November 16, 2004
From: British Soc Plant Pathol. New Disease Reports Vol. 10 [edited]

Severe leaf curl disease of cowpea - a new disease of cowpea in northern India caused by mungbean yellow mosaic India virus and a satellite DNA b
A Rouhibakhsh, VG Malathi, Advanced Centre for Plant Virology, Division of Plant Pathology, Indian Agricultural Research Institute, New Delhi 110012, India. Accepted for publication 11 Aug 2004.

In the 2004 crop season, severe leaf distortion symptoms were observed in cowpea (_Vigna unguiculata_) in agricultural fields in northern India. Affected plants showed a yellow mosaic with downward leaf curling, vein swelling, and vein enations.

The virus causing this leaf curl disease was easily transmitted to cowpea seedlings by whitefly (_Bemisia tabaci_), resulting in a combination of symptoms. Typically, a few leaves showed only mild leaf curl symptoms, others showed a yellow mosaic without enation); the majority of the leaves showed a yellow mosaic with severe leaf curl and enations.

In Southern analysis, total nucleic acid extracted from symptomatic plants hybridized to radiolabeled probes specific to DNA A and DNA B of mungbean yellow mosaic India virus (MYMIV-[Cp]). As veinal enation is a typical
symptom associated with the presence of satellite DNA b, hybridization was also performed with a radiolabeled probe to DNA b associated with cotton leaf curl disease (CLCuD).

A positive signal obtained with extracts from leaves showing leaf curl symptoms indicated the presence of DNA b. However, no hybridization was seen with an extract from leaves showing only a yellow mosaic. Satellite DNA b was amplified with universal DNA b primers b01/ b02 (Briddon et al, 2003) and the ca.1.3 kb amplicon was cloned and sequenced (Accession No. AY728263). The nucleotide sequence of DNA b showed 59 per cent identity with DNA b associated with cotton leaf curl Rajasthan virus (Accession No. AY083590).

While MYMIV-[Cp] alone causes yellow mosaic symptoms, in the presence of DNA b more severe symptom phenotypes of vein enation and leaf curl occur. Thus, the pathogenicity of the virus is increased by the presence of satellite DNA b. This is the 1st report of a satellite DNA b associated with a bipartite begomovirus infecting a leguminous host.

Reference:
Briddon RW, Bull SE, Amin I, Idres AM, Mansoor S, Bedford ID, et al. Diversity of DNA b, a satellite molecule associated with some monopartite begomoviruses. Virology 2003; 312: 106-21.

[The gist of this piece is that the interaction between MYIMV and satellite DNA-beta results in more severe symptoms compared to plants inoculated with MYINV alone. Similar results have been reported for other DNA-beta molecules and viruses. DNA-beta molecules are symptom-modulating, ss-DNA satellites associated with monopartite begomoviruses belonging to the _Geminiviridae_. Cotton leaf curl Multan virus causes Cotton leaf curl disease (CLCuD), a major constraint to cotton production on the Indian subcontinent. It has been shown to be caused by a monopartite begomovirus (Cotton leaf curl Multan virus [CLCuMV]) and a novel single-stranded DNA satellite molecule termed CLCuD DNA-beta. The satellite molecule is trans-replicated by CLCuMV but does not possess the iteron sequences of this virus. Field surveys across all the cotton-growing regions of Pakistan
indicate that dual and multiple infections are the norm for CLCuD with no evidence of synergism. Despite the diversity of begomoviruses associated with CLCuD, only a single class of DNA-beta has been detected, suggesting
that this satellite has the capacity to be recruited by unrelated begomoviruses.

As of 2003, an 26 additional DNA beta molecules, associated with diverse plant species obtained from different geographical locations, have been cloned and sequenced. They were shown to be widespread in the Old World,
where monopartite begomoviruses are known to occur. Analysis of the sequences revealed a highly conserved organization for DNA-beta molecules consisting of a single conserved open reading frame, an adenine-rich
region, and a region of high sequence conservation [the satellite conserved region (SCR)]. The SCR contains a potential hairpin structure with the loop sequence TAA/GTATTAC, similar to the origins of replication of geminiviruses and nanoviruses. 2 major groups of DNA-beta satellites were resolved by phylogenetic analyses. One group originated from hosts within the _Malvaceae_ and the 2nd from a more diverse group of plants within the _Solanaceae_ and _Compositae_. Within the 2 clusters, DNA-beta molecules showed relatedness based both on host and geographic origin. These findings strongly support coadaptation of DNA-beta molecules with their respective helper begomoviruses.

The begomovirus-satellite disease complexes are associated with economically important diseases and have been isolated from vegetable and fiber crops, ornamental plants, and weeds throughout Africa and Asia. Their
widespread distribution and diversity, coupled to the global movement of plant material and the dissemination of the whitefly vector, suggests that these disease complexes pose a serious threat to tropical and sub-tropical agro-ecosystems worldwide.

Additional links: <http://gemini.biosci.arizona.edu/viruses/mymiv/index.html>
<http://www.icac.org/icac/projects/CommonFund/Leaf/proj_04_final.pdf>
<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=pubmed&dopt=Abstract&list_uids=12890625> 

<http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?holding=npg&cmd=Retrieve&db=PubMed&list_uids=12663223&...>  <http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=12890625&dopt=Abstract> 
- Mod.DH]