Stomagen positively regulates stomatal density in Arabidopsis

As the portals through which carbon dioxide is exchanged between the epidermal leaf tissue of plants and the atmosphere, the stomata are key elements in plant physiology. As such they are the focus of genetic and environmental regulation in plants, but until now no positive signalling factor of stomata development has been identified. Now a protein with stomata-inducing properties has been identified in Arabidopsis. Termed stomagen, it is a cysteine-rich peptide found in the inner tissue (mesophyll) layers of young leaves where it initiates the stomatal lineage possibly by binding to the cell-surface receptor TMM. This discovery raises the prospect that stomagen might be used to develop crops and trees with high stomatal densities and hence high absorption capacity for CO2, not only by genetic engineering, but also by spraying with synthetic stomagen or related synthetic peptides. Stomata are specialized structures in the epidermal layer of leaves that regulate the exchange of gases between the plant and the atmosphere. One or more positive intercellular signalling factors are assumed to be involved in stomatal development, but their identities remain elusive. A novel secretory protein — named stomagen — is now shown to be just such a factor; it is conserved among vascular plants and positively regulates stomatal density. Stomata in the epidermal tissues of leaves are valves through which passes CO2, and as such they influence the global carbon cycle1. The two-dimensional pattern and density of stomata in the leaf epidermis are genetically and environmentally regulated to optimize gas exchange2. Two putative intercellular signalling factors, EPF1 and EPF2, function as negative regulators of stomatal development in Arabidopsis, possibly by interacting with the receptor-like protein TMM3,4,5,6. One or more positive intercellular signalling factors are assumed to be involved in stomatal development, but their identities are unknown7. Here we show that a novel secretory peptide, which we designate as stomagen, is a positive intercellular signalling factor that is conserved among vascular plants. Stomagen is a 45-amino--rich peptide that is generated from a 102-amino-acid precursor protein designated as STOMAGEN. Both an in planta analysis and a semi-in-vitro analysis with recombinant and chemically synthesized stomagen peptides showed that stomagen has stomata-inducing activity in a dose-dependent manner. A genetic analysis showed that TMM is epistatic to STOMAGEN (At4g12970), suggesting that stomatal development is finely regulated by competitive binding of positive and negative regulators to the same receptor. Notably, STOMAGEN is expressed in inner tissues (the mesophyll) of immature leaves but not in the epidermal tissues where stomata develop. This study provides evidence of a mesophyll-derived positive regulator of stomatal density. Our findings provide a conceptual advancement in understanding stomatal development: inner photosynthetic tissues optimize their function by regulating stomatal density in the epidermis for efficient uptake of CO2.