The family Geminiviridae is one of the largest and most important families of plant viruses. The small, single-stranded DNA genomes of geminiviruses encode 5-7 proteins that redirect host machineries and processes to establish a productive infection. These interactions reprogramme plant cell cycle and transcriptional controls, inhibit cell death pathways, interfere with cell signalling and protein turnover, and suppress defence pathways. This Review describes our current knowledge of how geminiviruses interact with their plant hosts and the functional consequences of these interactions.

Geminiviruses: masters at redirecting and reprogramming plant processes

The geminiviruses are a family of small, non-enveloped viruses with single-stranded, circular DNA genomes of 2500–5200 bases. Geminiviruses are transmitted by various types of insect (whiteflies, leafhoppers, treehoppers and aphids). Members of the genus Begomovirus are transmitted by whiteflies, those in the genera Becurtovirus, Curtovirus, Grablovirus, Mastrevirus and Turncurtovirus are transmitted by specific leafhoppers, the single member of the genus Topocuvirus is transmitted by a treehopper and one member of the genus Capulavirus is transmitted by an aphid. Geminiviruses are plant pathogens causing economically important diseases in most tropical and subtropical regions of the world. This is a summary of the International Committee on Taxonomy of Viruses (ICTV) Report on the taxonomy of the Geminiviridae which is available at www.ictv.global/report/geminiviridae.

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ICTV Virus Taxonomy Profile: Geminiviridae.

Geminiviruses are small DNA viruses that use plant replication machinery to amplify their genomes. Microarray analysis of the Arabidopsis (Arabidopsis thaliana) transcriptome in response to cabbage leaf curl virus (CaLCuV) infection uncovered 5,365 genes (false discovery rate <0.005) differentially expressed in infected rosette leaves at 12 d postinoculation. Data mining revealed that CaLCuV triggers a pathogen response via the salicylic acid pathway and induces expression of genes involved in programmed cell death, genotoxic stress, and DNA repair. CaLCuV also altered expression of cell cycle-associated genes, preferentially activating genes expressed during S and G2 and inhibiting genes active in G1 and M. A limited set of core cell cycle genes associated with cell cycle reentry, late G1, S, and early G2 had increased RNA levels, while core cell cycle genes linked to early G1 and late G2 had reduced transcripts. Fluorescence-activated cell sorting of nuclei from infected leaves revealed a depletion of the 4C population and an increase in 8C, 16C, and 32C nuclei. Infectivity studies of transgenic Arabidopsis showed that overexpression of CYCD3;1 or E2FB, both of which promote the mitotic cell cycle, strongly impaired CaLCuV infection. In contrast, overexpression of E2FA or E2FC, which can facilitate the endocycle, had no apparent effect. These results showed that geminiviruses and RNA viruses interface with the host pathogen response via a common mechanism, and that geminiviruses modulate plant cell cycle status by differentially impacting the CYCD/retinoblastoma-related protein/E2F regulatory network and facilitating progression into the endocycle.

https://www.jstor.org/stable/pdfplus/40066189.pdf

Global analysis of Arabidopsis gene expression uncovers a complex array of changes impacting pathogen response and cell cycle during geminivirus infection

Although viruses are most often studied as pathogens, many are beneficial to their hosts, providing essential functions in some cases and conditionally beneficial functions in others. Beneficial viruses have been discovered in many different hosts, including bacteria, insects, plants, fungi and animals. How these beneficial interactions evolve is still a mystery in many cases but, as discussed in this Review, the mechanisms of these interactions are beginning to be understood in more detail.

The good viruses: viral mutualistic symbioses

Plant viral movement proteins: agents for cell-to-cell trafficking of viral genomes.

The majority of plant-infecting viruses utilize an RNA genome, suggesting that plants have imposed strict constraints on the evolution of DNA viruses. The geminiviruses represent a family of DNA viruses that has circumvented these impediments to emerge as one of the most successful viral pathogens, causing severe economic losses to agricultural production worldwide. The genetic diversity reflected in present-day geminiviruses provides important insights into the evolution and biology of these pathogens. To maximize replication of their DNA genome, these viruses acquired and evolved mechanisms to manipulate the plant cell cycle machinery for DNA replication, and to optimize the number of cells available for infection. In addition, several strategies for cell-to-cell and long-distance movement of the infectious viral DNA were evolved and refined to be compatible with the constraints imposed by the host endogenous macromolecular trafficking machinery. Mechanisms also evolved to circumvent the host antiviral defense systems. Effectively combatting diseases caused by geminiviruses represents a major challenge and opportunity for biotechnology.

