PHOTOPERIODIC REGULATION OF RICE STEM ELONGATION BY THE GENE PINE1

In several plant species, inflorescence formation is accompanied by stem elongation. While these two processes need to progress harmoniously, until recently no genetic link had been found that could integrate and coordinate both pathways. Only a recent study has shed light on a new and unexplored pathway that, via the PREMATURE INTERNODE ELONGATION 1 (PINE1) gene, links flowering with stem elongation [1]. PINE1 is a zinc finger transcription factor able to repress stem elongation during vegetative growth. During floral induction, the florigenic signal can silence PINE1 expression, triggering internode elongation. This thesis aims to explore the mechanism of action of PINE1 and the potential exploitation of this pathway for rice breeding for agriculture. The hypothesis that PINE1 could be regulated by genes involved in the flowering machinery has been tested by generating overexpressors of the known flowering related genes OsMADS14, OsMADS15 and FTL1 to assess their phenotype regarding stem elongation. Indeed, FTL1, the expression of which is promoted by florigens, could be involved in PINE1 inhibition during flowering. Previous literature on genes similar to PINE1 reported interaction with co-repressors proteins. Thus, some of the recurring interactors were cloned to test whether they can effectively interact with PINE1. The data suggests that PINE1 is able to form a complex with TOPLESS (TPL) proteins, which may function in transcriptional repression through chromatin remodeling. The recognition of PINE1 by TPL depends on the presence of EAR-motifs, which are also important for PINE1 function overall. As PINE1 is a regulator of stem elongation and plant height is an important breeding trait, we attempted to modify plant height by modulating PINE1 expression by introducing mutations in its promoter region. Some of the generated plants did show strong dwarfism, indicating that some mutation in the promoter region lead do a PINE1 overexpression. For the same purpose of finding ways to control plant height, we also identified some candidate genes controlled by PINE1 using RNA sequencing and created knock-out mutations using the CRISPR/Cas9 system to further explore their role in stem elongation. Overall, this work establishes the framework for a new pathway that controls plant architecture and has implications for crop improvement. Further studies will be needed in the coming years to elucidate the entirety of this cascade of events from flowering to stem elongation and to explore the potential applications of this research for agriculture.