Manipulating phytochrome levels in tomato fruits (Solanum lycopersicum L.) and its impacts on plastid development, physiology and nutritional composition

Abstract

Increasing food nutritional quality has become an important goal of modern plant genetic improvement programs and, in this context, tomato is a trending model species for studies concerning biosynthesis and accumulation of nutrients in fleshy fruits. Recent evidence suggests that tomato carotenoid and flavonoid contents can be simultaneously increased by alterations in perception or transduction of light signals and its possible interaction with plant hormones, causing changes in biosynthesis and storage capacity of these compounds, which are closely associated with alterations in plastid (chloroplasts and chromoplasts) abundance, size and ultrastructure. Thus, this project aims to analyze the impacts of manipulating phytochrome-coding genes (PHYs) on plastid biogenesis, nutraceutical composition (carotenoids and flavonoids) and hormonal signaling (auxins, cytokinins and ethylene) in tomato. The study comprises two stages: (i) First, we will evaluate pytochrome action on mRNA levels of auxin signaling transcription factors during deetiolating of wild-type tomato seedlings and for PHYA, PHYB1 and PHYB2 mutants and its respective combinations; (ii) In the second stage, transgenic plants will be generated, in which specific tomato PHY-coding genes (PHYA, PHYB1 and PHYB2) will be overexpressed specifically in fruit tissues, thereby allowing to evaluate the impacts of manipulating particular PHYs in the fruits without affecting other plant tissues. Immature, breaker and ripe fruits of these lineages will be characterized in terms of total chlorophyll, carotenoids and flavonoids contents, antioxidant activity, carotenoids and flavonoids profile and plastid number and size for later comparison with the ultrastructure of these organelles aiming to check the influence of phytochrome overexpression on the biogenesis, differentiation and internal organization of plastids. Free and conjugated auxins content, intermediate components of ethylene biosynthesis pathway and the abundance of gene transcripts coding for auxin-, cytokinin- and ethylene-responsive factors will be determined, providing information about the relation between phytochrome overexpression and the metabolism and acitivity of these key phytohormones. At last, variations in the expression of genes related to the light signal transduction (regulatory proteins) and enzymes involved in flavonoids and carotenoids biosynthesis will be followed throughout the fruit development and ripening in order to increase our understanding regarding the interactions between these different factors. Our main hypothesis is that manipulating the light conditions under which the fruits ripe as well the endogenous levels of phytochromes will cause marked alterations in auxin, cytokinin and ethylene signaling pathways that, in turn, will result in significant changes in plastid development and nutrient composition of tomato fruits. The proposed goals in this project have the potential to improve our current knowledge regarding light influence on the plastid biogenesis and differentiation along with the phytonutrient metabolism in fleshy fruits as well as to provide a better understanding about the interaction network between phytochromes, phytohormones (focused on auxins) and key regulatory proteins involved in light signal transduction pathways, making possible the development of new techniques for the breeding of plant lineages harboring fruits with improved nutritional composition. (AU)