Involvement of an ethylene response factor in chlorophyll degradation during citrus fruit degreening

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 sinensis Osbeck, 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 that CitERF13 directly binds to the CitPPH promoter and enhances its activity. Transient and stable over-expression of CitERF13 resulted in rapid chlorophyll degradation in Nicotiana tabacum leaves and led to accumulation of pheophorbide (Pheide) a, a metabolite of pheophorbide hydrolase (PPH). Similar results were observed from transient transformation of CitERF13 in citrus fruit peel. Moreover, this function of CitERF13 was conserved within Arabidopsis and tomato, as the homologs AtERF17 and SlERF16 similarly acted as activators of PPH genes and accelerators of chlorophyll degradation.