SUMMARYViral movement proteins exploit host endomembranes and the cytoskeleton to move within the cell via routes that, in some cases, are dependent on the secretory pathway. For example, melon necrotic spot virus p7B, a type II transmembrane protein, leaves the endoplasmic reticulum (ER) through the COPII-dependent Golgi pathway to reach the plasmodesmata. Here we investigated the sequence requirements and putative mechanisms governing p7B transport through the early secretory pathway. Deletion of either the cytoplasmic N-terminal region (CR) or the luminal C-terminal region (LR) led to ER retention, suggesting that they are both essential for ER export. Through alanine-scanning mutagenesis, we identified residues in the CR and LR that are critical for both ER export and for viral cell-to-cell movement. Within the CR, alanine substitution of aspartic and proline residues in the DSSP b-turn motif (D 7 AP 10 A) led to movement of discrete structures along the cortical ER in an actin-dependent manner. In contrast, alanine substitution of a lysine residue in the LR (K 49 A) resulted in a homogenous ER distribution of the movement protein and inhibition of ER-Golgi traffic. Moreover, the ability of p7B to recruit Sar1 to the ER membrane is lost in the D 7 AP 10 A mutant, but enhanced in the K 49 A mutant. In addition, fluorescence recovery after photobleaching revealed that K 49 A but not D 7 AP 10 A dramatically diminished protein lateral mobility. From these data, we propose a model whereby the LR directs actin-dependent mobility toward the cortical ER, where the cytoplasmic DSSP b-turn favors assembly of COPII vesicles for export of p7B from the ER.
Unlike fungal and bacterial diseases, no direct method is available to control viral diseases. The use of resistance-inducing compounds can be an alternative strategy for plant viruses. Here we studied the basal response of melon to
Melon necrotic spot virus
(MNSV) and demonstrated the efficacy of hexanoic acid (Hx) priming, which prevents the virus from systemically spreading. We analysed callose deposition and the hormonal profile and gene expression at the whole plant level. This allowed us to determine hormonal homeostasis in the melon roots, cotyledons, hypocotyls, stems and leaves involved in basal and hexanoic acid-induced resistance (Hx-IR) to MNSV. Our data indicate important roles of salicylic acid (SA), 12-oxo-phytodienoic acid (OPDA), jasmonic-isoleucine, and ferulic acid in both responses to MNSV. The hormonal and metabolites balance, depending on the time and location associated with basal and Hx-IR, demonstrated the reprogramming of plant metabolism in MNSV-inoculated plants. The treatment with both SA and OPDA prior to virus infection significantly reduced MNSV systemic movement by inducing callose deposition. This demonstrates their relevance in Hx-IR against MNSV and a high correlation with callose deposition. Our data also provide valuable evidence to unravel priming mechanisms by natural compounds.
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