Characterization and functional validation of beta-carotene hydroxylase AcBCH genes in Actinidia chinensis

Carotenoids are the pigment substances of yellow-fleshed kiwifruit, and among them beta-cryptoxanthin has only been detected in the brighter yellow-fleshed variety 'Jinshi 1'. beta-Carotene hydroxylase (BCH) catalyzes the formation of beta-cryptoxanthin and zeaxanthin, but its molecular characteristics and functions have not been fully explained. Here we isolated two beta-carotene hydroxylase genes, AcBCH1 and AcBCH2 from kiwifruit (Actinidia chinensis), and their relative expression levels exhibited a close correlation with the content of beta-cryptoxanthin. AcBCH1 catalyzed the formation of beta-cryptoxanthin when transformed into beta-carotene-accumulating yeast cells. Moreover, silenced expression of AcBCH1 in kiwifruit caused decreases in the contents of zeaxanthin, lutein, and beta-cryptoxanthin, and an increase in beta-carotene content. The content of beta-carotene decreased significantly after the AcBCH1/2 genes were overexpressed in tomato. The content of zeaxanthin increased and beta-carotene decreased in transgenic kiwifruit seedlings. The results will enrich our knowledge of the molecular mechanisms of carotenoid biosynthesis in kiwifruit.