# Identification of the Begomoviruses Squash Leaf Curl Virus and Watermelon Chlorotic Stunt Virus in Various Plant Samples in North America

Rafaela S. Fontenele, Amulya Bhaskara, Ilaria N. Cobb, Lucas C. Majure, Andrew M. Salywon, Jesús A. Avalos-Calleros, Gerardo R. Argüello-Astorga, Kara Schmidlin, Philippe Roumagnac, Simone G. Ribeiro, et al.

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## ► To cite this version:

Rafaela S. Fontenele, Amulya Bhaskara, Ilaria N. Cobb, Lucas C. Majure, Andrew M. Salywon, et al. Identification of the Begomoviruses Squash Leaf Curl Virus and Watermelon Chlorotic Stunt Virus in Various Plant Samples in North America. Viruses, 2021, 13 (5), pp.810. 10.3390/v13050810. hal-03247581

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Article

# Identification of the Begomoviruses Squash Leaf Curl Virus and Watermelon Chlorotic Stunt Virus in Various Plant Samples in North America

Rafaela S. Fontenele 1,2 ID, Amulya Bhaskara 1,3, Ilaria N. Cobb 1,4, Lucas C. Majure 5 ID, Andrew M. Salywon 6,
Jesús A. Avalos-Calleros 7, Gerardo R. Argüello-Astorga 7, Kara Schmidlin 1, Philippe Roumagnac 8,9 ID,
Simone G. Ribeiro 10 ID, Simona Kraberger 1, Darren P. Martin 11, Pierre Lefeuvre 12 ID
and Arvind Varsani 1,2,13,14,* ID

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**Citation:** Fontenele, R.S.; Bhaskara, A.; Cobb, I.N.; Majure, L.C.; Salywon, A.M.; Avalos-Calleros, J.A.; Argüello-Astorga, G.R.; Schmidlin, K.; Roumagnac, P.; Ribeiro, S.G.; et al. Identification of the Begomoviruses Squash Leaf Curl Virus and Watermelon Chlorotic Stunt Virus in Various Plant Samples in North America. *Viruses* **2021**, *13*, 810. https://doi.org/10.3390/v13050810

Academic Editors: Jan Frederik Kreuze and Adrian Fox

Received: 5 April 2021
Accepted: 28 April 2021
Published: 30 April 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

**Abstract:** Geminiviruses are a group of plant-infecting viruses with single-stranded DNA genomes. Within this family, viruses in the genus *Begomovirus* are known to have a worldwide distribution causing a range of severe diseases in a multitude of dicotyledonous plant species. Begomoviruses are transmitted by the whitefly *Bemisia tabaci*, and their ssDNA genomes can be either monopartite or bipartite. As part of a viral survey, various plants including those in the families Alliaceae, Amaranthaceae, Apiaceae, Asteraceae, Brassicaceae, Cactaceae, Cucurbitaceae, Lamiaceae, Lauraceae, Malvaceae, Oleaceae and Solanaceae were sampled and screened for begomoviruses using both a high-throughput sequencing and a begomovirus-specific primer pair approach. Based on the sequences derived using these approaches, the full-length genome of various begomoviruses were amplified from plants using abutting primers. Squash leaf curl virus (SLCV) and watermelon chlorotic stunt virus (WCSV) were identified in Cactaceae (n = 25), Solanaceae (n = 7), Cucurbitaceae (n = 2) and Lamiaceae (n = 1) samples. WCSV is an Old World bipartite begomovirus that has only recently been discovered infecting watermelons in the Americas. Our discovery of WCSV in the USA is the first indication that it has reached this country and indicates that this virus might be widespread throughout North America. Phylogenetic analysis suggests WCSV was introduced to the New World twice. The detection of begomoviruses in cactus plants suggests possible spillover events from agricultural areas into native vegetation. Since WCSV and SLCV have previously been found in mixed infections, pseudo-recombination infection experiments were conducted. We demonstrate that WCSV DNA-B is successfully trans-replicated by SLCV DNA-A despite very low degree of similarity

<footnote n="1">¹ The Biodesign Center for Fundamental and Applied Microbiomics, Arizona State University, Tempe, AZ 85287, USA; rafasfontenele@asu.edu (R.S.F.); amulyabhaskara@gmail.com (A.B.); ilaria.cobb@gmail.com (I.N.C.); kara.schmidlin@asu.edu (K.S.); simona.kraberger@asu.edu (S.K.)</footnote>

<footnote n="2">² School of Life Sciences, Arizona State University, Tempe, AZ 85287, USA</footnote>

<footnote n="3">³ School of Behavioral and Brain Sciences, University of Texas at Dallas, 800 W Campbell Rd, Richardson, TX 75080, USA</footnote>

<footnote n="4">⁴ The University of British Columbia, Vancouver, BC V6T 1Z4, Canada</footnote>

<footnote n="5">⁵ Florida Museum of Natural History, University of Florida, Gainesville, FL 32611, USA; lmajure@floridamuseum.ufl.edu</footnote>

<footnote n="6">⁶ Desert Botanical Garden, Phoenix, AZ 85008, USA; asalywon@dbg.org</footnote>

<footnote n="7">⁷ División de Biología Molecular, Instituto Potosino de Investigación Científica y Tecnológica, A.C., Camino a la Presa de San José 2055, Lomas 4ta Secc, San Luis Potosi 78216, Mexico; jesus.avalos@ipicyt.edu.mx (J.A.A.-C.); grarguel@ipicyt.edu.mx (G.R.A.-A.)</footnote>

<footnote n="8">⁸ CIRAD, UMR PHIM, 34090 Montpellier, France; philippe.roumagnac@cirad.fr</footnote>

