A proline-, threonine-, and glycine-rich protein down-regulated by droughtis localized in the cell wall of xylem elements

A cDNA clone encoding a proline-, threonine-, and glycine-rich protein (PTG RP) was isolated from a wild tomato species (Lycopersicon chilense) (L.X. Y u, H. Chamberland, J.G. Lafontain, Z. Tabaeizadeh [1996] Genome 39: 1185-11 93). Northern-blot analysis and in situ hybridization studies revealed that PTGRP is downregulated by drought stress. The level of the mRNA in leaves and stems of 8-d drought-stressed plants decreased 5- to 10-fold compared w ith that in regularly watered plants. The mRNA reaccumulated when drought-s tressed plants were rewatered. Antibodies raised against a glutathione S-tr ansferase/PTGRP fusion protein were used to elucidate the subcellular local ization of the protein by immunogold labeling. In regularly watered L. chil ense plants, PTGRP protein was found to be localized in xylem pit membranes and disintegrated primary walls. Examination of sections from drought-stre ssed plants revealed a significant decrease in the levels of labeling. In t hese 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 follo wing 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-reg ulated protein that has been precisely localized in the cell wall.