AUTHOR=Zhao Shuang , Gao Hanbing , Jia Xumei , Wei Jiangtong , Mao Ke , Ma Fengwang 

TITLE=MdHB-7 Regulates Water Use Efficiency in Transgenic Apple (Malus domestica) Under Long-Term Moderate Water Deficit

JOURNAL=Frontiers in Plant Science

VOLUME=12

YEAR=2021

URL=https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2021.740492

DOI=10.3389/fpls.2021.740492

ISSN=1664-462X

ABSTRACT=<p>Improved water use efficiency (WUE) promotes plant survival and crop yield under water deficit conditions. Although the plant-specific HD-Zip I transcription factors have important roles in plant adaptation to various abiotic stresses, including water deficit, their functions in regulating WUE of apple (<italic>Malus domestica</italic>) are poorly understood. We characterized the role of <italic>MdHB-7</italic> in WUE regulation by subjecting <italic>MdHB-7</italic> transgenic plants to long-term moderate soil water deficit. The long-term WUE (WUE<sub>L</sub>) of transgenic apple plants with <italic>MdHB-7</italic> overexpression or <italic>MdHB-7</italic> RNA interference (RNAi) differed significantly from that of control plants. Upregulation of <italic>MdHB-7</italic> caused reduced stomatal density, whereas the suppression of <italic>MdHB-7</italic> increased stomatal density under both normal and long-term moderate soil water deficit conditions. Moderate reduction in stomatal density helped to improve the WUE of <italic>MdHB-7</italic> overexpression transgenic plants, especially under water deficit conditions. <italic>MdHB-7</italic> overexpression plants maintained high rates of photosynthesis that were conducive to the accumulation of biomass and the improvement of WUE<sub>L</sub>. <italic>MdHB-7</italic> overexpression also alleviated the inhibition of root growth caused by long-term moderate soil water deficit and improved root vitality and hydraulic conductivity, which were essential for improving plant WUE<sub>L</sub>. By contrast, <italic>MdHB-7</italic> RNA interference reduced the WUE<sub>L</sub> of transgenic plants by inhibiting these factors under normal and long-term moderate soil water deficit conditions. Taken together, our results provide solid evidence for a crucial role of <italic>MdHB-7</italic> in the regulation of apple WUE<sub>L</sub> and provide new insights for improving the WUE of apple plants under moderate soil water deficit.</p>