Response of transgenic cucumber expressing a rice class I chitinase gene to two fungal pathogens with different infectivities

A transgenic cucumber line (CR32) over-expressing the rice class I chitinase gene exhibited resistance to Phytophthora rot (Phytophthora nicotianae var. parasitica) but not to Fusarium wilt (Fusarium oxysporum f. sp. cucumerinum). The infection behavior of these fungi on CR32 and nontransgenic plants was examined with an optical microscope. In zoosporangia of P. nicotianae var. parasitica, the rates of germination and penetration on leaves of both CR32 and nontransgenic plants were almost equal. After infection, however, the growth of infection hyphae was markedly suppressed in CR32 compared with their growth in the nontransgenic plants. In F. oxysporum f. sp. cucumerinum, the infection hyphae localized in petiole vessels of both CR32 and nontransgenic plants, and growth did not differ in the two plants. We investigated the antifungal activity of a high-molecular-weight fraction (HF) and a low-molecular-weight fraction (LF) of crude leaf extracts from CR32 and from the nontransgenic line. CR32 HF, which included the rice chitinase, had antifungal activity only against F. oxysporum f. sp. cucumerinum. In contrast, CR32 LF, which did not have the rice chitinase, had strong antifungal activity against the two fungi. These results suggested that a low-molecular-weight antifungal substance(s) was induced in CR32 and might function as a factor of resistance to P. nicotianae var. parasitica, which has cell walls that almost never contain chitin. Because rice chitinase has already been demonstrated not to localize in vessels of CR32, the infection localization of F. oxysporum f. sp. cucumerinum in vessels might enable the fungus to avoid antifungal substance(s), resulting in Fusarium wilt in transgenic cucumber.