Poster: Membrane Transport

Abs # 1225: Vacuolar potassium-selective channel and its possible role under salt stress

Presenter:	Pottosin, Igor I, pottosin@cgic.ucol.mx
Authors	Pottosin, Igor I (A)   Martínez-Estévez, Manuel  (B)   Dobrovinskaya, Oxana R. (A)   Muńiz, Jesús  (A)
Affiliations:	(A): Universidad de Colima (B): Centro de Investigación Científica de Yucatán

Under salt stress plants need to ameliorate toxic effects of Na⁺ in cytosol and to maintain turgor. Therefore, salt-tolerant species accumulate large amounts of Na⁺ in vacuoles. Vacuolar Na⁺ uptake is mediated by electroneutral Na⁺/H⁺ exchange energized by tonoplast H⁺ pumps, whose operation is electrogenic. Thus, passive transport of a counter-ion different from H⁺ is additionally required to keep the whole process running. Dominant tonoplast ion channels, SV and FV, conduct Na⁺ and K⁺ indiscriminately. These channels have to be shut down under salt stress to avoid Na⁺ leak from the vacuole and futile, ATP-consuming Na⁺ cycling between vacuole and cytosol. Therefore, we have initiated the search of tonoplast K⁺-selective channels, which can mediate appropriate shunt conductance for H⁺ pumps under salinity. Novel channel with a conductance of 40 pS has been characterized in taproot vacuoles from salt-tolerant plant Beta vulgaris. In spite of a somewhat lower conductance value, this channel shared many common characteristics with the VK channel characterized in guard cell vacuoles. It was voltage-independent, required submicromolar cytosolic Ca²⁺ for activation, and was perfectly selective for K⁺ over Cl^- and Na⁺. Despite low expression (few tens of copies per vacuole) these K⁺-selective channels could readily compensate the currents generated by the whole vacuole population of H⁺ pumps. The joint action of H⁺-ATPase, Na⁺/H⁺ antiporter, and vacuolar K⁺ channel is functionally equivalent to that of Na⁺/K⁺ ATPase, a constitutive enzyme in animal cells, lacking in plants. Na⁺ uptake, accompanied by release of K⁺ from the vacuole, will improve cytosolic K⁺: Na⁺ ratio, thus contributing to the salt stress resistance. Supported by CONACyT grant 38181-N to IP.