Induction of a polyubiquitin gene (ubi1) by potato phytoalexins and heat shock in Gibberella pulicaris

Gibberella pulicaris, a causal agent of potato dry rot, infects potato tubers via wounds, where it is exposed to the phytoalexins rishitin and lubimin. Incubation of mycelium on agar supplemented with phytoalexins transiently induced the transcription of a polyubiquitin gene consisting of four ubiquitin units arranged head to tail; the fourth unit contains a 54-bp intron and an additional glutamine at the C-terminus of the encoded protein. Southern analysis of the G. pulicaris genome revealed one copy of the isolated polyubiquitin gene and one or two copies of other ubiquitin genes. Increased transcription of the gene was detectable above a threshold of 100 μg/ml of rishitin and at elevated temperatures, which indicates that exposure to phytoalexins causes a stress reaction of hyphal cells similar to that after heat shock.