Functional Analysis of the MdSGR1 Gene in Methyl Jasmonate-Regulated Chlorophyll Degradation in Apple

Fruit color is a key quality indicator for apples and directly influences their market value. The process of fruit ripening encompasses various physiological and biochemical changes, such as the breakdown of chlorophyll and the buildup of anthocyanins and carotenoids. This study investigated the mechanism of chlorophyll degradation in apple peels using ‘Granny Smith’ varieties. The experiments involving the treatment with methyl jasmonate (MeJA) indicated that a concentration of 10 µM MeJA led to a reduction in chlorophyll degradation, while a higher concentration of 1500 µM MeJA enhanced this degradation, which aligned with the variations observed in the expression of genes associated with chlorophyll degradation. The key chlorophyll degradation gene MdSGR1 was cloned and found to be induced by methyl jasmonate. MdSGR1 encodes a 283-amino-acid protein belonging to the stay-green superfamily. The promoter possesses inducible cis-acting elements that respond to methyl jasmonate, low temperature and light, while the protein is localized to chloroplasts. Overexpression and silencing vectors were constructed. Overexpression of MdSGR1 induced chlorosis in tobacco leaves and ‘Granny Smith’ apple peels, decreased chlorophyll content, and upregulated related gene expression. Conversely, silencing MdSGR1 produced opposite effects. Arabidopsis thaliana plants overexpressing MdSGR1 exhibited low chlorophyll content, reduced photosynthetic rate, upregulated expression of genes associated with chlorophyll degradation. The results of yeast one-hybrid and dual-luciferase reporter assays indicated that the MdMYC2 transcription factor interacts with the promoter region of MdSGR1. In conclusion, MdSGR1 is crucial for the degradation of chlorophyll in apple peel, and it is regulated both by the MdMYC2 transcription factor and different concentrations of MeJA. This study preliminarily elucidated the regulatory mechanism of methyl jasmonate on chlorophyll degradation in fruit peel, and these findings provide an important theoretical basis for controlling degreening and color quality in apple fruit.