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DEVELOPMENT AND HORMONE ACTION

Possible Involvement of Leaf Gibberellins in the Clock-Controlled Expression of XSP30, a Gene Encoding a Xylem Sap Lectin, in Cucumber Roots¹

Institute of Biological Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan (A.O., C.S., T.M., H.K., S.S.); and Noda Institute for Scientific Research, Noda, Chiba 278-0037, Japan (S.M.)

Root-produced organic compounds in xylem sap, such as hormones and amino acids, are known to be important in plant development. Recently, biochemical approaches have revealed the identities of several xylem sap proteins, but the biological functions and the regulation of the production of these proteins are not fully understood. XYLEM SAP PROTEIN 30 kD (XSP30), which is specifically expressed in the roots of cucumber (Cucumis sativus), encodes a lectin and is hypothesized as affecting the development of above-ground organs. In this report, we demonstrate that XSP30 gene expression and the level of XSP30 protein fluctuate in a diurnal rhythm in cucumber roots. The rhythmic gene expression continues for at least two or three cycles, even under continuous light or dark conditions, demonstrating that the expression of this gene is controlled by a circadian clock. Removal of mature leaves or treatment of shoots with uniconazole-P, an inhibitor of gibberellic acid (GA) biosynthesis, dampens the amplitude of the rhythmic expression; the application of GA negates these effects. These results suggest that light signals perceived by above-ground organs, as well as GA that is produced, possibly, in mature leaves, are important for the rhythmic expression of XSP30 in roots. This is the first demonstration of the regulation of the expression of a clock-controlled gene by GA.

Article, publication date, and citation information can be found at www.plantphysiol.org/cgi/doi/10.1104/pp.103.030742.

^* Corresponding author; e-mail pdp{at}sakura.cc.tsukuba.ac.jp; fax 81-29-853-4579.

Received July 23, 2003; returned for revision August 19, 2003; accepted August 21, 2003.