Plant Physiol, June 2002, Vol. 129, pp. 717-732

Evidence for a Ustilago maydis Steroid 5-Reductase by Functional Expression in Arabidopsis det2-1 Mutants¹

Institut für Genetik und Mikrobiologie der Universität München, Maria-Ward-Strasse 1a, D-80638 Munich, Germany (C.W.B., C.K., M.B., R.K.); and Max-Planck-Institut für Molekulare Pflanzenphysiologie, Karl Liebknecht Strasse 25, D-14476 Golm, Germany (T.A.)

We have identified a gene (udh1) in the basidiomycete Ustilago maydis that is induced during the parasitic interaction with its host plant maize (Zea mays). udh1 encodes a protein with high similarity to mammalian and plant 5-steroid reductases. Udh1 differs from those of known 5-steroid reductases by six additional domains, partially predicted to be membrane-spanning. A fusion protein of Udh1 and the green fluorescent protein provided evidence for endoplasmic reticulum localization in U. maydis. The function of the Udh1 protein was demonstrated by complementing Arabidopsis det2-1 mutants, which display a dwarf phenotype due to a mutation in the 5-steroid reductase encoding DET2 gene. det2-1 mutant plants expressing either the udh1 or the DET2 gene controlled by the cauliflower mosaic virus 35S promoter differed from wild-type Columbia plants by accelerated stem growth, flower and seed development and a reduction in size and number of rosette leaves. The accelerated growth phenotype of udh1 transgenic plants was stably inherited and was favored under reduced light conditions. Truncation of the N-terminal 70 amino acids of the Udh1 protein abolished the ability to restore growth in det2-1 plants. Our results demonstrate the existence of a 5-steroid reductase encoding gene in fungi and suggest a common ancestor between fungal, plant, and mammalian proteins.