The Titer of Citrus Yellow Vein Clearing Virus is Positively Associated with the Severity of Symptoms in Infected Citrus Seedlings

Abstract

Citrus yellow vein clearing virus is a new member of the genus Mandarivirus in the family Alphaflexiviridae. Citrus yellow vein clearing virus (CYVCV) is the causal agent of citrus yellow vein clearing disease and is widely distributed in Pakistan, India, Turkey, and China. CYVCV is transmitted from citrus to citrus by Dialeurodes citri, grafting, and contaminated knife blades, threatening citrus production. In this study, four infectious full-length cDNA clones of CYVCV (namely AY112, AY132, AY212, and AY221) derived from CYVCV isolate AY were obtained through yeast homologous recombination and inoculated to ‘Eureka’ lemon (Citrus limon Burm. f.) by Agrobacterium-mediated vacuum infiltration. Pathogenicity analysis indicated that the clones AY212 and AY221 caused more severe symptoms than AY112 and AY132. Northern blot and quantitative reverse transcription PCR (qRT-PCR) analyses showed that the titers of virulent clones (AY212 and AY221) were significantly higher than those of attenuated clones (AY112 and AY132) in the infected ‘Eureka’ lemon (Citrus limon Burm. f.) seedlings. Subsequent comparative studies of viral infectivity, accumulation, and symptoms induced by AY221 in nine citrus cultivars indicated that (i) the infectivity of AY221 varied from 25% to 100% among different cultivars; (ii) ‘Oota’ ponkan (C. reticulata L.) showed the lowest infection rate with mild symptoms, which might be a useful resource for CYVCY-resistance genes; (iii) CYVCV titer was positively associated with the symptom development in infected citrus seedlings. In general, this report revealed the biological properties of CYVCV, thus laying a foundation for further investigation of pathogenic mechanisms in this virus.