The expression of the pyk20 gene of Arabidopsis thaliana, which encodes a protein containing a glutamine-rich domain is up-regulated during Plasmodiophora brassicae infection. Transgenic A. thaliana plants harbouring a beta-glucuronidase (uidA) reporter gene under the control of the pyk20 gene promoter were grown in soil and infected with P. brassicae resting spores. GUS expression in non-infected plants was found in stipules, apical meristem, leaf vascular tissues, vascular tissue of roots and in the root tips. After infection with P. brassicae, GUS staining was observed in the root hairs during primary infection and in galls in roots and hypocotyls during secondary infection phase. GUS expression during primary infection was also detected in a small number (approx. 0.01 %) of zoosporangia. Sections of the GUS-stained galls showed reporter gene expression in infected and non-infected tissues. Northern-blot analysis using a pyk20 cDNA clone as a probe confirmed responsiveness of the pyk20 gene to P. brassicae infection. (C) 2000 Academic Press.