Differential function of TOL homologs

Climate change, which manifests itself in harsher environmental conditions, has put the food supply of a growing world population under considerable pressure. Since plants cannot simply move away from an unfavorable environment by themselves, they have developed intricate mechanisms in order to be able to react quickly and precisely to their surroundings (adaptation). The plasma membrane of a cell is the interface between the inside and the outside and is interspersed with proteins that are essential for the detection and transmission of internal and external stimuli. Strict control of the amount and positioning of these proteins is critical for adaptive processes. In the model plant Arabidopsis thaliana, we have shown that a protein family, the TOL proteins, is responsible for the initial steps in the transport of plasma membrane proteins to the vacuole for their degradation. In this project we want to investigate the function of individual members of this protein family in regulating the abundance of plasma membrane proteins in various signaling pathways. In addition to their general role in the degradation pathway of membrane proteins, individual TOLs can play a more specific role in different signaling pathways. One example is the role of distinct TOLs in the signaling pathway of the plant hormone abscisic acid, which plays an important role in germination and drought stress. Evidence that TOLs fulfill these differential functions comes from the analysis of their domains, their diverse expression patterns and their subcellular localizations. TOLs also appear to undergo dynamic rearrangements in response to certain environmental stimuli. The main aim of this project is to evaluate which networks and signal pathways the TOLs are involved in and to identify differences in their localization and their relevant interaction partners. In summary, these experiments should provide insights into the first steps in the degradation of plasma membrane proteins relevant in essential signaling pathways. This basic research project will establish previously unknown regulatory processes in the control of protein turnover at the plasma membrane. This will contribute to our understanding of how plants optimize and fine-tune their response to the constantly changing environment. Furthermore, results from the model plant Arabidopsis thaliana can be translated to assist in the generation of crops, which are more resilient to the harsher climate of the future.

Decoding Plant Adaptation: How Cells Control Their Surface Proteins Unlike animals, plants are sessile organisms, rooted in place, unable to flee from drought, heat, or other environmental threats. Over millions of years of evolution, they have developed highly specialized molecular mechanisms to perceive and respond wide range of environmental stimuli. Central to this adaptive capacity is the plasma membrane, the dynamic lipid bilayer that forms the outer boundary of every cell. Embedded within this membrane are proteins that serve dual roles: acting both as sensors of external signals and as effectors of the cellular responses they trigger. A fundamental question that remains is how plants regulate the abundance and precise subcellular localization of these critical membrane proteins. Part of the answers lies in a family of proteins known as TOLs (TOM1-like). These proteins function as molecular recognition factors, selectively identifying target membrane proteins and directing them towards the vacuole, where they are broken down. In the model plant Arabidopsis thaliana, nine distinct TOL proteins have been identified, yet their individual functions remain largely uncharacterized. Single TOL loss-of function mutant plant lines TOL typically display no overt phenotype, whereas higher-order mutants, lacking multiple TOL genes exhibit severe developmental phenotypes and severe growth defects. Notably, one such mutant shows hypersensitivity to abscisic acid (ABA), a key phytohormone mediating abiotic stress responses, suggesting that certain TOL proteins have acquired specialized roles in hormone signalling beyond their general function in protein turnover. Each TOL protein is structurally distinct and displays differential expression across tissues and subcellular compartments. Furthermore, TOL proteins undergo dynamic relocalization in response to stress conditions and chemical perturbations, implying that their function extends beyond membrane protein degradation to the active modulation of cellular signalling networks. In this study, we sought to determine where TOL proteins act within the cell, identify their molecular interaction partners, and establish whether they function independently or as part of larger protein complexes. Our findings reveal a previously uncharacterised role for TOL proteins in mediating plant responses to drought and salinity, two abiotic stresses of considerable agricultural relevance. Together, this work advances our mechanistic understanding of membrane protein homeostasis in plants, a process essential for environmental adaptation, and provides a foundation for developing stress-resilient crops through targeted molecular breeding strategies.