Limitations observed in the use of agroinoculation for geminivirus research

Bottle gourd IC-0262269, a super-susceptible genotype to tomato leaf curl Palampur virus

While conducting field trial of 82 genotypes of bottle gourd at Delhi during 2020-2021, a particular genotype, IC-0262269 was found to be affected by chlorotic curly stunt disease (CCSD). The affected plants were severely stunted and bearing very small chlorotic and crinkle leaves. The disease incidence in the said genotype was as high as 80% among different replicated trial blocks. The application of PCR using a generic primers specific to begomoviruses, as well as species-specific PCR diagnostics to six tomato-infecting begomoviruses: tomato leaf curl New Delhi virus (ToLCNDV), tomato leaf curl Palampur virus (ToLCPalV), tomato leaf curl Joydebpur virus (ToLCJoV), tomato leaf curl Gujrat virus (ToLCGuV), tomato leaf curl Bangalore virus (ToLCBV), and chilli leaf curl virus (ChiLCV) showed that, only ToLCPalV could be detected in the genotype IC-0262269. Following, rolling circle amplification, cloning and sequencing of full-length DNA-A and DNA-B genome of an isolate BoG1-ND from the genotype IC-0262269 revealed association of ToLCPalV with the disease. The successful agro-infection of the cloned genome of BoG1-ND (DNA-A and DNA-B) in the plants of Nicotiana benthamiana and bottle gourd demonstrated that ToLCPalV is the causal begomovirus of CCSD. The study provides the first evidence of the natural occurrence of ToLCPalV in bottle gourd crop and also showed that the bottle gourd genotype IC-0262269 is super-susceptible to ToLCPalV.

Supplementary Information

The online version contains supplementary material available at 10.1007/s13205-023-03838-y.

Characterization of a New Monopartite Begomovirus with a Betasatellite Associated with Leaf Curl, Yellow Vein, and Vein Enation in Hibiscus rosa-sinensis

A new begomovirus, tentatively named hibiscus yellow vein leaf curl virus (HYVLCV), was identified in Hibiscus rosa-sinensis plants showing symptoms of leaf curl, yellow vein, and vein enation on the undersides of the leaf in Taiwan. Sequence analysis of the full-length HYVLCV genome from the rolling cycle amplicon revealed a genome of 2,740 nucleotides that contains six open reading frames and a conserved sequence (5'-TAATATTAC-3') commonly found in geminiviral genomes. HYVLCV shares the highest nucleotide identity (88.8%) with cotton leaf curl Multan virus (CLCuMuV) genome, which is lower than the criteria (91%) set for species demarcation in the genus Begomovirus. No begomoviral DNA-B was detected; however, a begomovirus-associated DNA betasatellite (DNA-β) was detected. The DNA-β (1,355 nucleotides) shares the highest nucleotide identity (78.6%) with malvastrum yellow vein betasatellite (MaYVB). Because the identity is slightly higher than the criteria (78%) set for the species demarcation threshold for a distinct DNA-β species, the DNA-β of HYVLCV reported in this study is considered the same species of MaYVB and tentatively named MaYVB-Hib. An expected 1,498-bp fragment was amplified with two HYVLCV-specific primers from 10 of 11 field-collected samples. Four independent amplicons were sequenced, revealing 100% nucleotide identity with the HYVLCV genome. Agroinoculation of a dimer of the infectious monopartite genome alone to Nicotiana benthamiana resulted in mild symptoms at 28 days postinoculation (dpi); coagroinoculation with the DNA-β satellite resulted in severe symptoms at 12 dpi. HYVLCV could be transmitted to healthy H. rosa-sinensis by grafting, resulting in yellow vein symptoms at 30 dpi.

Maintenance of Cotton Leaf Curl Multan Betasatellite by Tomato Leaf Curl New Delhi Virus-Analysis by Mutation

Viruses of the genus Begomovirus (family Geminiviridae) are economically important phytopathogens that are transmitted plant-to-plant by the whitefly Bemisia tabaci. Most Old World (OW) begomoviruses are monopartite and many of these interact with symptoms and host range determining betasatellites. Tomato leaf curl New Delhi virus (ToLCNDV) is one of only a few OW begomoviruses with a bipartite genome (components known as DNA A and DNA B). Four genes [AV2, coat protein (CP), transcriptional-activator protein (TrAP), and AC4] of ToLCNDV were mutated and the effects of the mutations on infectivity, symptoms and the ability to maintain Cotton leaf curl Multan betasatellite (CLCuMuB) were investigated. Infectivity and virus/betasatellite DNA titer were assessed by Southern blot hybridization, PCR, and quantitative PCR. The results showed TrAP of ToLCNDV to be essential for maintenance of CLCuMuB and AV2 to be important only in the presence of the DNA B. AC4 was found to be important for the maintenance of CLCuMuB in the presence of, but indispensable in the absence of, the DNA B. Rather than being required for maintenance, the CP was shown to possibly interfere with maintenance of the betasatellite. The findings show that the interaction between a bipartite begomovirus and a betasatellite is more complex than just trans-replication. Clearly, multiple levels of interactions are present and such associations can cause additional significant losses to crops although the interaction may not be stable.

