Abscisic acid-inducible 25 kDa xylem sap protein abundant in winter poplar

To investigate the environmental regulation of root function, xylem sap was annually collected from Populus nigra and the proteins in the sap were analyzed. A 25 kDa xylem sap protein (XSP25) was found to be most abundant in the xylem sap in winter and mass spectrometry analyses showed its high similarity to abscisic acid (ABA)-inducible basic secretory protein reported in tobacco BY-2 cells. By utilizing the information of whole genome sequence of Populus, PmXSP25 was cloned from Populus maximowiczii, naturally growing poplar in Japan. The expression of PmXSP25 was abundant in root at December and February and strongly enhanced by ABA application to the autumn root. We suggest that dormancy- inducing short day length and low temperature promote the synthesis of XSP25 in root, possibly via ABA, as an adaptation to the winter environment.