AcHZP45 is a repressor of chlorophyll biosynthesis and activator of chlorophyll degradation in kiwifruit.

The degradation of chlorophyll during fruit development is essential in order to reveal a more 'ripe' color that signals readiness to wild dispersers of seeds and the human consumer. Here, comparative biochemical analysis of developing fruits of Actinidia deliciosa cv. Xuxiang ('XX', green-fleshed) and Actinidia chinensis cv. Jinshi No.1 ( 'JS', yellow-fleshed) indicated that variation in chlorophyll content is the major contributor to differences in flesh color. Four differentially expressed candidates, down-regulated genes AcCRD1 and AcPOR1 involved in chlorophyll biosynthesis, and up-regulated genes AcSGR1 and AcSGR2 driving chlorophyll degradation, were identified. Prochlorophyllide and chlorophyllide, the metabolites produced by AcCRD1 and AcPOR1, progressively reduced in 'JS', but not in 'XX', indicating that chlorophyll biosynthesis was less active in yellow-fleshed fruit. AcSGR1 and AcSGR2 were verified to be involved in chlorophyll degradation, using both transient expression in tobacco and stable over-expression in kiwifruit. Furthermore, a homeobox-leucine zipper (HD-Zip II) AcHZP45 showed significantly increased expression during 'JS' fruit ripening, which both repressed expression of AcCRD1 and AcPOR1 and activated expression of AcSGR1 and AcSGR2. Collectively, the present study indicated that contrary dynamics of chlorophyll biosynthesis and degradation coordinate the differences in chlorophyll content in kiwifruit flesh, which is orchestrated by the key transcription factor AcHZP45.