Analysis of chlorophyll degradation and related gene expression in peach peel

Abstract: 【Objective】Peach (Prunus persica), is one of the most important deciduous fruit trees in the world. The visual appeal of peach fruits, particularly their color, is a critical factor influencing consumer purchasing decisions. Chlorophyll content is important in determining the skin color and overall appearance of peaches. The precise role of chlorophyll degradation genes within the peach genome needs to fully understand, and their potential effects on chlorophyll breakdown in the peach peel and fruit ripening remain unclear. This study aimed to lay preliminary groundwork for understanding the ripening period and the underlying color differences in peach fruits by examining the color changes in the peach peels during the process of chlorophyll degradation, and the expression patterns of the genes related to the chlorophyll degradation.【Methods】Four peach varieties including Chunmei and 2-50 were chosen for this investigation. The color variations in the peach fruits prior to ripening were quantified using a colorimeter, and chlorophyll content was measured with a UV spectrophotometer. Although the primary chlorophyll degradation pathway has been identified, there are only limited researches on gene expression in the fruits. To fill this gap, quantitative transcription analysis was conducted, and the coding sequences (CDS) of relevant genes were sourced from genomic databases. The expression of the chloro-phyll degradation-associated genes was quantified using real-time quantitative polymerase chain reaction (qRT-PCR), and the correlation between the chlorophyll degradation and the gene expression was established through statistical analysis.【Results】The L* value of the peach fruit color difference index increased gradually from 30 to 16 days before ripening and then decreased as the fruit matured. All the varieties exhibited an S-type a* value trend, with a* experiencing a sharp rise from 23 to 12 days prior to ripening. During the 12 days before ripening, the green hue of the peach fruit faded, and changed to red and showed a continuous increase in the a* value. The a* value shifted from negative to positive or near zero from 12 to 8 days before ripening, signifying the pivotal period for color change. The b* value initially increased from 30 to 16 days before ripening and then decreased, and the yellow-fleshed varieties showed higher values than the white- fleshed ones. The chlorophyll content significantly decreased in all the varieties as the fruit developed, and a steep decline was observed in 30 and 16 days before maturity. From 12 to 8 days before ripening, the chlorophyll levels of the four varieties fell below 8 µg · g-1 , indicating visible coloring, though ripening was still not completed. The high expression levels of the PpCLH2 and the PpSGRL were detected 30 days before maturity when the fruit was entirely green. The expression levels decreased as the fruit ripened, while the PpCLH1 expression peaked during the critical period of incomplete coloring and color transformation, suggesting that it would play a pivotal role in the chlorophyll degradation. The PpPPH, PpPAO and PpRCCR genes showed increased expression from 8 to 0 days before ripening, while the PpSGR was significantly overexpressed as the fruit matured, indicating that it would play a primary role in the chlorophyll degradation. The gene cluster analysis and correlation studies further highlighted the significant and negative correlation between the PpSGR expression and the chlorophyll content, underscoring its key function during peach fruit ripening.【Conclusion】This research would provide novel insights into the expression of the genes related to the chlorophyll degradation and their relationship with the color change and the chlorophyll content in the peach peels. We identified that the period approximately 12 to 8 days before ripening would be critical for the color transition in the peach peels, and the PpCLH1 and PpSGR would be the key genes in the pre-ripening chlorophyll degradation process. These findings would not only enhance our understanding of the chlorophyll degradation throughout peach fruit development but also offer a valuable reference for future studies on the molecular mechanisms of the chlorophyll degradation in peaches.