MIRO1-mediated mitochondrial fusion is required for stomatal immunity in Arabidopsis.

Stomatal immunity is a critical first barrier in plant defence, yet the organelle-level mechanisms underpinning this process remain poorly understood. Here we show that the outer mitochondrial membrane protein MIRO1 is essential for flg22-triggered stomatal closure in Arabidopsis. Upon immune activation, MIRO1 promotes mitochondrial fusion in guard cells. This mitochondrial remodelling is necessary to maintain mitochondrial function, including membrane potential, ATP synthesis, mitochondrial reactive oxygen species production and the activation of organic acid metabolism. In miro1 mutants, these mitochondrial functions are compromised, which is associated with defective stomatal closure and increased bacterial entry. We further show that flg22 triggers MPK3/6-dependent phosphorylation of MIRO1 at Ser14. Phosphorylated MIRO1 displays enhanced oligomerization at mitochondrial contact sites to facilitate fusion. Mutations disrupting MIRO1 phosphorylation or oligomerization abolish its immune function. Collectively, our findings establish MIRO1 as a key molecular link between immune signalling and mitochondrial dynamics during stomatal defence regulation.