA binding proteins MhYTP1and MhYTP2 in apple WUE regulation under drought condition.The main results are shown below:1.MhYTP1 overexpression lines showed higher rates of photosynthesis and WUE under long-term moderate drought condition.Further analysis showed that the ABA content of transgenic plants was substantially higher than in the wild-type(WT)controls plants after 40and 80 days of drought treatment.In addition,transgenic plants showed increased stomatal density and greater stomatal closure than WT plants after 80 days cultivation under both well-watered and long-term moderate drought conditions.These data suggested that overexpression of MhYTP1 improved the WUE and drought resistance by increasing ABA levels under long-term moderate drought condition.2.Grafting experiments using transgenic lines overexpressing MhYTP1 indicated that the transpiration rate was lower and WUE_I was higher after 60 days cultivation when transgenic plants were used as scions rather than rootstocks and when transgenic plants were used as rootstocks than WT plants were used as rootstocks.This demonstrated that transgenic plants have better growth and drought tolerance as scions than rootstocks and show similar effect when transgenic rather than WT plants were used as rootstocks.3.MhYTP2 could improve apple WUE and drought tolerance,and increase organic matter accumulation by improving net photosynthetic rate,reducing transpiration rate and promoting root initiative water absorption.The reduction of transpiration rate is accomplished through the activation of ABA and ethylene signaling,which decreases the size of stomatal apertures under both well-watered and drought conditions.4.Grafting experiments were conducted using transgenic lines overexpressing MhYTP1and MhYTP2 either as scions or rootstocks.From this,the net photosynthetic rate,WUE_I and root water potential were higher after 60 days cultivation when transgenic plants were used as rootstocks rather than scions.This demonstrated that MhYTP2 could be a good candidate for breeding apple rootstocks or rootstocks of other graft-suitable crops with greater WUE.In addition,among the five grafting combinations,it was found that the combination of MhYTP1 transgenic plants as scions and MhYTP2 transgenic plants as rootstocks showed the strongest drought resistance.