Host range and genetic diversity of croton yellow vein mosaic virus, a weed-infecting monopartite begomovirus causing leaf curl disease in tomato

Croton yellow vein mosaic virus (CYVMV) is a widely occurring begomovirus in Croton bonplandianum, a common weed in the Indian subcontinent. In this study, CYVMV (genus Begomovirus, family Geminiviridae) was transmitted by whiteflies (Bemisia tabaci) to as many as 35 plant species belonging to 11 families, including many vegetables, tobacco varieties, ornamentals and weeds. CYVMV produced bright yellow vein symptoms in croton, whereas in all the other host species, the virus produced leaf curl symptoms. CYVMV produced leaf curl in 13 tobacco species and 22 cultivars of Nicotiana tabacum and resembled tobacco leaf curl virus (TobLCV) in host reactions. However, CYVMV was distinguished from TobLCV in four differential hosts, Ageratum conyzoides, C. bonplandianum, Euphorbia geniculata and Sonchus bracyotis. The complete genome sequences of four isolates originating from northern, eastern and southern India revealed that a single species of DNA-A and a betasatellite, croton yellow vein mosaic betasatellite (CroYVMB) were associated with the yellow vein mosaic disease of croton. The sequence identity among the isolates of CYVMV DNA-A and CroYVMB occurring in diverse plant species was 91.8-97.9 % and 83.3-100 %, respectively. The CYVMV DNA-A and CroYVMB generated through rolling-circle amplification of the cloned DNAs produced typical symptoms of yellow vein mosaic and leaf curling in croton and tomato, respectively. The progeny virus from both the croton and tomato plants was transmitted successfully by B. tabaci. The present study establishes the etiology of yellow vein mosaic disease of C. bonplandianum and provides molecular evidence that a weed-infecting monopartite begomovirus causes leaf curl in tomato.

Diversity and distribution of begomoviruses infecting tomato in India

Leaf curl begomoviruses cause serious yield losses to Indian tomato crops. Total DNAs were extracted from leaves of 69 tomato plants and 34 weeds or neighbouring crops collected from all the major tomato producing areas of India. Eighty-one of the 103 samples were positive by PCRs using begomovirus genus-specific primers. Coat protein (CP) genes from 29 samples were PCR amplified, cloned and sequenced. Phylogenetic analyses of the CP sequences revealed five different tomato leaf curl begomovirus (TLCB) clusters each <88% identity to the others. Four clusters represented known Indian TLCBs, whereas one cluster contained sequences originating from Haryana State with most identity (89%) to the provisional Begomovirus species Croton yellow vein mosaic virus.Sixty-five begomovirus positive samples were characterised further by PCR with DNA-beta, DNA-B, four Indian TLCB species, PALIc1960/PARIv722 (universal begomovirus primers), and by sequencing. The majority of samples represented monopartite TLCBs associated with DNA-beta components. All four known TLCBs appeared to be present throughout India. TLCBs were also present in chilli, cowpea, okra and tobacco crops, as well as in some common weeds. Papaya leaf curl virus and Pepper leaf curl Bangladesh virus sequences were detected in tomato. Mixed begomovirus infections, a prerequisite for recombination, were evident in 13 samples.

Sterility Mosaic Disease-the "Green Plague" of Pigeonpea: Advances in Understanding the Etiology, Transmission and Control of a Major Virus Disease

Pigeonpea (Cajanus cajan), is a grain legume that is a very important subsistence crop in marginal farming systems adopted by millions of smallholder farmers in the Indian subcontinent. It is grown for its seed for human consumption and for income generation by trading surpluses in local and commercial markets, but is widely used for diverse purposes, including as animal fodder and for soil conservation. Sterility mosaic (SMD) is the most damaging disease of pigeonpea endemic in the Indian subcontinent. It causes yield losses of >US$300 million per annum in India and Nepal alone. SMD-affected plants show severe stunting and mosaic symptoms on leaves, with complete or partial cessation of flowering. The SMD causal agent is spread by the arthropod mite vector Aceria cajani (Acari: Eriophyidae). Cultivating SMD-resistant genotypes is the most viable way to manage this serious disease of pigeonpea. Progress in developing broad-based SMD resistant material has been hindered by the lack of knowledge of the causal agent, the absence of diagnostic tools, and factors influencing host-plant resistance. After seven decades of research, vital breakthroughs made on the identification, detection, transmission, and epidemiology of the SMD causal agent, Pigeonpea sterility mosaic virus (PPSMV), are enabling the development of broad-based durable resistant pigeonpea cultivars. These breakthroughs will contribute greatly to sustainable pigeonpea production and enhance the income and livelihood of poor farmers in the semi-arid tropics of the Indian subcontinent.

