Aquaporins in poplar: What a difference a symbiont makes!
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The formation of ectomycorrhizas, a tight association between fine roots of trees and certain soil fungi, improves plant nutrition in a nutrient-limited environment and may increase plant survival under water stress conditions. To investigate the impact of mycorrhiza formation on plant water uptake, seven genes coding for putative water channel proteins (aquaporins) were isolated from a poplar ectomycorrhizal cDNA library. Four out of the seven genes were preferentially expressed in roots. Mycorrhiza formation resulted in an increased transcript level for three of these genes, two of which are the most prominently expressed aquaporins in roots. When expressed in Xenopus laevis oocytes, the corresponding proteins of both genes were able to transport water. Together, these data indicate, that the water transport capacity of the plasma membrane of root cells is strongly increased in mycorrhized plants. Measurements of the hydraulic conductance of intact root systems revealed an increased water transport capacity of mycorrhized poplar roots. These data, however, also indicate that changes in the properties of the plasma membrane as well as those of the apoplast are responsible for the increased root hydraulic conductance in ectomycorrhizal symbiosis.
KeywordsEctomycorrhiza Water transport Aquaporin Root hydraulic conductance Poplar phylogeny
Membrane intrinsic protein
Plasma membrane intrinsic protein
Tonoplast intrinsic protein
Small intrinsic protein
Osmotic permeability coefficient
Polymerase chain reaction
We are indebted to Margret Ecke, Tawfik Muhsin and Andrea Bock for excellent technical assistance and to Dr. Nina Grunze for critical reading of the manuscript. This work was financed by the Deutsche Forschungsgemeinschaft (Ha 970/10-1 and Ne 332/7-1) as part of the focus program SSP1084 ”Molekulare Grundlagen der Mykorrhiza Symbiose”.
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