Resistance to fungi

The improvement of resistance to phytopathogenic fungi is one of the most crucial objectives in cucurbit cultivation. However, the problem is complex because highly resistant sources must be available for the breeding programmes. For example, Fusarium wilt-resistant materials have been found in genetic resources in cucumber and used in breeding programmes.34 On the contrary, breeding materials for resistance against grey mould (Botrytis cinerea), one of the most serious cucumber diseases have not been found.

Transformation techniques have been used to produce resistant transgenic plants by expressing chitinase genes aimed at inhibiting fungal development in the plant. In cucumber, the response of transgenic plants expressing different chitinase genes originating from petunia, tobacco or bean to inoculation with fungal pathogens including Alternaria radicini, Botrytis cinerea, Colletotrichumlagenarium, and Rhizoctonia solani has shown that chitinase over expression failed to reduce the development of the disease.35 In opposition, Tabei et al.5 have succeeded in obtaining B. cinerea-resistant lines of cucumber by expressing a rice endochitinase gene. Different responses for disease resistance were observed including inhibition of appressorium formation and penetration of hyphae as well as restriction of invasion of the infection hyphae. Furthermore, disease resistance against grey mould was confirmed to be inheritable and chitinase over expression did not affect the morphological development of the plants. Although the effectiveness of the fungal resistance must be confirmed by field trials, this approach could be used for other cucurbits species to fight against plant diseases caused by phytopathogenic fungi. However, the extent of disease tolerance appears to be highly dependent on the type of chitinase protein expressed and the characteristics of the fungal pathogen. On the other hand, chitinase over expression in combination with a-glucanase, PR proteins or other enzymes involved in the synthesis of antifungal compounds could provide a broader spectrum of activity against fungal pathogens.