Chlorophyll is a type of green pigment contained in higher plants and all other organisms that can perform photosynthesis. There are many types of chlorophylls, such as chlorophylls a, b, c and d, as well as bacterial chlorophylls and green bacteria chlorophylls. The common feature of their structure is that they include a porphyrin ring composed of four pyrroles, and the four pyrroles are combined with the metal magnesium element. Chlorophyll absorbs most of the red and purple light but reflects green light, so chlorophyll appears green, and it plays a core role in the light absorption of photosynthesis. Chlorophyll exists on the thylakoid membrane in the chloroplasts of leaves.
Citrus is one of the most important fruit trees in the world and has great industrial and economic value. Color is one of the important qualities of citrus fruits, and the composition and content of chlorophyll and carotenoids determine its appearance color. The two metabolic pathways of chlorophyll degradation and carotenoid biosynthesis are very complex and are regulated by multiple genes, multiple nodes and multiple factors. During the ripening process of citrus fruit, chlorophyll degradation and carotenoid biosynthesis occur almost simultaneously. However, the coordinated regulation of these two metabolic processes at the transcriptional level is not yet fully understood. Recently, a study has analyzed the molecular mechanism by which the transcription factor CrWRKY42 promotes citrus fruit coloring by co-regulating chlorophyll degradation and carotenoid biosynthesis pathways. The results of this study provide a theoretical basis and important gene resources for the genetic improvement of citrus fruit color quality.
This study discovered a WRKY family transcription factor CrWRKY42 involved in the color change process of citrus fruit. Overexpression of CrWRKY42 in citrus callus significantly upregulated the expression of multiple carotenoid biosynthesis genes such as phytoene dehydrogenase (CrPDS), lycopene β-cyclase (CrLCYB2) and β-carotene hydroxylase (CrBCH1), and the carotenoid content increased significantly. Interference with CrWRKY42 in citrus callus significantly reduced the total carotenoid content and inhibited the expression levels of multiple carotenoid synthesis-related genes. In addition, transient overexpression and interference of the CrWRKY42 gene in citrus fruits revealed that CrWRKY42 can simultaneously activate the expression of key genes for chlorophyll degradation and carotenoid biosynthesis, synergistically regulate two metabolic pathways, and thus promote the color change of citrus fruits. The results of biochemical experiments showed that CrWRKY42 can directly bind to the W-box element on the CrBCH1 promoter and activate its expression. Further studies found that CrWRKY42 can also directly bind to and activate the promoters of multiple chlorophyll degradation and carotenoid biosynthesis genes, including NONYELLOW COLORING (CrNYC), STAY-GREEN (CrSGR), CrPDS, and CrLCYB2.
In summary, CrWRKY42, as a direct positive regulator of chlorophyll degradation and carotenoid biosynthesis, plays an important role in the coloring process of citrus fruits. The results of this study help to further understand the complex transcriptional regulatory mechanisms of chlorophyll and carotenoid metabolism during fruit ripening, and have guiding significance for the improvement and regulation of fruit color quality.
