P3N-PIPO, a Frameshift Product from the P3 Gene, Pleiotropically Determines the Virulence of Clover Yellow Vein Virus in both Resistant and Susceptible Peas

Peas carrying the cyv1 recessive resistance gene are resistant to clover yellow vein virus (ClYVV) isolates No. 30 (Cl-No. 30) and 90-1 (Cl-90-1) but can be infected by a derivative of Cl-90-1 (Cl-90-1 Br2). The main determinant for the breaking of cyv1 resistance by Cl-90-1 Br2 is P3N-PIPO produced from the P3 gene via transcriptional slippage, and the higher level of P3N-PIPO produced by Cl-90-1 Br2 than by Cl-No. 30 contributes to the breaking of resistance. Here we show that P3N-PIPO is also a major virulence determinant in susceptible peas that possess another resistance gene, Cyn1, which does not inhibit systemic infection with ClYVV but causes hypersensitive reaction-like lethal systemic cell death. We previously assumed that the susceptible pea cultivar PI 226564 has a weak allele of Cyn1. Cl-No. 30 did not induce cell death, but Cl-90-1 Br2 killed the plants. Our results suggest that P3N-PIPO is recognized by Cyn1 and induces cell death. Unexpectedly, heterologously strongly expressed P3N-PIPO of Cl-No. 30 appears to be recognized by Cyn1 in PI 226564. The level of P3N-PIPO accumulation from the P3 gene of Cl-No. 30 was significantly lower than that of Cl-90-1 Br2 in a Nicotiana benthamiana transient assay. Therefore, Cyn1-mediated cell death also appears to be determined by the level of P3N-PIPO. The more efficiently a ClYVV isolate broke cyv1 resistance, the more it induced cell death systemically (resulting in a loss of the environment for virus accumulation) in susceptible peas carrying Cyn1, suggesting that antagonistic pleiotropy of P3N-PIPO controls the resistance breaking of ClYVV. IMPORTANCE Control of plant viral disease has relied on the use of resistant cultivars; however, emerging mutant viruses have broken many types of resistance. Recently, we revealed that Cl-90-1 Br2 breaks the recessive resistance conferred by cyv1, mainly by accumulating a higher level of P3N-PIPO than that of the nonbreaking isolate Cl-No. 30. Here we show that a susceptible pea line recognized the increased amount of P3N-PIPO produced by Cl-90-1 Br2 and activated the salicylic acid-mediated defense pathway, inducing lethal systemic cell death. We found a gradation of virulence among ClYVV isolates in a cyv1-carrying pea line and two susceptible pea lines. This study suggests a trade-off between breaking of recessive resistance (cyv1) and host viability; the latter is presumably regulated by the dominant Cyn1 gene, which may impose evolutionary constraints upon P3N-PIPO for overcoming resistance. We propose a working model of the host strategy to sustain the durability of resistance and control fast-evolving viruses.