The Role of Potassium in Wood Formation of Poplar

The influence of potassium (K) supply on wood formation and the molecular mechanisms of K+-dependent xylogenesis were studied in poplar, one of the most important tree species in the field of wood biotechnology. Structural analyses revealed that the wood elongation zone as well as the vessel lumina were significantly reduced upon K+ starvation. In contrast to plants grown under optimal K+ supply, plants grown under limiting K+ concentrations also showed low and equally distributed K+ contents in vessels, fibres and cambial cells. To study the molecular basis of K+ transport during wood formation, the EST database from the cambial region of poplar was searched for sequence homologies to known K+ transporters from Arabidopsis. By quantitative RT-PCR we found that especially the P. tremula outward rectifying K+ channel (PTORK) and P. tremula K+ channel 2 (PTK2) correlated with seasonal cambial activity. The transcripts of both channels coincided with the seasonal K+ variations in the wood formation zone. The investigation of the biophysical properties showed that PTORK mediates K release upon membrane depolarization, while PTK2 is almost voltage-independent and might play a role in K+ uptake. By using immunofluorescence microscopy PTORK was localised in the plasma membrane of sieve elements and xylem ray-cells indicating a function in K+ release from these cells. In addition, a PM H+-ATPase which generates the necessary H+-gradient for the uptake of K+ and other nutrients into cambial cells could be localised in active cambial and differentiating xylem cells as well as in ray cells surrounding vessels. Since the ion channels are involved in the regulation of wood formation an important biotechnological challenge would be the generation of transgenic trees with modified K+ channels in order to change wood structure and thereby optimise wood properties.