Tombusviruses Target a Major Crossroad in the Endocytic and Recycling Pathways via Co-opting Rab7 Small GTPase

RAB7 GTPases as coordinators of plant endomembrane traffic

The ras gene from rat brain (RAB) family of small GTPases is highly conserved among eukaryotes and regulates endomembrane trafficking pathways. RAB7, in particular, has been linked to various processes involved in regulating endocytic and autophagic pathways. Plants have several copies of RAB7 proteins that reflect the intricacy of their endomembrane transport systems. RAB7 activity regulates different pathways of endomembrane trafficking in plants: (1) endocytic traffic to the vacuole; (2) biosynthetic traffic to the vacuole; and (3) recycling from the late endosome to the secretory pathway. During certain developmental and stress related processes another pathway becomes activated (4) autophagic trafficking towards the vacuole that is also regulated by RAB7. RAB7s carry out these functions by interacting with various effector proteins. Current research reveals many unexplored RAB7 functions in connection with stress responses. Thus, this review describes a comprehensive summary of current knowledge of plant RAB7's functions, discusses unresolved challenges, and recommends prospective future research directions.

A sword or a buffet: plant endomembrane system in viral infections

The plant endomembrane system is an elaborate collection of membrane-bound compartments that perform distinct tasks in plant growth and development, and in responses to abiotic and biotic stresses. Most plant viruses are positive-strand RNA viruses that remodel the host endomembrane system to establish intricate replication compartments. Their fundamental role is to create optimal conditions for viral replication, and to protect replication complexes and the cell-to-cell movement machinery from host defenses. In addition to the intracellular antiviral defense, represented mainly by RNA interference and effector-triggered immunity, recent findings indicate that plant antiviral immunity also includes membrane-localized receptor-like kinases that detect viral molecular patterns and trigger immune responses, which are similar to those observed for bacterial and fungal pathogens. Another recently identified part of plant antiviral defenses is executed by selective autophagy that mediates a specific degradation of viral proteins, resulting in an infection arrest. In a perpetual tug-of-war, certain host autophagy components may be exploited by viral proteins to support or protect an effective viral replication. In this review, we present recent advances in the understanding of the molecular interplay between viral components and plant endomembrane-associated pathways.

Manipulation of the Cellular Membrane-Cytoskeleton Network for RNA Virus Replication and Movement in Plants

Viruses infect all cellular life forms and cause various diseases and significant economic losses worldwide. The majority of viruses are positive-sense RNA viruses. A common feature of infection by diverse RNA viruses is to induce the formation of altered membrane structures in infected host cells. Indeed, upon entry into host cells, plant-infecting RNA viruses target preferred organelles of the cellular endomembrane system and remodel organellar membranes to form organelle-like structures for virus genome replication, termed as the viral replication organelle (VRO) or the viral replication complex (VRC). Different viruses may recruit different host factors for membrane modifications. These membrane-enclosed virus-induced replication factories provide an optimum, protective microenvironment to concentrate viral and host components for robust viral replication. Although different viruses prefer specific organelles to build VROs, at least some of them have the ability to exploit alternative organellar membranes for replication. Besides being responsible for viral replication, VROs of some viruses can be mobile to reach plasmodesmata (PD) via the endomembrane system, as well as the cytoskeleton machinery. Viral movement protein (MP) and/or MP-associated viral movement complexes also exploit the endomembrane-cytoskeleton network for trafficking to PD where progeny viruses pass through the cell-wall barrier to enter neighboring cells.

Co-opted membranes, lipids, and host proteins: what have we learned from tombusviruses?

Positive-strand RNA viruses replicate in intracellular membranous structures formed after virus-driven intensive manipulation of subcellular organelles and membranes. These unique structures are called viral-replication organelles (VROs). To build VROs, the replication proteins coded by (+)RNA viruses co-opt host proteins, including membrane-shaping, lipid synthesis, and lipid-modification enzymes to create an optimal microenvironment that (i) concentrates the viral replicase and associated host proteins and the viral RNAs; (ii) regulates enzymatic activities and spatiotemporally the replication process; and (iii) protects the viral RNAs from recognition and degradation by the host innate immune defense. Tomato bushy stunt virus (TBSV), a plant (+)RNA virus, serves as an advanced model to study the interplay among viral components, co-opted host proteins, lipids, and membranes. This review presents our current understanding of the complex interaction between TBSV and host with panviral implications.