Exploiting chinks in the plant's armor: evolution and emergence of geminiviruses

A strong recovery response occurs in cantaloupe (Cucumis melo) and watermelon (Citrullus lanatus) infected with the bipartite begomovirus Cucurbit leaf crumple virus (CuLCrV). This response is characterized by initially severe symptoms, which gradually become attenuated (almost symptomless). An inverse relationship was detected between viral DNA levels and recovery, indicating that recovered tissues had reduced viral titers. Recovered tissues also were resistant to reinfection with CuLCrV; i.e., recovered leaves reinoculated with the virus did not develop symptoms or have an increased level of viral DNA. In contrast, infection of CuLCrV-recovered leaves with the RNA virus, Cucumber mosaic virus (CMV), disrupted recovery, resulting in the development of severe disease symptoms (more severe than those induced by CMV or CuLCrV alone) and increased CuLCrV DNA levels. Small RNAs with homology to CuLCrV DNA were detected in recovered and nonrecovered tissues; as well as in phloem exudates from infected, but not uninfected plants. Levels of these small RNAs were positively correlated with viral titer; thus, recovered tissues had lower levels than symptomatic tissues. In addition, viral DNA from a host that undergoes strong recovery (watermelon) was more highly methylated compared with that from a host that undergoes limited recovery (zucchini). Furthermore, inoculation of CuLCrV-infected zucchini with a construct expressing an inverted repeat of the CuLCrV common region enhanced recovery and reduced viral symptoms and viral DNA levels in newly emerged leaves. Taken together, these results suggest that recovery from CuLCrV infection is an adaptive antiviral defense mechanism, most likely mediated by gene silencing.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PHYTO-98-9-1029

Recovery from Cucurbit leaf crumple virus (family Geminiviridae, genus Begomovirus) infection is an adaptive antiviral response associated with changes in viral small RNAs.

The present invention provides gene targets, constructs and methods for the genetic control of plant disease caused by multiple plant viruses. The present invention relates to achieving a plant protective effect through the identification of target coding sequences and the use of recombinant DNA technologies for post-transcriptionally repressing or inhibiting expression of the target coding sequences of plant-parasitic viruses. Protein-expression based approaches may also be utilized to augment phenotype resistance. Thus, transcription of a single transgenic event comprising one or more plant expression cassettes can allow for broad spectrum resistance of a plant to multiple plant viral strains and species among the geminiviruses, tospoviruses, and potexviruses.

Multiple virus resistance in plants

In a virus-infected plant, small interfering RNAs (siRNAs) corresponding to the viral genome form a large proportion of the small RNA population. It is possible to reassemble significant portions of the virus sequence from overlapping siRNA sequences and use these to identify the virus. We tested this technique with a resistance-breaking and a non-resistance-breaking strain of tomato spotted wilt virus (TSWV). We were able to assemble contigs covering 99% of the genomes of both viruses. The abundance of TSWV siRNAs allowed us to detect TSWV at early time points before the onset of symptoms, at levels too low for conventional detection. Combining traditional and bioinformatic detection methods, we also measured how replication of the resistance-breaking strain differed from the non-resistance-breaking strain in susceptible and resistant tomato varieties. We repeated this technique in identification of a squash-infecting geminivirus and also used it to identify an unspecified tospovirus.

Using small RNA sequences to diagnose, sequence, and investigate the infectivity characteristics of vegetable-infecting viruses.

Hagen, C., Rojas, M. R., Sudarshana, M. R., Xoconostle-Cazares, B., Natwick, E. T., Turini, T. A., and Gilbertson, R. L. 2008. Biology and molecular characterization of Cucurbit leaf crumple virus, an emergent cucurbit-infecting begomovirus in the Imperial Valley of California. Plant Dis. 92:781-793. Cucurbit leaf crumple virus (CuLCrV) is an emergent and potentially economically important bipartite begomovirus first identified in volunteer watermelon plants in the Imperial Valley of southern California in 1998. Field surveys indicated that CuLCrV has become established in the Imperial Valley; and field plot studies revealed that CuLCrV primarily infects cucurbits, including cantaloupe, squash, and watermelon. Full-length DNA-A and DNA-B clones of an Imperial Valley isolate of CuLCrV were obtained by polymerase chain reaction (PCR) with overlapping primers. These clones were infectious in various cucurbits and common bean (cv. Topcrop); symptoms included stunted growth and leaf crumple, curl, and chlorosis. CuLCrV was not saptransmissible, and immunolocalization and DNA in situ hybridization studies revealed that it is phloem-limited. A CuLCrV agroinoculation system was generated, and host range studies revealed differential susceptibility in cucurbits, with squash, watermelon, cantaloupe, and honeydew melon being most to least susceptible, respectively. Germplasm screening studies identified a number of resistant cantaloupe and honeydew melon cultivars. The genome organization of this CuLCrV isolate (CuLCrV-CA) is similar to other bipartite begomoviruses, and phylogenetic analysis placed CuLCrV in the Squash leaf curl virus (SLCV) cluster of New World bipartite begomoviruses. A CuLCrV-specific PCR test was developed which allows for differentiation from other begomoviruses, including SLCV.

https://apsjournals.apsnet.org/doi/pdf/10.1094/PDIS-92-5-0781

Charles Hagen

Papers

Exploiting chinks in the plant's armor: evolution and emergence of geminiviruses

Recovery from Cucurbit leaf crumple virus (family Geminiviridae, genus Begomovirus) infection is an adaptive antiviral response associated with changes in viral small RNAs.

Multiple virus resistance in plants

Using small RNA sequences to diagnose, sequence, and investigate the infectivity characteristics of vegetable-infecting viruses.

Biology and Molecular Characterization of Cucurbit leaf crumple virus, an Emergent Cucurbit-Infecting Begomovirus in the Imperial Valley of California