Emerging evidence of seed transmission of begomoviruses: implications in global circulation and disease outbreak.

Begomoviruses (family Geminiviridae) are known for causing devastating diseases in fruit, fibre, pulse, and vegetable crops throughout the world. Begomoviruses are transmitted in the field exclusively through insect vector whitefly (Bemisia tabaci), and the frequent outbreaks of begomoviruses are attributed largely due to the abundance of whitefly in the agri-ecosystem. Begomoviruses being phloem-borne were known not be transmitted through seeds of the infected plants. The recent findings of seed transmission of begomoviruses brought out a new dimension of begomovirus perpetuation and dissemination. The first convincing evidence of seed transmission of begomoviruses was known in 2015 for sweet potato leaf curl virus followed by several begomoviruses, like bhendi yellow vein mosaic virus, bitter gourd yellow mosaic virus, dolichos yellow mosaic virus, mungbean yellow mosaic virus, mungbean yellow mosaic India virus, pepper yellow leaf curl Indonesia virus, tomato leaf curl New Delhi virus, tomato yellow leaf curl virus, tomato yellow leaf curl Sardinia virus, and okra yellow mosaic Mexico virus. These studies brought out two perspectives of seed-borne nature of begomoviruses: (i) the presence of begomovirus in the seed tissues derived from the infected plants but no expression of disease symptoms in the progeny seedlings and (ii) the seed infection successfully transmitted the virus to cause disease to the progeny seedlings. It seems that the seed transmission of begomovirus is a feature of a specific combination of host-genotype and virus strain, rather than a universal phenomenon. This review comprehensively describes the seed transmitted begomoviruses reported in the last 9 years and the possible mechanism of seed transmission. An emphasis is placed on the experimental results that proved the seed transmission of various begomoviruses, factors affecting seed transmission and impact of begomovirus seed transmission on virus circulation, outbreak of the disease, and management strategies.

Evaluation of sowing dates for managing yellow mosaic disease caused by mungbean yellow mosaic India virus in mungbean.

Yellow mosaic disease, a most important destructive disease of mungbean production caused by Mungbean yellow mosaic India virus (MYMIV) under North Indian conditions. However, management of this deadly disease is still becoming the biggest challenge due to breaking of resistance under changing climatic conditions. Hence, a field experiment was conducted at IARI, New Delhi, India during Kharif 2021 and Spring-Summer 2022 to understand the sowing date influence on incidence of MYMIV in mungbean resistant (Pusa 1371) and susceptible (Pusa 9531) cultivars. The results revealed the higher disease incidence percentage (PDI) in the first sowing (15-20th July) of Kharif and third sowing (5-10th April) of Spring-Summer season. The mean PDI ranged from 25-41% to 11.80-13.54% for resistant followed by 23.13-49.84% and 14.40-21.45% in susceptible cultivar during Kharif and Spring-Summer season respectively. The detection of MYMIV through DAC-ELISA at 405 nm showed the absorbance values of 0.40-0.60 in susceptible and < 0.45 in resistant cultivar during the Kharif and 0.40-0.45 in Spring-Summer season. The PCR analysis with MYMIV and MYMV specific primers indicated the presence of only MYMIV and absence of MYMV in the present studied mungbean cultivars. The PCR analysis with DNA-B specific primers resulted in the amplification of 850 bp from both susceptible and resistant cultivars during the first sowing of Kharif whereas amplification was observed only in susceptible cultivar with second and third sowings of Kharif and all the three sowings of Spring-Summer season. The experiment results revealed that the most suitable date of sowing for mungbean will be before 30th March during Spring-Summer and after third week of July (30th July to 10th August) during the Kharif season under Delhi conditions. Supplementary Information: The online version contains supplementary material available at 10.1007/s13205-023-03621-z.

Molecular and biological characterization of soybean yellow mottle mosaic virus severe strain infecting soybean in India.

The complete nucleotide sequence and genome organization of soybean yellow mottle mosaic virus severe strain causing bright yellow mosaic, mottling and puckering symptoms in soybean (Glycine max) from India was determined. The monopartite single stranded genomic RNA is 3974 nuclotides long and has the potential to encode six viral proteins viz., p25, p83, p8, p10, p39 and p25. The SYMMV-Sb isolate differed from mungbean strain with 69 nucleotides and nine aminoacids dispersed over the various ORFs. Comparative sequence analysis revealed that SYMMV-Sb shared 98% nt sequence identity at complete genome level and 96-100% at all ORFs level with SYMMV mungbean strain from India and 71-92% identity with SYMMV Korean soybean isolate, whereas it showed very low sequence identity with other tombusviridae members (2-53%). The phylogenetic analysis showed the clustering of SYMMV-Sb along with other members of genus Gammacarmovirus. The SYMMV-Sb isolate produced chlorotic blotches, mild and veinal mottling, necrosis and puckering symptoms in various leguminous host plants. The symptomatalogy of the soybean isolate was differed from mungbean strain as earlier induced severe symptoms on soybean and mild symptoms on mungbean. SUPPLEMENTARY INFORMATION: The online version contains supplementary material available at 10.1007/s13205-021-02925-2.

Seed transmission of a distinct soybean yellow mottle mosaic virus strain identified from India in natural and experimental hosts.

