Cucumber Green Mottle Mosaic Virus Decreases Chlorophyll a Content in Cucurbit Crops by Upregulating the Key Gene in Chlorophyll Catabolic Pathway, Chlorophyllase 1

Cucumber green mottle mosaic virus (CGMMV, Tobamovirus viridimaculae) is a tobamovirus that induces leaf green mottling, mosaic patterns, bleaching, fruit sponginess, rotting, and malformation symptoms in various cucurbit crops. The underlying mechanisms by which CGMMV elicits these symptoms have yet to be elucidated. In the present study, we observed that the infection of CGMMV in bottle gourd, but not in N. benthamiana, led to the significant upregulation of a key gene involved in chlorophyll degradation, Chlorophyllase 1 (CLH1). This induction may be closely linked to chlorophyll degradation, particularly that of chlorophyll a (Clh a) in bottle gourd plants. Phylogenetic analysis showed that the amino acid sequence of BgCLH1 has a closer relationship with those of CLH1 from other cucurbit crops and has a relatively farther relationship with those of the well-studied CLH1 from Arabidopsis thaliana and Citrus sinensis. Further, confocal microscopy analysis indicated that BgCLH1 may be localized to the cytoplasm instead of the chloroplast. Moreover, silencing of the BgCLH1 gene not only reduced viral accumulation but also resulted in an increase in chlorophyll content. Similar results were also observed in watermelon, suggesting that this regulatory mechanism may be conserved across cucurbit crops. Our findings thus reveal a complex and intricate interplay between viral infection and the chlorophyll metabolic pathway.