Poplar potassium transporters capable of controlling K+ homeostasis and K+-dependent xylogenesis
2002
Langer, K. | Ache, P. | Geiger, D. | Stinzing, A. | Arend, M. | Wind, C. | Regan, S. | Fromm, J. | Hedrich, R.
The cambial K(+) content of poplar increases during the growth period in a K(+) supply dependent manner. Upon K(+) starvation or application of tetraethylammoniumchloride (TEA(+)), a K(+) channel blocker, the average vessel lumen and expansion zone area were significantly reduced. In search for the molecular basis of potassium-dependent xylogenesis in poplar, K(+) transporters homologous to those of known function in Arabidopsis phloem- and xylem-physiology were isolated from a poplar wood EST library. The expression profile of three distinct K(+) channel types and one K(+) transporter, Populus tremula K(+) uptake transporter 1 (PtKUP1), was analysed by quantitative RT-PCR. Thereby, we found P. tremula outward rectifying K(+) channel (PTORK) and P. tremula K(+) channel 2 (PTK2) correlated with the seasonal wood production. K(+) transporter P. tremula 1 (KPT1) was predominantly found in guard cells. Following the heterologous expression in Xenopus oocytes the biophysical properties of the different channels were determined. PTORK, upon membrane de-polarization mediates potassium release. PTK2 is almost voltage independent, carrying inward K(+) flux at hyperpolarized potential and K(+) release upon de-polarization. PtKUP1 was expressed in a K(+) uptake-deficient Escherichia coli strain, where this K(+) transporter rescued K(+)-dependent growth. In order to link the different K(+) transporters to the cambial activity and wood production, we compared the expression profiles to seasonal changes in the K(+) content of the bark as well as xylem vessel diameter. Thereby, we found PTORK and PTK2 transcripts to follow the annual K(+) variations in poplar branches. PtKUP1 was expressed at a low level throughout the year, suggesting a housekeeping function. From these data, we conclude that K(+) channels are involved in the regulation of K(+)-dependent wood production.
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