The PstICL1 gene, which encodes isocitrate lyase, a key enzyme in the glyoxylate cycle, was cloned and characterized in the biotrophic wheat pathogen Puccinia striiformis f. sp. tritici (Pst). Expression analyses of PstICL1 exhibited high levels of transcripts in ungerminated urediniospores, which showed low isocitrate lyase enzyme activity. In planta, PstICL1 expression was continuously down-regulated upon germination. During the later stages of the infection of wheat, the level of PstICL1 expression was extremely low. The function of PstICL1 was identified via mutant complementation. The expression of PstICL1 in Saccharomyces cerevisiae can complement the defects of the △ICL mutant. Using 3-nitropropionate, we observed that inactivation of isocitrate lyase greatly reduced the germination rate of urediniospores, indicating that PstICL1 plays a key role during Pst germination. Furthermore, analysis of lipid bodies revealed that lipid components continuously enter the germ tube from the urediniospore cell during germ tube elongation. Moreover, during this period, the lipid contents continuously decreased, and the total carbohydrates markedly increased, demonstrating that the lipids are being converted into carbohydrates. These results suggest that PstICL1 is required for Pst germination.