First Report of Maize Yellow Mosaic Virus on Zea mays in South Korea

Experimental Host and Vector Ranges of the Emerging Maize Yellow Mosaic Polerovirus

Maize yellow mosaic virus (MaYMV) is an emerging polerovirus that has been detected in maize, other cereal crops, and weedy grass species in Asia, Africa, and the Americas. Disease symptoms in maize include prominent leaf tip reddening and stunting. Infection by MaYMV has been reported to reduce plant growth and yields by 10 to 30% in some instances. In this study, an experimental host range for MaYMV among agronomically important cereal crops and common grasses was established. Additional aphid species were assessed as potential vectors for MaYMV, and their transmission efficiencies were determined. Here, we report oats, foxtail millet, barley, and rye as new experimental cereal crop hosts of MaYMV in addition to confirming the previously reported hosts of corn, sorghum, wheat, and broom millet. Four of the nine other grass species evaluated were also identified as suitable experimental hosts for MaYMV: ryegrass, switchgrass, green foxtail, and sand love grass. Interestingly, no visible symptoms were present in any of the infected hosts besides the susceptible maize control. Vector range studies identified the greenbug aphid Schizaphis graminum as a new vector of MaYMV, though transmission efficiency was lower than the previously reported Rhopalosiphum maidis vector and similar to the other known aphid vector R. padi. Given MaYMV's global ubiquity, ability to evade detection, and broad host range, further characterization of yield impacts and identification of viable control strategies are desirable.

Yellow Dwarf Viruses of Cereals: Taxonomy and Molecular Mechanisms

Yellow dwarf viruses are the most economically important and widespread viruses of cereal crops. Although they share common biological properties such as phloem limitation and obligate aphid transmission, the replication machinery and associated cis-acting signals of these viruses fall into two unrelated taxa represented by Barley yellow dwarf virus and Cereal yellow dwarf virus. Here, we explain the reclassification of these viruses based on their very different genomes. We also provide an overview of viral protein functions and their interactions with the host and vector, replication mechanisms of viral and satellite RNAs, and the complex gene expression strategies. Throughout, we point out key unanswered questions in virus evolution, structural biology, and genome function and replication that, when answered, may ultimately provide new tools for virus management.

First report of maize yellow mosaic virus causing maize reddening in Henan,China

A novel polerovirus maize yellow mosaic virus (MaYMV) has been discovered in Asia (Chen et al. 2016; Lim et al. 2018; Sun et al. 2019; Wang et al. 2016), East Africa (Guadie et al. 2018; Massawe et al. 2018) and South America (Gonçalves et al. 2017). MaMYV was first reported to infect maize (Zea mays L.) showing yellow mosaic symptoms on the leaves in Yunnan, Guizhou, and yellowing and dwarfing symptoms on the leaves in Anhui provinces of China in 2016 (Chen et al. 2016; Wang et al. 2016). An East African isolate of MaYMV has recently been shown to induce leaf reddening in several maize genotypes (Stewart et al. 2020). To our knowledge the leaf reddening symptoms in maize was not reported in China and MaYMV was not reported in Henan province, China. A survey of viral diseases on maize was carried out during the autumn of 2021 in Zhengzhou (Henan province), China. During the survey, the leaves showing reddening symptoms were observed on maize plants in all four fields investigated. Symptomatic leaves of 12 plants from four fields of Xingyang county, Zhengzhou (n=12) were collected and mixed for metatranscriptomics sequencing, and total RNA was extracted and subjected to an rRNA removal procedure using a Ribo-zero Magnetic kit according to the manufacturer's instructions (Epicentre, an Illumina® company). cDNA libraries were constructed using a TruSeq™ RNA sample prep kit (Illumina). Barcoded libraries were paired-end sequenced on an Illumina HiSeq X ten platform at Shanghai Biotechnology Co., Ltd. (Shanghai, China) according to the manufacturer's instructions (www.illumina.com). In total 67607392 clean reads were de novo assembled using CLC Genomics Workbench (version:6.0.4). 105796 contigs were obtained. The assembled contigs were queried by homology search tools (BLASTn and BLASTx) against public database(GenBank). One 5,457 nucleotide (nt) long contig with the most reads of 558826 was obtained and blast analysis showed it shared 99.3% nt sequence identity (99% coverage) with MaYMV Yunnan4 isolate (KU291100).. According to the sequencing data no other plant viruses except MaYMV were present in the sequencing data. To confirm the presence of this virus, twelve leaf samples showing reddening symptoms were detected by RT-PCR using specific primer pairs for CP full length open reading frame (F: ATGAATACGGGAGGTAGAAA, R: CTATTTCGGGTTTTGAACAT). Amplicons with expected size of 594 bp were gained in seven samples and three of them were cloned into pMD18T vector and sequenced. The three isolates (OM417795, OM417796, and OM417797) shared 99.16% to 99.83% nt sequence identity with MaYMV-Yunnan3 isolate (KU291100). Further P0 sequence analysis of the three samples (OM417798, OM417799, and OM417800) with primer pairs F: ATGGGGGGAGTGCCTAAAGC/R: TCATAACTGATGGAATTCCC showed they shared 99.5% to 99.62% nt sequence identity with MaYMV-Yunnan3 isolate.To our knowledge, this is the first report of the occurrence of MaYMV infecting maize in Henan, China. Besides, our finding firstly discovered reddening symptoms caused by MaYMV on maize in China which is different from the previous symptoms observed in the other three provinces of China possibly due to the different maize varieties grown in different areas. According to our investigation, maize showing reddening symptoms was common in the fields. Henan province is the main corn production area in China. Corn leaf aphid (Rhopalosiphum maidis), the insect vector of MaYMV, is an important pest of corn in Henan province, thereby the occurrence of MaYMV might cause potential threat to maize production in China.

