Plants undergoing the onslaught of wound-causing agents activate mechanisms directed to healing and further defence. Responses to mechanical damage are either local or systemic or both and hence involve the generation, translocation, perception, and transduction of wound signals to activate the expression of wound-inducible genes. Although the central role for jasmonic acid in plant responses to wounding is well established, other compounds, including the oligopeptide systemin, oligosaccharides, and other phytohormones such as abscisic acid and ethylene, as well as physical factors such as hydraulic pressure or electrical pulses, have also been proposed to play a role in wound signalling. Different jasmonic acid-dependent and -independent wound signal transduction pathways have been identified recently and partially characterized. Components of these signalling pathways are mostly similar to those implicated in other signalling cascades in eukaryotes, and include reversible protein phosphorylation steps, calcium/calmodulin-regulated events, and production of active oxygen species. Indeed, some of these components involved in transducing wound signals also function in signalling other plant defence responses, suggesting that cross-talk events may regulate temporal and spatial activation of different defences.

Plant growth and development depends on the activity of a continuously replenished pool of stem cells within the shoot apical meristem to supply cells for organogenesis. In Arabidopsis, the stem cell-specific protein CLAVATA3 (CLV3) acts cell nonautonomously to restrict the size of the stem cell population, but the hypothesis that CLV3 acts as an extracellular signaling molecule has not been tested. We used genetic and immunological assays to show that CLV3 localizes to the apoplast and that export to the extracellular space is required for its function in activating the CLV1/ CLV2 receptor complex. Apoplastic localization allows CLV3 to signal from the stem cell population to the organizing center in the underlying cells. INTRODUCTIONPlants, unlike animals, continuously produce organs from their growing tips, which are called apical meristems. In the shoot apical meristem (SAM), which generates all of the aboveground structures of the plant, a few cells at the apex are maintained as a pluripotent stem cell population. As the stem cells divide, their progeny are displaced toward the flanks of the meristem, where they become incorporated into organ primordia. The maintenance of a functional SAM requires precise coordination between the loss of stem cells from the meristem through organogenesis and their replacement through cell division.The Arabidopsis CLAVATA1 ( CLV1 ), CLV2 , and CLV3 genes are required to restrict the amount of stem cell accumulation in both shoot and floral meristems. Plants with loss-of-function mutations in any of the CLV genes form greatly enlarged shoot and floral meristems, causing stem overgrowth and the production of extra flowers and floral organs (Clark et al., 1993(Clark et al., , 1995Kayes and Clark, 1998). The CLV1 gene encodes a Leu-rich repeat receptor Ser/Thr kinase (Clark et al., 1997), a member of a large class of proteins found in both plants and animals, many of which are involved in cell signaling. CLV2 encodes a Leu-rich repeat receptor-like transmembrane protein with a short cytoplasmic tail (Jeong et al., 1999). Together, CLV1 and CLV2 form components of a membrane-bound receptor signal transduction complex (Trotochaud et al., 2000). CLV3 encodes a 96-amino acid predicted extracellular protein (Fletcher et al., 1999) that can act in a cell nonautonomous manner (Fletcher et al., 1999;Brand et al., 2000). CLV3 binds to CLV1 and CLV2 and is required for the formation of the active receptor complex, indicating that CLV3 acts as a ligand that signals through CLV1 and CLV2 (Trotochaud et al., 2000).CLV3 and CLV1 expression is restricted to subsets of shoot and floral meristem cells. CLV3 mRNA is expressed in stem cells, which are found primarily in the epidermal and subepidermal cell layers of shoot and floral meristems (Fletcher et al., 1999). CLV1 is expressed in the underlying cells, in a domain partially overlapping that of CLV3 (Clark et al., 1997). It has been hypothesized that CLV3 is made in the overlying cell layers and moves to the underlying cells to activate the C...

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.

SummaryPlants react to mechanical damage by activating a set of genes, the products of which are thought to serve defensive functions. In solanaceous plants, cell wallderived oligosaccharides and the plant hormones jasmonic acid and ethylene participate in the signalling network for wound-induced expression of proteinase inhibitors and other defence-related genes, both in the locally damaged and in the systemic non-damaged leaves. Here we show that in Arabidopsis thaliana, these signalling components interact in novel ways to activate distinct responses. In damaged tissues, oligosaccharides induce the expression of a speci®c set of woundresponsive genes while repressing jasmonic acidresponsive genes that are activated in the systemic tissues. The oligosaccharide-mediated repression of the jasmonic acid-dependent signalling pathway is exerted through the production and perception of ethylene in the locally damaged tissue. This cross-talk between separate wound signalling pathways thus allows the set up of different responses in the damaged and the systemic tissues of plants reacting to injury.