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Signal integration of stomatal stress responses

Final Report Summary - STORM (Signal integration of stomatal stress responses)

The vast majority of environmental challenges to multicellular organisms occur at body surface tissues. Plants maintain a healthy body surface barrier through the dynamic closure of stomata, leaf epidermal pores that represent entry gates for phytopathogens when open. The stomata-forming guard cells are highly immunomodulatory to microbial patterns such as bacterial flagellin and fungal chitin, which are perceived by immune receptor complexes and upon signalling, induce stomatal closure. Although the molecular components conferring stomatal closure are emerging, the immune signalling pathways in guard cells is still unclear, in particular the events that link receptor signalling with components of the core guard cell pathway that result in closure.
In this project, we established a high-throughput imaging platform combined with computational image analysis suited to quantify stomatal apertures. Genetic screening led to the identification of critical components in stomatal regulation. In particular, we could show that a kinase of the chitin immune receptor complex activates a guard cell-expressed S-type anion channel at a specific phospho-site, thereby inducing stomatal closure. This highlights a short signal transduction module, in which the immune receptor complex directly regulates a core component that executes stomatal closure. Consistently, we found that immune signalling resulting in stomatal closure occurs in a guard cell-autonomous manner and independent of guard cell signalling in response to abscisic acid, a drought stress hormone promoting stomatal closure.
We therefore focussed on molecular components required for stomatal closure to microbial patterns but playing no roles in abscisic acid-induced closure of stomata. Interestingly, these included critical regulators of endocytic trafficking. We observed that impaired flagellin-induced stomatal closure correlated with reduced immune receptor removal in major endocytic trafficking mutants. Yet, mutants in Rab GTPases that mediate endocytic trafficking showed no correlation between stomatal closure and immune receptor endocytosis. We further demonstrate that a Rab7 GTPase mediates flagellin-induced stomatal closure through activation-dependent interaction with its effector to potentially regulate cytoskeleton-membrane contact formation. Whilst this pathway is not required for the acute induction of stomatal closure, it is important for sustaining the closure of stomata, a process likely essential for effective leaf surface immunity.
The importance of stomatal closure in plant immunity is supported by the finding that infectious pathogens evolved virulence factors that target guard cell immune signalling. In addition to pathogen-induced re-opening of closed stomata, we identified a virulence factor that inhibited the closure by targeting downstream immune signalling events.
To open and close the stomatal pore, guard cells dynamically adjust their shape. We developed a computational framework that combines 3D biomechanical model with parameter optimization, which revealed that proper stomatal dynamics is determined by two key properties of the cell wall.
Taken together, our studies revealed important molecular mechanisms of stomatal aperture regulation, such as sustained stomatal closure, to promote plant tissue surface immunity.