Developmental regulated mechanisms affect the ability of a fungal pathogento infect and colonize tobacco leaves

During tobacco development, a transition state from susceptibility to resis tance to fungal pathogen infection is observed. Leaves acquire resistance t o Phytophthora parasistica when the plant becomes committed to flowering. T he ability to develop resistance does not imply pathogen-induced defence re sponses as for the onset of systemic acquired resistance (SAR). Throughout flowering growth, fungal establishment is restrained at two levels. The fir st level is the control of infection effectiveness. Using the salicylic aci d non-accumulating NahG plants, we demonstrate that this control does not r equire salicylic acid accumulation. The intercellular fluids (IFs) from tob acco leaves committed to flowering exhibit a cytotoxic activity on fungal z oospore cells based on in vitro germination assays. Its accumulation is cor related to the control of infection effectiveness that occurs during flower ing growth. The expression of this activity appears to constitute a develop mental regulated mechanism that inhibits early steps of fungal pathogen ins tallation. A second level of fungal growth control is the restriction of fu ngal hyphae expansion. In contrast to infection initiation, fungal hyphae s preading appears to be restricted by similar mechanisms induced during SAR as it is attested by the requirement of salicylic acid accumulation and by the correlating apoplastic accumulation of PR1 proteins. These results prov ide evidence for the activation of a set of at least two regulatory pathway s during flowering growth. This activation leads to the induction of mechan isms which control fungal development by affecting the ability of the fungu s to both infect and colonise plant tissues.