It is well established in animal systems that compartmentalization of signaling receptors within the endocytic pathway contributes to signaling specificity and regulation (Miaczynska et al. 2004; Fischer et al. 2006). Multiple examples have been reported in vertebrates that demonstrate this concept. Epidermal growth factor receptor (EGFR) is found at the plasma membrane, but when clathrin-mediated endocytosis is impaired, downstream signaling components such as mitogen-activated protein kinases (MAPKs) have reduced activity (Viera et al. 1996). In endosomes, EGFR has been shown to interact with other signaling components (Sorkin et al. 2000). In fact, treatment with a chemical that causes the internalization of inactive (dephosphorylated) ligand-bound receptors, followed by chemical washout, reveals that the endosomal pool of EGFRs is able to promote signaling and a biological response (Pennock and Wang 2003). As important, the plasma membrane and endosomal pools of EGFR appear to be functionally distinct in that the pools show selectivity in their association with other signaling components (Burke et al. 2001). Other examples of active receptor association with endosomes include nerve growth factor association with its receptor TrkA and phospholipase C (Grimes et al. 1996) and the G-protein-coupled 2-adrenergic receptor (Daaka et al. 1998). The TGFreceptor forms heteromeric complexes that undergo endocytosis and phosphorylation. This endosomal complex in turn phosphorylates and activates the transcription factor R-Smad2, which is targeted to the nucleus. A central modulator of endosomal signaling appears to be SARA (Smad anchor for receptor activation) (Tsukazaki et al. 1998), which serves as an adaptor between the TGFcomplex and R-Smad2 (Fig. 1). Endocytosis and endosomal signaling pathways have also been characterized in Drosophila, where they are critical for development via the establishment of morphogen gradients and signaling involving endosomal complexes. In Drosphila, the TGF-like morphogen Decapentaplegic (Dpp) is involved in wing disc formation via gradients originating from secretory cells. Endocytosis, as well as control of extracellular diffusion (via heparin sulfate proteoglycans), contribute to establishment of the intracellular and extracellular components of the gradient, respectively. As with TGFin vertebrates, a SARA-like homolog has been identified (Bennett and Alphey 2002) and several lines of evidence suggest that Dpp may be involved in signaling that is dependent upon endocytosis and endosomes in a TGF-analogous manner (Bennett and Alphey 2002; for review, see Fischer et al. 2006). Although there are as many, if not more, endomembrane trafficking components known in plants, the specific involvement of the endocytic pathway in plant receptor signaling has not been well documented. Potential receptors have also been identified. The largest class of plasma membrane receptors (most of which are orphan) appears to be the receptor-like kinases (RLKs), of which there are >600 in Arabidopsis and at least 1100 in rice (Morillo and Tax 2006). In spite of this, little is known about trafficking of RLKs, and recent examples in plants have largely pointed toward endocytosis as supporting either the recycling of transporters or as a route for protein turnover. The small molecule plant hormone, auxin, is essential for many aspects of plant development and response to environmental cues such as light and the directional growth of roots and stems in response to gravity. The movement of auxin is controlled by a family of plasma membrane efflux transporters known as PIN (PINFORMED), whose major role is to establish concentration gradients of auxin across organs that ultimately respond to the hormone via asymmetric growth (Tanaka et al. 2006; Teale et al. 2006; Kerr and Bennett 2007; Zazimalova et al. 2007). For example, differences in lateral growth rates across organs result in the familiar downward bending of roots toward gravity. In roots, the PIN1 transporter is recycled between endosomes and the plasma membrane, and this mechanism along with protein turnover in the vacuole, is thought to be important for establishing and controlling auxin gradients. Moreover, auxin can block endocytosis of PIN1 providing a feedback mechanism, whereby the morphogen itself can influence transporter abundance at the plasma membrane, and thus, efflux and gradient orientation (Paciorek et al. 2005). Interestingly, it is not known in this case Corresponding author. E-MAIL Natasha.raikhel@ucr.edu; FAX (951) 827-2155. Article is online at http://www.genesdev.org/cgi/doi/10.1101/gad.1577607.