Plant Biology Two Paths from ABA to K⁺ Channels

In plants, K⁺ fluxes regulate gas exchange by controlling the stromal aperature in guard cells and, in most other plant cells, are involved in osmoregulation. K⁺ currents in guard cells are regulated by abscisic acid (ABA). Sutton et al. studied the regulation of an outward K⁺ current from mesophyll cells and found that ABA inhibited this current in both patch-clamped mesophyll protoplasts and voltage-clamped Xenopus oocytes injected with mesophyll mRNA. To determine whether the pathways from ABA to guard cell K⁺ channels (such as KAT1) and mesophyll K⁺ channels are the same, KAT1 was expressed in Xenopus oocytes with either guard cell mRNA or mesophyll cell mRNA, and ABA sensitivity was measured. Expression of the mRNA for the guard cell channel KAT1 alone in the oocytes produced an inward K⁺ current that was not affected by ABA. Injection of the mRNA for KAT1 and mRNA from guard cells reconstituted an inward ABA-inhibited current. Coinjection of mRNA for KAT1 and mesophyll cell mRNA resulted in an inward K⁺ current that was not inhibited by ABA, suggesting that the signal transduction pathways from ABA to the K⁺ channels expressed by guard cells and mesophyll are distinct and cannot substitute for one another.

Sutton, F., Paul, S.S., Wang, X.-Q., and Assmann, S.M. (2000) Distinct abscisic acid signaling pathways for modulation of guard cell versus mesophyll cell potassium channels revealed by expression studies in Xenopus laevis oocytes. Plant Physiol. 124: 223-230. [Abstract] [Full Text]