Fruits are divided into two classes: climacteric and nonclimacteric. By definition, nonclimacteric fruit ripens without the peak in ethylene and increase in respiration associated with ripening of climacteric fruits. Symons et al. provide evidence that brassinosteroids stimulate ripening of grape berries, specifically field-grown Cabernet Sauvignon. The abundance of castasterone (CS), a bioactive brassinosteroid, and its precursor increased during the period of berry ripening. Two enzymes involved in brassinosteroid biosynthesis (VvDWF1 and VvBR6OX1) and the brassinosteroid receptor (VvBRI1) were cloned from grape. Transcript analysis showed that VvDWF1 and VvBRI1 increased during berry ripening, whereas the abundance of VvBR6OX1 transcripts decreased. The decrease in VvBR6OX1 was consistent with negative feedback regulation by CS. This type of feedback by brassinosteroids has been reported in other species of plants. Application of exogenous brassinosteroid enhanced the rate of berry ripening, whereas application of an inhibitor brassinosteroid biosynthesis delayed berry ripening. Thus, brassinosteroids appear to serve a role in stimulating ripening of nonclimacteric fruits. Understanding nonclimacteric fruit ripening has important economic and agricultural implications, because several nonclimacteric fruits are grown for food or beverage production, including strawberry, grape, and citrus.
G. M. Symons, C. Davies, Y. Shavrukov, I. B. Dry, J. B. Reid, M. R. Thomas, Grapes on steroids: Brassinosteroids are involved in grape berry ripening. Plant Physiol. 140, 150-158 (2006). [Abstract] [Full Text]