<footnote n="9">⁹ PHIM Plant Health Institute, Univ Montpellier, CIRAD, INRAE, Institut Agro, IRD, 34090 Montpellier, France
10 Embrapa Recursos Genéticos e Biotecnologia, Brasília 70770-917, Brazil; simone.ribeiro@embrapa.br</footnote>

<footnote n="11">¹¹ Computational Biology Division, Department of Integrative Biomedical Sciences, Institute of Infectious Diseases and Molecular Medicine, University of Cape Town, Observatory, 7925 Cape Town, South Africa; darrenpatrickmartin@gmail.com</footnote>

<footnote n="12">12 CIRAD, UMR PVBMT, F-97410 St. Pierre, France; pierre.lefeuvre@cirad.fr</footnote>

<footnote n="13">13 Center for Evolution and Medicine, Arizona State University, Tempe, AZ 85287, USA</footnote>

<footnote n="14">¹⁴ Structural Biology Research Unit, Department of Integrative Biomedical Sciences, University of Cape Town, 7925 Cape Town, South Africa</footnote>

<footnote>* Correspondence: Arvind varsani@asu.edu</footnote>
between the replication-associated iterative sequences present in their common region, an essential feature for binding of the replication associated protein. This study highlights the importance of viral surveys for the detection of spillover events into native vegetation, but also suggests the need for more surveillance of WCSV in the USA, as this virus is a serious threat to watermelon cultivation in the Middle East.

**Keywords:** *Geminiviridae; begomovirus; pseudo-recombination; recombination*

# 1. Introduction

The family *Geminiviridae* is a group of plant-infecting viruses with circular single-stranded DNA genomes encapsidated in twinned semi-icosahedral particles [1]. Within this family, the members of the genus *Begomovirus* have a worldwide distribution, infecticotyledonous plants and are transmitted by whiteflies (*Bemisia tabaci*). With over 424 species, the genus *Begomovirus* is the largest geminivirus genus. Begomoviruses have emerged as severe pathogens causing high yield losses on a variety of economically important crops [2,3]. Begomovirus genomes have either one (monopartite) or two (bipartite) genomic components named DNA-A and DNA-B. Monopartite begomoviruses are mainly found in the Old World, whereas New World begomoviruses are predominantly bipartite. In bipartite begomoviruses, the DNA-A component encodes five proteins, the replication associated protein (Rep), replication enhancer protein (Ren), transactivator protein (Trap), a capsid protein (CP) and C4, a symptom determinant protein. The DNA-B encodes a movement protein (MP) and a nuclear shuttle protein (NSP); hence, the DNA-B component is responsible for cell-to-cell and long distance movement within the infected plant. For bipartite begomoviruses, cognate DNA-A and DNA-B components share a common region (CR) in the intergenic region, having a high-enough degree of similarity to allow the Rep encoded by DNA-A to replicate both components.

Squash leaf curl virus (SLCV) is a typical New World bipartite begomovirus that was initially identified and characterized infecting squash in the USA [4,5] and since then has been shown to be widespread throughout North America [6–8]. In 2002, SLCV was identified infecting squash plants in Israel [9], which was the first time a New World begomovirus was documented in the Old World. SLCV is now widely spread in the greater Middle East (Egypt, Jordan, Lebanon, Oman, Pakistan and Palestine) [10–15]. Although SLCV mainly infects squash plants, causing severe curling and yellow mottling of leaves, the host range of this begomovirus includes all major cultivated crops from the family Cucurbitaceae. In addition, it has also been identified infecting some Solanaceae species including tomato and pepper, two Malvaceae species (cotton and *Malva parviflora*) and an uncultivated Brassicaceae species (*Sinapis arvensis*). SLCV is also commonly found in mixed infections with tomato yellow leaf curl virus, cucurbit leaf crumple virus and watermelon chlorotic stunt virus, mixtures which usually increase disease severity [7,16,17].

Watermelon chlorotic stunt virus (WCSV) is a bipartite begomovirus of the Old World that was initially identified infecting watermelon in Yemen [18] but has been extensively described in several Middle East countries/regions (Iran, Israel, Jordan, Lebanon, Oman, West bank, Palestine and Saudi Arabia) and Sudan in Africa [15,19–25]. WCSV can also infect other cucurbits but mainly causes severe damage to watermelon crops [15]. Recently, WCSV has been found infecting watermelon in Sonora, Mexico [26] causing the typical symptoms of leaf curling and yellowing, which was the first identification of this begomovirus species in the Western hemisphere (i.e., New World).

Here, we describe and characterize isolates of SLCV and WCSV found infecting plants in the Cactaceae, Cucurbitaceae, Lamiaceae and Solanaceae families in the USA and Mexico. The three WCSV isolates reported in this study are the first identification of this virus in the USA, and their presence shows that this virus is likely more widespread in North America than previously known. The identification of both SLCV and WCSV infecting
Cactaceae species provides evidence of spillover events from agro-ecosystems into native vegetation. The biological implications of begomoviruses infecting cactus plants need to be further investigated. The demonstration that a cactus-derived SLCV DNA-A was able to replicate a cactus-derived WCSV DNA-B in *Nicotiana benthamiana* plants and cause mild curling symptoms suggests that these invasive pathogens may pose new ecological and/or agricultural threats.

## 2. Materials and Methods

### 2.1. *Sample Collection and Processing*

Cactaceae plants were sampled as part of a viral survey and processed as described in [27]. In addition, Alliaceae, Amaranthaceae, Apiaceae, Asteraceae, Brassicaceae, Cucurbitaceae, Lamiaceae, Lauraceae, Malvaceae, Oleaceae and Solanaceae plants were sampled from five different locations in Arizona (USA); a community garden (*n* = 3); three farms (*n* = 20) and an herb garden at the Desert Botanical Garden in Phoenix, AZ (*n* = 25) (Supplementary Table S1). Total DNA was extracted from non-cactus samples using either the GeneJET Plant Genomic DNA Purification Kit (Thermo Fisher Scientific, Waltham, MA, USA) or the GenCatch Plant Genomic DNA Purification Kit (Epoch Life Science, Missouri City, TX, USA) according to the manufacturer's instructions.

