Resistance to viruses

More than 30 viruses can infect cucurbits, but only twelve cause economically important losses.^{20, 21} The following viruses are the most commonly encountered in warm and temperate areas and capable of infecting several cucurbit species: (i) CMV (Cucumber Mosaic Virus); (ii) WMV (Watermelon Mosaic Virus); (iii) ZYMV (Zucchini Yellow Mosaic Virus); (iv) PRSV (Papaya Ring Spot Virus) and (v) SqMV (Squash Mosaic Virus). The incorporation of virus resistance genes into cucurbits has been the goal of many breeding programmes. However, because resistance genes are derived from wild species, they are not simply inherited and/or are recessive, so introgression of these genes into horticulturally acceptable genotypes is not an easy task. Genetic engineering has allowed breeders rapidly to develop virusresistant varieties by introducing dominant coat protein or replicase viral genes into inbred parents of existing commercial hybrids.

Viral coat protein-mediated protection has allowed the generation of CMVresistant cucumber plants²² that can exhibit a high level of CMV-resistance under field conditions.²³ Similarly, melon plants over-expressing CMV or ZYMV coat proteins have been generated.^24-26 For Clough and Hamm,²⁶ a significant reduction in disease incidence in the transgenic lines occurred in field conditions. Expression of the coat protein gene delayed virus disease development and the subsequent systemic spread of virus in the transgenic plants. However, because most cucurbits are susceptible to several viruses, multiple virus resistance has been sought, instead of single virus resistance.

Transgenic lines of yellow crookneck squash (Cucurbita pepo) containing multiple coat protein constructs of CMV, WMV and ZYMV have been generated and used for the production of hybridvarieties.²⁷ Field evaluation was performed for two transgenic lines: (i) CZW-30, transformed with the triple coat protein gene construct that exhibited resistance to all three viruses²⁸ and (ii) ZW-20, transformed with the coat protein genes of WMV and ZYMV that displayed excellent resistance to the two viruses.²⁹ The ZW-20 line and subsequent generations were approved for commercial distribution by As grow in the USA in 1995 and were the first disease resistance transgenic plants to be approved for commercialisation.²⁷ Similarly, two experimental transgenic summer squash hybrids, possessing resistance to ZYMV and WMV and to ZYMV, WMV and CMV exhibited outstanding resistance in field conditions to the corresponding viruses as compared to the non-transgenic virus-susceptible hybrid ‘Pavo’.³⁰

Another strategy to enhance virus tolerance in melon, based on the over expression of polyribosime directed toward CMV coat protein, has been reported by Plages.³¹ A further approach to reduce development of the virus in cucurbits is to use a cDNA copy of RNA1 or an altered form of the 2areplicase gene from CMV which have been found to enhance virus tolerance in transgenic tobacco.^{32, 33}