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 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 with that in regularly watered plants. The mRNA reaccumulated 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 droughtregulated protein that has been precisely localized in the cell wall.

An emerging family of bcl-2-like genes has been identified from nematode to humans. These genes play a role in the maintenance of homeostasis. Its members have highly conserved domains that are important for their dimerization. Since nothing is known about the importance of these genes in plant cells, we have investigated their presence in an alga as well as in three higher plants both by Western analysis and by immunocytochemistry. Immunoblots revealed the presence of a protein immunoreacting with the anti-bcl-2 polyclonal antibody in leaves of tobacco plants. Furthermore, immunocytochemical localization has shown that this protein is mainly associated with mitochondria, plastids, and nuclei of plant cells. Taken together, our results suggest that bcl-2 is a protein highly conserved throughout evolution.