Functional characterization of VAL B3 domain proteins in Arabidopsis thaliana development

Abstract

VIVIPAROUS1 (VP1)/ ABSCISIC ACID INSENSITIVE3 (ABI3)-LIKE (VAL) proteins function as transcriptional regulators, playing a pivotal role in repressing transcriptional programs through epigenetic modifications. Over recent years, accumulating evidence has demonstrated that VAL enable plants to coordinate developmental transitions and respond to environmental changes by interacting with Polycomb Repressive Complex (PRC1 and PRC2). In this thesis, we explore the molecular mechanisms underlying VAL activity at different developmental stages of the model plant Arabidopsis thaliana, mainly focusing on seed germination, young seedling growth, and the transition to reproductive development. In the introduction, we systematically reviewed recent progress in defining the molecular mechanisms whereby VAL proteins mediate transcriptional silencing through chromatin remodeling, histone modifications and interactions with PRC1 and PRC2 during the plant life cycle. This summary provides a comprehensive background of the extent of VAL-mediated regulation. Additionally, we point out current limitations in the study of VAL proteins. In chapter 1 of the results section, we combined a genetic and transcriptomic approach to investigate the requirement of VAL for the activation of flowering at different day-length conditions. We found that VAL1, and not its sister protein VAL2, is required to induce the floral transition both under long- and short-day photoperiods. We have been able to demonstrate that VAL1 induction of flowering occurs via the direct epigenetic repression of the organ boundary genes BLADE ON PETIOLE1 (BOP1) and BOP2. Our work thus expands the repertoire of VAL target genes and further demonstrates the pleiotropic role of VAL factors in the regulation of Arabidopsis development. In chapter 2 of the results section, we characterized the function of VAL in response to the plant phytohormone abscisic acid (ABA) in the transition from seed to seedling. We found that val1 mutant is hyposensitive to exogenous ABA during seed germination. In contrast, val1 becomes hypersensitive to exogenous ABA at the onset of seedling development, confirming the essential and complex function of VAL1 in the modulation of ABA signaling in Arabidopsis. In addition, our analyses revealed that VAL1 defines the developmental interval during post-germination in which Arabidopsis plants remain susceptible to ABA-induced growth arrest. Our results demonstrate that VAL1 triggers the reduction of chromatin accessibility of ABA-responsive promoters thus diminishing plant ability to respond to ABA. Overall, this thesis offers significant new insights into the scope and mechanisms of VAL-mediated regulation in Arabidopsis.

Date of Award	2 Jul 2025
Original language	English
Supervisor	Julia Irene Questa (Director)