Transmission of Pigeon pea sterility mosaic virus by the Eriophyid Mite, Aceria cajani (Acari: Arthropoda)

The transmission characteristics of Pigeon pea sterility mosaic virus (PPSMV) to pigeon pea (Cajanus cajan) by its eriophyid mite vector, Aceria cajani, were studied. Nonviruliferous A. cajani colonies were established on detached healthy leaflets of a PPSMV-immune pigeon pea cultivar floating on water. The transmission efficiency of single A. cajani was up to 53% but was 100% when >5 mites per plant were used. A. cajani acquired PPSMV after a minimum acquisition access period (AAP) of 15 min and inoculated virus after a minimum inoculation access period (IAP) of 90 min. No latent period was observed. Starvation of A. cajani prior to, or following, PPSMV acquisition reduced the minimum AAP and IAP periods to 10 min and 60 min, respectively, and mites retained virus for up to 13 h. None of the mites that developed from eggs taken from PPSMV-infected leaves transmitted the virus, indicating that it is not transmitted transovarially. Taken together, these data suggest a semipersistent mode of transmission of PPSMV by A. cajani.

First Report of the Bemisia tabaci B Biotype in India and an Associated Tomato leaf curl virus Disease Epidemic

In May 1999, in the Kolar district of Karnataka State, Bemisia tabaci numbers on tomato increased by approximately 1,000-fold that observed previously (3). This was associated with an epidemic of severe tomato leaf curl disease that caused complete crop failure. DNAs extracted from 35 symptomatic tomato leaf samples collected within the epidemic region all gave the expected 500 to 600 bp amplicon with begomovirus-specific primers A/B (1). These primers amplify from the conserved nonanucleotide TAATATTAC in the common region of DNA-A to the conserved amino acid sequence CEGPCKYG within the coat protein gene. AluI and TaqI restriction patterns of all 35 polymerase chain reaction (PCR) products were identical. One PCR product from an epidemic (GenBank no. AF321929) and a non-epidemic (AF321930) site (Bangalore) were cloned and sequenced. The two 531-bp inserts showed 96% nucleotide identity to each other and 94% nucleotide identity to the equivalent region of Tomato leaf curl Bangalore virus (ToLCBV-Ban-4) (AF165098), suggesting that the epidemic was caused by an indigenous ToLCBV strain. Adult B. tabaci were collected from tomato plants at nine sites within the epidemic. DNA was extracted from 9 to 13 individuals per site and analyzed by RAPD-PCR using primers OpB20 and OpB11. Eighty to 100% of individuals per site had identical patterns to those of B biotype individuals from Israel and Florida, which were different to the patterns produced by the indigenous Indian B. tabaci. Adult B. tabaci from the epidemic and nonepidemic (Bangalore) regions were cultured separately on zucchini plants (n = 20) vars. Fordhook and Ambassador. Distinct silverleaf symptoms appeared in all plants fed on by the epidemic B. tabaci, but not on those fed on by the nonepidemic whiteflies. Irregular ripening of tomatoes was also a widespread problem in the epidemic area. Cytochrome oxidase I (COI) (720 bp) gene sequences were obtained for epidemic (AF321927) and nonepidemic (AF321928) B. tabaci, which had only 80% nucleotide identity to each other. A GenBank BLAST search showed that the former were most similar to B biotype whitefly from Israel (AF164667; 97%) and Texas (AF164675; 99%). The B biotype transmits Indian ToLCBV (2) and its introduction into India is of great concern as it is already associated with a devastating plant-disease epidemic. References: (1) D. Deng et al. Ann. App. Biol. 125:327, 1994. (2) P. F. McGrath and B. D. Harrison. Ann. App. Biol. 126:307, 1995. (3) H. K. Ramappa et al. Ann. App. Biol. 133:187, 1998.

Tomato leaf curl virus from Bangalore (ToLCV-Ban4): sequence comparison with Indian ToLCV isolates, detection in plants and insects, and vector relationships

