Functional characterization of PIF proteins (Phytochrome-Interacting Factors) in tomato, Solanum lycopersicum

Abstract

Light signals are fundamental for plant physiology, providing not only energy for photosynthesis, but also environmental cues for the maintenance of daily rhythms and developmental progression through the life cycle. Photoreceptors, such as phytochromes, perceive and transduced through a complex pathway aiming the induction of adaptive responses. Phytochrome-Interacting Transcription Factors (PIFs) are members of a multigenic family that harbors between four and ten members in Viridiplantae and control light-driven responses. As integrators of light and hormone signaling, PIF proteins are involved in different developmental processes from germination to flowering in Arabidopsis thaliana. After germination, PIF proteins accumulate in the dark inducing hypocotyl elongation and inhibiting chlorophyll biosynthesis and chloroplast development. When exposed to light, as PIF levels decrease, seedlings turn green and photomorphogenic growth begins. In this context, PIF proteins are important to repress an initial over-reaction to light marked by the accumulation of highly reactive chlorophyll precursors, which could lead to tissue photo-oxidative damage. In adult plants, PIFs induce nocturnal daily growth and regulate dark- and age-induced leaf senescence. Regarding the later, PIFs directly and indirectly activate many senescence associated genes, leading to chlorophyll breakdown and plastid degradation. Chloroplast maintenance in source-leaves and chloroplast to chromoplast transition during fruit ripening are important to determine crop yield and quality of fleshy fruits. As most functional studies were performed in Arabidopsis thaliana, PIF role in fruit development and ripening remains unknown. Additionally, functional specificity has not been addressed for all of the multigenic family members. Aiming to fill these gaps, this project proposes to functionally characterize the PIF genes in the model species Solanum lycopersicum. First, by performing a phenetic analysis, we aim to identify all of the PIF encoding genes in tomato and establish their orthology relation to the A. thaliana counterparts. Next, a comprehensive expression pattern study will be performed and the roles of tomato PIF proteins will be discussed in the context of de-etiolation, leaf senescence and fruit development. Based on these results, at least three genes differentially expressed in at least one of the three conditions will be submitted to a further study using a reverse-genetics approach. Transgenic plants silenced for each of the chosen genes will be generated via RNAi and evaluated by different parameters, such as growth, carbon metabolism, crop yield and fruit nutritional quality. Further analyzes will be performed as needed to address the regulatory mechanisms involved in any phenotypic and metabolic alteration found. Thereby, this project aims to understand the role of PIF proteins in the regulation of photomorphogenic and metabolic processes affecting the yield and nutritional quality of tomato fruits. (AU)