Molecular and Biological Characterization of Distinct Strains of Jatropha mosaic virus from the Dominican Republic Reveal a Potential to Infect Crop Plants

In the Dominican Republic (DO), jatropha plants with yellow mosaic symptoms are commonly observed in and around fields of various crop plants. Complete nucleotide sequences of DNA-A and DNA-B components of four bipartite begomovirus isolates associated with symptomatic jatropha plants collected from three geographical locations in the DO were determined. Sequence comparisons revealed highest identities (91 to 92%) with the DNA-A component of an isolate of Jatropha mosaic virus (JMV) from Jamaica, indicating that the bipartite begomovirus isolates from the DO are strains of JMV. When introduced into jatropha seedlings by particle bombardment, the cloned components of the JMV strains from the DO induced stunting and yellow mosaic, indistinguishable from symptoms observed in the field, thereby fulfilling Koch's postulates for the disease. The JMV strains also induced disease symptoms in Nicotiana benthamiana, tobacco, and several cultivars of common bean from the Andean gene pool, including one locally grown in the DO. Asymmetry in the infectivity and symptomatology of pseudorecombinants provided further support for the strain designation of the JMV isolates from the DO. Thus, JMV in the DO is a complex of genetically distinct strains that have undergone local evolution and have the potential to cause disease in crop plants.

Infectious cDNA clones of the crinivirus Tomato chlorosis virus are competent for systemic plant infection and whitefly-transmission

Tomato chlorosis virus (ToCV) (genus Crinivirus, family Closteroviridae) causes important emergent diseases in tomato and other solanaceous crops. ToCV is not transmitted mechanically and is naturally transmitted by whiteflies. The ToCV genome consists of two molecules of linear, positive-sense RNA encapsidated into long flexuous virions. We present the construction of full-length cDNA clones of the ToCV genome (RNA1 and RNA2) fused to the SP6 RNA polymerase promoter and under the control of the CaMV 35S promoter. RNA1 replicated in the absence of RNA2 in Nicotiana benthamiana and tomato protoplasts after inoculation with cDNA-derived in vitro transcripts. Agroinfiltration of RNA1 and RNA2 under the 35S promoter resulted in systemic infection in N. benthamiana plants. In addition, tomato plants were infected by grafting with agroinfected N. benthamiana scions, showing the typical ToCV symptoms. The viral progeny generated in tomato was transmissible by the whitefly Bemisia tabaci.

Molecular characterization of two sweepoviruses from China and evaluation of the infectivity of cloned SPLCV-JS in Nicotiana benthamiana

Sweepoviruses are important begomoviruses that infect Ipomoea plants worldwide and cause sweet potato yield losses and cultivar decline. Two sweepoviruses, sweet potato leaf curl virus-Jiangsu (SPLCV-JS) and sweet potato leaf curl China virus-Zhejiang (SPLCCNV-ZJ), were cloned from diseased sweet potato plants collected in the Jiangsu and Zhejiang provinces of China. Sequence characterization and phylogenetic analysis demonstrated that both are typical monopartite begomoviruses and have close relationships to several reported SPLCV and SPLCCNV isolates, respectively, from Asian countries. Analysis of the protein alignments and subcellular localizations of the six SPLCV-JS proteins was also conducted to verify their putative functions. In Nicotiana benthamiana, an infectivity assay of the infectious SPLCV-JS clone resulted in mild symptoms and weak viral DNA accumulation. Interestingly, SPLCV-JS, together with a heterologous betasatellite DNA (tomato yellow leaf curl China virus isolate Y10 [TYLCCNV-Y10] DNA-β), showed a synergistic effect on enhanced symptom severity and viral DNA accumulation. This is the first reported infectious SPLCV clone.

Roles and interactions of begomoviruses and satellite DNAs associated with okra leaf curl disease in Mali, West Africa

Okra leaf curl disease (OLCD) is a major constraint on okra (Abelmoschus esculentus) production in West Africa. Two monopartite begomoviruses (okra virus-1 and okra virus-2), a betasatellite and a DNA1 satellite are associated with OLCD in Mali. Okra virus-1 is an isolate of okra yellow crinkle virus (OYCrV), okra virus-2 is a recombinant isolate of cotton leaf curl Gezira virus (CLCuGV) and the betasatellite is a variant of cotton leaf curl Gezira betasatellite (CLCuGB). Cloned DNA of OYCrV and CLCuGV were infectious and induced leaf curl symptoms in Nicotiana benthamiana plants, but did not induce OLCD in okra. However, when these clones were individually co-inoculated with the cloned CLCuGB DNA, symptom severity and viral DNA levels were increased in N. benthamiana plants and typical OLCD symptoms were induced in okra. The CLCuGB was also replicated by, and increased symptom severity of, three monopartite tomato-infecting begomoviruses, including two from West Africa. The sequence of the DNA1 satellite was highly divergent, indicating that it represents a distinct West African lineage. DNA1 replicated autonomously, and replication required the DNA1-encoded Rep protein. Although DNA1 reduced helper begomovirus DNA levels, symptoms were not attenuated. In the presence of CLCuGB, DNA levels of the helper begomoviruses and DNA1 were substantially increased. Together, these findings establish that OLCD in Mali is caused by a complex of monopartite begomoviruses and a promiscuous betasatellite with an associated parasitic DNA1 satellite. These findings are discussed in terms of the aetiology of OLCD and the evolution of new begomovirus/satellite DNA complexes.