Alteration of the post-translational modification pathway shed light on the importance of peptide hormones in plant growth and fruit development

Plant growth mechanisms, along with fruit development and ripening, are complex processes determined by both genetic and environmental factors. Vegetative growth is fundamental to plant development and enables the subsequent development of flowers and fruit and their ripening. These latter processes are essential for successful plant reproduction and the production of economically valuable food sources. Different plant species share common regulatory networks that rely on hormones, transcription factors (TFs), and various signaling molecules. Among these, peptide hormones (PHs) play crucial roles in both long- and short-distance signaling and fulfill numerous functions, making them strong candidates for regulating various developmental processes. Recent studies have highlighted the involvement of PHs in different plant processes, yet some of their functions, particularly in fruit development, remain underexplored. To address this gap, my PhD project aimed to investigate the role of PHs, leveraging the fact that many PH families share common steps in their biosynthetic pathways. This project focused on impairing the synthesis of multiple PHs by interfering with the post-translational modifications necessary for their biological activity. Specifically, the research targeted two types of post-translational modifying enzymes: subtilase family proteases and Tyrosyl Protein Sulfotransferase (TPST). For the subtilase targets, two approaches were employed: overexpression of microbial protease inhibitors and silencing via amiRNAs, the latter also being used for TPST silencing. By using different promoters, PH synthesis was impaired in a fruit-specific manner in tomato and, for validation of the experimental approach, constitutively in tobacco. Once the mutant plants were generated, phenotypic analyses were conducted. In tomato, flower and fruit development were assessed through two separate experiments under different conditions. In the first experiment, flower/fruit load was controlled and kept low to monitor fruit development and ripening over time without metabolic constraints. In the second experiment, plants were not pruned, allowing observation of fruit growth under conditions of high metabolic demand, where resources were distributed among competing sinks. In tobacco, overall growth was examined with a focus on specific organ development. This approach demonstrated the significant roles of secreted PHs in various plant processes, particularly in the development of different organs and the regulatory dynamics involved in fleshy fruit development and ripening. A parallel complementary gain-of-function approach was also employed to address the original objective of the work. This involved overexpressing promising CLE and GLV genes candidates, based on their expression profiles, at the fruit-specific level in Solanum lycopersicum.