Abstract
Improving plant drought tolerance and water-use efficiency (WUE) is important for plant survival and crop yield under conditions of water shortage. HD-Zip transcription factors (TFs) have been shown to help plants respond to drought. Here, MdHB7-like (an HD-Zip I TF) was identified as a drought-induced gene in apple plants. Transgenic apple with upregulated expression of MdHB7-like showed enhanced drought tolerance, whereas MdHB7-like RNA interference (RNAi) lines were more sensitive to drought. Further analysis showed that overexpression of MdHB7-like reduced stomatal density, but the stomatal density of MdHB7-like RNAi lines was increased. Transcriptome analysis showed that MdHB7-like overexpressing plants may reduce stomatal density by affecting the expression of stomatal development related genes, such as MdEPFL9.1 and MdEPFL9.2. The decrease in stomatal density reduced leaf water loss, helping to enhance the drought tolerance of overexpression lines. In addition, MdHB7-like overexpression enhanced drought tolerance of apple plants by promoting stomatal closure. Further study showed that the overexpression of MdHB7-like significantly increased WUE under long-term moderate drought conditions, which also related to the reduction of stomatal density. These results suggest that MdHB7-like is a valuable genetic resource for enhancing drought tolerance and increasing the WUE of apple germplasm.
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