| Summary: | 7-hydroxymethyl chlorophyll (Chl) <i>a</i> reductase (HCAR) plays critical roles in the Chl cycle and degradation during leaf senescence, however, its function in horticultural crops remains unknown. Here, we identified an <i>HCAR</i> gene (<i>CsHCAR</i>) from cucumber (<i>Cucumis sativus</i> L.) and investigated its roles in response to dark-induced Chl degradation. <i>CsHCAR</i> encoded 459 amino acids, which were orthologous to Arabidopsis HCAR, had the conserved domains, and localized in the chloroplast. Gene expression analysis showed that <i>CsHCAR</i> expression was the highest in senescent leaves and was responsive to different stresses and phytohormone treatments. Overexpression of <i>CsHCAR</i> in tobacco accelerated dark-induced Chl degradation through enhancing the expression of Chl catabolic genes. After 10 d of darkness treatment, the biomass of <i>CsHCAR</i> overexpression plants was reduced. Furthermore, the value of net photosynthetic rate, maximum quantum yield of photosystem II, and effective quantum yield of photosystem II in <i>CsHCAR</i> overexpression plants was significantly reduced in comparison to that in wild-type (WT) plants. The photosynthetic protein content, including Lhcb1, Lhcb2, Lhcb4, RbcS, and RbcL in <i>CsHCAR</i> overexpression plants exhibited a lower level as compared to that observed in WT plants. In addition, the expression of genes encoding these proteins in <i>CsHCAR</i> overexpression plants was significantly lower than that in WT plants. Moreover, <i>CsHCAR</i> overexpression plants inhibited the dark-induced accumulation of reactive oxygen species (ROS). These results indicate that <i>CsHCAR</i> affects the stability of photosynthetic proteins in chloroplasts, positively regulates Chl degradation, and plays an important role in maintaining ROS homeostasis in leaves.
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