Characterization of Chlorophyll Degradation Genes Reveals Gene Cluster HuSGR2 and HuSGR3 Promoting Chlorophyll Degradation in Pitaya Peel

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Background: Chlorophyll degradation is a characteristic sign of fruit ripening. However, the chlorophyll degradation pathway during pitaya fruit development remains unexplored. Methods and Results: Here, chlorophyll contents showed a downward trend across the five developmental stages of ‘Jindu No.1’ pitaya peels. Based on the pitaya genome data, twenty chlorophyll degradation genes were identified, including two NYCs, three CLHs, five SGRs, six PAOs, and four RCCRs, spread across eight pitaya chromosomes. In addition, their phylogenetic relationships, conserved motifs, and domains were analyzed using homologous genes from beet and Arabidopsis species. Transcriptomic data and RT-qPCR analyses of these genes suggested that three HuSGRs demonstrated a significant upward trend during pitaya peel maturation. Indeed, the HuSGR1 has the complete gene structure, including the chloroplast transit peptide, SGR domain, and variable C-terminal region. However, HuSGR2 and HuSGR3 contained the N- and C-terminal sequences, respectively, of HuSGR1. They were separated by a 690 bp distance on chromosome 8, forming a gene cluster. Overexpressed HuSGR2 or HuSGR3 alone resulted in a significant decrease in chlorophyll contents in tobacco leaves. Notably, a more obvious reduction of chlorophyll contents was observed when overexpressing them together. Conclusions: Our results show that HuSGR2 and HuSGR3 were involved in accelerating the chlorophyll degradation process, providing new insights into the molecular basis of color formation in pitaya peels.

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