Chlorophyll a/b binding protein plays a key role in natural and ethylene-induced degreening of Ponkan (Citrus reticulata Blanco)

Abstract

Chlorophyll content in peel gradually declines during citrus fruit development, and this can be accelerated by applying ethylene. In order to understand the molecular regulation of chlorophyll loss, the expression of several chlorophyll-related genes was determined in Ponkan (Citrus reticulata Blanco) peel during fruit maturation and ethylene-induced degreening. During fruit development, the transcript level of pheophorbide a oxygenase (CitPaO) and stay-green protein (CitSGR) was stable, and no obvious change of chlorophyll b reductase (CitNYC) mRNA was found. In addition, chlorophyllase (CitChlase) mRNA was decreased, indicating the decline of chlorophyll degradation capacity in this process. Only the reduced expression of Mg-chelatase (CitCHLH) and chlorophyll a/b binding protein (CitCAB1, 2) was found to be correlated with the reduction in chlorophyll content. Chlorophyll loss was greatly accelerated by postharvest ethylene fumigation. In this process, the expression of CitCHLH, CitPaO and CitSGR was not affected. However, it greatly increased the expression of CitNYC and CitChlase, and accelerated the decline in CitCAB expression. Taken together, these results indicate that the decrease in expression of CitCAB was highly associated with chlorophyll loss, no matter whether during natural or ethylene-induced degreening. However, the increase in CitChlase and CitNYC transcript abundance was only related to accelerated chlorophyll degradation in ethylene-induced degreening. In conclusion, the higher availability of free chlorophylls for degradation, resulting from down regulated expression of CitCABs, is likely to be a main reason for chlorophyll reduction during natural and ethylene-induced degreening.