Abstract
BackgroundOkra (Abelmoschus esculentus; family Malvaceae) is grown in temperate as well as subtropical regions of the world, both for human consumption as a vegetable and for industrial uses. Okra yields are affected by the diseases caused by phyopathogenic viruses. India is the largest producer of okra and in this region a major biotic constraint to production are viruses of the genus Begomovirus. Begomoviruses affecting okra across the Old World are associated with specific, symptom modulating satellites (beta satellites). We describe a comprehensive analysis of the diversity of beta satellites associated with okra in India.ResultsThe full-length sequences of 36 beta satellites, isolated from okra exhibiting typical begomovirus symptoms (leaf curl and yellow vein), were determined. The sequences segregated in to four groups. Two groups correspond to the beta satellites Okra leaf curl beta satellite (OLCuB) and Bhendi yellow vein beta satellite (BYVB) that have previously been identified in okra from the sub-continent. One sequence was distinct from all other, previously isolated beta satellites and represents a new species for which we propose the name Bhendi yellow vein India beta satellite (BYVIB). This new beta satellite was nevertheless closely related to BYVB and OLCuB. Most surprising was the identification of Croton yellow vein mosaic beta satellite (CroYVMB) in okra; a beta satellite not previously identified in a malvaceous plant species. The okra beta satellites were shown to have distinct geographic host ranges with BYVB occurring across India whereas OLCuB was only identified in northwestern India. Okra infections with CroYVMB were only identified across the northern and eastern central regions of India. A more detailed analysis of the sequences showed that OLCuB, BYVB and BYVIB share highest identity with respect βC1 gene. βC1 is the only gene encoded by beta satellites, the product of which is the major pathogenicity determinant of begomovirus-beta satellite complexes and is involved in overcoming host defenses based on RNAi.ConclusionThe diversity of beta satellites in okra across the sub-continent is higher than previously realized and is higher than for any other malvaceous plant species so far analyzed. The beta satellites identified in okra show geographic segregation, which has implications for the development and introduction of resistant okra varieties. However, the finding that the βC1 gene of the major okra beta satellites (OLCuB, BYVB and BYVIB) share high sequence identity and provides a possible avenue to achieve a broad spectrum resistance.
Highlights
Okra (Abelmoschus esculentus; family Malvaceae) is grown in temperate as well as subtropical regions of the world, both for human consumption as a vegetable and for industrial uses
Betasatellites are associated with leaf curl and yellow vein disease symptoms of okra in India A total of 36 leaf samples from okra plants showing yellow vein and/or leaf curl symptoms typical of begomoviruses (Figure 1) were collected from widely separated locations across India between 2005 and 2007 (Table 1, Figure 2)
Amplifications from non-symptomatic plants were uniformly negative. This result showed a beta satellite to be consistently associated with yellow vein and leaf curl symptoms of okra
Summary
Okra (Abelmoschus esculentus; family Malvaceae) is grown in temperate as well as subtropical regions of the world, both for human consumption as a vegetable and for industrial uses. India is the largest producer of okra and in this region a major biotic constraint to production are viruses of the genus Begomovirus. Begomoviruses affecting okra across the Old World are associated with specific, symptom modulating satellites (beta satellites). Geminiviruses are small plant-infecting, arthropodborne viruses with single-stranded (ss)DNA genomes that are encapsidated in twinned (geminate) quasi-isometric particles. These viruses are found in tropical to warm temperate geographical zones and infect a wide range of plants including crops, ornamental plants and weeds [1]. A small number of bipartite begomoviruses occur in the Old World, the majority are monopartite, having genomes consisting of only a homolog of the DNA A components of the bipartite viruses. It has become evident that, there are a few truly monopartite begomoviruses (such as Tomato yellow leaf curl virus [3], which has become globally widespread [4]), the majority are monopartite and associate with additional ssDNA molecules [5]
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