Genetic control of carotenoid accumulation in kiwifruit

Abstract

Actinidia macrosperma is a kiwifruit species that accumulates 40-fold more carotenoids in the fruit compared with green (A. deliciosa) or gold (A. chinensis) kiwifruit cultivars, giving it bright orange colour. Recently, in a number of species, a protein called ‘ORange’ (OR) was shown to facilitate carotenoid accumulation through regulating chloroplast to chromoplast transition, as well as post-transcriptionally increasing the enzyme activity of the carotenoid biosynthesis rate-limiting enzyme, phytoene synthase (PSY). How carotenoids are accumulated in kiwifruit is poorly understood. Therefore, it is of interest to determine whether OR regulates plastid transition and increase PSY activity to accumulate carotenoids in kiwifruit. Two OR and two OR-like genes were identified from the sequenced A. chinensis genome and A. macrosperma homologues were analysed. A fruit ripening series in three temperature conditions, and transient expression of AmOR genes in tobacco leaves, were used to elucidate the effect of AmOR and AmOR-like gene expression on metabolite change and plastid transition and to observe if temperature affects their expression. These were measured using a combination of metabolite analytical instruments, qPCR and visualized with light and fluorescence microscopy. AmOR-like genes were more highly expressed than AmOR genes. In particular, AmORL1 expression correlated with chlorophyll and chloroplast degradation during fruit ripening. In tobacco, here was no indication that AmORL1 facilitates carotenoid accumulation and therefore, ‘true’ chromoplasts (carotenoid storing plastids) are not produced in transient experiments. It appears that PSY expression is a pre-requisite for carotenoid accumulation. This study provides support for the hypothesis that ORL1 is a key regulator of chloroplast transition and expression of this gene in conjunction with PSY can increase carotenoid content.