Emerging geminivirus problems: A serious threat to crop production

Geminiviruses form the second largest family of plant viruses, the Geminiviridae, represented by four genera: Mastrevirus, Curtovirus, Topocuvirus and Begomovirus. During the last two decades these viruses have emerged as devastating pathogens, particularly in the tropics and subtropics, causing huge economic losses and threatening crop production. Epidemics caused by re‐emerging and newly emerging geminiviruses are becoming frequent even in regions that were earlier free from these viruses. Compared to mastreviruses and curtoviruses, begomoviruses have emerged as more serious problems in a variety of crops, for example, cassava, cotton, grain legumes and vegetables. Major contributory factors for the emergence and spread of new geminivirus diseases are the evolution of variants of the viruses, the appearance of the whitefly ‘B’ biotype and the increase in the vector population. Variability in geminiviruses has arisen through mutations, recombination and pseudorecombination. Genomic recombination in geminiviruses, not only between the variants of the same virus but also between species and even between genera, has resulted in rapid diversification. From the disease point of view, most virulent variants have developed through recombination of viral genomes such as those associated with cassava mosaic, cotton leaf curl, and tomato leaf curl diseases. Heterologous recombinants containing parts of the host genome and/or sequences from satellite‐like molecules associated with monopartite begomoviruses provide unlimited evolutionary opportunities. Human activity has also played an important role in the emergence of serious geminivirus diseases across the globe, like the changes in cropping systems, the introduction of new crops, the movement of infected planting materials and the introduction of host susceptibility genes through the exchange of germplasm.     

Application of Vascular Puncture for Evaluation of Curtovirus Resistance in Chile Pepper and Tomato

Curtoviruses cause severe damage to tomatoes and peppers. Functional field resistance to curtoviruses in these plants is desirable but difficult to produce and difficult to screen for because it is time‐consuming and resistance could be achieved by developing resistance either to the virus or to insect feeding. To improve and speed curtovirus resistance testing in tomato (Solanum lycopersicum) and pepper (Capsicum annuum) plants, two puncture methods were developed and compared to leafhopper inoculation and feeding preference assays. The two puncture methods were adapted to introduce a modified Agrobacterium tumefaciens plasmid carrying a recombinant curtovirus into the meristem tissue of tomato plants and into newly germinated chile pepper seedlings. The puncture techniques were used to screen for resistance to curtoviruses in chile pepper and tomato breeding lines and varieties. Similarly, the peppers and tomatoes were assayed for curtovirus resistance using leafhopper inoculation and feeding preference, which was assessed by stylet sheath staining. Virus infection by puncture and leafhopper feeding was monitored using PCR and ELISA. ELISA was performed using an antibody to bacterially expressed coat protein. While pepper cvs Tabasco, NuMex Las Cruces cayenne and New Mexico 6‐4 were infected using both puncture and leafhopper inoculation methods, New Mexico 6‐4 had higher infection rates than the other two cultivars. Stylet sheath staining results suggest that leafhoppers prefer to feed on New Mexico 6‐4 rather than Tabasco and NuMex Las Cruces cayenne. Eight tomato cultivars were infected using meristem removal injection inoculation. Three tomatoes cultivars (CVF‐11, Saladmaster and Supersteak) were infected using leafhopper inoculation, although stylet sheath staining results suggested that the first two cultivars were not preferred by the insect vector. Our results suggest that puncture methods and leafhopper inoculation are successful in resistance screening, and both methods should be used as part of screening, because they assess different types of resistance.     

The unconventional geminivirus Beet curly top Iran virus: satisfying Koch's postulates and determining vector and host range

Beet curly top Iran virus (BCTIRV) is a geminivirus with unusual genomic organisation, recently reported in Iran, infecting sugarbeet and a few other plant species. Although three BCTIRV sequences have been reported, demonstration that BCTIRV DNA is the causal agent of the disease was missing. A full‐length genomic DNA was obtained from symptomatic leaves of sugarbeet collected in the Sivand area of Iran, and its nucleotide sequence was determined (BCTIRV‐Siv, 2845 nt). To satisfy Koch's postulates, an infectivity assay was developed by inserting a 1.4‐mer of BCTIRV‐Siv DNA in Agrobacterium tumefaciens and using it in agroinoculation experiments. The cloned viral DNA was capable of infecting sugarbeets, reproducing the leaf curling and vein enations observed in the field. These results demonstrate that the single DNA component of BCTIRV is sufficient for infectivity. Host range studies indicated that some economically important crops can be affected, such as spinach, tomato and sweet pepper, as well as important laboratory plants including Nicotiana benthamiana, Arabidopsis thaliana and Jimson weed. Circulifer haematoceps, the dominant leafhopper species present in sugarbeet fields in Iran, was successfully used to transmit the disease. The availability of an infectious clone will facilitate extended host range studies, to determine the potential risks to other crops, as well as genetic studies on this unusual member of the family Geminiviridae.     

Trade-off between foliage and tuber resistance to Phthorimaea operculella in wild potatoes

Beet leafhopper, Circulifer tenellus (Baker) (Homoptera: Cicadellidae), is the only known North American vector of beet curly top virus (Geminiviridae), which causes major economic losses in a number of crops including sugar beet, tomato, beans, and peppers. Beet curly top virus is a phloem-limited, persistently transmitted, circulative geminivirus. The strain/species of curly top virus used in this study is the CFH strain, also referred to as beet severe curly top virus (BSCTV). The direct current (DC) electrical penetration graph technique was used to determine the specific stylet penetration behavior associated with inoculation of BSCTV. Viruliferous leafhoppers were allowed to feed on healthy 3–4-week-old sugar beet plants until specific electrical penetration graph waveforms were produced, at which point feeding was artificially terminated. A series of comparisons between leafhoppers that produced different combinations of waveforms clearly implicated waveform D1 as the only waveform correlated with inoculation of BSCTV. All successful inoculations contained waveform D1, and 56 out of 64 leafhoppers that produced waveform D1 successfully inoculated test plants. Eighty-five leafhoppers did not produce waveform D1 and none of these inoculated BSCTV. While the occurrence of waveform D1 appears to be necessary for BSCTV inoculation, there was no correlation between duration of waveform D1 and inoculation success rate. The correlation of waveform D1 and BSCTV inoculation found in this study implies that waveform D1 is associated with phloem salivation.     

Toward understanding the molecular mechanism of a geminivirus C4 protein

Geminiviruses are ssDNA plant viruses that cause significant agricultural losses worldwide. The viruses do not encode a polymerase protein and must reprogram differentiated host cells to re-enter the S-phase of the cell cycle for the virus to gain access to the host-replication machinery for propagation. To date, 3 Beet curly top virus (BCTV) encoded proteins have been shown to restore DNA replication competency: the replication-initiator protein (Rep), the C2 protein, and the C4 protein. Ectopic expression of the BCTV C4 protein leads to a severe developmental phenotype characterized by extensive hyperplasia. We recently demonstrated that C4 interacts with 7 of the 10 members of the Arabidopsis thaliana SHAGGY-like protein kinase gene family and characterized the interactions of C4 and C4 mutants with AtSKs. Herein, we propose a model of how C4 functions.