Viruses Reshape Their Host Cellular Machinery for Their Own Needs
By: Shrish Tariq
Tags: Plant Pathology, Science, Plant Virus, Agriculture, Phytopathgens

The infection of a plant by pathogens leads to severe losses in the form of yield and production. These microbial pathogens are of various kinds. Some are multicellular like fungi, nematodes, unicellular organisms like bacteria, and also non-cellular viruses. Among all phytopathogens, plant viruses are the most important plant pathogens that convert host cellular components into proviral factors. And then use these components for their RNA or DNA synthesis.

Viruses are obligate, intracellular organisms consist of DNA or RNA as their genome and that can only be synthesized inside host cells. In plants, the access to host cell cytoplasm is only accomplished through insect vectors (mainly characterized are whiteflies, leafhoppers, aphids, etc) or wounds. But in animals, the cell cytoplasm is accessed through natural openings. After entry into the host cell cytoplasm, the virus initiates the manipulation of host factors to complete its infection cycle which is consist of translation, multiplication of the genome of the virus, cell to cell movement, and long-distance transport of virion (complete virus particle consist of a protein coat and DNA or RNA as the genome) through vascular bundles (xylem and phloem).

A representation of the infection cycle of the soybean mosaic virus:

For the completion of their infection cycle, the virus genome appoints the host proteins or factors, and then remodel their functions to assist in every step of the virus life cycle. For example, positive-sense RNA viruses manipulate the endomembrane system of the host for their replication and intercellular movement.

The endoplasmic reticulum (ER), Golgi apparatus, trans-Golgi network (TGN), and vacuole constitute the endomembrane system. The endoplasmic reticulum is the largest membranous network in eukaryotic cells and is present throughout the cell cytoplasm. This network carries out the synthesis and modifications of proteins, lipids, and all kinds of polysaccharides. On the other hand, the Golgi network is also present everywhere in the cytoplasm and functions as the transporter of proteins and lipids to their main destination that is received from ER.

Numerous research studies on the tobacco mosaic virus showed that how viruses skilfully hijack the endomembrane system of plants. During early infection stages, the tobacco mosaic virus causes severe alterations in the endoplasmic reticulum structure and converting it into an irregular shape and also induced their proliferation. Later on, these modified structures are used for viral RNA synthesis (vRNA) plus intracellular movement and termed viral factories or virsome, viroplasm, virus replication complex (VRC), etc.

Similarly, the cowpea mosaic virus (CPMV)induces the proliferation of ER near the cell nucleus, and potato virus X (PVX) also forms modified ER structures known as X bodies that are used for replication. And for intracellular transport, these biotroph parasites use Golgi bodies (transport proteins to their final destinations that may be a plasma membrane or apoplast, or to where they needed) that are connected with the endoplasmic reticulum.

After finishing their final assembly of the virion particles, they must travel to the plasmodesmata (PD)to infect neighboring cells to travel the whole plant system for systemic infections. Therefore, to access the neighboring cells, viruses must encode a protein, dedicated to their movement and is called movement protein.

The movement of virion particles across cells is completely facilitated by MP that also encourages viral systemic infection. The infection in plants appears as the visible alterations or symptoms mostly on foliage.

The foliage symptoms are the consequence of the hijacking of the endomembranous system.