Chlorophyll degradation naturally occurs during plant senescence. However, in fruit such as citrus, it is a positive characteristic, as degreening is an important colour development contributing to fruit quality. In the present work,Citrus sinensisOsbeck, cv. Newhall fruit was used as a model for chlorophyll degradation. An ethylene response factor,CitERF13, was isolated and its transcriptional changes were closely correlated with fruit peel degreening during development or in response to ethylene. Dual-luciferase and yeast one-hybrid assays, as well as motif mutation, indicated thatCitERF13directly binds to theCitPPHpromoter and enhances its activity. Transient and stable over-expression ofCitERF13resulted in rapid chlorophyll degradation inNicotiana tabacumleaves and led to accumulation of pheophorbide (Pheide)a, a metabolite of pheophorbide hydrolase (PPH). Similar results were observed from transient transformation ofCitERF13in citrus fruit peel. Moreover, this function ofCitERF13was conserved withinArabidopsisand tomato, as the homologsAtERF17andSlERF16similarly acted as activators ofPPHgenes and accelerators of chlorophyll degradation.Significance Statement
Chlorophyll degradation in the fruit is a desired trait in citrus, so understanding the underlying molecular mechanisms is important for citrus breeding. Here we show that an ethylene response factor directly binds to the promoter of PPH, a gene encoding pheohorbide hydrolase, a key mark of chlorophyll degradation.