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As one kind of endogenous non-coding small RNA, microRNAs (miRNAs) have shown their pivotal roles in post-transcriptional gene silencing.miRNAs have recently been discovered to participate in plant-pathogen interactions, especially behaving as effectors.By secreting effectors into Solanaceae hosts, the oomycetes pathogen of Phytophthora infestans (P.infestans) can cause devastating crop diseases.Although miRNAs have recently been discovered in P.infestans, the diversity and versatility of miRNAs in this pathogen compel additional exploration.Based on our previous study and bioinformatic strategy, key regulatory miRNAs were systematically identified in P.infestans, from which the existence of two new miRNAs (miRH6 and miR4472) was validated via qRT-PCR.P.infestans grown to 7 days on cellophane covered oat medium was used to infect tomato.The cellophane was peeled from medium and immediately covered by tomato leaves on the lower surface.The expression level of the two miRNAs was further detected and manifested to change significantly during infection for different time (0, 3, 12,24, 48 and 72h).For miRH6 the relative quantities compared to the control (Oh) were 1.00, 0.79,1.84, 2.19, 2.81 and 1.58 folds, and for miR4472 the numbers were 1.00, 1.47, 2.76, 2.57,3.01 and 1.18.In general, peak of the expression fluctuation appeared at 48h.There was a tendency of ascent and decline respectively before and after 48h.Such results correspond to the previous research that most genes induced during infection changed most distinctly at 48h.Adding the fact that days 2 ~4 of infection correlate with formation of infectious structures called haustoria, it is implicated that miRH6a and miR4472a contribute to P.infestans infecting tomato leaves.The work reported here was a preliminary supplement of deprived pathogenic mechanism in P.infestans, in particular a providing of new ideas for plant protection.