Engineering drought-tolerant apple by knocking down six genes and potential application of transgenic apple as a rootstock.

Drought poses a major threat to apple fruit production and quality. Because of the apple's long juvenile phase, developing varieties with improved drought tolerance using biotechnology approaches is needed. Here, we used the RNAi approach to knock down six genes in the apple. Under prolonged drought stress, the RNAi plants performed better than wild-type plants and had stronger root systems, higher root-to-shoot ratio, greater hydraulic conductivity, increased photosynthetic capacity, and increased water use efficiency. Moreover, RNAi plants promoted the drought tolerance of the scion when they were used as rootstock, compared with wild-type and M9-T337 rootstocks. Scions grafted onto RNAi plants showed increased plant height, stem diameter, photosynthetic capacity, specific leaf weight, and water use efficiency. The use of RNAi plants as rootstocks can also increase the C/N ratio of the scion and achieve the same effect as the M9-T337 rootstock in promoting the flowering and fruiting of the scion. Notably, using RNAi plants as rootstocks did not reduce fruit weight and scion quality compared with using M9-T337 rootstock. Our research provides candidate genes and demonstrates a general approach that could be used to improve the drought tolerance of fruit trees without sacrificing the yield and quality of scion fruits.