Potassium transport in developing fleshy fruits: the grapevine inward K+ channel VvK1.2 is activated by CIPK- CBL complexes and induced in ripening berry flesh cells.

The grape berry provides a model for investigating the physiology of non-climacteric fruits. Increased K⁺ accumulation in the berry has a strong negative impact on fruit acidity (and quality). In maturing berries, we identified a K⁺ channel from the Shaker family, Vv K1.2, and two CBL-interacting protein kinase ( CIPK)/calcineurin B-like calcium sensor ( CBL) pairs, Vv CIPK04- Vv CBL01 and Vv CIPK03- Vv CBL02, that may control the activity of this channel. Vv CBL01 and Vv CIPK04 are homologues of Arabidopsis AtCBL1 and AtCIPK23, respectively, which form a complex that controls the activity of the Shaker K⁺ channel AKT1 in Arabidopsis roots. Vv K1.2 remained electrically silent when expressed alone in Xenopus oocytes, but gave rise to K⁺ currents when co-expressed with the pairs Vv CIPK03-Vv CBL02 or Vv CIPK04- Vv CBL01, the second pair inducing much larger currents than the first one. Other tested CIPK- CBL pairs expressed in maturing berries were found to be unable to activate Vv K1.2. When activated by its CIPK- CBL partners, Vv K1.2 acts as a voltage-gated inwardly rectifying K⁺ channel that is activated at voltages more negative than -100 mV and is stimulated upon external acidification. This channel is specifically expressed in the berry, where it displays a very strong induction at veraison (the inception of ripening) in flesh cells, phloem tissues and perivascular cells surrounding vascular bundles. Its expression in these tissues is further greatly increased upon mild drought stress. Vv K1.2 is thus likely to mediate rapid K⁺ transport in the berry and to contribute to the extensive re-organization of the translocation pathways and transport mechanisms that occurs at veraison. [ABSTRACT FROM AUTHOR]