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The P6 protein of cauliflower mosaic virus (CaMV) is a multifunctional protein that forms the electron dense, amorphous inclusion bodies that accumulate in the cytoplasm and has been shown to physically interact with all other CaMV proteins, including the CaMV movement protein (P1). In this study, we have investigated the subcellular localization of the P6 and P1 proteins in transient expression assays in Nicotiana benthamiana , as well as the influence of P6 on the expression and subcellular localization of P1. A version of P6 tagged with RFP was shown to envelop the endoplasmic reticulum (ER), whereas P1 tagged with RFP was shown to induce the fragmentation of the ER. Co-expression of P6 with P1 led to an enhancement of the spatial and temporal expression of P1, with a shift from expression through the plasma membrane and interior of the cell to punctate spots associated with the cell wall. • The CaMV P1 protein induces the fragmentation of the endoplasmic reticulum. • This new function could explain how tubules that form from P1 lack a desmotubule. • By contrast, CaMV P6 envelops the ER leaving it intact within P6 inclusion bodies. • P1 expression is spatially enhanced and sustained when co-expressed with P6. [ABSTRACT FROM AUTHOR]

Abstract Arabidopsis thaliana ecotype En-2 is resistant to several strains of Cauliflower mosaic virus (CaMV), including strain W260, but is susceptible to strain NY8153. Resistance in En-2 is conditioned by a single, semi-dominant gene called CAR 1. We constructed several recombinant infectious clones between W260 and NY8153 and evaluated their capability to infect En-2. This analysis showed that the capacity of NY8153 to break resistance in En-2 was conditioned by mutations within the CaMV gene 1, a gene that encodes a protein dedicated to cell-to-cell movement (P1), and conversely, that P1 of W260 is responsible for eliciting the plant defense response. A previous study had shown that P6 of W260 was responsible for overcoming resistance in Arabidopsis ecotype Tsu-0 and that P6 of CaMV strain CM1841 was responsible for triggering resistance. The present study now shows that a second gene of CaMV is targeted by Arabidopsis for plant immunity. Highlights • The Arabidopsis CAR1 resistance gene in ecotype En-2 specifies resistance to CaMV. • Unlike most CaMV strains, NY8153 is able to infect En-2 systemically. • We now show that gene 1 of CaMV strain NY8153 overcomes resistance of CAR1. • Thus, gene 1 of other CaMV strains must either be defective of elicit plant defenses. [ABSTRACT FROM AUTHOR]