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

Abstract

IMPORTANCEControl 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.H ost plants protect themselves from virus infection by activating defense systems mediated by immune receptors (e.g., nucleotide-binding-site-leucine-rich-repeat [NB-LRR] proteins) (1). Plants have many NB-LRR immune receptors, each of which recognizes specific viral proteins. The activated immune response is referred to as a hypersensitive response (HR) and is often accompanied by cell death. When an HR is induced, the virus is localized in and around the infection locus. NB-LRR immune receptors are encoded by resistance genes that are genetically dominant.Another important defense against plant virus infection is genetically recessive resistance (2). The viral life cycle totally relies on host cells, and viruses require host factors in order to multiply within cells and move to neighboring cells. Therefore, the lack of a specific host-coopted factor required for the viral life cycle leads to host resistance against the virus, which would be recessively inherited. Many natural recessive resistance genes against viruses have been identified in diverse crops (2). Extensive studies have been carried out on viruses belonging to the Potyvirus genus, the major genus of the Potyviridae family, which is one of the two largest plant virus genera and is found in most climatic regions worldwide (3). These viruses infect a broad range of host plants, including both monocots and dicots. They cause considerable crop damage, resulting in severe economic losses. Most of the recessive

Role of the Genetic Background in Resistance to Plant Viruses

“…Owing to their large population size and short generation time, viruses have a great potential to quickly evolve and adapt under selection pressures [ 102 ]. Host factors (including genetic background), in combination with environmental factors, likely affect virus evolution and adaptation, helping viruses to invade new hosts and potentially form more virulent strains [ 50 , 103 , 104 ]. The durability of deployed resistance genes ultimately depends on the overall evolutionary potential of viruses (which is determined by the genetic structure of viral populations), on the functional flexibility of the pathogen avirulence factors targeted by the resistance gene(s), and on ecological effects which may increase or slow down the emergence of resistance-breaking isolates [ 103 ].…”

“…Evolutionary constraints upon viral resistance-breaking determinant can be imposed by a trade-off between the breakdown of a recessive resistance gene and host viability regulated by a dominant resistance gene [ 50 , 104 ]. In the pea/ Clover yellow vein virus (ClYVV) pathosystem, Atsumi et al [ 104 ] showed that the recessive gene cyv1 confers resistance to ClYVV and the major viral resistance-breaking determinant is the P3N-PIPO protein. After overcoming the cyv1 resistance, the RB variants accumulate efficiently and produce a more abundant P3N-PIPO that is also a viral effector recognized by the dominant gene Cyn1 , which induces systemic cell-death and the loss of host viability that is unfavorable for the virus.…”

“…After overcoming the cyv1 resistance, the RB variants accumulate efficiently and produce a more abundant P3N-PIPO that is also a viral effector recognized by the dominant gene Cyn1 , which induces systemic cell-death and the loss of host viability that is unfavorable for the virus. The authors propose that a trade-off for the virus in overcoming paired defense mechanisms may sustain the durability of resistance against ClYVV [ 50 , 104 ].…”

Virus-Mediated Targeted DNA Methylation Illuminates the Dynamics of Methylation in an Endogenous Plant Gene

“…The leaf tissues of N. benthamiana were homogenized in liquid nitrogen. The total RNA was isolated using the acid guanidinium thiocyanate-phenol-chloroform (AGPC) extraction method [44], then purified on a FARB minicolumn (Favorgen Biotech Corp., Ping-Tung, Taiwan) [45]. Total RNA was digested with Turbo DNase (Thermo Fisher Scientific, Waltham, MA, USA) and reverse transcribed using random hexamer by PrimeScript II reverse transcriptase (TaKaRa Bio), according to the manufacturer's instructions.…”

Protein Nucleotidylylation in +ssRNA Viruses

“…Potyvirus genomes encode an ORF which is translated to a 340–370 kDa polyprotein, which is then co- and/or post-translationally processed by three forms of internally encoded protease into cleavage intermediates or mature protein products ( Figure 1 ). A fusion protein, P3N-PIPO, is read off an alternative open reading frame as a result of a frameshift caused by RNA polymerase slippage on the first ORF [ 133 , 134 ].…”