Mitsuhiro Sugiyama

Identification of quantitative trait loci for downy mildew resistance in cucumber (Cucumis sativus L.)

Downy mildew, caused by Pseudoperonospora cubensis (Berk. & Curt.) Rostov, is one of the most economically important foliar diseases in cucumber (Cucumis sativus L.). Cucumber line CS-PMR1, derived from self-pollination of USDA Plant Introduction 197088, has a high level of resistance to downy mildew and is considered to be promising breeding material. In this study, we performed quantitative trait locus (QTL) analysis for downy mildew resistance using 111 recombinant inbred lines (RILs) derived from a cross between CS-PMR1 and the old Japanese cultivar Santou, which exhibits moderate resistance. The resistance of the RILs and their parents was evaluated by diverse methods using different plant organs (cotyledons, true leaves), stages (seedlings and adult plants), and evaluation criteria (lesion expansion and extent of sporulation). The high resistance of CS-PMR1 was associated with many QTLs with relatively small effects, whereas the moderate resistance of Santou was associated with one major QTL and possibly two others with relatively small effects. In all assays, the major QTL at which the Santou allele was associated with increased resistance had the largest effect. This QTL allele from Santou and several of the most effective QTL alleles identified in CS-PMR1 should be highest priority for selection to efficiently breed new cultivars that carry adequate levels of downy mildew resistance.

Resistance in melon to Cucurbit chlorotic yellows virus, a whitefly-transmitted crinivirus

Fifty-one melon (Cucumis melo) accessions that originated from India, Pakistan and Bangladesh were evaluated for resistance to Cucurbit chlorotic yellows virus (CCYV), a newly emerged species in the genus Crinivirus. CCYV was inoculated using sweet potato whitefly (Bemisia tabaci) biotype Q. Accessions, JP 138332, JP 216154, JP 216155, JP 216751 and JP 91204, showed no or faint symptoms, although CCYV was detected from the non-inoculated upper leaves by reverse transcription-polymerase chain reaction (RT-PCR). The five accessions were subjected to quantitative RT-PCR to analyze relative accumulation of CCYV RNA. All accessions except JP 138332 had levels of CCYV RNA accumulation comparable to the commercial variety, ‘Earl’s Seine’, which was used as a control. JP 138332 showed a much lower CCYV RNA accumulation. Numbers of B. tabaci biotype Q on JP 138332 did not differ from ‘Earl’s Seine’, in antixenosis tests, and the result suggested the resistance to CCYV was not due to antixenosis. Consequently, five accessions are of interest for development of resistant varieties. In particular, JP 138332 possesses a promising resistant trait for CCYV, which might be associated with inhibition of virus multiplication.

Effect of temperature on symptom expression and viral spread of Melon yellow spot virus in resistant cucumber accessions

Melon yellow spot virus (MYSV), a member of the genus Tospovirus, is a devastating thrips-transmitted virus of cucurbits in Japan. Recently, we reported that cucumber accessions originating from South Asia, in particular Southeast Asia, had moderate resistance to MYSV. Here, we investigated the effect of three temperatures (20°C, 25°C, and 30°C) on symptom expression and viral spread of MYSV in plants of resistant cucumber accessions. No systemic infection developed in resistant cucumber plants after inoculation with melon isolate MYSV-S at low temperature (20°C); viral spread of MYSV-S and cucumber isolate MYSV-FuCu05P in inoculated cotyledons was suppressed. In contrast, higher incubation temperatures (25°C and 30°C) facilitated viral spread in inoculated cotyledons and systemic infection of MYSV-S. These data suggest that the resistance to MYSV of resistant cucumber accessions is temperature dependent.

A Variant of Cucumber mosaic virus Is Restricted to Local Lesions in Inoculated Tobacco Leaves with a Hypersensitive Response

A variant of Cucumber mosaic virus, CMV(Y/GM2), was isolated from a tobacco plant with mild green mosaic symptoms that was regenerated in vitro from a yellow strain of CMV [CMV(Y)]-infected tobacco leaves by tissue culture. CMV(Y/GM2) has two amino acid substitutions at 36 and 111 positions in the coat protein encoded on RNA3. CMV, assembled by mixing invitro transcribed CMV(Y) RNA1 and RNA2 plus infectious RNA3 transcribed in vitro from cDNA to RNA3 of CMV(Y/GM2), was prepared and designated as CMV(Y/GM2)tr. When tobacco (Nicotiana tabacum cv. Xanthi nc) plants were inoculated with CMV(Y/GM2)tr, large necrotic local lesions in which the virus was localized, developed on the inoculated leaves. This host response unique to CMV(Y/GM2)tr was similar to the hypersensitive response (HR), which is a common resistance response to avirulent pathogens and was observed in five cultivars of Nicotiana tabacum and eight Nicotiana species. The revertant virus, however, accumulated to quite different levels in the various hosts. CMV(Y/GM2)tr induced pathogenesis-related 1 (PR-1) protein accumulation and systemic acquired resistance (SAR) which were generally observed in the HR. However, when tobaccos were inoculated with CMV(S36P)tr and CMV(V111I)tr, which have an amino acid substitution at either the 36 or 111 position in the coat protein of CMV(Y), respectively, CMV(S36P)tr was restricted to the primary infection site without necrotic local lesion formation and PR-1 protein and SAR induction. CMV(V111I)tr, however, systemically spread and induced mild green mosaic symptoms, while the host had the HR to CMV(Y/GM2)tr. The localization of CMV(Y/GM2)tr at the primary infection site may not only be caused by the HR, but also by the restriction of virus systemic movement resulting from the amino acid substitution at position 36 in the coat protein of CMV(Y).