Inhibition of Arabidopsis stomatal development by plastoquinone oxidation

Stomata are the pores in the epidermal surface of plant leaves that regulate the exchange of water and CO₂ with the environment thus controlling leaf gas exchange.¹ In the model dicot plant Arabidopsis thaliana, the transcription factors SPEECHLESS (SPCH) and MUTE sequentially control formative divisions in the stomatal lineage by forming heterodimers with ICE1.² SPCH regulates entry into the stomatal lineage and its stability or activity is regulated by a mitogen-activated protein kinase (MAPK) signaling cascade, mediated by its interaction with ICE1.^3-6 This MAPK pathway is regulated by extracellular epidermal patterning factor (EPFs) peptides, which bind a transmembrane receptor complex to inhibit (EPF1 and EPF2) or promote (STOMAGEN/EPFL9) stomatal development.^7-9 MUTE controls the transition to guard mother cell identity and is regulated by the HD-ZIP transcription factor HDG2, which is expressed exclusively in stomatal lineage cells.^10,11 Light signals acting through phytochrome and cryptochrome photoreceptors positively regulate stomatal development in response to increased irradiance.^12,13 Here we report that stomatal development is also regulated by the redox state of the photosynthetic electron transport chain (PETC). Oxidation of the plastoquinone (PQ) pool inhibits stomatal development by negatively regulating SPCH and MUTE expression. This mechanism is dependent on MPK6 and forms part of the response to lowering irradiance, which is distinct to the photoreceptor dependent response to increasing irradiance. Our results show that environmental signals can act through the PETC, demonstrating that photosynthetic signals regulate the development of the pores through which CO2 enters the leaf.