Apple necrotic mosaic virus (ApNMV) is significantly associated with apple mosaic disease in China. However, the mechanisms of ApNMV infection, as well as host defense against the virus, are still poorly understood. Mitochondrial ATP synthase plays a fundamental role in the regulation of plant growth and development. However, the mechanism of mitochondrial ATP synthase in response to virus infection remains to be defined. In the present study, a yeast two-hybrid (Y2H) screening revealed that the apple mitochondrial ATP synthase OSCP subunit (MdATPO) interacts with ApNMV coat protein (CP). It was further verified that overexpression of MdATPO in Nicotiana benthamiana inhibited viral accumulation. In contrast, silencing of NbATPO facilitated viral accumulation, indicating that ATPO plays a defensive role during ApNMV infection. Further investigation demonstrated that ApNMV infection accelerated abscisic acid (ABA) accumulation, and ABA negatively regulated ATPO transcription, which was related to the capacity of ABA insensitive 5 (ABI5) to bind to the ABA-responsive elements (ABREs) of the ATPO promoter. Altogether, our results indicated that transcription factor ABI5 negatively regulated ATPO transcription by directly binding its promoter, leading to the sensitivity of apple and N. benthamiana to ApNMV infection. The current presentation facilitates our comprehensive understanding of the intricate responses of host to ApNMV.

The fungus Colletotrichum fructicola (a species of C. gloeosporioides complex) causes devastating anthracnose in strawberry. Like other species of the genus Colletotrichum, it uses a composite strategy including both the biotrophic and necrotrophic processes for pathogenesis. Host-derived hormones are central regulators of immunity, among which salicylic acid (SA) is the core defense one against biotrophic and hemibiotrophic pathogens. However, the manner and timing of pathogen manipulation of SA are largely elusive in strawberry. To achieve better understanding of the early challenges that SA-mediated defense experiences during strawberry/C. fructicola interaction, dynamic changes of SA levels were followed through the high-performance liquid chromatography method. A very early burst of free SA at 1 h postinoculation (hpi) followed by a fast quenching during the next 12 h was noticed, although rhythm variations were present in two hosts. Transcriptional characterization of genes related to SA pathway in two varieties on C. fructicola inoculation revealed that these genes were differentially expressed, although they were all induced at different time points. At the same time, three types of genes encoding homologous effectors interfering with SA accumulation were found to be first inhibited but sequentially activated during the first 24 hpi. Furthermore, subcellular localization analysis suggests that CfShy1 is a weapon of C. fructicola for strawberry invasion. Based on these results, we propose that the infection strategy that C. fructicola utilizes on strawberry is specialized, which is implemented through the optimized expression of a specific set of effector genes. Transcriptional characterization of host genes supports that de novo SA biosynthesis and the free SA release from methyl salicylate might contribute equally to the intricate control of SA homeostasis in strawberry. C. fructicola manipulation of SA-dependent resistance in strawberry might be closely related to multihormonal interplay among SA, jasmonic acid, abscisic acid, and cytokinin.

