Plant Physiol, December 2002, Vol. 130, pp. 1967-1973

Germanium Does Not Substitute for Boron in Cross-Linking of Rhamnogalacturonan II in Pumpkin Cell Walls¹

Forestry and Forest Products Research Institute, P.O. Box 16, Tsukuba Norin Kenkyu Danchinai, Ibaraki 305-8687, Japan (T.I.); National Agricultural Research Center for Kyushu Okinawa Region, Nishigoshi, Kumamoto 861-1192, Japan (T.M.); Institute of Biological Sciences, University of Tsukuba, Tsukuba, Ibaraki 305-8572, Japan (H.I., S.S.); and Shimadzu Co., Kanda Nishikicho, Chiyoda-ku, Tokyo 101-8448, Japan (J.T.)

Boron (B)-deficient pumpkin (Cucurbita moschata Duchesne) plants exhibit reduced growth, and their tissues are brittle. The leaf cell walls of these plants contain less than one-half the amount of borate cross-linked rhamnogalacturonan II (RG-II) dimer than normal plants. Supplying germanium (Ge), which has been reported to substitute for B, to B-deficient plants does not restore growth or reduce tissue brittleness. Nevertheless, the leaf cell walls of the Ge-treated plants accumulated considerable amounts of Ge. Dimeric RG-II (dRG-II) accounted for between 20% and 35% of the total RG-II in the cell walls of the second to fourth leaves from Ge-treated plants, but only 2% to 7% of the RG-II was cross-linked by germanate (dRG-II-Ge). The ability of RG-II to form a dimer is not reduced by Ge treatment because approximately 95% of the monomeric RG-II generated from the walls of Ge-treated plants is converted to dRG-II-Ge in vitro in the presence of germanium oxide and lead acetate. However, dRG-II-Ge is unstable and is converted to monomeric RG-II when the Ge is removed. Therefore, the content of dRG-II-Ge and dRG-II-B described above may not reflect the actual ratio of these in muro. ¹⁰B-Enriched boric acid and Ge are incorporated into the cell wall within 10 min after their foliar application to B-deficient plants. Foliar application of ¹⁰B but not Ge results in an increase in the proportion of dRG-II in the leaf cell wall. Taken together, our results suggest that Ge does not restore the growth of B-deficient plants.