Plant Physiol, October 1999, Vol. 121, pp. 557-564

A Proline-, Threonine-, and Glycine-Rich Protein Down-Regulated by Drought Is Localized in the Cell Wall of Xylem Elements¹

Department of Biological Sciences, University of Quebec, P.O. Box 8888, Station Centre Ville, Montreal, Quebec, Canada H3C 3P8 (H.H., Z.T.); and Departments of Biology (H.C., M.P., J.G.L.) and Biochemistry (G.B.), Laval University, Sainte-Foy, Quebec, Canada G1K 7P4

A cDNA clone encoding a proline-, threonine-, and glycine-rich protein (PTGRP) was isolated from a wild tomato species (Lycopersicon chilense) (L.X. Yu, H. Chamberland, J.G. Lafontain, Z. Tabaeizadeh [1996] Genome 39: 1185-1193). Northern-blot analysis and in situ hybridization studies revealed that PTGRP is down-regulated by drought stress. The level of the mRNA in leaves and stems of 8-d drought-stressed plants decreased 5- to 10-fold compared with that in regularly watered plants. The mRNA re-accumulated when drought-stressed plants were rewatered. Antibodies raised against a glutathione S-transferase/PTGRP fusion protein were used to elucidate the subcellular localization of the protein by immunogold labeling. In regularly watered L. chilense plants, PTGRP protein was found to be localized in xylem pit membranes and disintegrated primary walls. Examination of sections from drought-stressed plants revealed a significant decrease in the levels of labeling. In these samples, only a few scattered gold particles were detected in the same areas. In the leaf tissues of plants that had been rewatered for 3 d following an 8-d drought stress, the labeling pattern was similar to that of the regularly watered plants. To our knowledge, PTGRP is the first drought-regulated protein that has been precisely localized in the cell wall.