Guard Cell Inward K+Channel Activity inArabidopsisInvolves Expression of the Twin Channel Subunits KAT1 and KAT2*

Stomatal opening, which controls gas exchanges between plants and the atmosphere, results from an increase in turgor of the two guard cells that surround the pore of the stoma. KAT1 was the only inward K+channel shown to be expressed in Arabidopsisguard cells, where it was proposed to mediate a K+influx that enables stomatal opening. We report that another ArabidopsisK+channel, KAT2, is expressed in guard cells. More than KAT1, KAT2 displays functional features resembling those of native inward K+channels in guard cells. Coexpression inXenopusoocytes and two-hybrid experiments indicated that KAT1 and KAT2 can form heteromultimeric channels. The data indicate that KAT2 plays a crucial role in the stomatal opening machinery.