The dynamic trans-Golgi network/early endosome (TGN/EE) facilitates cargo sorting and trafficking and plays a vital role in plant development and environmental response. Transport protein particles (TRAPPs) are multi-protein complexes acting as guanine nucleotide exchange factors and possibly as tethers, regulating intracellular trafficking. TRAPPs are essential in all eukaryotic cells and are implicated in a number of human diseases. It has been proposed that they also play crucial roles in plants; however, our current knowledge about the structure and function of plant TRAPPs is very limited. Here, we identified and characterized AtTRAPPC11/RESPONSE TO OLIGOGALACTURONIDE2 (AtTRAPPC11/ROG2), a TGN/EE-associated, evolutionarily conserved TRAPP protein in Arabidopsis (Arabidopsis thaliana). AtTRAPPC11/ROG2 regulates TGN integrity, as evidenced by altered TGN/EE association of several residents, including SYNTAXIN OF PLANTS61, and altered vesicle morphology in attrappc11/rog2 mutants. Furthermore, endocytic traffic and brefeldin A body formation are perturbed in attrappc11/rog2, suggesting a role for AtTRAPPC11/ROG2 in regulation of endosomal function. Proteomic analysis showed that AtTRAPPC11/ROG2 defines a hitherto uncharacterized TRAPPIII complex in plants. In addition, attrappc11/rog2 mutants are hypersensitive to salinity, indicating an undescribed role of TRAPPs in stress responses. Overall, our study illustrates the plasticity of the endomembrane system through TRAPP protein functions and opens new avenues to explore this dynamic network.
The cell wall, a crucial cell compartment, is composed of a network of polysaccharides and proteins, providing structural support and protection from external stimuli. While the cell wall structure and biosynthesis have been extensively studied, very little is known about the transport of polysaccharides and other components into the developing cell wall. This review focuses on endomembrane trafficking pathways involved in cell wall deposition. Cellulose synthase complexes are assembled in the Golgi, and are transported in vesicles to the plasma membrane. Non-cellulosic polysaccharides are synthesized in the Golgi apparatus, whereas cellulose is produced by enzyme complexes at the plasma membrane. Polysaccharides and enzymes that are involved in cell wall modification and assembly are transported by distinct vesicle types to their destinations; however, the precise mechanisms involved in selection, sorting and delivery remain to be identified. The endomembrane system orchestrates the delivery of Golgi-derived and possibly endocytic vesicles carrying cell wall and cell membrane components to the newly-formed cell plate. However, the nature of these vesicles, their membrane compositions, and the timing of their delivery are largely unknown. Emerging technologies such as chemical genomics and proteomics are promising avenues to gain insight into the trafficking of cell wall components.
Cellulose synthase complexes (CSCs) at the plasma membrane (PM) are aligned with cortical microtubules (MTs) and direct the biosynthesis of cellulose. The mechanism of the interaction between CSCs and MTs, and the cellular determinants that control the delivery of CSCs at the PM, are not yet well understood. We identified a unique small molecule, CESA TRAFFICKING INHIBITOR (CESTRIN), which reduces cellulose content and alters the anisotropic growth of Arabidopsis (Arabidopsis thaliana) hypocotyls. We monitored the distribution and mobility of fluorescently labeled cellulose synthases (CESAs) in live Arabidopsis cells under chemical exposure to characterize their subcellular effects. CESTRIN reduces the velocity of PM CSCs and causes their accumulation in the cell cortex. The CSC-associated proteins KORRIGAN1 (KOR1) and POM2/CELLULOSE SYNTHASE INTERACTIVE PROTEIN1 (CSI1) were differentially affected by CESTRIN treatment, indicating different forms of association with the PM CSCs. KOR1 accumulated in bodies similar to CESA; however, POM2/CSI1 dissociated into the cytoplasm. In addition, MT stability was altered without direct inhibition of MT polymerization, suggesting a feedback mechanism caused by cellulose interference. The selectivity of CESTRIN was assessed using a variety of subcellular markers for which no morphological effect was observed. The association of CESAs with vesicles decorated by the trans-Golgi network-localized protein SYNTAXIN OF PLANTS61 (SYP61) was increased under CESTRIN treatment, implicating SYP61 compartments in CESA trafficking. The properties of CESTRIN compared with known CESA inhibitors afford unique avenues to study and understand the mechanism under which PM-associated CSCs are maintained and interact with MTs and to dissect their trafficking routes in etiolated hypocotyls.
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