### 2.2. *Genome Assembly and Annotation*

Using previously acquired high-throughput sequencing data from cactus samples [27], the contigs were mined for those with similarities to begomoviruses. Pairs of abutting primers were designed to recover the full-length DNA-A and DNA-B components of watermelon chlorotic stunt virus (WCSV) and squash leaf curl virus (SLCV) from cactus and other sampled plants (Supplementary Table S1). Samples were also screened with the universal primer pair PAL1v1978/PAR1c715 for the DNA-A component of begomoviruses [28] to identify begomovirus positive samples, these amplicons were cloned and sequenced to confirm their identity.

PCR with primers listed in Supplementary Table S2 (designed based on de novo assembled high throughput sequencing data) was preformed using KAPA HiFi HotStart DNA polymerase (KAPA Biosystems, Wilmington, MA, USA). Amplicons were resolved in a 0.7% agarose gel and amplicon with a size of ~2.5–3 kb were excised, gel purified and cloned in the pJET1.2 cloning vector (Thermo Fisher Scientific, Waltham, MA, USA). The recombinant plasmids were Sanger sequenced by primer walking at Macrogen Inc. (Seoul, South Korea), and assembly and annotations were carried out using Geneious 11.1.5 (Biomatters Ltd., Auckland, New Zealand).

