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.

The Arabidopsis genome contains a family of v-SNAREs: VTI11 , VTI12 , and VTI13 . Only VTI11 and VTI12 are expressed at appreciable levels. Although these two proteins are 60% identical, they complement different transport pathways when expressed in the yeast vti1 mutant. VTI11 was identified recently as the mutated gene in the shoot gravitropic mutant zig . Here, we show that the vti11 zig mutant has defects in vascular patterning and auxin transport. An Arabidopsis T-DNA insertion mutant, vti12 , had a normal phenotype under nutrient-rich growth conditions. However, under nutrient-poor conditions, vti12 showed an accelerated senescence phenotype, suggesting that VTI12 may play a role in the plant autophagy pathway. VTI11 and VTI12 also were able to substitute for each other in their respective SNARE complexes, and a doublemutant cross between zig and vti12 was embryo lethal. These results suggest that some VTI1 protein was necessary for plant viability and that the two proteins were partially functionally redundant.

The Sec1p family of proteins are thought to be involved in the regulation of vesicle fusion reactions through interaction with t-SNAREs (target soluble N-ethylmaleimide-sensitive factor attachment protein receptors) at the target membrane. AtVPS45 is a member of this family from Arabidopsis thaliana that we now demonstrate to be present on the trans-Golgi network (TGN), where it colocalizes with the vacuolar cargo receptor AtELP. Unlike yeast Vps45p, AtVPS45 does not interact with, or colocalize with, the prevacuolar t-SNARE AtPEP12. Instead, AtVPS45 interacts with two t-SNAREs, AtTLG2a and AtTLG2b, that show similarity to the yeast t-SNARE Tlg2p. AtTLG2a and -b each colocalize with AtVPS45 at the TGN; however, AtTLG2a is in a different region of the TGN than AtTLG2b by immunogold electron microscopy. Therefore, we propose that complexes containing AtVPS45 and either AtTLG2a or -b define functional subdomains of the TGN and may be required for different trafficking events. Among other Arabidopsis SNAREs, AtVPS45 antibodies preferentially coprecipitate AtVTI1b over the closely related isoform AtVTI1a, implying that AtVTI1a and AtVTI1b also have distinct functions within the cell. These data point to a functional complexity within the plant secretory pathway, where proteins encoded by gene families have specialized functions, rather than functional redundancy.