The wild-type strain (Cl-WT) of Clover yellow vein virus (ClYVV) systemically induces cell death in pea cv. Plant introduction (PI) 118501 but not in PI 226564. A single incompletely dominant gene, Cyn1, controls systemic cell death in PI 118501. Here, we show that activation of the salicylic acid (SA) signaling pathway enhances ClYVV virulence in susceptible pea cultivars. The kinetics of virus accumulation was not significantly different between PI 118501 (Cyn1) and PI 226564 (cyn1); however, the SAresponsive chitinase gene (SA-CHI) and the hypersensitive response (HR)-related gene homologous to tobacco HSR203J were induced only in PI 118501 (Cyn1). Two mutant viruses with mutations in P1/HCPro, which is an RNA-silencing suppressor, reduced the ability to induce cell death and SA-CHI expression. The application of SA and of its analog benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH) partially complemented the reduced virulence of mutant viruses. These results suggest that high activation of the SA signaling pathway is required for ClYVV virulence. Interestingly, BTH could enhance Cl-WT symptoms in PI 226564 (cyn1). However, it could not enhance symptoms induced by White clover mosaic virus and Bean yellow mosaic virus. Our report suggests that the SA signaling pathway has opposing functions in compatible interactions, depending on the virus-host combination. Additional keywords: resistance gene.A simple and cost-effective way of controlling plant virus disease is the use of resistant cultivars. Many researchers have studied the resistance mechanisms that confine viruses to primary infected tissue. One of the most studied examples is that mediated by a direct or indirect interaction between a resistance (R) gene product and the product of a pathogen avirulence (Avr) gene. This results in the hypersensitive response (HR) in which the pathogen is arrested in and around the site of primary infection. The HR is often accompanied by host cell death. The HR is associated with changes in ion fluxes, the generation of reactive oxygen species and nitric oxide, the activation of mitogen-activated protein kinase cascades, the alteration of phytohormone balance, dynamic transcriptional change, and the induction of pathogenesis-related (PR) genes (Soosaar et al. 2005). Salicylic acid (SA) plays a critical signaling role in the activation of disease resistance. Arabidopsis thaliana mutants that are disrupted in SA biosynthesis and signaling pathways enhance susceptibility to pathogen infection (e.g., eds1, pad4, sid2, and eds5 mutants) (Durrant and Dong 2004). However, SA appears to have prodeath and antideath functions in HR-expressing tissues (Alvarez 2000).Although there are some instances in which no resistancebreaking (virulent) virus emerged, many resistances can be overcome by virulent strains (Lecoq et al. 2004). Thus, it is very important to investigate events at the molecular level to understand how disease is expressed after infection; however, this area is poorly understood (Whitham et al. 2006)....

Clover yellow vein virus (ClYVV) elicits lethal tip necrosis in the pea line PI 118501. Pea line PI 118501 develops necrotic lesions and veinal necrosis on inoculated leaves, followed by systemic necrosis, leading to plant death. To understand the genetic basis of this lethal tip necrosis, we crossed lines PI 226564 and PI 250438, which develop mosaic symptoms in response to ClYVV inoculation. In reciprocal crosses of PI 118501 with PI 226564, all F 1 plants had mosaic symptoms with slight stem necrosis and early yellowing of upper leaves. Essentially the same symptom was manifested in PI 118501 × PI 250438 F 1 plants. In F 2 populations from the cross between PI 118501 and PI 226564, the observed ratios of necrosis, mosaic with slight stem necrosis, and mosaic fi t the expected 1 : 2 : 1 ratio. These results indicate that a single incompletely dominant gene confers the induction of necrosis in PI 118501. This locus in pea, conferring necrosis induction to ClYVV infection, was designated Cyn1 (Clover yellow vein virus-induced necrosis). A linkage analysis using 100 recombinant inbred lines derived from a cross of PI 118501 and PI 226564 demonstrated that Cyn1 was located 7.5 cM from the SSR marker AD174 on linkage group III.

Secondary metabolites in plants play important roles in defence against biotic and abiotic stresses. Although the biosynthesis pathways of secondary metabolites have been extensively studied, the regulatory mechanism of gene expression involved in these pathways remains poorly understood. In this study, we develop a virus-induced gene silencing (VIGS) system that enables a rapid analysis of the regulatory mechanism of genes involved in the biosynthesis of isoprenoids, one of the largest groups in secondary metabolites, using hydroponically-grown Nicotiana benthamiana. Using VIGS, we successfully reduced the transcript levels of 3-hydroxy-3-methylglutaryl-CoA reductase 1 (HMGR1), cycloartenol synthase 1 (CAS1), sterol side chain reductase 2 (SSR2) and S-adenosyl-L-Met-dependent C-24 sterol methyltransferase 1 (SMT1) in leaf, stem and root tissues in approximately 2 weeks. We identified novel feedback and feed-forward regulation of isoprenoid biosynthesis genes when CAS1, which encodes a key enzyme involved in the biosynthesis of sterols and steroidal glycoalkaloids, was down-regulated. Furthermore, the regulation of these genes differed among different tissues. These results demonstrate that our system can rapidly analyse the regulatory mechanisms involved in the biosynthesis of secondary metabolites.

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