### 2.3. *Infectivity Assays*

Infectivity assays were performed with isolates of DNA-A and DNA-B from both WCSV and SLCV. For the WCSV, the DNA-A component isolate LCM_52 (GenBank accession MW588390) and the DNA-B component isolate LCM_53 (GenBank accession MW588417) were used to generate two infectious clones. For the infectious clones of the SLCV, the DNA-A component isolate LCM_89_SP41 (GenBank accession MW588381) and the DNA-B isolate LCM_89_SP58 (GenBank accession MW588407) were used. These DNA-A and DNA-B components were recovered from cactus samples (Table 1). Specific primers were designed to amplify two copies of each cactus-derived begomoviruses component that were cloned in tandem to the binary vector pJL-89 [29], with the 35S promoter excluded as previously described by Ferro et al. [30], using Gibson assembly [31] (New England Biolabs, Ipswich, MA, USA). Each clone was transformed into competent *Escherichia coli* XL1 Blue cells. To further confirm the correct orientations of the cloned tandem genomes, clones were analyzed by restriction enzyme digestion: SLCV DNA-A and DNA-B, WCSV DNA-B using EcoRV and WSCV DNA-A using Sall. Clones containing the two tandemly cloned copies of DNA-A or DNA-B from SLVC or WCSV were used to transform *Rhizobium radiobacter* GV3101.
<table><caption>**Table 1.** Summary of the begomovirus components identified in each plant host, including the host family species, country and year of collection.</caption><thead><tr><th>Begomovirus</th><th>Accession #</th><th>Host</th><th>Isolate</th><th>Region of Collection</th><th>Collection Year</th><th>Component</th></tr></thead><tbody><tr><td>Squash leaf curl virus KPB1 SB</td><td>MW588396</td><td>Capsicum annuum Jalapeño</td><td>KPB1_SB</td><td>USA</td><td>2019</td><td>B</td></tr><tr><td rowspan="2">Squash leaf curl virus SF6</td><td>MW588388</td><td rowspan="2">Capsicum sp.</td><td>SF6_SA</td><td>USA</td><td>2019</td><td>A</td></tr><tr><td>MW588413</td><td>SF6_SB</td><td>USA</td><td>2019</td><td>B</td></tr><tr><td>Squash leaf curl virus SF3 SA</td><td>MW588385</td><td>Solanum melongena</td><td>SF3_SA</td><td>USA</td><td>2019</td><td>A</td></tr><tr><td rowspan="2">Squash leaf curl virus SF4</td><td>MW588386</td><td rowspan="2">Cucurbita pepo</td><td>SF4_SA</td><td>USA</td><td>2019</td><td>A</td></tr><tr><td>MW588411</td><td>SF4_SB</td><td>USA</td><td>2019</td><td>B</td></tr><tr><td>Squash leaf curl virus LCM95 SP41</td><td>MW588383</td><td>Cylindropuntia whipplei</td><td>LCM95_SP41</td><td>USA</td><td>2015</td><td>A</td></tr><tr><td rowspan="2">Squash leaf curl virus LCM93</td><td>MW588409</td><td rowspan="2">Ferocactus cylindraceus</td><td>LCM93_SP58</td><td>USA</td><td>2016</td><td>B</td></tr><tr><td>MW588410</td><td>LCM93_SP2634</td><td>USA</td><td>2016</td><td>B</td></tr><tr><td>Squash leaf curl virus LCM90 SP344</td><td>MW588382</td><td>Leptocereus quadricostatus</td><td>LCM90_SP344</td><td>USA</td><td>2015</td><td>A</td></tr><tr><td>Squash leaf curl virus LCM67 SP209</td><td>MW588400</td><td>Opuntia andersoni</td><td>LCM67_SP209</td><td>Mexico</td><td>2009</td><td>B</td></tr><tr><td rowspan="4">Squash leaf curl virus LCM65</td><td>MW588372</td><td rowspan="4">Opuntia arenaria</td><td>LCM65_SP41</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>MW588373</td><td>LCM65_SP344_1</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>MW588374</td><td>LCM65_SP344_2</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>MW588398</td><td>LCM65_SP209</td><td>USA</td><td>2009</td><td>B</td></tr><tr><td rowspan="3">Squash leaf curl virus LCM69</td><td>MW588375</td><td rowspan="3">Opuntia atrispina</td><td>LCM69_SP41</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>MW588376</td><td>LCM69_SP341</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>MW588377</td><td>LCM69_SP344</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>Squash leaf curl virus LCM66 SP209</td><td>MW588399</td><td>Opuntia aureispina</td><td>LCM66_SP209</td><td>USA</td><td>2006</td><td>B</td></tr><tr><td rowspan="3">Squash leaf curl virus LCM89</td><td>MW588381</td><td rowspan="3">Opuntia basilaris var. longiareolata</td><td>LCM89_SP41</td><td>USA</td><td>2015</td><td>A</td></tr><tr><td>MW588407</td><td>LCM89_SP58</td><td>USA</td><td>2015</td><td>B</td></tr><tr><td>MW588408</td><td>LCM89_SP2634</td><td>USA</td><td>2015</td><td>B</td></tr><tr><td>Squash leaf curl virus LCM68 SP209</td><td>MW588401</td><td>Opuntia boldinghii</td><td>LCM68_SP209</td><td>USA</td><td>2009</td><td>B</td></tr><tr><td>Squash leaf curl virus LCM74 SP1332</td><td>MW588402</td><td>Opuntia bravoana</td><td>LCM74_SP1332</td><td>Mexico</td><td>2009</td><td>B</td></tr><tr><td rowspan="2">Squash leaf curl virus LCM76</td><td>MW588380</td><td rowspan="2">Opuntia caracassana</td><td>LCM76_SA</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>MW588404</td><td>LCM76_SP1332</td><td>USA</td><td>2009</td><td>B</td></tr><tr><td rowspan="3">Squash leaf curl virus LCM75</td><td>MW588378</td><td rowspan="3">Opuntia carstenii</td><td>LCM75_SP41</td><td>Mexico</td><td>2009</td><td>A</td></tr><tr><td>MW588379</td><td>LCM75_SP344</td><td>Mexico</td><td>2009</td><td>A</td></tr><tr><td>MW588403</td><td>LCM75_SP1332</td><td>Mexico</td><td>2009</td><td>B</td></tr></tbody></table>
<table><caption>Table 1. *Cont.*</caption><thead><tr><th>Begomovirus</th><th>Accession #</th><th>Host</th><th>Isolate</th><th>Region of Collection</th><th>Collection Year</th><th>Component</th></tr></thead><tbody><tr><td>Squash leaf curl virus LCM78 SP1332</td><td>MW588405</td><td>Opuntia chaffeyi</td><td>LCM78_SP1332</td><td>Mexico</td><td>2006</td><td>B</td></tr><tr><td>Squash leaf curl virus LCM79 SP1332</td><td>MW588406</td><td>Opuntia chaffeyi</td><td>LCM79_SP1332</td><td>Mexico</td><td>2006</td><td>B</td></tr><tr><td>Squash leaf curl virus LCM56 SP341</td><td>MW588369</td><td>Opuntia guatemalensis</td><td>LCM56_SP341</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>Squash leaf curl virus LCM58 SP41</td><td>MW588370</td><td>Opuntia hondurensis</td><td>LCM58_SP41</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>Squash leaf curl virus LCM60 SP341</td><td>MW588371</td><td>Opuntia inaperta</td><td>LCM60_SP341</td><td>USA</td><td>2009</td><td>A</td></tr><tr><td>Squash leaf curl virus LCM62 SP8848</td><td>MW588397</td><td>Opuntia karwinskiana</td><td>LCM62_SP8848</td><td>USA</td><td>2009</td><td>B</td></tr><tr><td>Squash leaf curl virus LCM29 SP41</td><td>MW588368</td><td>Opuntia puberula</td><td>LCM29_SP41</td><td>Mexico</td><td>2009</td><td>A</td></tr><tr><td>Squash leaf curl virus DBG34 SP15973</td><td>MW588395</td><td>Opuntia robusta</td><td>DBG34_SP15973</td><td>USA</td><td>2018</td><td>B</td></tr><tr><td>Squash leaf curl virus LCM96 SP41</td><td>MW588384</td><td>Pereskiopsis kellermannii</td><td>LCM96_SP41</td><td>USA</td><td>2016</td><td>A</td></tr><tr><td rowspan="2">Squash leaf curl virus SF5</td><td>MW588387</td><td rowspan="2">Raphanus sativus</td><td>SF5_SA</td><td>USA</td><td>2019</td><td>A</td></tr><tr><td>MW588412</td><td>SF5_SB</td><td>USA</td><td>2019</td><td>B</td></tr><tr><td rowspan="2">Squash leaf curl virus CG5</td><td>MW588366</td><td rowspan="2">Solanum lycopersicum</td><td>CG5_SA</td><td>USA</td><td>2018</td><td>A</td></tr><tr><td>MW588393</td><td>CG5_SB</td><td>USA</td><td>2018</td><td>B</td></tr><tr><td rowspan="2">Squash leaf curl virus CG6</td><td>MW588367</td><td rowspan="2">Solanum lycopersicum</td><td>CG6_SA</td><td>USA</td><td>2018</td><td>A</td></tr><tr><td>MW588394</td><td>CG6_SB</td><td>USA</td><td>2018</td><td>B</td></tr><tr><td rowspan="2">Squash leaf curl virus SWAT</td><td>MW588389</td><td rowspan="2">Citrullus lanatus</td><td>SWAT_SA</td><td>USA</td><td>2019</td><td>A</td></tr><tr><td>MW588414</td><td>SWAT_SB</td><td>USA</td><td>2019</td><td>B</td></tr><tr><td>Watermelon chlorotic stunt virus HERB15</td><td>MW588391</td><td>Solanum sp.</td><td>HERB15</td><td>USA</td><td>2019</td><td>A</td></tr><tr><td>Watermelon chlorotic stunt virus HERB28</td><td>MW588392</td><td>Stachys byzantina</td><td>HERB28</td><td>USA</td><td>2019</td><td>A</td></tr><tr><td>Watermelon chlorotic stunt virus LCM53</td><td>MW588417</td><td>Opuntia cochenillifera</td><td>LCM53</td><td>USA</td><td>2006</td><td>B</td></tr><tr><td>Watermelon chlorotic stunt virus LCM 52</td><td>MW588390</td><td>Opuntia auberi</td><td>LCM_52</td><td>Mexico</td><td>2006</td><td>A</td></tr><tr><td>Watermelon chlorotic stunt virus LCM51</td><td>MW588416</td><td>Opuntia auberi</td><td>LCM51</td><td>Mexico</td><td>2006</td><td>B</td></tr><tr><td>Watermelon chlorotic stunt virus LCM50</td><td>MW588415</td><td>Consolea spinosissima</td><td>LCM50</td><td>USA</td><td>2006</td><td>B</td></tr></tbody></table>

