Mutations in Viral Movement Protein Alter Systemic Infection and Identify an Intercellular Barrier to Entry into the Phloem Long-Distance Transport System

Abstract

Viral systemic infection of a plant host involves two processes, cell-to-cell movement and long-distance transport. Molecular determinants associated with these two processes were probed by investigating the effects that alanine scanning mutations in the movement protein (MP) of red clover necrotic mosaic virus (RCNMV) had on viral infection in the plant hosts Nicotiana edwardsonii, Vigna unguiculata (cowpea), and the experimental plant Nicotiana benthamiana. Plants were inoculated with RCNMV expressing wild-type and mutant forms of the MP. Immunocytochemical studies at the light and electron microscope levels were performed on these plants, using a polyclonal antibody raised against the RCNMV capsid protein to identify the cells/tissues that RCNMV could infect. These experiments demonstrated that one cellular boundary at which the RCNMV MP functions to facilitate entry into the phloem long-distance transport system is located at the interfaces between the bundle sheath and phloem parenchyma cells and the companion cell-sieve element complex. Interestingly, in Nicotiana tabacum, a host that only allows a local infection, RCNMV cell-to-cell movement was found to be blocked at this same intercellular boundary. Four mutants that were able to systemically infect N. benthamiana were partially or completely defective for systemic infection of N. edwardsonii and cowpea, which indicated that these MP mutants exhibited host-specific defects. Thus, the roles of the RCNMV MP in cell-to-cell movement and in long-distance transport appear to be genetically distinct. These results are discussed in terms of the mechanism by which RCN MV enters the phloem to establish a systemic infection.

Developmental regulation and significance of KNOX protein trafficking inArabidopsis

“…Previous studies underline the importance that viral MPs play in long distance viral movement and infection (Deom et al, 1994;Wang et al, 1998;Itaya et al, 2002). Since we saw a high level of GFP fluorescence from the GFP~TVCV MP fusion in mesophyll and vascular tissues of the shoot, we investigated how far GFP~TVCV MP could traffic in the plant, by imaging roots of pAtML1-GFP~TVCV MP plants.…”

“…Many studies on trafficking of macromolecules relate to virus infection, as some plant viruses move from cell to cell through plasmodesmata (PD). However, the smallest viruses or viral nucleic acids are estimated to be larger than the channel size of PDs (Ding et al, 1992b), and viruses traffic via an active pathway that requires virus encoded movement proteins (MPs) (Gibbs, 1976;Wang et al, 1998). For example, tobacco mosaic virus (TMV) encodes a 30 kDa MP that interacts with PDs to increase their size exclusion limit (SEL), traffics itself and facilitates spread of the virus (Deom et al, 1987;Wolf et al, 1989;Waigmann et al, 1994).…”

Plasmodesma-Mediated Selective Protein Traffic between “Symplasmically Isolated” Cells Probed by a Viral Movement Protein

“…The traffic of 3a MP:GFP between SE and CC and then out of the SE-CC complex into surrounding tissues suggests that 3a MP also is involved in viral exit out of the SE-CC complex. Some viruses are restricted to the phloem (Sanger et al, 1994;Wang et al, 1996;Ghoshroy et al, 1998), whereas others cannot enter the phloem (Ding et al, 1998;Wang et al, 1998). It will be revealing to test MPs from these viruses for their ability to traffic through PD at the boundary between the SE-CC complex and neighboring cells.…”

Analysis of protein transport in theBrassica oleraceavasculature reveals protein-specific destinations

“…[23][24][25] Furthermore, the insect infecting flock house virus can spread systemically in plants if aided by a plant viral movement protein. 25 These data suggest that there is a mechanism in plants that specifically regulates viral proteins in the phloem.…”