Received: 13 March, 2007. Accepted: 2 May, 2007. Invited Mini-Review Plant Viruses ©2007 Global Science Books Watermelon mosaic virus Asad Ali 1* • Tomohide Natsuaki 2** 1 Plant Pathology Department, Faculty of Crop Protection Sciences, NWFP Agricultural University, Peshawar, Pakistan 2 Faculty of Agriculture, Utsunomiya University, Utsunomiya 321-8505, Japan Corresponding author: * asad72pk@yahoo.com, ** natsuaki@cc.utsunomiya-u.ac.jp ABSTRACT Potyviruses have been badly affecting crop yields in most parts of the world, with Zucchini yellow mosaic virus (ZYMV), Watermelon mosaic virus (WMV) and Papaya ringspot virus (PRSV) being of particular economic importance. Watermelon mosaic virus (WMV) causes severe economic losses in cucurbitaceous, leguminous, malvaceous and chenopodiaceous plants in temperate and Mediterranean regions. It produces chlorosis, mottling, blisters on leaves and fruits, leaf distortion and stunting in watermelon, muskmelon, squash, pumpkin and cucumber. WMV has been shown to infect experimentally, more than 170 plant species belonging to 27 plant families. The biological variability of WMV has been well-documented. Serologically, it is close to Soybean mosaic virus (SMV) and Papaya ringspot virus (PRSV), but distantly related to Potato virus Y (PVY) and Bean yellow mosaic virus (BYMV). The genome of the reported WMV isolates is more than 10kb, flanked by untranslated regions at both the ends. The large open reading frame (ORF) encodes a putative polyprotein of 3217 aa, with a calculated Mr. of 366,904. Sequence analyses of WMV isolates revealed close relationship with the reported isolates of SMV (84.7% to 85.8% aa identity). However, the N-terminal P1 protein encoding region was substantially different, presenting less than 35% identity. SimPlot analysis revealed that WMV arose through an ancestral event of interspecific recombination between SMV and Bean common mosaic virus (BCMV)/ Peanut stripe virus (PSV)-related potyviruses. Very little genetic material resistant to WMV-2 is available. Cultural practices, crop rotation, cross-protection and genetic resistance have effectively been used against WMV. Coat protein transgenic resistance to WMV has also been reported in squash and cantaloupe. _____________________________________________________________________________________________________________ Keywords: WMV, cucurbit, potyvirus, recombination, sequence CONTENTS INTRODUCTION........................................................................................................................................................................................ 80 ECONOMIC IMPORTANCE ...................................................................................................................................................................... 81 SYMPTOMATOLOGY ............................................................................................................................................................................... 81 MOLECULAR CHARACTERIZATION .................................................................................................................................................... 81 CONTROL................................................................................................................................................................................................... 83 FUTURE PERSPECTIVES ......................................................................................................................................................................... 83 REFERENCES............................................................................................................................................................................................. 84 _____________________________________________________________________________________________________________ INTRODUCTION The genus Potyvirus is by far the largest of the known plant virus groups and contains nearly 200 definite and tentative species (Fan et al. 2003; Fauquet et al. 2005). Viruses in this genus are 680-900 nm in length, 11-13 nm in diameter and encapsidate a genome of about 10 kb comprising mul- tiple copies of a single protein species of 30-47 kDa (Shuk- la et al. 1994). They are transmitted by aphids in a non-per- sistent manner using helper components. Some of the mem- bers are seed-transmitted. Flexuous particles contain (+)- sense ssRNA with a 5 VPg, 5 non-coding region, single open reading frame (ORF) encoding a single poly-protein and 3 untranslated region (UTR). The polyprotein is later processed into 10 functional proteins by virus-encoded proteinases (Shukla et al. 1994). On a worldwide basis, potyviruses are badly affecting crop yields, with Zucchini yellow mosaic virus (ZYMV), Watermelon mosaic virus (WMV) and Papaya ringspot virus (PRSV) being of par- ticular economic importance (Lecoq et al. 2001). WMV, a pathogen of worldwide importance, causes economic losses in Cucurbitaceous, Leguminous, Malvace- ous and Chenopodiaceous plants (Purcifull et al. 1984). The biological variability of WMV is well documented (Purcifull and Hiebert 1979; van Regenmortel 1971). Lind- berg et al. (1956) carried out the first detailed study of viral isolates from cucurbits and classified the isolates into two groups, namely melon mosaic and squash mosaic groups. Subsequently, WMV isolates were classified as WMV-1 and WMV-2 (Purcifull and Hiebert 1979; Yeh et al. 1984). Specifically, isolates that failed to infect non-cucurbitaceous plants were designated WMV-2, while isolates that infected plants outside the Cucurbitaceae were designated WMV-1, although the latter is now considered to be a strain of PRSV (Purcifull et al. 1984). Purcifull and Heibert (1979) also re- ported a third isolate that did not react with antisera against either WMV-1 or WMV-2, and which has now been given the status of a distinct potyvirus species, the Moroccan watermelon mosaic virus (MWMV). Soybean mosaic virus (SMV) and Blackeye cowpea mosaic virus (BlCMV) are both closely related to, but distinct from, WMV (Fauquet et al. 2005). Although WMV has long been known in many parts of the world, it has not yet been characterized exten- sively at the molecular level. The complete genome sequen- ces of only three isolates have been reported very recently from France, Pakistan and China.