Infection assays were performed in *Nicotiana benthamiana* plants. In each experiment ten *N. benthamiana* plants were inoculated with WCSV DNA-A/DNA-B or SLCV DNA-A/DNA-B. To investigate whether the DNA-B of each virus could be efficiently replicated by the DNA-A of the other, infection assays were also performed with either SLCV DNA-A/WCSV DNA-B and WCSV DNA-A/SLCV DNA-B combinations. In all inoculations, *R. radiobacter* was grown for 20 h in Luria broth with kanamycin (50 µg/mL)
and rifampicin (50 µg/mL). The culture was then centrifuged for 10 min at 4600 rpm to pellet the cells before resuspension in MES buffer (10 mM MES hydrate and 10 mM MgSO4•7H2O) with 150 µM of acetosyringone to an OD600nm of 1.0. Equal volumes of each component (OD600nm 1.0) were mixed together prior to inoculation. Systemic infection was tested by collecting newly emerged leaves. Total DNA extractions from leaf material were performed using the GenCatch Plant Genomic DNA Purification Kit (Epoch Life Science, Missouri City, TX, USA) according to the manufacturer's instructions. Total DNA was tested by PCR with pairs of abutting primers for each virus component (Supplementary Table S2) using KAPA HiFi HotStart DNA polymerase (KAPA Biosystems, Wilmington, MA, USA).

## 2.4. Sequence Analyses

The full-length sequence of each component from the SLCV and WCSV together with available genomes from the same species (downloaded from GenBank on 16 December 2020) were used to generate a dataset for each virus component. For each dataset of SCLV and WCSV components, sequences were aligned using MAFFT v.7 [32] and subsequently used to detect recombination using the program RDP v.5.5 [33]. The methods RDP [34], GENECONV [35], BOOTSCAN [36], MAXCHI [37], CHIMERA [38], SISCAN [39] and 3SEQ [40] were used with default parameters for the recombination analysis. Only recombination events that were detected by more than three methods with a *p*-value <0.05 were accepted.

Alignments with recombinant regions removed were used for phylogenetic analysis. Following ModelFinder [41] analyses, the nucleotide substitution models used were TIM + F+G4, TIM3 + F+G4, TIM2 + F+G4 and HKY + F+G4 for WCSV DNA-A, WCSV DNA-B, SLCV DNA-A and SLCV DNA-B, respectively. The Maximum-Likelihood (ML) phylogenetic tree for each dataset was inferred using IQ-TREE [42] with 1000 bootstrap replicates for branch support from which branches with <60% support were collapsed using TreeGraph2 [43]. With the exception of the SLCV DNA-A ML phylogenetic tree that was rooted with SLCV DNA-A sequences from Pakistan (MF504011 and MF504010); all other ML phylogenetic trees were rooted with other begomovirus sequences. The ML phylogenetic trees were visualized and edited using iTOL [44]. All pairwise identities were determined using SDTv1.2 [45].

# 3. Results and Discussion

After a viral survey on cactus samples using high-throughput sequencing [27], 23 contigs with similarities to WCSV and SLCV were identified. Amplification and cloning of possible full-length genomes using abutting primers designed from these contigs showed the presence of the begomovirus squash leaf curl virus (SLCV) and watermelon chlorotic stunt virus (WCSV) in 25 cactus plants. Additional non-cactus plants sampled from five locations in the state of Arizona were also screened for the presence of SLCV and WCSV, and from those, 10 plants were found to be infected. In some cases, only one component from each virus could be detected (Table 1). In samples where only DNA-B could be identified, no other begomovirus was detected using the degenerate begomoviruses DNA-A component primer pair, PAL1v1978/PAR1c715 [28]. However, for the *Opuntia robusta* (Lab ID DBG34) sample in which the SLCV DNA-B was identified, Opuntia virus 1 had already been characterized through high throughput sequencing based approaches [27]. In addition, two other samples (Table 1) that only contained DNA-B components also had sub-genomic molecules with similarity to becurtoviruses and Opuntia virus 1. Therefore, it is highly likely for the plant where only the DNA-B was identified, that other geminiviruses were present but we were unable to detect them. Importantly, these might be able to replicate the DNA-B component.
### 3.1. Squash Leaf Curl Virus

A total of 24 SLCV DNA-A and 22 SLCV DNA-B component sequences from 29 plants including cactus (Cactaceae) (n = 21), eggplant (Solanaceae) (n = 1), tomato (Solanaceae) (n = 3), zucchini (Cucurbitaceae) (n = 1), watermelon (Cucurbitaceae) (n = 1) and pepper (Solanaceae) (n = 2) (Table 1) were determined. Both SLCV DNA-A and DNA-B were identified in 10 samples, and only DNA-A was identified and recovered from nine samples and only DNA-B from 10 samples (Table 1). To our knowledge, SLCV had been previously identified in plants from the families Brassicaceae, Cucurbitaceae, Malvaceae and Solanaceae but never from Cactaceae. From the 21 cactus plants analyzed, four presented both DNA-A and DNA-B, eight presented only DNA-A and the other nine samples only DNA-B.

