The
ipiB and
ipiO genes of the potato late blight fungus
Phytophthora infestans (Mont.) de Bary were isolated from a genomic library in a screen for genes induced in planta. Expression of these genes was studied during pathogenesis on various host tissues and different host plants, some of which show specific resistance against
P. infestans infection. During pathogenesis on leaves and tubers of the fully susceptible potato cultivar (cv.) Ajax and on leaves of the fully susceptible tomato cv. Moneymaker, the
P. infestans ipiB and
ipiO genes show a transient expression pattern with highest mRNA levels in the early stages of infection. During the interaction with leaves of the partially resistant potato cv. Pimpernel, the expression is also transient but accumulation and disappearance of the mRNAs is delayed. Also in
P. infestans inoculated onto a race-specific resistant potato cultivar and onto the nonhost
Solanum nigrum, ipiB and
ipiO mRNA is detectable during the initial stages of infection. Apparently, the expression of the
ipiB and the
ipiO genes is activated in compatible, incompatible and nonhost interactions. In encysted zoospores,
ipiB and
ipiO mRNA accumulation was not detectable, but during cyst germination and appressorium formation on an artificial surface the genes are highly expressed. Expression studies in mycelium grown in vitro revealed that during nutrient starvation the expression of the
ipiB and
ipiO genes is induced. For
ipiO gene expression, carbon deprivation appeared to be sufficient. The
ipiO gene promoters contain a sequence motif that functions as a glucose repression element in yeast and this motif might be involved in the regulation of
ipiO gene expression.
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