Asakura Makoto, Okuno Tetsuro, Takano Yoshitaka
Laboratory of Plant Pathology, Graduate School of Agriculture, Kyoto University, Kyoto 606-8502, Japan.
Appl Environ Microbiol. 2006 Sep;72(9):6345-54. doi: 10.1128/AEM.00988-06.
In Colletotrichum lagenarium, which is the causal agent of cucumber anthracnose, PEX6 is required for peroxisome biogenesis and appressorium-mediated infection. To verify the roles of peroxisome-associated metabolism in fungal pathogenicity, we isolated and functionally characterized ICL1 of C. lagenarium, which encodes isocitrate lyase involved in the glyoxylate cycle in peroxisomes. The icl1 mutants failed to utilize fatty acids and acetate for growth. Although Icl1 has no typical peroxisomal targeting signals, expression analysis of the GFP-Icl1 fusion protein indicated that Icl1 localizes in peroxisomes. These results indicate that the glyoxylate cycle that occurs inside the peroxisome is required for fatty acid and acetate metabolism for growth. Importantly, in contrast with the pex6 mutants that form nonmelanized appressoria, the icl1 mutants formed appressoria that were highly pigmented with melanin, suggesting that the glyoxylate cycle is not essential for melanin biosynthesis in appressoria. However, the icl1 mutants exhibited a severe reduction in virulence. Appressoria of the icl1 mutants failed to develop penetration hyphae in the host plant, suggesting that ICL1 is involved in host invasion. The addition of glucose partially restored virulence of the icl1 mutant. Heat shock treatment of the host plant also enabled the icl1 mutants to develop lesions, implying that the infection defect of the icl1 mutant is associated with plant defense. Together with the requirement of PEX6 for appressorial melanization, our findings suggest that peroxisomal metabolic pathways play functional roles in appressorial melanization and subsequent host invasion steps, and the latter step requires the glyoxylate cycle.
在引起黄瓜炭疽病的瓜炭疽菌中，过氧化物酶体生物发生和附着胞介导的感染需要PEX6。为了验证过氧化物酶体相关代谢在真菌致病性中的作用，我们分离并对瓜炭疽菌的ICL1进行了功能表征，ICL1编码参与过氧化物酶体乙醛酸循环的异柠檬酸裂解酶。icl1突变体无法利用脂肪酸和乙酸盐进行生长。尽管Icl1没有典型的过氧化物酶体靶向信号，但GFP-Icl1融合蛋白的表达分析表明Icl1定位于过氧化物酶体。这些结果表明，过氧化物酶体内发生的乙醛酸循环是脂肪酸和乙酸盐代谢以支持生长所必需的。重要的是，与形成非黑化附着胞的pex6突变体不同，icl1突变体形成的附着胞高度黑色素化，这表明乙醛酸循环对于附着胞中的黑色素生物合成不是必需的。然而，icl1突变体的毒力严重降低。icl1突变体的附着胞未能在宿主植物中发育穿透菌丝，这表明ICL1参与宿主入侵。添加葡萄糖部分恢复了icl1突变体的毒力。对宿主植物进行热激处理也使icl1突变体能够形成病斑，这意味着icl1突变体的感染缺陷与植物防御有关。与PEX6对附着胞黑化的需求一起，我们的研究结果表明，过氧化物酶体代谢途径在附着胞黑化和随后的宿主入侵步骤中发挥功能作用，而后者需要乙醛酸循环。