Soybean yellow mottle mosaic virus (SYMMV) is a newly identified member of the genus Gammacarmovirus from grain legumes in India. As the modes of transmission of this virus have not been described, we assessed the possibility of SYMMV to be transmitted through seed collected from field infected mungbean plants and mechanically sap inoculated French bean plants using serological and molecular techniques followed by progeny assays. Direct antigen coated enzyme linked immunosorbent assay (DAC-ELISA) and reverse transcription polymerase chain reaction (RT-PCR) results are inconsistent with field infected mungbean seed tissues to ensure seed transmissibility irrespective of seed number used. Seed from mechanical sap inoculated French bean showed higher absorbance values in DAC-ELISA and amplification corresponding to replicase, movement and coat protein regions of SYMMV genome. The relative accumulation of SYMMV was higher in pod walls, immature seed and stamens and stigma of mechanical sap inoculated French bean. Progeny assays with infected seed revealed the seed transmissibility of SYMMV at the rate of 63.33% in mungbeanand 73.33% in French bean. Mechanical sap inoculation of mungbean progeny seedlings on French bean cv. Pusa Parvati produced characteristic symptoms of SYMMV. The results obtained from this study demonstrate that SYMMV is seed borne in nature and can be transmitted to next generation seedlings. This is the first report of seed transmission of SYMMV in mungbean and French bean.

Development of Soybean Yellow Mottle Mosaic Virus-Based Expression Vector for Heterologous Protein Expression in French Bean.

Plant virus-based vectors provide attractive and valuable tools for rapid production of recombinant protein in large quantities as they produce systemic infections in differentiated plant tissues. In the present study, we engineered the Soybean yellow mottle mosaic virus (SYMMV) as a gene expression vector which is a promising candidate for systemic expression of foreign proteins in French bean plants. Full virus vector strategy was exploited for insertion of foreign gene by inserting MCS through PCR in the circular pJET-SYMMV clone. To examine the ability of the SYMMV vector system, GFP gene was cloned after the start codon of coat protein (CP) so that its expression was driven by the SYMMV-CP subgenomic promoter. When in vitro run off SYMMV-GFP transcript was mechanically inoculated to French bean leaves, good level of GFP expression was observed through confocal microscopy up to 40 dpi. Expression of heterologous protein was also confirmed through ISEM, DAC-ELISA and RT-PCR with specific primers at 20 dpi. The recombinant SYMMV construct was stable in in vitro runoff transcript inoculated plants but the inserted GFP was lost in progeny virion inoculated plants. The system developed here will be useful for further studies of SYMMV gene functions and exploitation of SYMMV as a gene expression vector.

Genome characterization, infectivity assays of in vitro and in vivo infectious transcripts of soybean yellow mottle mosaic virus from India reveals a novel short mild genotype.

Nucleotide sequence of a distinct soybean yellow mottle mosaic virusisolate from Vignaradiata (mungbean isolate, SYMMV-Mb) from India was determined and compared with othermembers of the family Tombusviridae. The complete monopartite single-stranded RNA genome of SYMMV-Mb consisted of 3974nt with six putative open reading frames and includes 5' and 3' untranslated regions of 35 and 254nt, respectively. SYMMV-Mb genome shared 75% nt sequence identity at complete genome level and 67-92% identity at all ORFs level with SYMMV Korean and USA isolates (soybean isolates) followed by CPMoV, whereas it shared very low identity with other tombusviridae members (5-41%). A full-length infectious cDNA clone of the SYMMV-Mb placed under the control of the T7 RNA polymerase and the CaMV35S promoters was generated and French bean plants on mechanical inoculation with in vitro RNA transcripts, p35SSYMMV-O4 plasmid and agroinoculation with p35SSYMMV-O4 showed symptoms typical of SYMMV-Mb infection. The infection was confirmed by DAC-ELISA, ISEM, RT-PCR and mechanical transmission to new plant species. Further testing of different plant species with agroinoculation of p35SSYMMV-O4 showed delay in symptoms but indistinguishable from mechanical sap inoculation and the infection was confirmed by DAC-ELISA, RT-PCR and mechanical transmission to new plants. The system developed here will be useful for further studies on pathogenecity, viral gene functions, plant-virus-vector interactions of SYMMV-Mb and to utilize it as a gene expression and silencing vector.

Diagnosis of a new variant of soybean yellow mottle mosaic virus with extended host-range in India.

Soybean yellow mottle mosaic virus (SYMMV, genus Carmovirus) was previously known to occur in South Korea and USA causing bright yellow mosaic in soybean. In this study, SYMMV (Car-Mb14 isolate) was isolated from mungbean (Vigna radiata) exhibiting mild mottling and puckering symptoms in the experimental field at Indian Agricultural Research Institute, New Delhi during 2012. The virus isolate, Car-Mb14 induced veinal mottling, mild mottling, chlorotic blotching, local and systemic necrosis in soybean, mungbean, blackgram, French bean and guar bean, respectively. The symptomatology of the present isolate of SYMMV was different from the previously reported South Korean isolate, as the later did not induce symptoms in any of the above legumes other than soybean. The present isolate was phylogenetically distinct and shared 90-93 % sequence identity in coat protein (CP) of 52 SYMMV isolates reported from Korea and USA. In order to know the serological relationships, the CP gene of the present isolate was over expressed as a 39 kDa protein in E. coli and an antiserum of 1:16,000 titer against the recombinant CP was produced. Serological cross reactivity analysis revealed that SYMMV was serologically related to blackgram mottle virus but not to cowpea mottle virus, the other legume infecting carmoviruses. The antiserum was used to detect prevalence of SYMMV in legume crops by ELISA. Out of 145 field samples of legumes (mungbean, blackgram, French bean and soybean) collected from different places in India, SYMMV was detected only in 16 samples of mungbean and one sample of blackgram. The natural infection of SYMMV in mungbean and blackgram was further confirmed based on CP gene sequence. This study provides evidence of occurrence of a new variant of SYMMV with distinct symptom phenotype and extended host-range in India.