The role of BBX proteins in tomato fruit development and ripening

Abstract

Tomato (Solanum lycopersicum L.) is an important source of nutraceutical compounds for human diet, mostly synthesized in the plastids (e.g. carotenoids and tocopherols). During ripening, the fruits suffer remarkable physiological and biochemical changes associated with chloroplast-to-chromoplast conversion. Thus, the number and activity of fruit chloroplasts, besides the role in carbon fixation and, consequently, yield; is determinant for the nutritional quality of the edible fruit. The main role of light in plastid biogenesis and activity and, even, in ripening has been extensively addressed; however, new molecular players are constantly being identified. Fruit-localized phytochromes (SlPHYs) regulate plastid biogenesis and maturation, modulate sugar metabolism in early developing fruits and control carotenoid biosynthesis during fruit ripening. Additionally, SlPHYB2 negatively regulates chlorophyll accumulation by controlling the expression of the master transcription factor of fruit chloroplast development and maintenance, SlGLK2. Recently, our group has begun to study tomato BBX encoding genes, so far, exclusively studied in Arabidopsis thaliana, many of which are involved in light signalling. These are particularly interesting to explore in tomato due to their putative effect on plastid biogenesis and function and, consequently, their impact on crop quality. The mRNA accumulation pattern of SlBBX revealed six genes that showed a clear ripening associated profile: SlBBX19 (Solyc01g110370), SlBBX20 (Solyc12g089240), SlBBX16 (Solyc12g005750), SlBBX26 (Solyc10g006750), SlBBX28 (Solyc12g005660) and SlBBX29 (Solyc02g079430). Interestingly, the promoter regions of these six genes showed binding motifs for light signal factors and SlRIN, the master ripening transcription factor. This work hypothesizes that the BBX proteins that participate in light signalling affect tomato vegetative and reproductive development. In this context, this project aims to characterize the role of the six SlBBX protein encoding genes, whose mRNA profile increase or decrease along chloroplast-chromoplast transition, during tomato fruit development and ripening. In order to achieve this objective, this study will: (I) investigate whether the ripening-dependent transcriptional profile of the selected SlBBX genes is determined by SlRIN master transcription factor regulation; (II) investigate the effect of PHY-mediated light perception and SlGLK2 plastidial master transcription factor on the expression of the selected SlBBX genes and (III) obtain and phenotypically characterize CRISPR-mediated knockout tomato plants for the selected SlBBX genes. (AU)