Abstract
7-hydroxymethyl chlorophyll (Chl) a 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 HCAR gene (CsHCAR) from cucumber (Cucumis sativus L.) and investigated its roles in response to dark-induced Chl degradation. CsHCAR encoded 459 amino acids, which were orthologous to Arabidopsis HCAR, had the conserved domains, and localized in the chloroplast. Gene expression analysis showed that CsHCAR expression was the highest in senescent leaves and was responsive to different stresses and phytohormone treatments. Overexpression of CsHCAR in tobacco accelerated dark-induced Chl degradation through enhancing the expression of Chl catabolic genes. After 10 d of darkness treatment, the biomass of CsHCAR 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 CsHCAR 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 CsHCAR overexpression plants exhibited a lower level as compared to that observed in WT plants. In addition, the expression of genes encoding these proteins in CsHCAR overexpression plants was significantly lower than that in WT plants. Moreover, CsHCAR overexpression plants inhibited the dark-induced accumulation of reactive oxygen species (ROS). These results indicate that CsHCAR affects the stability of photosynthetic proteins in chloroplasts, positively regulates Chl degradation, and plays an important role in maintaining ROS homeostasis in leaves.
Highlights
Chlorophyll (Chl) plays a central role in the photosynthetic system and is actively synthesized by glutamate during plant development and degraded into non-fluorescent Chl catabolites during senescence [1]
Further analysis revealed that the CsHCAR gene was located on chromosome 3, and the length of coding DNA sequence (CDS) was 1380 bp, encoding 459 amino acids (Table 1)
We identified one hydroxymethyl Chl a reductase (HCAR) gene in the cucumber genome, which was orthologous with other species, including Arabidopsis, melon, and rice
Summary
Chlorophyll (Chl) plays a central role in the photosynthetic system and is actively synthesized by glutamate during plant development and degraded into non-fluorescent Chl catabolites during senescence [1]. The Chl cycle is more than a reciprocal conversion of Chl a and Chl b; it plays an important role in the degradation of light-harvesting Chl a/b protein complex of photosystem II (LHCII) [15,16]. Arabidopsis SGR1 physically interacts with the Chl catabolic enzymes (CCEs) and LHCII, forming a multi-protein complex that is likely important for rapid detoxification of Chl catabolic intermediates in senescing chloroplasts [17,18]. Arabidopsis and rice (Oryza sativa) HCAR gene mutants show a stay-green phenotype during dark-induced leaf senescence, and the accumulation of Chl decomposition intermediates 7-HMChl a and Pheide a [14,21]. HCAR physically interacts with LHCII and other CCEs, such as SGR1, NYC1, NOL, and RCCR, indicating that HCAR is a component of the Chl degradation complex [18]. The research on HCAR is mainly focused on Arabidopsis and rice, it has not been reported in horticultural plants
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