The SLCV DNA-A component sequences share 92.5–98.3% pairwise identity with other SLCV DNA-A isolates and 98.6–100% identity amongst themselves (Supplementary Data S1). The SLCV DNA-B component sequences share 88.7–95.3% pairwise identity with other SLCV DNA-B isolates and 94.5–100% identity amongst themselves (Supplementary Data S1).

Recombination analysis of the SLCV DNA-A and DNA-B sequences from this study and others available in GenBank resulted in the identification of nine putative recombination events for DNA-A and 10 for DNA-B (Figure 1A,B). In the DNA-A dataset (n = 162), eight sequences were found to be recombinants, one of which presented two recombination events (MG763920). The recombinantionally derived genomic fragments ranged in size from 138 nts to 1292 nts with the majority occurring in the virion sense genes. For the SLCV DNA-B dataset (n = 44) eleven sequences were identified as recombinants with one sequence (MG763921) presenting four recombination events. Recombination event #6 was identified in five sequences and event #4 represents a recombination that occurred between sequences recovered from this study (USA). The transferred fragments ranged from 35 to 1000 nts.

Phylogenetic analysis of the SLCV DNA-A and DNA-B sequences indicated a degree of geographical clustering (Figure 2). The SLCV DNA-A sequences identified in this study all cluster within the same clade (Figure 2A. Clade A<sub>IV</sub>). In this clade all the cactus-derived sequences cluster together and are collectively most closely related to SLCV isolates from tomato plants (Figure 2A. Clade A<sub>IV</sub>). There is a distinct clade containing SLCV DNA-A sequences from the Middle East (Figure 2A. Clade A<sub>V</sub>). Interestingly, there is one SLCV DNA-A sequence recovered from zucchini in Mexico that stands out from this clade (Clade A<sub>V</sub>). Basal to clades A<sub>IV</sub> and A<sub>V</sub> are other SLCVs from the USA that infect watermelon and squash (including whiteflies found feeding on squash) (Figure 2A. Clades A<sub>II</sub> and A<sub>III</sub>) and two more diverse sequences from Pakistan found infecting cotton plants (Figure 2A. Clade A<sub>I</sub>). The geographical clustering observed for the Middle East sequences is similar to that which has been reported previously [46]. Based on the phylogeny it seems like SLCV could have been introduced once into the Middle East. The Pakistan isolates share ~92% pairwise identity with other SLCV isolates. Due to sampling bias, it is difficult to identify any associations between different SLCV clades and the host plants from which they were sampled.

Phylogenetic analysis of the SLCV DNA-B sequences revealed three distinct clades (Figure 2B). Clade B<sub>II</sub> contains SLCV DNA-B sequences from the Middle East and Clade B<sub>III</sub> contains all but one of the North American sequences, two sequences from Pakistan and one from Jordan. Lastly, one sequence from the USA identified infecting watermelon [47] sits outside those two clades (Clade B<sub>I</sub>). This pattern suggests that DNA-B may have been introduced to the Middle-East at least twice. These apparently distinct DNA-A and DNA-B evolutionary histories highlight once again the importance of component reassortments for bipartite begomoviruses [48,49].
<figure><img src="image_5.png" /><figcaption>**Figure 1.** Summary of the recombination events detected in the datasets of (A). SLCV DNA-A, (B). SLCV DNA-B, (C). WCSV DNA-A and (D). WCSV DNA-B by RDP5 v.5.5 [33]. The methods used to detect recombination are RDP (R) GENCONV (G), BOOTSCAN (B), MAXCHI (M), CHIMERA (C), SISCAN (S) and 3SEQ (T). The method with the highest *p*-value for each recombination event is in bold text. Sites where the actual breakpoint is undetermined are marked with *. Recombinant sequences marked with ^ indicated that recombinant sequence may have been misidentified (one of the identified parents might be the recombinant). On the right hand side of the table there is a graphical representation of each genome. Each recombination event is represented in orange relative to the reference genome.</figcaption></figure>
<figure><img src="image_6.png" /><figcaption>**Figure 2.** Maximum-Likelihood phylogenetic tree of the SCLV (A). DNA-A and (B). DNA-B component sequences. The country from which the sequences have been identified are color coded and the host/isolation source is marked by symbols. Clades containing the sequences from this study are colored in grey.</figcaption></figure>
## 3.2. *Watermelon Chlorotic Stunt Virus*

Three WCSV DNA-A isolates and three DNA-B isolates were recovered from six plants from the Cactaceae, Lamiaceae and Solanaceae families (Table 1). To our knowledge, with the exception of one other Solanaceae plant (*Datura innoxia*) [50], this is the first identification of WCSV from plants in the Cactaceae and Lamiaceae.

The three WCSV DNA-A sequences share >97.7% genome pairwise identity amongst themselves and 96.5–99.7% identity with other WCSV DNA-A sequences available in GenBank (Supplementary Data S2). The WCSV DNA-B sequences share 92–99% genome pairwise identity with other WCSV DNA-B sequences and 99.9% sequence identity amongst them. Similar to what was observed for the SLCV components, the WCSV DNA-B sequences are more diverse than the WCSV DNA-A sequences.

A single putative recombinant region was identified in the WCSV DNA-A dataset (n = 144) (Figure 3). The recombinant sequence was isolated from a watermelon plant in Israel. The recombinationally derived genome fragment was 315 nucleotides long and includes a portion of the *rep* and *ac4* coding regions. Meanwhile, three recombinant regions were identified in the WCSV DNA-B dataset (Figure 3). The recombinationally transferred genome fragments ranged in size from 343 nts to 1113 nts (~40% of the genome). No recombination was detected in any of the WCSV sequences recovered in this study.