Strawberry (Fragaria × ananassa Duch.) is one of the most important horticultural plants worldwide with high economic and nutritional value. Strawberry associated virus 1 (SaV1) is a putative Cytorhabdovirus isolated from strawberry in Fujian province, China (Ding et al., 2019). Strawberry virus 1 (StrV-1) is another putative Cytorhabdovirus characterized from F. ananassa and F. vesca in Czech Republic (Fránová et al., 2019). The complete genomes of isolates of SaV1 and StrV-1 share 79 to 98% nucleotide (nt) identities. In August 2020, foliar chlorotic spots or streaks were observed in four strawberry cultivars (cv. Honeoye, Mibao, 8128 and All Star) in Yantai, Shandong province, China. To identify the associated viruses, symptomatic leaves from two plants of each cultivar (8 samples) were pooled for high-throughput sequencing (HTS). Total RNA was extracted from the composite sample and used for constructing a cDNA library after ribosomal RNA (rRNA)-depletion. Sequencing was carried out on Illumina Hiseq 4000 (Novogene, China). Raw reads were filtered, trimmed and de novo assembled as described previously (Grabherr et al., 2013; Zhou et al. 2020). The resulting contigs were screened by BLASTn and BLASTx against GenBank database. Subsequent analyses indicated the presence of strawberry vein banding virus, strawberry pallidosis associated virus and strawberry mottle virus in the analyzed sample, which had been reported previously in strawberry (Martin and Tzanetakis, 2013; Shi et al., 2018; Bhagwat et al., 2016). Besides, five contigs ranging from 266 to 6,057 nt were obtained. They shared 87 to 91% nt sequence identity with StrV-1 isolate B (GenBank accession no. MK211271). To confirm StrV-1 infection in the strawberry plants, total RNA was isolated from all eight samples using RNAprep Pure Plant Plus Kit (Tiangen, China). Reverse transcription polymerase chain reaction (RT-PCR) was conducted with two pairs of specific primers StrVp1 (Forward: 5'-CATTACTGAAGCATTCCGTG-3'/Reverse: 5'-AGATATCACGCACAGTGAC-3'), and StrVp2 (Forward: 5'-TTGCGCGAAGCGGATGTCCG-3'/Reverse: 5'-GGCTGCCAGAGCGTTGGATG-3'), targeting nt positions 70-1,231 and 7,825-9,348 of StrV-1 isolate B, respectively. Fragments with the expected sizes were amplified from two samples of cv. All Star. The amplicons were cloned, sequenced, and deposited in GenBank under accession no. MW419123-124 and MW645247-248. Both protein encoding sequences shared 91 to 92% and 80 to 84% nt identities with the corresponding sequences of StrV-1 isolate B and SaV1, respectively, indicating that the isolates from this study are genetic variants of StrV-1 and distantly related to SaV1. Crude sap was prepared by homogenizing leaf tissues of StrV-1 infected strawberry in 0.02 mol/L sodium phosphate buffer with 0.45% (w/v) sodium diethyldithiocarbamate thihydrate, then gently rubbed onto five healthy Nicotiana benthamiana plants. Neither the inoculated leaves nor the systemically infected leaves showed obvious symptoms seven days post inoculation. However, StrV-1 was detected by RT-PCR in all five N. benthamiana plants as described above. In addition, a survey of strawberry greenhouses was conducted in August 2020 and approximately 10% of plants in a 667 m2 greenhouse in Yantai had StrV-1-like symptoms. To the best of our knowledge, this is the first report of the occurrence of StrV-1 infecting strawberry in Shandong province, China. Our findings expand the geographic range and genetic diversity of StrV-1 and indicate it could be a potential virus threat to strawberry production in China.

The early-ripening peach industry has undergone rapid development in the Panxi region of the Sichuan Basin in recent years. However, after the introduction of some new peach varieties to the high-altitude peach-producing areas in Panxi, the titratable acid content in peach fruit has significantly increased. This study compared the fruit quality indicators of early-ripening peach varieties cultivated in Xide County (a high-altitude peach-producing area) and Longquanyi District (a low-altitude peach-producing area) in Sichuan Province and analyzed the differences in organic acid metabolism by combining primary metabolomic and transcriptomic approaches. The results showed that the ‘Zhongtaohongyu’ fruit from the high-altitude peach-producing area had a much higher accumulation of malic acid and, accordingly, a significantly higher organic acid content than the other samples. The lower annual average temperature and stronger ultraviolet radiation in high-altitude peach-producing areas may lead to the increased expression of genes (PpNAD-ME1, PpNADP-ME3, and PpPEPC1) in the organic acid synthesis pathway and the decreased expression of genes (PpACO2, PpNAD-MDH2/3/4/5, and PpPEPCK2) in the organic acid degradation pathway in peach fruit, ultimately resulting in the accumulation of more organic acids. Among them, the downregulation of the key genes PpNAD-MDH3/4/5 involved in malic acid metabolism may be the main reason for the higher malic acid accumulation in peach fruit from high-altitude peach-producing areas. Overall, this study elucidates the mechanism by which environmental factors enhance the accumulation of organic acids in peach fruit from high-altitude peach-producing areas from a multi-omics perspective, as well as providing a theoretical basis for screening key genes involved in organic acid metabolism in peach fruit.

The natural host range for brassica yellows virus (BrYV) is generally limited to Cruciferae. However, we found that BrYV can naturally infect strawberry. The full-length genome sequences of BrYV-MB (accession No. MZ666129) and BrYV-HY (accession No. ON060762) identified in strawberry from Yantai and Beijing, China, were obtained by high-throughput sequencing (HTS) combined with the RT-PCR and RACE techniques. The complete genome sequences of BrYV-MB and BrYV-HY are 5666 nt and contain six open reading frames (ORFs). The two isolates have the highest nucleotide (nt) sequence identity of 99.0%. The infectious cDNA clone of BrYV-HY was constructed through homologous recombination and used to agroinfiltrate Nicotiana benthamiana and Arabidopsis thaliana. The inoculated leaves of N. benthamiana showed necrotic symptoms after 4 days of inoculation (dpi), and the systematic leaves of A. thaliana exhibited purple symptoms at 14 dpi. To develop a rapid and high-sensitive method for the detection of BrYV, a TaqMan real-time fluorescence quantitative RT-PCR method (TaqMan RT-qPCR) was established. Under optimum reaction conditions, the sensitivity of the detection was as low as 100 fg and approximately 100-fold more sensitive than the conventional RT-PCR, so it can be used in large-scale testing.