On the discovery of an endomembrane compartment in plants

To maintain homeostasis and to react to external stimuli, eukaryotic cells have evolved a complex internal membrane system. Among them, lytic compartments are hallmarks of eukaryotic cells: Animals possess lysosomes, whereas fungi and plants build up vacuoles. Although largely molecularly conserved, the plant endomembrane system displays unique features, among them the presence of large vacuoles. The central vacuole can occupy up to 90% of the cellular volume in mature vegetative tissues and is involved in numerous essential functions. These include degradation of cellular waste, storage of ions and proteins, plant growth, defense against pathogens, and intracellular pH homeostasis. Given the importance of vacuoles for plants, it is not surprising that trafficking to this compartment has been intensively studied. Despite significant progress toward the identification of different trafficking routes, the underlying molecular machinery is far from being understood and is, in part, controversially discussed in the field. In PNAS, Delgadillo et al. (1) elegantly identify and characterize a set of mutants impaired in vacuolar transport. Most identified mutant alleles were known trafficking factors, but subsequent localization analysis proposes the identification of a previously unknown plant compartment.

Soluble vacuolar cargoes require positive sorting mechanisms to reach the vacuole. Vacuolar sorting receptors (VSRs) are responsible for this function, but, although extensively studied (2–6), it is still not entirely clear where they initially bind and subsequently release their cargoes in plants. In yeast, VSRs have been demonstrated to be recycled for further rounds of cargo binding by so-called retromer vesicles (7). The Golgi Associated Retrograde Protein (GARP) complex and golgins function in membrane tethering, which could also contribute to selective cargo retrieval mechanisms at the trans-Golgi network (TGN) (8–12). In plants, retromer vesicles are likely to recycle VSRs as well (13), but to which compartment is heavily debated. Whereas one model proposes the TGN as acceptor compartment for retromer, the other favors the endoplasmic reticulum (ER)/Golgi (14–16).

To shed light on the molecular players, Delgadillo et al. (1) use an elegant genetic screen based on the artificial vacuolar cargo VAC2 (17), which is a powerful tool to identify and analyze components of vacuolar transport (18, 19). If vacuolar trafficking is affected, VAC2 is, by default, secreted to the apoplast, where it terminates the shoot apical meristem in a WUSCHEL-dependent manner (Fig. 1A). This approach allows the screening of a large mutant population, thereby facilitating the identification of vascular trafficking mutants. Overall, the authors present here 17 modified transport to the vacuole (mtv) mutants in Arabidopsis thaliana. Most of the MTV proteins localize to the TGN−multivesicular body (MVB) interface, except two members of the GARP tethering complex, VPS51 and VPS54. Unexpectedly, these proteins are absent from the Golgi, the TGN, and MVBs, but are present at a compartment that the authors term “ER- and microtubule-associated compartment” (EMAC). While some of the findings require further confirmation using stable and functional marker lines, the identification of the EMAC could solve certain controversies in the field of plant cell biology. Both EMAC markers VPS51 and VPS54 colocalize with microtubules and kinesin motor proteins. In addition, VPS51 directly interacts with several Arabidopsis kinesins through their motor domains. Different pharmacological treatments reveal that EMACs are relatively stationary compartments, a feature shared with the TGN in mammalian systems. Here, retromer vesicles are guided along microtubules to their acceptor compartment, the late Golgi/TGN (20). In plants, the microtubule-associated protein CLASP interacts with the retromer subunit sorting nexin 1 (21), and could thereby provide a mechanism to guide retromer vesicles to EMACs where they could be tethered by the GARP complex. In addition, the close association of EMACs with the ER also could hint at the role of the retromer complex in anterograde transport. Recycled VSRs could bind here to newly synthesized soluble cargo destined for the vacuole (Fig. 1B). The proposed plant model for the highly dynamic endomembrane system is well in accordance with the findings that VSRs are capable of binding their ligands already in the ER (22, 23). Thus, in contrast to mammals, retromer-dependent VSR recycling in plants might occur between the TGN (6) and EMACs. In conflict with this, a recent study identified the Golgi as the main acceptor compartment for recycled VSRs and identified the ER only for the binding of newly synthesized receptors (23). However, the narrow interface between Golgi and ER at ER import sites (24), together with the tight ER association of the compartment, may have hampered the depiction of the precise localization based on conventional microscopic assessments. Ultrastructural analysis and confirmation of VSR cargo binding at EMACs could solve the longstanding puzzle of VSR transport and recycling. Moreover, retromer transport along microtubules would substantiate the currently emerging role of these filaments within plant cells (25).