Abstract. Mutants and wild type plants of Arabidopsis thaliana were analysed for dierences in glucosinolate accumulation patterns, indole-3-acetic acid (IAA) bio- synthesis and phenotype. A previously identi®ed series of mutants, termed TU, with altered glucosinolate patterns was used in this study. Only the line TU8 was aected in shoot phenotype (shorter stems, altered branching pattern). Synthesis of IAA and metabolism were not much aected in the TU8 mutant during seedling development, although the content of free IAA peaked earlier in TU8 during plant development than in the wild type. Indole glucosinolates and IAA may, however, be involved in the development of clubroot disease caused by the obligate biotrophic fungus Plasm- odiophora brassicae since the TU3 line had a lower infection rate than the wild type, and lines TU3 and TU8 showed decreased symptom development. The decline in clubroot formation was accompanied by a reduced number of fungal structures within the root cortex and slower development of the fungus. Indole glucosinolates were lower in infected roots of TU3 and TU8 than in control roots of these lines, whereas in wild-type plants the dierences were not as prominent. Free IAA and indole-3-acetonitrile (IAN) were increased in infected roots of the wild type and mutants with normal clubroot symptoms, whereas they were reduced in infected roots of mutants TU3 and TU8. These results indicate a role for indole glucosinolates and IAN/IAA in relation to symptom development in clubroot disease. Key words: Arabidopsis ± Auxin ± Clubroot disease ± Glucosinolate ± Plasmodiophora Introduction Glucosinolates are secondary plant products synthesized by members of the Brassicaceae but also occurring in several other plant families (Rodman 1991). The gluco- sinolates have been implicated in plant defense against bacterial and fungal pathogens as well as insect preda- tors (Bennett and Wallsgrove 1994), but they may also play a role in host/pathogen recognition. Glucosinolates can be grouped into three dierent classes, depending on the amino acid from which they are derived: aliphatic/alkenyl glucosinolates derived from methionine, aromatic glucosinolates derived from phenylalanine and tyrosine, and indole glucosinolates derived from tryptophan (Bennett and Wallsgrove 1994; Bennett et al. 1995). The glucosinolate content is dependent on the tissue type and age (Porter et al. 1991) and can be induced after wounding and pathogen attack (Doughty et al. 1991; Bodnaryk 1992). The ®rst step in glucosinolate biosynthesis, namely the conversion of the amino acid precursor to the aldoxime, is catalyzed by at least four dierent enzyme systems, each speci®c for its precursor (Bennett et al. 1995, 1997). The ®rst step in indole glucosinolate biosynthesis, the con- version of tryptophan to indole-3-acetaldoxime (IAOX), is catalyzed by basic peroxidase isoenzymes (Ludwig- MuÈ ller and Hilgenberg 1988; Ludwig-MuÈ ller et al. 1990). This reaction might also be the ®rst step in indole-3-acetic acid (IAA) biosynthesis via a tryptophan-dependent pathway in the Brassicaceae (Ludwig-MuÈller and Hi- lgenberg 1988), involving IAOX and indole-3-acetonitrile (IAN) as intermediates. The interaction of microbial and plant secondary-product metabolism in relation to infec- tion has not often been reported; however, the infection of Brassica species by the clubroot fungus provides an excellent model for evaluating such relationships. The clubroot disease of the Brassicaceae, caused by the obligate biotroph Plasmodiophora brassicae, is probably the most damaging disease within this plant family. Not only are cultivars used widely in agriculture susceptible to the fungus, but weed crucifers such as Arabidopsis thaliana are also suitable hosts. As a result Abbreviations: dpi = days post inoculation; IAN = indole-3- acetonitrile; IMG = indole-3-methylglucosinolate; 1-MeO-IMG = 1 (or N)-methoxy-indole-3-methylglucosinolate; 4-MeO-IMG = 4-methoxy-indole-3-methylglucosinolate; TrpOxE = tryptophan- oxidizing enzyme Correspondence to: J. Ludwig-MuÈller; E-mail: Ludwig-Mueller@em.uni-frankfurt.de; Fax: 49 (69) 79824822 Planta (1999) 208: 409±419 Indole glucosinolate and auxin biosynthesis in Arabidopsis thaliana (L.) Heynh. glucosinolate mutants and the development of clubroot disease Jutta Ludwig-MuÈller 1 , Kerstin Pieper 1 , Manfred Ruppel 1 , Jerry D. Cohen 2 , Ephraim Epstein 2 , Guy Kiddle 3 , Richard Bennett 3 1 Botanisches Institut, J.W. Goethe-UniversitaÈt, Siesmayerstr. 70, D-60054 Frankfurt, Germany 2 Horticultural Crops Quality Laboratory, Beltsville Agricultural Research Center, Agricultural Research Service, United States Department of Agriculture, Beltsville, MD 20705-2350, USA 3 Department of Biochemistry and Physiology, IACR Rothamsted, Harpenden, Herts AL5 2JQ, UK Received: 23 July 1998 / Accepted: 12 January 1999