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.
