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Research focus:  

Lehrstuhl für Botanik I
Julius-von-Sachs-Platz 2, 97082 Würzburg

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Research foci (and basic equipment-based research projects):
DFG project "The apoplast of higher plants: storage, transport and reaction". Topic: Potassium dependent wood formation

We were able to show, that the cambial K+ content of poplar increases during the growth period in a K+ supply-dependent manner. Upon K+ starvation or application of 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 Arabidopis 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, PtKUP1 (Populus tremula K+ uptake-transporter 1), was analyzed by quantitative RT-PCR. Thereby we found PTORK (Populus tremula outward rectifying K+ channel) and PTK2 (Populus tremula K+ channel 2) correlated with the seasonal wood production. KPT1 (K+ transporter Populus tremula 1) 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 depolarization mediates potassium release. PTK2 is almost voltage-independent, carrying inward K+ flux at hyperpolarized potential and K+ release upon depolarization. PtKUP1 was expressed in a K+ uptake-deficient E. 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.