Potato mop-top virus (PMTV) causes spraing (unsightly brown arcs and rings in tubers of susceptible cultivars) and yellow chevrons or shortened internodes (mopping) in the leaves and stems of plants grown from infected tubers. Economic losses are due to poor tuber quality particularly in salad potato crops and PMTV is prohibited in seed potatoes exported to some countries.

It is transmitted in nature by a soil-borne plasmodiophorid  (Spongospora subterranea) that itself causes the disease powdery scab on tubers. Both diseases are prevalent in cool and damp conditions, and are of particular importance to the Scottish potato (and seed potato) harvest. It is therefore highly relevant that research into this disease is carried out at SCRI with financial support from the Scottish Government.

The potato mop-top virus (PMTV) genome consists of three RNAs that encode proteins used to effect the main viral functions of replication, encapsidation, movement and transmission. PMTV used a conserved modular set of three proteins known as triple gene block proteins (TGB) to allow movement of the genome through the plant, a feature common to viruses across eight different genera. PMTV, similar to other hordei-like viruses does not require coat protein for cell-cell movement. The three TGB movement proteins act in a coordinated manner, and all are required for cell-to-cell and systemic movement of the virus genome.

Current research suggests that the largest of the three TGB proteins, TGB1, interacts with viral RNA, forming a ribonucleoprotein (vRNP) movement complex that moves between cells. The TGB1-vRNP complex requires the integral membrane proteins TGB2 and TGB3 for localisation to the plasma membrane and plasmodesmata. Our previous research has shown that TGB2 and TGB3 act together to deliver the vRNP complex to PD by a vesicle-mediated transport process.

Studies using fluorescent protein-tagged TGB2 and TGB3 have revealed that they co-localise in cellular membranes and mobile granules, and utilise the actin-ER network to facilitate movement to the cell periphery and PD. TGB3 contains a PD-targeting signal and TGB2 contains a vesicle-targeting signal that enables them to associate with components of the endocytic pathway (Haupt et al., 2005). Although TGB2 and TGB3 can both increase the size exclusion limit of PD, there is no evidence that either of them are capable of independently trafficking between cells (Haupt et al., 2005).

Furthermore, when a fusion of TGB2 to monomeric red fluorescent protein (mRFP) was expressed independently in cells, it co-localised with Ara7 (AtRabF2b), a Rab GTPase involved in endosome trafficking. TGB2 is also known to interact in vitro with a DnaJ-like plant chaperone belonging to the receptor-mediated endocytosis family RME-8 (Haupt et al., 2005).

Some of the current and ongoing research on PMTV at SCRI is focused on investigating the association with the endocytic pathway, with a particular emphasis on the role of TGB2 in the movement process. Recent work at SCRI on the related Hordeivirus, barley stripe mosaic virus (BSMV; Torrance et al., 2006) has shown that replication is associated with cytoplasmic invaginations in chloroplasts, and that the BSMV TGB2 movement protein localised to chloroplast inclusions in the presence of viral RNA, providing evidence that these proteins may play a role at the sites of virus replication, possibly in anchoring or assembly of the replication complexes. We have recently discovered that PMTV TGB2  co-localises with a number of subcellular structures suggesting that PMTV TGB2 too may play additional roles.