The ML phylogenetic tree of the WCSV DNA-A sequences has four clades displaying some degree of geographical clustering (Figure 3A). In Clade A<sub>I</sub>, there are two sequences from Sudan while Clade A<sub>II</sub> includes sequences from Saudi Arabia and Yemen. Clade A<sub>III</sub> includes a sequence from Saudi Arabia in addition to sequences from Iran and Oman, and two of the three sequences from this study. Clade A<sub>IV</sub> is the largest and is mainly composed of sequences of isolates from watermelon plants. There are two groups within this clade, one composed mainly of sequences of isolates from Jordan and the other includes those from Israel, Lebanon, Palestine and two from Mexico recovered from watermelon [26] and cactus from this study. Based on this analysis, WCSV DNA-A seems to have been introduced to North America twice. The WCSV DNA-A sequence from cactus sits closely together with the Mexico isolate and those isolates share 99.7% pairwise identity (Supplementary Data S2).

The ML phylogenetic analysis of the WCSV DNA-B does not reveal clear clade structuring but shows similar geographical trends to the DNA-A phylogenetic analysis (Figure 3B). The three WCSV DNA-B sequences recovered from this are closely related to the WCSV DNA-B obtained from watermelon plants from Mexico in 2012, which was the first identification of WCSV in North America [26].
<figure><img src="image_7.png" /><figcaption>**Figure 3.** Maximum-Likelihood phylogenetic tree of the WCSV (A). DNA-A and (B). DNA-B component sequences. The country from which the sequences have been identified are color coded and the host/isolation source is marked by symbols. Clades containing the sequences from this study are colored in grey.</figcaption></figure>
### 3.3. Infectivity Assays for Pseudo-Recombination

WCSV and SLCV have been previously found in mixed infection [20] and in some cases synergistically interact to cause more severe symptoms than either virus alone [15,17]. During mixed infection, begomoviruses can also undergo pseudo-recombination (or reassortment), i.e., DNA-A and DNA-B components from two different viruses form a new association where the DNA-B is successfully replicated by the DNA-A encoded Rep protein. Both WCSV and SLCV have been, respectively, been found to pseudo-recombine (also referred to as reassortment) with the begomoviruses tomato leaf curl Palampur virus and cucurbit leaf curl virus [6,51]. However, to our knowledge pseudo-recombination between WCSV and SLCV have never been found in nature and have never been experimentally tested.

Infectivity assays with a combination of DNA-A and DNA-B components from both viruses were conducted. For the control experiment with both components of SLCV 10/10, *N. benthamiana* plants were systemically infected 15 days post inoculation as determined by PCR on total DNA of newly emerged leaves. The infected plants displayed symptoms of leaf curling (Figure 4). Meanwhile, 8/10 plants inoculated with SLCV DNA-A and WCSV DNA-B were found to be co-infected. Both SLCV DNA-A and WCSV DNA-B were detected by PCR in newly emerged leaves, which indicates systemic infection. The plants showed mild symptoms of leaf curling when compared to the symptoms displayed by the plants inoculated with both SLCV components (Figure 4). This demonstrates that SLCV DNA-A is able to pseudo-recombine with WCSV DNA-B causing mild symptoms in *N. benthamiana* plants. However, the ability of this SLCV-DNA-A/WCSV-DNA-B combination to infect plants in the family Cucurbitaceae needs to be investigated since these would presumably be the natural hosts of such combinations.

<figure><img src="image_8.png" /><figcaption>**Figure 4.** *Nicotiana benthamiana* plants infected with SLCV DNA-A/DNA-B; SLCV DNA-A/WCSV DNA-B showing leaf curling symptoms. As a control, an image of a noninoculated *N. benthamiana* plant.</figcaption></figure>

On the other hand, infectivity assays with both components of WCSV did not yield any infections. None of the inoculated plants displayed symptoms and neither of the WCSV components were detected by PCR in any of the *N. benthamiana* plants inoculated at 15 dpi. It is plausible that the infection assay did not work efficiently; however, the components used for these experiments were isolated from different plants since none of the plants screened in this study had both components of WCSV (Table 1). No infection could be
detected by PCR for the plants inoculated with WCSV DNA-A and SLCV DNA-B, but since the infection from WCSV components did not yield an infection, it is possible that the WCSV DNA-A clones used for these experiments were in some way defective.

The similarity of the common region (CR) between a pair of DNA-A and DNA-B components is cardinal for the capacity of DNA-A to trans-replicate DNA-B. Besides a conserved nonanucleotide sequence within a stem-loop structure at the origin of virion replication, the CR also contains TATA boxes and replication-associated iterative sequences called "iterons". Iteron sequence similarity between DNA-A and DNA-B components is essential for effective trans-replication. The SLCV CR has TATA box and iteron sequences that are typical of WCSV isolates (TGGTGTCC) that occur as four repeats and two inverted repeats [52] (Figure 5). In comparison, the WCSV CR has the iteron sequence typical of Old World begomoviruses, containing three previously reported directly repeated "TGGAGAC" sequences upstream of the TATA box [19,20,53] (Figure 5).

