Highlights d 1.6 million tests identified 1,388 SARS-CoV-2 infections in Guangdong by 19 March d Virus genomes can be recovered using a variety of sequencing approaches d Analyses reveal multiple viral importations with limited local transmission d Effective control measures helped reduce and eliminate chains of viral transmission

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SARS-CoV-2 is a novel coronavirus first identified in December 2019. Notable features make SARS-CoV-2 distinct from most other previously-identified Betacoronaviruses, including the receptor binding domain of SARS-CoV-2 and a unique insertion of twelve nucleotide or four amino acids (PRRA) at the S1/S2 boundary. In this study, we identified two deletion variants of SARS-CoV-2 that either directly affect the polybasic cleavage site itself (NSPRRAR) or a flanking sequence (QTQTN). These deletions were verified by multiple sequencing methods. In vitro results showed that the deletion of NSPRRAR likely does not affect virus replication in Vero and Vero-E6 cells, however the deletion of QTQTN may restrict late phase viral replication. The deletion of QTQTN was detected in 3 of 68 clinical samples and half of 24 in vitro isolated viruses, whilst the deletion of NSPRRAR was identified in 3 in vitro isolated viruses. Our data indicate that (i) there may be distinct selection pressures on SARS-CoV-2 replication or infection in vitro and in vivo, (ii) an efficient mechanism for deleting this region from the viral genome may exist, given that the deletion variant is commonly detected after two rounds of cell passage, and (iii) the PRRA insertion, which is unique to SARS-CoV-2, is not fixed during virus replication in vitro. These findings provide information to aid further investigation of SARS-CoV-2 infection mechanisms and a better understanding of the NSPRRAR deletion variant observed here. Important notes The spike protein determines the infectivity and host range of coronaviruses. SARS-CoV-2 has two unique features in its spike protein, the receptor binding domain and an insertion of twelve nucleotides at the S1/S2 boundary resulting a furin-like cleavage site. Here, we identified two deletion variants of SARS-CoV-2 that either directly affect the furin-like cleavage site itself (NSPRRAR) or a flanking sequence (QTQTN) and investigated these deletions in cell isolates and clinical samples. The absence of the polybasic cleavage site in SARS-CoV-2 did not affect virus replication in Vero or Vero-E6 cells. Our data indicate the PRRAR and its flanking sites are not fixed in vitro, thus there appears to be distinct selection pressures on SARS-CoV-2 sequences in vitro and in vivo. Further investigation of the mechanism of generating these deletion variants and their infectivity in different animal models would improve our understanding of the origin and evolution of this virus.

Effects of naphthaleneacetic acid (NAA), aminoethoxyvinylglycine (AVG), and 1-methylcyclopropene (1-MCP) alone or in combination on fruit ethylene production, preharvest fruit drop, fruit quality, and fruit maturation were examined in ‘Golden Supreme’ and ‘Golden Delicious’ apples (Malus ×domestica Borkh.). In ‘Golden Supreme’ apples, the combination of two applications of AVG and one application of NAA 3 and 1 week, respectively, before the anticipated optimum harvest date synergistically inhibited fruit ethylene production and delayed fruit drop and ripening. Compared with one or two applications of AVG, the combination of one application of AVG and two applications of NAA had much lower preharvest fruit drop, although there was no significant difference in fruit ethylene production among these treatments. In ‘Golden Delicious’ apples, 1-MCP at 396 mg·L−1 had a better effect in delaying fruit drop than did AVG at 125 mg·L−1 or NAA at 20 mg·L−1 when they were applied a week before the optimum harvest date. The combination of NAA and 1-MCP or AVG was more effective in delaying fruit drop than were NAA, 1-MCP, or AVG alone. Fruit ethylene production was inhibited by 1-MCP and AVG but not by NAA. 1-MCP and AVG delayed fruit ripening, whereas NAA increased fruit ripening as determined by fruit firmness and starch.

BackgroundNaphthaleneacetic acid (NAA), a synthetic auxin analogue, is widely used as an effective thinner in apple orchards. When applied shortly after fruit set, some fruit abscise leading to improved fruit size and quality. However, the thinning results of NAA are inconsistent and difficult to predict, sometimes leading to excess fruit drop or insufficient thinning which are costly to growers. This unpredictability reflects our incomplete understanding of the mode of action of NAA in promoting fruit abscission.ResultsHere we compared NAA-induced fruit drop with that caused by shading via gene expression profiling performed on the fruit abscission zone (FAZ), sampled 1, 3, and 5 d after treatment. More than 700 genes with significant changes in transcript abundance were identified from NAA-treated FAZ. Combining results from both treatments, we found that genes associated with photosynthesis, cell cycle and membrane/cellular trafficking were downregulated. On the other hand, there was up-regulation of genes related to ABA, ethylene biosynthesis and signaling, cell wall degradation and programmed cell death. While the differentially expressed gene sets for NAA and shading treatments shared only 25% identity, NAA and shading showed substantial similarity with respect to the classes of genes identified. Specifically, photosynthesis, carbon utilization, ABA and ethylene pathways were affected in both NAA- and shading-induced young fruit abscission. Moreover, we found that NAA, similar to shading, directly interfered with leaf photosynthesis by repressing photosystem II (PSII) efficiency within 10 minutes of treatment, suggesting that NAA and shading induced some of the same early responses due to reduced photosynthesis, which concurred with changes in hormone signaling pathways and triggered fruit abscission.ConclusionsThis study provides an extensive transcriptome study and a good platform for further investigation of possible regulatory genes involved in the induction of young fruit abscission in apple, which will enable us to better understand the mechanism of fruit thinning and facilitate the selection of potential chemicals for the thinning programs in apple.