Citrus yellow mottle-associated virus (CiYMaV) belonging to the subgenus Mandarivirus within the genus Potexvirus, was first identified in 2018 from Pakistan (CiYMaV-PK), where it is endemic in several regions.  Here, three full-length cDNA clones (pCiYMaV-FL-1, pCiYMaV-FL-18, and pCiYMaV-FL-22) corresponding to the genomic RNA of CiYMaV were constructed and then agroinfiltrated on Chandler pummelo (Citrus grandis) seedlings using the vacuum infiltration method.  All the inoculated plants developed severe vein yellowing, leaf mottling, and dwarfing symptoms by 40 days post-infiltration (dpi).  The results of a direct tissue blot immunoassay and reverse transcription polymerase chain reaction detection showed 94.7–100% infection rates of pCiYMaV-FL at 60 dpi.  Despite there being no observed difference among the three clones in the severity of symptom, pCiYMaV-FL-22 showed the highest accumulation levels of viral RNA and coat proteins.  Moreover, pCiYMaV-FL-22 successfully infected seven other citrus varieties and induced symptoms in five of them.  Transmission electron microscopy identified the presence of filamentous virus particles in extracts from systemic leaves of the plants infected with pCiYMaV-FL-22 at 6-months post-infiltration.  Taken together, the results indicate that Koch’s postulates were fulfilled for CiYMaV in citrus plants.  This is the first report of full-length infectious cDNA clones of CiYMaV, and thus, the data provide a basis for further study of the molecular mechanisms of virus infection and the development of a viral vector to express foreign genes in citrus plants.

Although it is usually latent on citrus, apple, and pear, apple stem grooving virus (ASGV) poses a great risk to many sensitive cultivars.  Since severe leaf yellow mottle mosaic (LYMM) symptoms have been observed on Huangjinmiyou (HJY) pummelos (Citrus grandis cv. Huangjinmiyou), a commercial variety that is widely cultivated in South China, high throughput sequencing (HTS) was used to find potential pathogens and only three divergent ASGV variants were identified.  The three ASGV variants shared 81.03–82.34% genome-wide pairwise identities with each other, and were separately closest to other ASGV variants from different hosts and/or geographical regions, as indicated by viral phylogenies.  However, these new variants may have developed from viral interstrain interactions, based on the results of recombination analysis.  A large-scale survey using reverse transcription-PCR (RT-PCR) protocols designed for the three ASGV variants revealed a high incidence (92.7–100%) of ASGV in symptomatic HJY trees from 11 major citrus-producing regions in China.  None of ASGV were detected in asymptomatic trees.  Temperature treatments applied to the symptomatic HJY plants showed that ASGV is sensitive to high temperatures (30–35°C), at which not only the plants recovered, but also the viruses were not detected by RT-PCR, while at low temperatures (20–24°C), both the symptoms and viruses remained detectable.  These data show that ASGV is associated with the LYMM disease prevalent on HJY in China, and this is the significant basis especially of taking appropriate measures timely to manage the disease.  

Seedlessness in grape (Vitis vinifera) is an important commercial trait for both the fresh and drying markets.  However, despite numerous studies, the mechanisms and key genes regulating grape seedlessness are mostly unknown.  In this study, we sequenced the genomes of the V. vinifera seeded cultivar ‘Red Globe’, the seedless cultivar ‘Centennial Seedless’, and the derived hybrids.  Nonsynonymous single nucleotide polymorphisms (SNPs) were identified by genome sequencing and analyzed using published transcriptome data.  Nonsynonymous SNPs occurred in genes related to seed development, which were identified as protein kinases, transcription factors, and cytochrome P450s and showed differential expression during ovule development in both seeded and seedless grapes.  These nonsynonymous SNP-associated genes were mainly involved in biological processes such as hormone balance, seed coat and endosperm development, reproductive organ development, oxidation and reduction, senescence and cell death.  A potential quantitative trait locus (QTL) region associated with seed size was characterized based on the SNP-index, and expression analysis of candidate genes in the QTL region during ovule development in multiple seeded and seedless grape cultivars were conducted.  Three SNPs were further subjected to SNaPshot analysis and one SNP in G8 showed 67.5% efficiency in the grape progeny validation.  Overall, the data obtained in this study shed light on the differences in seed development between seeded and seedless progeny at the genomic level, which provides valuable resources for future functional studies and grape breeding.

Citrus leaf blotch virus (CLBV) is a member of the genus Citrivirus, in the family Betaflexiviridae.  It has been reported CLBV could infect kiwi, citrus and sweet cherry in China.  Of 289 citrus samples from six regions of China, 15 were detected to be infected with CLBV in this study.  The complete genome of four isolates of CLBV was obtained from Reikou in Sichuan (CLBV-LH), Yura Wase in Zhejiang (CLBV-YL), Bingtangcheng in Hunan (CLBV-BT), Fengjie 72-1 in Chongqing (CLBV-FJ), respectively.  While they all represented 8 747 nucleotides in monopartite size, excluding the poly(A) tail, each of the isolates coded three open reading frames (ORFs).  Identity of the four isolates ranged from 98.9 to 99.8% to each other and from 96.8 to 98.1% to the citrus references in GenBank by multiple alignment of genomes.  A phylogenetic tree based on the genome sequences of available CLBV isolates indicated that the four isolates were clustered together, suggesting that CLBV isolates from citrus in China did not have obvious variation.  This is the first report of the complete nucleotide sequences of CLBV isolates infecting citrus in China.