Interaction between cucumber green mottle mosaic virus MP and CP promotes virus systemic infection

The movement protein (MP) and coat protein (CP) of tobamoviruses play critical roles in viral cell-to-cell and long-distance movement, respectively. Cucumber green mottle mosaic virus (CGMMV) is a member of the genus Tobamovirus. The functions of CGMMV MP and CP during viral infection remain largely unclear. Here, we show that CGMMV MP can interact with CP in vivo, and the amino acids at positions 79-128 in MP are vital for the MP-CP interaction. To confirm this finding, we mutated five conserved residues within the residue 79-128 region and six other conserved residues flanking this region, followed by in vivo interaction assays. The results showed that the conserved threonine residue at the position 107 in MP (MPT107) is important for the MP-CP interaction. Substitution of T107 with alanine (MPT107A) delayed CGMMV systemic infection in Nicotiana benthamiana plants, but increased CGMMV local accumulation. Substitutions of another 10 conserved residues, not responsible for the MP-CP interaction, with alanine inhibited or abolished CGMMV systemic infection, suggesting that these 10 conserved residues are possibly required for the MP movement function through a CP-independent manner. Moreover, two movement function-associated point mutants (MPF17A and MPD97A) failed to cause systemic infection in plants without impacting on the MP-CP interaction. Furthermore, we have found that co-expression of CGMMV MP and CP increased CP accumulation independent of the interaction. MP and CP interaction inhibits the salicylic acid-associated defence response at an early infection stage. Taken together, we propose that the suppression of host antiviral defence through the MP-CP interaction facilitates virus systemic infection.