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

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

doi:10.3389/fpls.2021.740492

Shuang Zhao, Ke Mao + Show 4 more

Open Access

https://doi.org/10.3389/fpls.2021.740492

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Abstract

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 (Malus domestica) are poorly understood. We characterized the role of MdHB-7 in WUE regulation by subjecting MdHB-7 transgenic plants to long-term moderate soil water deficit. The long-term WUE (WUEL) of transgenic apple plants with MdHB-7 overexpression or MdHB-7 RNA interference (RNAi) differed significantly from that of control plants. Upregulation of MdHB-7 caused reduced stomatal density, whereas the suppression of MdHB-7 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 MdHB-7 overexpression transgenic plants, especially under water deficit conditions. MdHB-7 overexpression plants maintained high rates of photosynthesis that were conducive to the accumulation of biomass and the improvement of WUEL. MdHB-7 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 WUEL. By contrast, MdHB-7 RNA interference reduced the WUEL 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 MdHB-7 in the regulation of apple WUEL and provide new insights for improving the WUE of apple plants under moderate soil water deficit.

Highlights

Global climate change and the increasing use of groundwater for agriculture have caused severe water shortages for crops in many parts of the world (Shao et al, 2008; Falkenmark, 2013; Wang et al, 2018)
At the beginning of the experiment, there were no significant differences in plant height, stem diameter, and dry weight between GL-3 and MdHB-7 transgenic plants (Supplementary Figure S1)
To eliminate the possibility that MdHB-7 affected plant growth by regulating the expression of other homeodomain–leucine zipper (HD-Zip) genes, we examined the expression of MdHD-Zips in GL-3 and MdHB-7 transgenic plants

Summary

Introduction

Global climate change and the increasing use of groundwater for agriculture have caused severe water shortages for crops in many parts of the world (Shao et al, 2008; Falkenmark, 2013; Wang et al, 2018). Water use efficiency (WUE) is defined as the carbon fixed or biomass produced per unit of water used. MdHB-7 Regulates Water Use Efficiency due to soil water deficit (Condon et al, 2004; Ali and Talukder, 2008). Apple (Malus domestica) is one of the most widely grown and economically important fruits in temperate regions (Zhao et al, 2020b). The Loess Plateau of Shaanxi Province is one of China’s main apple growing regions (Liu et al, 2019). Plants face continuous moderate soil water deficit, which limit the sustainable development of apples in this area (Yan et al, 2015). Genes related to WUE regulation whose manipulation could improve apple WUE under soil water deficit conditions are important targets for apple breeding

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