Development of an RT-LAMP assay for the detection of maize yellow mosaic virus in maize

Maize is one of the most widely cultivated cereal crops worldwide. Maize yellow mosaic virus (MaYMV) (species Maize yellow mosaic virus, genus Polerovirus and family Luteoviridae) was first reported in maize from China. In this study, a one-step reverse-transcription loop-mediated isothermal amplification (RT-LAMP) assay was developed for detecting MaYMV. The optimal concentrations of betaine, Mg²⁺ and dNTPs for the assay were 0 M, 1.4 mM and 6 mM, respectively, and the optimal reaction time was 50 min. Using total plant RNA as the template, the detection limit of the RT-LAMP assay for MaYMV was 1 pg, while that of RT-PCR was 100 pg, indicating that the RT-LAMP assay developed was 100 times more sensitive than RT-PCR. Importantly, the RT-LAMP assay successfully detected MaYMV using rapidly extracted crude RNA from infected maize as a template. In conclusion, the RT-LAMP assay developed was a rapid, specific, sensitive and low-cost method for the detection of MaYMV in field samples of maize.

Genetic Variability and Molecular Evolution of Maize Yellow Mosaic Virus Populations from Different Geographic Origins

Maize yellow mosaic virus (MaYMV) hosted in various gramineous plants was assigned to the genus Polerovirus (family Luteoviridae) in 2018. However, little is known about its genetic diversity and population structure. In this study, 509 sugarcane leaf samples with mosaic symptoms were collected in 2017 to 2019 from eight sugarcane-growing provinces in China. Reverse-transcription PCR results revealed that four positive-sense RNA viruses were found to infect sugarcane, and the incidence of MaYMV among samples from Fujian, Sichuan, and Guangxi Provinces was 52.1, 9.8, and 2.5%, respectively. Based on 82 partial MaYMV sequences and 46 whole-genome sequences from different host plants, phylogenetic analysis revealed that MaYMV populations are very closely associated with their source geographical regions (China, Africa, and South America). Pairwise identity analysis showed significant variability in genome sequences among MaYMV isolates with genomic nucleotide identities of 91.1 to 99.9%. In addition to codon mutations, insertions or deletions also contributed to genetic variability in individual coding regions, especially in the readthrough protein (P3-P5 fusion protein). Low gene flow and significant genetic differentiation of MaYMV were observed among the three geographical populations, suggesting that environmental adaptation is an important evolutionary force that shapes the genetic structure of MaYMV. Genes in the MaYMV genome were subject to strong negative or purification selection during evolution, except for the movement protein (MP), which was under positive selection pressure. This finding suggests that the MP may play an important role in MaYMV evolution. Taken together, our findings provide basic information for the development of an integrated disease management strategy against MaYMV.

Occurrence, genetic diversity, and recombination of maize lethal necrosis disease-causing viruses in Kenya

Maize is the most important food crop in Kenya accounting for more than 51 % of all staples grown in the country. Out of Kenya's 5.3 million ha total crops area, more than 2.1 million ha is occupied by maize which translates to 40 % of all crops area. However, with the emergence of maize lethal necrosis (MLN) disease in 2011, the average yields plummeted to all-time lows with severely affected counties recording 90-100% yield loss in 2013 and 2014. The disease is mainly caused by Maize chlorotic mottle virus (MCMV) in combination with Sugarcane mosaic virus (SCMV) or other potyviruses. In this study, a country-wide survey was carried out to assess the MLN causing viruses in Kenya, their distribution, genetic diversity, and recombination. The causative viruses of MLN were determined by RT-PCR using virus-specific primers and DAS-ELISA. Next-generation sequencing (NGS) data was generated, viral sequences identified, genetic diversity of MLN viruses was determined, and recombination was evaluated. MCMV and SCMV were detected in all the maize growing regions at varying levels of incidence, and severity while MaYMV, a polerovirus was detected in some samples through NGS. However, there were some samples in this study where only MCMV was detected with severe MLN symptoms. SCMV Sequences were highly diverse while MCMV sequences exhibited low variability. Potential recombination events were detected only in SCMV explaining the elevated level of diversity and associated risk of this virus in Kenya and the eastern Africa region.

Maize lethal necrosis viruses and other maize viruses in Rwanda

Maize lethal necrosis (MLN) is emergent in East Africa, first reported in 2011 in Kenya, and is devastating to maize production in the region. MLN is caused by coinfection of maize with the emergent maize chlorotic mottle virus (MCMV) and any of several maize-infecting potyviruses endemic in East Africa and worldwide. Here, we examined the distribution of MCMV and sugarcane mosaic virus (SCMV), the major viruses contributing to MLN in Rwanda. These and other viruses in maize across Rwanda were further characterized by deep sequencing. When identified, MCMV had high titres and minimal sequence variability, whereas SCMV showed moderate titres and high sequence variability. Deep sequencing also identified maize streak virus and other maize-associated viruses, including a previously described polerovirus, maize yellow mosaic virus, and barley yellow dwarf virus, diverse maize-associated totiviruses, maize-associated pteridovirus, Zea mays chrysovirus 1, and a maize-associated betaflexivirus. Detection of each virus was confirmed in maize samples by reverse transcription polymerase chain reaction.