Many soluble plant vacuolar proteins are sorted away from secreted proteins into small vesicles at the trans-Golgi network by transmembrane cargo receptors. Cleavable vacuolar sorting signals include the NH2-terminal propeptide (NTPP) present in sweet potato sporamin (Spo) and the COOH-terminal propeptide (CTPP) present in barley lectin (BL). These two proteins have been found to be transported by different mechanisms to the vacuole. We examined the ability of the vacuolar cargo receptor AtELP to interact with the sorting signals of heterologous and endogenous plant vacuolar proteins in mediating vacuolar transport in Arabidopsis thaliana. AtELP extracted from microsomes was found to interact with the NTPPs of barley aleurain and Spo, but not with the CTPPs of BL or tobacco chitinase, in a pH-dependent and sequence-specific manner. In addition, EM studies revealed the colocalization of AtELP with NTPP-Spo at the Golgi apparatus, but not with BL-CTPP in roots of transgenic Arabidopsis plants. Further, we found that AtELP interacts in a similar manner with the NTPP of the endogenous vacuolar protein AtALEU (Arabidopsis thaliana Aleu), a protein highly homologous to barley aleurain. We hypothesize that AtELP functions as a vacuolar sorting receptor involved in the targeting of NTPP-, but not CTPP-containing proteins in Arabidopsis.

Plants respond to various stresses by expressing distinct sets of genes. The effects of multiple stresses on plants and their interactions are not well understood. We have discovered that salt stress causes the accumulation of proteinase inhibitors and the activation of other wound-related genes in tomato (Lycopersicon esculentum) plants. Salt stress was also found to enhance the plant's response to wounding locally and systemically. The tomato mutant (def-1), which has an impairment in the octadecanoid pathway, displayed a severe reduction in the accumulation of proteinase inhibitors under salt stress, indicating that salt stress-induced accumulation of proteinase inhibitors was jasmonic acid dependent. The analysis of salt stress in another tomato mutant, spr-1, which carries a mutation in a systemin-specific signaling component, and transgenic tomato plants that express an antisense-prosystemin cDNA, showed that prosystemin activity was not required for the salt-induced accumulation of proteinase inhibitors, but was necessary to achieve maximal levels. These results suggest that a prosystemin independent-but jasmonic acid-dependent pathway is utilized for proteinase inhibitor accumulation in response to salt stress.Plants in the field are exposed to multiple stresses, and their response to these various stresses determines their capacity to survive. Plants can use multiple signaling pathways and signals to mediate their response; for example, at least four different signaling pathways have been identified for water-deficit stress (Shinozaki and Yamaguchi-Shinozaki, 1997; Xiong et al., 2002). Different forms of stress may activate or utilize the same components, including proteins and other signaling molecules. Transgenic tobacco (Nicotiana tabacum) plants expressing constitutively active NPK1, a mitogen-activated protein kinase kinase kinase (MAPKKK) showed an enhanced tolerance to cold, heat shock, and salt stress (Kovtun et al., 2000). Signaling molecules such as jasmonic acid (JA) are involved in multiple stress responses and development in plants (Creelman and Mullet, 1995, 1997; Turner et al., 2002). However, it is the specific combination of various components of the signaling network coupled with spatial and temporal factors that allows the plant to mount a directed response to any given stress factor.The phenomenon of cross-tolerance where a plant's resistance to one stress resulted in resistance to another form of stress is a growing area of research (Genoud and Metraux, 1999; Bowler and Fluhr, 2000).Increased tolerances to multiple stresses can involve the interaction between abiotic and biotic stresses. For example, plants pretreated with a sub-lethal dose of ozone or UV irradiation exhibited tolerance to a virulent pathogen (Yalpani et al., 1994; Sharma et al., 1996). Inoculation of Arabidopsis with a rhizobacterium made the plants more tolerant to drought and the pathogen Erwinia carotovora (Timmusk and Wagner, 1999). It has been shown that drought and UV radiation act synergistically to induc...

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We have previously demonstrated that the carboxyl-terminal propeptide of barley lectin is both necessary and sufficient for protein sorting to the plant vacuole. Specific mutations were constructed to determine which amino acid residues or secondary structural determinants of the carboxyl-terminal propeptide affect proper protein sorting. We have found that no consensus sequence or common structural determinants are required for proper sorting of barley lectin to the vacuole. However, our analysis demonstrated the importance of hydrophobic residues in vacuolar targeting. In addition, at least three exposed amino acid residues are necessary for efficient sorting. Sorting was disrupted by the addition of two glycine residues at the carboxyl-terminal end of the targeting signal or by the translocation of the glycan to the carboxy terminus of the propeptide. These results suggest that some components of the sorting apparatus interact with the carboxy terminus of the propeptide.