<figure><img src="image_9.png" /><figcaption>**Figure 5.** Alignment of the common region from SLCV and WCSV genomic components. Highlighted in purple is the TATA box, in light orange is the nonanucleotide sequence, in green are the identified "iteron" sequences for each begomovirus and the orange line indicate possible shared "iterons" between SLCV and WCSV. Graphical representation of the genomic organization of the DNA-A and DNA-B from SLCV and WCSV showing the position of the common region (CR) on the genome. *rep*—replication associated protein; *cp*—capsid protein; *ren*—replication enhancer protein; *trap*—transactivator protein; *v2*—movement proteins; *c4*—symptom determinant protein; *mp*—movement protein and *nsp*—nuclear shuttle protein.</figcaption></figure>

Since the SLCV DNA-A is able to trans-replicate the WCSV DNA-B, it was expected that the iteron sequences of WCSV DNA-B would be similar to those of SLCV. However, the alignment of the CR of the SLCV and WCSV components show that there are only two regions flanking the TATA box in the WCSV CR that have any observable similarity to the SLCV iterons: an inverted repeat, "GGACACgt" and a direct repeat "TGGaGaCt" (Figure 5). Nevertheless, our inoculation assay results demonstrate that the Rep of SLCV DNA-A is able to bind to the CR of the WCSV DNA-B to initiate replication. Similarly, viable pseudo-recombination has also been detected for the DNA-A and DNA-B of two tomato-infecting begomoviruses with different "iteron" sequences [54]. In previous reports of successful pseudo-recombination between WCSV and tomato leaf curl Palampur virus (ToLCPMV; [51]) and between SLCV and cucurbit leaf crumple virus (CuLCV; [6] the viruses shared similar iterons. It is possible that the iteron sequences are not the only factor contributing to the successful trans-replication of the WCSV DNA-B.

**4. Concluding Remarks**

This study reports the detection of SLCV (24 DNA-A and 22 DNA-B) and WCSV (3 DNA-A and 3 DNA-B) in 23 cactus plants, as well as 10 other plants from the USA and two cactus plants from Mexico (Table 1). Both SLCV and WCSV were also identified in
cultivated plants sampled in the state of Arizona (USA) showing those viruses are actively
circulating in the cropping systems of the area. Interestingly, that diverse begomoviruses
are associated with cactus may represent a spillover event from agricultural areas into
native vegetation. A broad diversity of native Cucurbitaceae plants are also found in
North America and could further act as intermediate reservoirs of SLCV and WCSV in
agro-ecological areas. It is important to point out that some of the cactus plants from
the USA originated from other countries but had been cultivated at the Desert Botanical
Garden in Phoenix (USA) for years before being sampled for this study. Hence, it cannot
be ruled out that those plants might have been infected before coming to the USA.

The identification of WCSV in the USA and Mexico suggests that this begomovirus
may be broadly distributed in North America. Phylogenetic analysis suggests that WCSV
may have been introduced to the New World on two different occasions. It is unclear
whether WCSV, an Old World begomovirus, will have as severe an impact in North
America as it has had in the Middle East [15,20,46]. Future surveys need to be conducted to
determine the distribution and spread of the virus in North America, especially given that
the insect vector *Bemisia tabaci* for begomoviruses is widespread on the continent. The Old
World begomovirus tomato yellow leaf curl virus (TYLCV) is an example of a successful
"invasive" species in the New World [55–57].

Here, we also demonstrate that SLCV DNA-A is able to replicate the WCSV DNA-B,
causing mild leaf curling symptoms. Although the exact molecular mechanism for this is
unclear, since these molecules do not share a similar common region or iteron sequences,
this demonstrates that encounters between those two viruses in mixed infection can lead
to pseudo-recombination in nature. Previous reports of pseudo-recombination for WCSV
with tomato leaf curl Palampur virus [51] and SLCV with cucurbit leaf crumple virus [6]
exist. However, in both cases, the viruses shared similar iterons. The fact that begomovirus
components with no iteron similarity were able to pseudo-recombine brings into question
what other molecular interactions are necessary for the initiation of replication.

Taken together, our results suggest that future surveillance studies are essential to
determine the distribution of the SLCV and WCSV globally. Even though the cactus
plants with begomovirus infection did not display any apparent symptoms, there is no
information on any long-term threat from begomoviruses to the local cactus populations.
Additionally, the vegetative propagation of several economically important cacti, such as
*Opuntia ficus-indica* and *Selenicereus undatus*, may represent a disregarded means by which
begomoviruses, as well as other geminiviruses that we have recently identified [27,58] in
cactus plant, could be spread throughout the world.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10.3390/v13050810/s1, Supplementary Data S1: Genomewide pairwise identity comparison of the SLCV DNA-A and DNA-B sequences from this study together with SLCV sequences in GenBank; Supplementary Data S2. Genomewide pairwise identity comparison of the WCSV DNA-A and DNA-B sequences in this study together with WCSV sequences in GenBank; Supplementary Table S1: Details of the noncactus plants sampled as part of this study including plant species, family, collection location, country and date; Supplementary Table S2: Pairs of abutting primers used in this study.

**Author Contributions:** Conceptualization, R.S.F. and A.V.; methodology, R.S.F. and A.V.; software, R.S.F., D.P.M. and A.V.; validation, R.S.F. and A.V.; formal analysis, R.S.F., A.B., I.N.C., J.A.A.-C., D.P.M., P.L., A.V.; investigation, R.S.F., A.B., I.N.C., L.C.M., A.M.S., J.A.A.-C., G.R.A.-A., K.S., P.R., S.G.R., S.K., D.P.M., P.L., A.V.; resources, R.S.F., A.B., I.N.C., L.C.M., A.M.S., J.A.A.-C., G.R.A.-A., K.S., P.R., S.G.R., S.K., D.P.M., P.L., A.V.; data curation, R.S.F. and A.V.; writing—original R.S.F. and A.V.; writing—review and editing, R.S.F., A.B., I.N.C., L.C.M., A.M.S., J.A.A.-C., G.R.A.-A., K.S., P.R., S.G.R., S.K., D.P.M., P.L., A.V.; visualization, R.S.F. and A.V.; supervision, A.V.; project administration, A.V.; funding acquisition, A.V. All authors have read and agreed to the published version of the manuscript.

**Funding:** The molecular work was supported by a start-up grant awarded to AV from Arizona State University, USA.

**Institutional Review Board Statement:** Not applicable.
**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Sequence determined as part of this study have been deposited in GenBank under accession #s MW588366-MW588417.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

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