Suppression of phenylalanine ammonia lyase expression in sugar beet by the fungal pathogen Cercospora beticola is mediated at the core promoter of the gene

The suppression of plant defence reactions plays a crucial role in causing plant diseases. In this report, we show that inducible plant defences are repressed during the development of Cercospora leaf spot disease. In the early phase of infection of sugar beet (Beta vulgaris L.) leaves with the phytopathogenic fungus Cercospora beticola, a reduction in the expression of the phenylalanine ammonia lyase (BvPAL) and cinnamic acid 4-hydroxylase (BvC4H) genes was observed. BvPAL reduction was found at the transcript and enzyme activity levels. In order to analyse the signal transduction process responsible for suppression, the BvPAL promoter was isolated. An abbreviated 5'- and 3'-deletion series of the promoter was effected using transient biolistic assays, which showed that the activity of a truncated promoter from positions -34 to +246, relative to the transcriptional starting site, retains approximately 30% of the activity of the full-length promoter. The region within the BvPAL promoter required for the reduction in transcription was identified as being positions -34 to +45, with respect to the start of the transcription. This region is equivalent to the core promoter, characterised by the TATA-box, an initiator (Inr) and an unknown downstream element in the region between +7 and +45. These data indicate that (1) plant defence responses are repressed during the development of Cercospora leaf spot disease and (2) the PAL core promoter is involved in the detection of the repression signal.