Role of Autophagy in Plant Nutrient Deficiency - Nutrient Use Efficiency in Plants - page 175

Fig. 7.3 Initiation of autophagy and phagophore formation. When nutrients are available, TOR is
active in the complex TORC1 (see text for details) and keeps ATG1 dissociated from ATG13 (and
associated proteins) trough hypo- and hyper-phosphorylation, respectively. During nutrient star-
vation the TORC2 complex is formed and the phosphorylation status reverses, allowing the
complex ATG1-ATG13-ATG17-ATG101 to promote the autophagosome nucleation to the PAS
by C-I and ATG8-PE docking to the phagopore inner surface. The C-I is composed of ATG6,
hypothetically ATG14 and their kinase subunits VPS15 and VPS34. The Atg17 scaffold protein is
a recruiting point for further ATG proteins as ATG9 (in complex with ATG2-ATG18-ATG27),
which mediates membrane delivery contributing to the phagophore elongation. Once the
autophagosome closes it can fuse to the plant vacuole for cargo degradation. Dashed arrows
passing through proteins indicate the proteins, which are governed by the protein present in the tail
of the arrow. TOR Target of rapamycin, PAS phagophore assembly site, C-I phosphatidylinositol
3-kinase complex I, Vps vacuolar protein sorting
conditions, an inactive TOR (the TORC2 complex) includes RICTOR (rapamycin-
insensitive companion of TOR) instead of RAPTORs allowing it to swap the ATG1
and ATG13 phosphorylation status to the active one (autophosphorylation of ATG1
and dephosphorylation of ATG13). This, in turn, promotes the nucleation of the
autophagosome by involvement of a downstream hypothetical (not yet identified)
phosphorylation target. Arabidopsis contains four ATG1 proteins (ATG1a-c and a
truncated ATG1t, present only in plants) and two ATG13 proteins (ATG13a and
ATG13b). The phosphorylation-dependent regulatory mechanism has been con-
firmed for ATG1a and ATG13a (Suttangkakul et al. 2011 ; Li and Vierstra 2012b ).
The activated ATG1-ATG13 kinase complex relocates to the phagophore assembly
site (PAS) and promotes ATG9-mediated membrane delivery to the expanding
phagophore (Fig. 7.3 ). The ATG1-ATG13 kinase complex also docks to ATG8-
PE bound to the inner surface of the phagophore, and is captured and delivered to