Research Article
A Study of Dicotyledonous Weed Species as Hosts of Potato Yellow Mosaic Trinidad Virus (PYMTV)
Department of Life Sciences, The University of the West Indies, St. Augustine, The Republic of Trinidad and Tobago
PYMTV (Family: Geminiviridae; genus: Begomovirus) is ubiquitous in tomato (Lycopersicon esculentum Mill.) in Trinidad[1]. Several factors may contribute to the epidemic status of begomovirus disease including the extent and distribution of cultivated and non-cultivated host species[2]. Apart from their epidemiological importance, weeds that harbor dual or multiple begomovirus infections may facilitate recombination between the constituent begomoviruses, resulting in the emergence of recombinant viruses[3]. The specific objectives of this study were (I) to detect PYMTV in dicotyledonous weed species by dot blot hybridization and IC-PCR amplification and (ii) to identify other begomoviruses that may be present in dicotyledonous weeds.
Two surveys were conducted. The first was an intensive one that targeted six commercial tomato-growing areas between March-April, 2000. The fields were heavily infected (80 to 100% infection) with PYMTV and in close proximity to each other thereby maintaining a high level of primary inoculum in the tomato-growing areas. The second survey was conducted between April and September, 2000 in non-tomato-growing areas.
Detection of PYMTV by dot blot hybridization and IC-PCR amplification: Dot blot hybridization was carried out to detect the presence of PYMTV with a PYMTV DNA-A specific probe using a DIG (digoxigenin-dUTP) DNA labeling and detection kit (Roche Diagnostics/Boehringer-Mannheim Inc., Germany) based on the method of Rampersad and Umaharan[1]. Immunocapture-PCR (IC-PCR) amplification of infected leaf extracts for subsequent sequencing was carried out using degenerate primers as described by Rampersad and Umaharan[4]. Extracts of PYMTV-infected tomato leaves and healthy leaf extracts of weeds were used as positive and negative controls, respectively.
Dot blot hybridization and IC-PCR assays: Samples collected from tomato growing areas (681 samples: 81 dicotyledonous weed species belonging to 30 families) were all negative for PYMTV detection in both dot blot hybridization and IC-PCR assays (Table 1). This indicated that the virus was not present within or outside the tomato growing areas. IC-PCR detected two distinct begomoviruses in only two weed species, Sida rhombifolia (L.) and Rhynchosia minima (L.).
Nucleotide sequence comparisons: Target sequences for alignment (using DNASTAR and CLUSTALW and BLASTN-http://www.ncbi.nlm.nih.gov/blast/) were selected from the major representative phylogenetic clusters of New World begomoviruses.
Table 1: | Locations visited, weed species collected and tested for PYMTV infection |
Key: aLocation - Tomato-growing areas: Aranguez (North and South) = A; Pasea/Macoya = B; Cunupia = C; Tabaquite = D; Chaguaramas = E. Non-tomato-growing areas: Valencia = F; Sangre grande = G; Brazil = H; San Rafael = I; Talparo = J; Guaico Tamana = K; Todds Road = L; Princes Town = M; New Grant = N; Rio Claro = O; Mayaro = P; Manzanilla = Q; Maracas Bay area/ Blanchisseusse = R; Santa Cruz = S; San Juan = T; Maraval = U; Toco Main Road = V; Montserrat Hills = W; Mt Hope = X. bNT- Not tested with PCR. c Symptoms: NS No symptoms; S symptomatic: Chl chlorosis of leaves; YMc Yellow mosaic on leaves; Y/pink Mc yellow or pink mosaic on leaves; dNumber tested: () numerator indicates the number of symptomatic plants, denominator indicates the total number of plants sampled. |
The core cp region (AV1 ORF) was comparatively analyzed because of high percentage of similarity shared among geminiviruses, the availability of these sequences in GenBank[5,6]. Comparisons of the core cp region show that the Trinidad isolate (AY217344) is highly homologous to Rhynchosia mosaic virus (RhMV) that infects pigeon pea in Puerto Rico (AY062025) at 88.0%. The virus infecting S. rhombifolia (AY217345) seems to be closely related to Sida golden mosaic virus (SiGMV) (AF070923) at 89.2%. Based on sequence data and the absence of polymorphic bands in IC-PCR, the results also indicated that there were no mixed infections of PYMTV with other begomoviruses.
RhMV and SiGMV have been identified to infect R. minima and S. rhombifolia in Trinidad. RhMV has recently been implicated as a disease pathogen in tobacco in Mexico and in pigeon pea in Puerto Rico[7,8]. While these findings may not yet be critical to disease epidemics in tomato, they serve to highlight the importance of certain virus reservoirs in weed species to potential disease epidemics in other commodity crops. Sida spp. is a common roadside weed in the region and it is not surprising that SiGMV has also been found in Central America[9].
According to the results of this study, the absence of PYMTV in any of the dicotyledonous weed species sampled, even in the presence of high primary inoculum, suggests that PYMTV may have a limited naturally-infected host range. These findings are congruent with other host range studies conducted for Tomato mottle virus (ToMoV) in Florida[10] and Bean golden mosaic virus (BGMV) in the wider Caribbean[9]. PYMTV is an emergent virus that exclusively infects tomato which, in turn, is the only known inoculum source. Its absence in dicotyledonous weeds indicates rapid diversification due to high selection pressure if the virus is indigenous or introduction of the virus from an external source.
The author wishes to thank Dr. Brian Cockburn, Ms. Veronica Paz, Mr. Rawlins Rampersad and Dr. P. Umaharan for their contributions.