Résumé
Control of stomatal aperture at the leaf surface allows the plant to prevent excessive transpirational water loss and desiccation. The aperture depends on the turgor of the two guard cells that form the stomata, an increase or a decrease in turgor leading to stomatal opening or closure, respectively. Rapid transport of K+ into or from guard cells plays a crucial role in this osmotically driven movements. Inward or outward K+ channels from the Shaker family dominate the membrane conductance to K+. In Arabidopsis thaliana, we show that a single Shaker gene, GORK, encodes the outward conductance. GORK activity allows rapid stomatal closure in response to dark or to ABA, and plays an important role in plant water saving under drought conditions. The inward K+ conductance relies upon at least 4 Shaker genes, among which KAT1 and KAT2 display the higher expression levels. Taking advantage from the fact that Shaker channels are homo- or hetero-tetrameric proteins, we engineered a mutant plant totally deprived of inward K+ channel activity in guard cell (GCKin activity), by transforming a dominant negative kat2 construct into a kat2-KO background. This allowed to show that GCKin activity plays an important role in stomatal opening in response to light or to a decrease in evaporative demand or in CO2 availability. It is also shown to be central to the mechanisms that underlie the circadian rhythm of stomatal opening and to prevent deleterious of Na+ on stomatal control. Absence of GCKin activity resulted in a strong decrease in biomass production in a meteorological scenario resulting in decreased photosynthetic activity after the first two hours of illumination. Finally, we show that KAT1 and KAT2 subunits are preferentially incorporated into heterotetrameric channels and that this phenomenon affects channel functional properties and channel targeting to and/or stability in the plasma membrane.