Biosynthesis of Indole-3-Acetic Acid by the Gall-inducing Fungus Ustilago esculenta

Abstract

Ustilago esculenta incites the formation of an edible gall and prevents inflorescence and seed production in the aquatic perennial grass, Zizania latifolia. As compared to the healthy tissues, the edible galls had higher amounts of indole-3-acetic acid (IAA), which could be synthesized from the host plant and/or the fungal pathogen. In this study we investigated the ability for IAA production by U. esculenta. The production of IAA in c ulture by U. esculenta was solely dependent on the presence of tryptophan. The addition of thiamine (vitamin B) to medium greatly enhanced fungal growth, whereas IAA production was completely 1 inhibited. Maximum amount of IAA (ca.1.0 µg mL ) was obtained after 8-day incubation. The production of 1 IAA was highly correlated with the amount of tryptophan. The optimum temperature for IAA production ranged from 20 to 25oC. However, IAA production was significantly reduced when fungus was grown in the fluctuated temperatures, indicating that a constant temperature has a profound effect on IAA production. The corn smut pathogen U. maydis also synthesized IAA using tryptophan and accumulated a maximum amount of IA A (ca.1.2 µg mL ) at day 3, then production was declined steadily thereafter. In contrast, the sugarcane smut 1 pathogen U. scitaminea produced less amount of IAA (ca.0.53 µg mL ) compared to other two species. In 1 addition to tryptophan, U. esculenta apparently could convert indole-3-acetamide (IAAld), indole-pyruate (IPA) and indole-lactic acid (ILA) into IAA. However, indole-3-acetamide (IAM) was evidently not a suitabl e precursor for IAA production. The results suggest that biosynthesis of IAA in U. esculenta from tryptophan proceeds through IPA and IAAld.