Getting to the Right Side

Unlike most animal cells, plant cells exhibit a minimum of four polar plasma membrane (PM) domains, commonly designated as apical, basal, inner lateral (proximal), and outer lateral (distal; Kania et al., 2014). The outer lateral domain of the root epidermis has received special attention because it provides a filter for selective uptake of nutrients and the extrusion of toxic compounds and signaling molecules and thus was named the “root-soil interface” (Langowski et al., 2010).

In order to deliver polar cargoes during the secretory pathway to the PM, plants and animals employ in principle evolutionary conserved routes and core components. Also, exocytosis, which is mediated by the exocyst complex providing vesicle tethering to the PM, was apparently kept during evolution (Kania et al., 2014). Our advanced understanding of apical-basal polarity in plant cells is mainly based on the investigation of PIN-FORMED (PIN) proteins that are thought to gain polarity by nonpolar secretion and subsequent endocytotic recycling (Kleine-Vehn et al., 2011). In contrast, initial evidence was provided that lateral localization of BOR4, PIS1/PDR9/ABCG37, and PEN3/PDR8/ABCG36 is established at first place by polar secretion and is dependent on the actin but not the microtubule cytoskeleton (Łangowski et al., 2010, 2016).

In this issue, work by Mao et al. (2016) extends our knowledge of how outer lateral PM trafficking and tethering is achieved. By means of a microscopy-based genetic screen for PEN3-GFP mislocalization, the authors identified three novel ACTIN7 (ACT7) loss-of-function alleles that revealed large vesicle agglomerations resembling both trans-Golgi network (TGN) stacks and brefeldin A compartments (Mao et al., 2016). By using photoswitched PEN3-Eos2, it was shown that these “act7 compartments” are rapidly filled with endocytic marker FM4-64 but surrounded by outer lateral cargo proteins, such as PEN3 or the boric acid uptake channel NIP5;1. Interestingly, apical/basal and nonpolar proteins, such as auxin transporters of the PIN and ABCB family, also were recently reported on these compartments (Zhu et al., 2016), suggesting that ACT7 fulfils a generic function during TGN-PM trafficking to all four polar domains. Pharmacological (de)stabilization of the actin cytoskeleton leads to endomembrane agglomeration of PM cargoes, supporting the concept that balanced actin cytoskeleton function, with ACT7 being the major relevant isoform, is required for proper plant development.

Using a candidate approach, the authors further show that the exocyst tethering factor, Exo84b, colocalizes with PEN3 on the outer lateral domain and is required for polarity of outer lateral proteins. Lateral EXO84b-GFP location was dependent on ACT7 but, unlike PEN3, did not accumulate in TGN compartments. In summary, these data support a dual, independent function for ACT7 both in trafficking and tethering of outer lateral PM cargoes.

As usual, new findings raise new questions: One of the issues is if lateral polarity is provided by polar secretion as suggested by Łangowski et al. (2010) or by endocytotic recycling as supported here (Mao et al., 2016). In any case, it seems now to be clear that the main sorting hub for both lateral PEN3 secretion and endocytosis is the TGN, which suggests that both mechanisms might contribute to outer lateral polarity. A related problem is which factors determine lateral polarity of the tethering factor Exo84b itself. Moreover, it is unknown how focal PEN3 accumulation during pathogen infection is achieved.

A last problem concerns the substrate specificity of PEN3, which is currently thought to export defense compounds of the indole glucosinolate class (Lu et al., 2015). However, PEN3 was also shown to act in abiotic stress responses to heavy metals and as an exporter of the indole-3-acetic acid precursor indole-butyric acid (Ruzicka et al., 2010). Currently it is entirely unclear how such multifunctionality on a limited polar domain is regulated.