Arabidopsis Exocyst Subcomplex Containing Subunit EXO70B1 Is Involved in Autophagy‐Related Transport to the Vacuole

Abstract

Autophagic transport to the vacuole represents an endomembrane trafficking route, which is widely used in plants, not only during stress situations, but also for vacuole biogenesis and during developmental processes. Here we report a role in autophagic membrane transport for EXO70B1 -one of 23 paralogs of Arabidopsis EXO70 exocyst subunits. EXO70B1 positive compartments are internalized into the central vacuole and co-localize with autophagosomal marker ATG8f. This internalization is boosted by induction of autophagy. Loss of function (LOF) mutations in exo70B1 cause reduction of internalized autopagic bodies in the vacuole. Mutant plants also show ectopic hypersensitive response (HR) mediated by salicylic acid (SA) accumulation, increased nitrogen starvation susceptibility and anthocyanin accumulation defects. Anthocyanin accumulation defect persists in npr1x exo70B1 double mutants with SA signaling compromised, while ectopic HR is suppressed. EXO70B1 interacts with SEC5 and EXO84 and forms an exocyst subcomplex involved in autophagy-related, Golgiindependent membrane traffic to the vacuole. We show that EXO70B1 is functionally completely different from EXO70A1 exocyst subunit and adopted a specific role in autophagic transport.

Endosidin2 targets conserved exocyst complex subunit EXO70 to inhibit exocytosis

“…As an important component of the exocyst complex that mediates exocytosis, EXO70 regulates, for example, neurite outgrowth, epithelial cell polarity establishment, cell motility, and cell morphogenesis in animal cells (2)(3)(4)(5)(6). In plants, EXO70 proteins participate in polarized pollen tube growth, root hair growth, deposition of cell wall material, cell plate initiation and maturation, defense, and autophagy (7)(8)(9)(10)(11)(12). In humans, EXO70 mediates the trafficking of the glucose transporter Glut4 to the plasma membrane that is stimulated by insulin and involved in the development of diabetes (13).…”

RNA silencing of exocyst genes in the stigma impairs the acceptance of compatible pollen in Arabidopsis

“…Essential roles for other exocyst subunits include Arabidopsis SEC5a/SEC5b, SEC6, SEC8, and SEC15a/SEC15b in male gametophyte development and pollen tube growth (Cole et al, 2005;Hála et al, 2008;Wu et al, 2013), SEC8 in seed coat deposition (Kulich et al, 2010), SEC5a, SEC8, EXO70A1, and EXO84b in root meristem size and root cell elongation (Cole et al, 2014), and a maize (Zea mays) SEC3 homolog in root hair elongation (Wen et al, 2005). Finally, the Arabidopsis EXO70B1, EXO70B2, and EXO70H1 subunits have been implicated in plant defense responses (Pecenková et al, 2011;Stegmann et al, 2012;Kulich et al, 2013;Stegmann et al, 2013).…”

EXPO and Autophagosomes are Distinct Organelles in Plants

“…Indeed, whereas an Exo70A1-type exocyst is required for conventional secretory processes (Hála et al, 2008;Zárský et al, 2009), an Exo70E2-type exocyst appears to be characteristic for unconventional protein secretion (Wang et al, 2010;Ding et al, 2012Ding et al, , 2014aDing et al, , 2014b. Furthermore, Exo70B1 seems to be specifically needed for the fusion of autophagosomes with the vacuole (Kulich et al, 2013).…”

“…Unlike autophagosomes, which ultimately fuse with the lytic compartment of the cell (lysosome or vacuole), EXPO deliver the luminal contents into the apoplast first by fusion of the outer membrane with the PM, then by degradation of the inner boundary membrane (Wang et al, 2010;Ding et al, 2012). The authenticity of EXPO has been challenged by Kulich et al (2013), who have claimed that "overexpression of exocyst subunits itself may cause stimulation of biogenesis of new exocyst positive compartments that are normally absent." Although it cannot be ruled out that overexpression of Exo70E2 may result in an increase in EXPO numbers, indeed overexpression of Arabidopsis Exo70E2 (AtExo70E2) in mammalian cells causes EXPO-like structures to be formed (Ding et al, 2014b), and similar EXPO densities are obtained in Arabidopsis protoplasts when the expression is driven by the Exo70E2 native promoter (Ding et al, 2014b).…”