A Tradeoff Between the Escape from N′-Mediated Resistance and Virulence in Pepper Mild Mottle Virus Through Reduced Virus Accumulation

<i>N</i>' resistance is intrinsically broken by tobacco mosaic virus but is still effective against pepper mild mottle virus (PMMoV), including those breaking <i>L</i> resistance in peppers. To evaluate the durability of <i>N</i>' resistance to PMMoV, we performed random mutagenesis of the coat protein (CP) gene of PMMoV. We isolated 11 CP mutants with two to six amino acid changes that escaped the <i>N</i>'-mediated resistance response in <i>Nicotiana sylvestris</i>. Some mutants and their derivatives, which had minimal mutations to escape <i>N</i>'-mediated resistance, exhibited reduced accumulation in inoculated leaves and loss of systemic infectivity in a susceptible pepper (<i>Capsicum annuum</i>) cultivar, as determined by RT-PCR analysis. Although the mutant CPs also escaped recognition by <i>L</i>³ and <i>L</i>⁴ resistance proteins from pepper in transient expression assays, the loss of systemic infectivity suggests that the mutants are unlikely to overcome <i>L</i>-mediated resistance. In <i>Nicotiana benthamiana</i>, a highly susceptible systemic host of PMMoV, ELISA and RT-qPCR indicated that the mutants consistently infected the host systemically, albeit with attenuated virulence and reduced virus accumulation, especially in younger leaves. The results collectively suggest that the reduced virus accumulation enabled the mutant PMMoV to escape <i>N</i>'-mediated resistance, and as a trade-off, compromised its virulence. The results also suggest that PMMoV CP modulates the systemic symptoms.