In Vitro Virus Elimination from Taro (Colocasia esculenta) for Conservation and the Safe International Exchange
Taro is a very important Pacific Island crop and is widely cultivated. It plays an important cultural role and is a widely distributed food crop in the humid tropics and subtropics (Xu, Yang & Pu, 2001). It is a major staple food crop with both the foliage and root utilized as a food source. Although traditionally grown in gardens for local use, it has shifted into the domestic and export market. It has become a major export commodity for some Pacific Island Countries. It is estimated that the export of taro by Fiji in 2005 increased by 3% from 2004, with revenue of $FJ 19million.
Taro being a vegetatively propagated crop is prone to viral infection. Of the five viruses infecting taro, Dasheen Mosaic Virus (DsMV) and Taro Bacilliform Virus (TaBV) are the most widespread across the pacific (Revill et al 2005). Revill et al (2005) in their work found that DsMV was present in all of the survey sites and TaBV to be present in all except one. Following these two viruses was TaVCV which was the next most common virus but was more unevenly distributed. It was found in just over half of the countries surveyed (including Fiji but excluding Samoa). Colocasia bobone disease virus, CBDV and taro reovirus, TaRV have been found to be present in only certain Melanesian countries. CBDV is only found in PNG and the Solomon Islands while TaRV is considered to be of little or no importance is found in these two countries and also in Vanuatu.
Although documented studies on the loss of yield of Colocasia spp. caused by DsMV and TaBV is scarce, it can be assumed that production is lower from virus infected plants. Studies on Xanthosoma sagittifolia have showed that yield gains of more than seven times are possible with virus-free plants (Valverde et al, 1997). In another study by Reyes et al (2006) on the impact of DsMV on X. sagittifolia it was found that yield of virus free planting material was greater than that of virus infected planting material. Studies involving impact of TaBV on yield is even rarer compared to DsMV. The little data that is available on the benefits of using virus-free planting materials point in the direction of increased yields.
In vitro techniques allow for the production of plant material that is free from fungus, bacteria and other pests but often viruses remain a problem. The availability of sensitive techniques allow for the detection of viruses. . Infection with DsMV and TaBV is commonplace, and many of the taro accessions in the CePaCT collection are infected with one or both of these viruses. Countries often want to share varieties, making comparison between what they have and the best from elsewhere. Unfortunately, the movement of taro between countries is hindered due to viruses recorded on the crop (Jackson et al, 2001; Jackson, 1980). These varieties may be high yielding or may have either characteristics that make them attractive to other countries but they cannot be shared since they are infected with viruses. These desirable properties could be salt or drought tolerance, thereby providing plants with which to manage climate change and to help ensure food and nutritional security. If a crop accession has a virus it cannot be distributed, thus reducing its value because it cannot be shared and utilized.
The elimination of plant viruses have been successfully achieved based on their mode of replication and mechanism of movement within the plant (Awan et al, 2007). Five methods are currently in use, meristem culture, thermotherapy, chemotherapy cryotherapy and electrotherapy. Elimination of viruses has been attempted from many different crops using these techniques. They have been used to eliminate viruses from plants such as banana, yam, potato, plums, sweet potato etc to different degrees of success. These have been used alone or in combination. Only a single study was found that had worked exclusively on Colocasia spp. (Li, Xu & Chen, 2002). Li et al. utilized thermotherapy in vivo followed by meristem culture to evaluate DsMV elimination. Buds were shot from corms under 25°C, 24h light photoperiod and 70% relative humidity and used for meristem culture after treating at 38°C for 30 days, until 2-3cm high. They report a loss of 68% of the meristem cultures and a 10% success rate for DsMV elimination. Various other works have reported on the success of meristem culture in eliminating viruses from Colocasia spp. but none of them provide a success rate. DsMV was successfully eliminated from Xanthosoma spp. using electrotherapy. The authors report a disinfection rate of 100% (Castro, Pérez & Castellanos, 2001).
This study hopes to use different techniques individually and in combination to evaluate their effectiveness in virus elimination from Colocasia spp. in vitro. Meristem culture, thermotherapy electrotherapy, cryotherapy and chemotherapy will be used to eliminate from in vitro cultures the two most common viruses, DsMV and TaBV. Many of the in vitro accessions of Colocasia spp. at CePaCT are either infected with DsMV and/or TaBV. These accessions are valuable in their genetic identity but cannot be distributed as they are virus positive. The present study will evaluate different techniques of virus eradication and will outline a protocol for the routine eradication of virus namely, DsMV and TaBV.
Awan A. R., Mughal S. M., Iftikhar Y., Khan H. Z., 2007. In vitro elimination of potato leaf roll polerovirus from Potato varieties. European Journal of Scientific Research 18(1), 155-164.
Castro, Pérez & Castellanos, 2001. La electroterapia como alternativa parala eliminación del virus DMV en malanga. Manejo Integrado de Plagas (Costa Rica) 60, 57-6 0. (in Spanish)
Li Y., Xu C., and Chen J., 2002. Establishment of virus free taro (Colosia esulenta cv. Fenghuayunaitou) by meristem-tip culture with thermotherapy. Pakistan Journal of Plant Pathology, 1(2-4), 40-43.
Revill P., Jackson G., Hafner G., Yang I., Maino M., Dowling M., Devitt L., Dale J., and Harding R., 2005. PCR-based surveys of taro viruses in the South Pacific Islands. Australasian Plant Pathology 34: 327-331.
Reyes G., Rönnerg-Wästljung A. C. and M. Nyman, 2006. Comparison of Field Performance between Dasheen Mosaic virus-free and Virus-Infected In vitro Plants of Cocoyam (Xanthosoma SPP.) in Nicaragua. Experimental agriculture 42, 1-10.
Xu J. C., Yang Y.P. and Pu Y. D., 2001. Genetic Diversity in taro (Colocasia esculenta Schott, Araceae) in China: An ethnobotanical and genetic approach. Economic-Botany 55, 14-31.
Valverde R., Gόmez L., Saborio F., Torres S., Arias O. & Thorpe T., 1997. Field evaluation of Dasheen Mosaic virus-free cocoyam plants produced by in vitro techniques. Scientia Horticulturae 68, 37-47