Tomato leaf curl virus (ToLCV) is a whitefly (Bemisia tabaci) transmitted geminivirus (family Geminiviridae, genus Begomovirus) causing a destructive disease of tomato in many regions of India, East Asia and Australia. While ToLCV isolates from Australia and Taiwan have a single genomic component (designated DNA-A), those from Northern India have two components (DNA-A and DNA-B). The ToLCV isolates from Southern India (Bangalore) previously cloned seem to have a DNA-A-like monopartite genome. We have used degenerate DNA-A-specific PCR primers to clone the genome of a ToLCV isolate (named ToLCV-Ban4) from field-infected tomato plants growing in Bangalore, India, in 1997. Degenerate DNA-B-specific PCR primers have not allowed to amplify a putative DNA-B from infected tomato, at the time when DNA-B fragments were amplified from plants infected by known bipartite begomoviruses. The full-length 2759 nucleotide-long DNA-A-like viral genome was sequenced. Similarly to other monopartite ToLCV and TYLCV isolates, ToLCV-Ban4 contains six open reading frames, two on the virion strand and four on the complementary strand. Sequence comparisons indicated that ToLCV-Ban4 is similar to the other three isolates from Bangalore previously sequenced, and is closely related to ToLCV-Ban2 (approximately 91% nucleotide sequence identity). Phylogenetic analysis showed that the ToLCV isolates from Bangalore constitute a group of viruses separated from those of Northern India. ToLCV-Ban4 was detected in tomato and in its whitefly vector Bemisia tabaci by one or by a combination of ELISA, Southern blot hybridization and PCR. Parameters of virus acquisition, retention and transmission by the whitefly vector were investigated in the laboratory. Single whiteflies were able to acquire ToLCV-Ban4 from infected tomato and to transmit the virus to tomato test plants, but five insects were necessary to achieve 100% transmission. Minimum acquisition access and inoculation access periods were 10 min and 20 min, respectively. A latent period of 6 h was required for B. tabaci to efficiently infect tomato test plants. Following a 24 h acquisition access period the insect retained its ability to infect tomato test plants for 12 days, but not for its entire life. In one insect/one plant inoculation tests, female whiteflies were more efficient (approximately 95%) than males (approximately 25%) in transmitting the virus.

Comparative epidemiology of three tropical whitefly-transmitted geminiviruses

Spatial and temporal patterns of spread of African cassava mosaic, okra leaf curl and tobacco leaf curl viruses in West Africa, East Africa and India share some general characteristics. By comparing the results and running new analyses on available data, it is shown that the epidemiology of these viruses is controlled by the same key variables. For instance, spatial spread is characterised by strong border effects due to accumulation of whitefly vectors (Bemisia tabaci) on the wind-exposed field borders under the influence of the prevailing wind. This results in pronounced environmental gradients of disease. Temporal patterns of virus spread are driven by the sinusoidal fluctuation of temperature over the year which correspond with changes of whitefly populations.

Resistance of Lycopersicon species to Bemisia tabaci, a tomato leaf curl virus vector

More than 1200 cultivars, breeding lines, and wild species of Lycopersicon were screened for resistance to the tomato leaf curl virus under field conditions. All accessions of Lycopersicon esculentum were found susceptible to tomato leaf curl virus. Three lines of Lycopersicon hirsutum and one of Lycopersicon peruvianum accessions showed apparent resistance to the tomato leaf curl virus owing to the failure of virus transmission by whiteflies. Morphological analysis by light and scanning electron microscopy revealed distributions of different types of trichomes on leaf surfaces. Lycopersicon hirsutum proved to be the least affected by the leaf curl virus because it did not support whitefly infestation to any extent. Lycopersicon hirsutum differed from other species by the presence of glandular trichomes designated type VIc. Whiteflies became entrapped in the exudate of type VIc trichome glands before they could transmit the virus. Thus, it may be possible to control tomato leaf curl virus transmission by breeding plants with certain types of trichomes, especially trichome type VIc. Key words: leaf curl virus, nonpreference, physical resistance, glandular exudate, trichomes, whitefly.

Ultrastructural changes in tomato infected with tomato leaf curl virus, a whitefly-transmitted geminivirus

The ultrastructural modifications in nucleus of tomato infected with tomato leaf curl virus (TLCV) included hypertrophy of the nucleolus, segregation of nucleolar components into discrete granular and fibrillar regions, appearance of electron-dense particles associated ribbon-like structures, and presence of virus particles as either loosely compacted or hexagonally close-packed symmetrical arrays. The virus particles were isometric, about 18–20 nm in diameter. In the lumen of sieve elements, virus particles occasionally formed aggregates that were cylindrically arranged and occurred in pairs. Among the organelles other than the nucleus, virus particles were found in the plastids of sieve elements. In the chloroplasts of TLCV-infected cells, considerable disturbances in the internal organization were observed. In the most severe form of degeneration, the thylakoid system was fragmented and disorganized. In some chloroplasts starch grains were abnormally large. Excessive accumulation of osmiophilic bodies in degenerating chloroplasts was prominent. Key words: leaf curl virus, geminate particles, nucleolar hypertrophy, virus particles, osmiophilic body.