Diurnal and circadian regulation of putative potassium channels in a leaf moving organ

In a search for potassium channels involved in light- and clock-regulated leaf movements, we cloned four putative K channel genes from the leaf-moving organs, pulvini, of the legume Samanea saman. The S. saman SPOCK1 is homologous to KCO1, an Arabidopsis two-pore-domain K channel, the S. saman SPORK1 is similar to SKOR and GORK, Arabidopsis outward-rectifying Shaker-like K channels, and the S. saman SPICK1 and SPICK2 are homologous to AKT2, a weakly-inward-rectifying Shaker-like Arabidopsis K channel. All four S. saman sequences possess the universal K-channel-specific pore signature, TXXTXGYG, strongly suggesting a role in transmembrane K⁺ transport. The four S. saman genes had different expression patterns within four leaf parts: 'extensor' and 'flexor' (the motor tissues), the leaf blades (mainly mesophyll), and the vascular bundle ('rachis'). Based on northern blot analysis, their transcript level was correlated with the rhythmic leaf movements: (a) all four genes were regulated diurnally (Spick2, Spork1, and Spock1 in extensor and flexor, Spick1 in extensor and rachis); (b) Spork1 and Spock1 rhythms were inverted upon the inversion of the day-night cycle; and (c) in extensor and/or flexor, the expression of Spork1, Spick1, and Spick2 was also under a circadian control. These findings parallel the circadian rhythm shown to govern the resting membrane K⁺ permeability in extensor and flexor protoplasts and the susceptibility of this permeability to light stimulation (Kim et al., 1993). Thus, Samanea pulvinar motor cells are the first described system combining light and circadian regulation of K channels at the level of transcript and membrane transport.