Function Analysis Of MdNAC1 In Apple Under Salt Stress

China is one of the countries with the largest cultivated area and production of apple（Malus domestica Borkh.）in the world,and the Loess Plateau is the eugenic area for apple production.With the frequent occurrence of global extreme climate in recent years,the salinization problem of soil in this region has become increasingly serious.NAC is a kind of specific transcription factor in plant kingdom,which participates in plant growth and development by regulating the transcription level of relevant genes to promote stress tolerance of plants.In this study,Md NAC1 in apple was taken as the research object.Based on the three obtained Md NAC1-OE apple lines,the salt stress treatment of self-root and grafted seedlings were carried out,in order to clear the functional mechanism of Md NAC1affecting the salt tolerance of apple root and shoot were preliminarily analyzed.The main results are as follows:1.Overexpression of Md NAC1 weakened the salt tolerance of transgenic apple plants.In the 100 m M Na Cl treatment via hydroponic system,compared with the wild type‘GL-3’,REL and MDA of transgenic plants were significantly increased,while the measure of root activity was decreased;in addition,the total chlorophyll content,CO₂assimilation rate and the maximum quantum efficiency of photochemistry of photosynthesisⅡsystem decreased significantly,these indicated that overexpression of Md NAC1 aggravated the damage of salt stress on plants.Then,the activity of antioxidant enzymes in transgenic plants decreased;however,a large amount of ROS was accumulated in the plant cells,these might declare the antioxidant system of plants was damaged.At last,the ion equilibrium steady state was seriously damaged resulted in an increase of accumulation of Na⁺and a significant decrease of K⁺content in shoots of apples.2.Overexpression of Md NAC1 resulted in dwarfing and affected salt tolerance of apple plants,which was closely related to root growth and development.Compared with GL-3,Md NAC1-OE apple lines were significantly dwarfed with weak roots.However,the grafted seedlings with variety of‘Malus hupenensis Rhed.’or wild-type‘GL-3’as rootstock grew well without dwarfing.Furthermore,the grafted seedlings which the rootstocks were‘Malus hupenensis Rhed.’and the scions were‘GL-3’and Md NAC1-OE apple lines were performed with 200 m M Na Cl,the salt injures of transgenic plants were less than that of‘GL-3’,and that reflected in the decrease of REL and MDA.The damage of salinity stress to photosynthetic system,antioxidant system and ion homeostasis was alleviated as well.Therefore,Md NAC1 might be involved in the negative regulation of root growth and development of apple,and thus weaken the salt tolerance of apple.3.Taking the measurement that combines hormone metabolome and RNA-seq together,the mechanism of Md NAC1 negatively regulating root growth and salt tolerance of apple was preliminarily revealed.When Md NAC1 was overexpressed,the transcription levels of tryptophan-synthesis genes in roots were down-regulated,which resulted in the decrease of auxin and tryptophan contents,as a result,the growth of root was hindered.Besides,exogenous application of auxin and tryptophan significantly improved root growth and salt tolerance of Md NAC1-overexpression plants.In the end,through co-expression analysis of genes,it was found that Md NAC1 may indirectly regulate the synthesis and accumulation of tryptophan and auxin in roots of apple via ubiquitination.