Regulatory role of the membrane nanodomain-localized proteins HIR in the aquaporin-dependent plant immunity – HIRAQIM

Plant immunity is of critical importance in the context of climate change that favors the emergence of new plant diseases by suppressing plant defenses and modifying the geographical distribution of plant pathogens. These processes threaten food quality, human health, economy and ecosystems. Recently, a strong link has emerged in the scientific literature between two physiological traits: immunity and plant water status. However, the underlying molecular mechanisms remain elusive. The HIRAQIM project aims at filling this gap.

To face pathogen attacks, plants have evolved sophisticated and diverse mechanisms. Hypersensitive Induced Reaction proteins (HIR), that organize in plasma membrane (PM) nanodomains corresponding to nanometric scale structures containing specific lipids and proteins and acting as signaling/regulation platforms, participate in plant immunity via unknown molecular mechanisms. However, our recent results suggest that Arabidopsis HIR proteins are involved in H2O2 signaling that is essential for defense mechanisms. In addition, we showed that HIR proteins physically interact with and positively regulate Arabidopsis Plasma membrane Intrinsic Protein (PIP) aquaporins that facilitate water and H2O2 transport across the PM. Given that PIP aquaporins are regulated through phosphorylation but also by their lipid environment in membranes, we hypothesized that the activation of PIP in HIR2-containing nanodomains may rely on a specific lipid composition and/or on the presence in these domains of PIP-activating kinases. Besides their historical role in the maintenance of water status, PIP proteins emerge as important components of the defense against pathogens in plants. Interestingly, the water status of tissues is another primordial determinant of plant immunity. We believe that HIR may modulate plant immunity by regulating PIP proteins in specific PM nanodomains.

In this project, we therefore propose to:

- Decipher the HIR-dependent regulation of Arabidopsis PIP aquaporins by combining state of the art microscopy and biochemical approaches. We will analyze the influence of biotic stimuli on HIR2-PIP co-localization in PM nanodomains and determine whether the nanodomain-resident proteins HIR may actively recruit PIP in nanodomains with a specific lipid and/or protein composition, in order to ensure their proper functioning.
- Investigate how HIR proteins are involved in the defense against pathogens in Arabidopsis thaliana by: (i) characterizing the HIR-dependent H2O2 signaling using H2O2 biosensors, (ii) investigating how HIR-mediated regulation of PIP can influence plant immunity by modulating water and/or H2O2 transport, comparing two bacterial pathogens which pathogenicity relies on those physiological parameters.

The HIRAQIM project is based on the collaboration between two groups that present a strong thematic complementarity making it possible to combine for instance cutting-edge cell biology approaches with a fine mastery of pathosystems. This project should lead to new paradigms related to the regulation mechanisms of transport through the PM and to the connections between physiological processes that are a priori distant: plant water status and immunity.