Figure 5.

ROS-induced formation of membrane protrusion, intercellular and interorganelle bridge. (A) At physiologically higher ROS levels, TNTs are formed between cells via cytoskeleton-based (e.g., actin, myosin) membrane protrusion. ROS-induced diversified pathways including p38 MAPK, PI3k-Akt-mTOR signaling, ERK1/2 signaling, Rho GTPases family, and M-Sec-RalA-Exocyst complex, are shown to promote membrane protrusions mainly via Arp2/3-mediated actin cytoskeletal remodeling. The cooperative association between myosin and actin that is activated by ERK signaling plays an important role in mobilizing various related components and mediating membrane-cytoskeleton coordination. (B) In plant cells, the membrane-encircled organelles, e.g., the chloroplast and peroxisome, can form inter-organellar bridge between the same type of organelle (the stromule and peroxule) and different types of organelles under higher ROS condition. Repression of NTRC leads to increased stromules. The signaling mechanism is currently unknown. Arrows may not denote direct activation. Several key nodes in the signaling networks are colored in red, and Arp1/2 complex, Exocyst complex, NOX, ERK, p38 MAPK, Rac and Rho are also encoded in plant genomes. p38 MAPK, p38 mitogen-activated protein kinase; Pkc, protein Kinase C; PI3k, phosphatidylinositol-3-kinase; AKT, protein kinase B; mTOR, mammalian target of rapamycin; M-Sec, Myeloid and M cells-expressing Sec6 homolog, also known as TNF alpha-induced protein 2, or Primary response gene B94; RalA, Ras-related protein Ral-A; RalBP1, RalA-binding protein 1; Cdc42, Cell division control protein 42 homolog; WASP, the Wiskott–Aldrich Syndrome protein; ERK1/2, extracellular signal-regulated kinase 1; Exocyst, an octameric complex; RSK, ribosomal S6 kinase; SH3P2, Src homology-3 (SH3) domain-containing protein 2; Arp2/3, actin-related complex 2/3; WRC, WAVE2 Regulatory Complex; NOX, NADPH oxidase; WAVE, WASP family verprolin-homologous protein; NTRC, NADP-thioredoxin reductase C.