Induction of hypersensitive response in tobacco by 90 kD extracellular elicitor protein from Phytophthora boehmeriae

Induction of hypersensitive cell death, changes of phenylalanine ammonia-lyase (PAL) activity and activation of PR5 (osmotin) in tobacco by 90 kD extracellular elicitor protein, a hypersensitive response (HR) elicitor, isolated from Phytophthora boehmeriae were studied. At 24 hours after infiltration of the elicitor into tobacco (cv. W38) leaves at dosage of 10 nmol/L, a bright blue autofluorescence became visible under UV light in a ring of cells surrounding the necrosis tissue, which indicated compounds derived from the phenylpropanoid pathway were accumulated during HR. By Evans blue and trypan blue staining, it was observed that cell death occurred only in infiltrated tissue and was completed within 20 hours after treatment. Comparing with water-treated leaves, the activity of PAL, a key enzyme in plant defense response was increased in elicitor-treated leaves and in the immediate neighbor leaves. The elevated level of PAL was positively correlated with the enhancement in disease resistance in tobacco induced by elicitor that we reported previously. Moreover, the elicitor could rapidly induced transcription of PR5 in elicitor-treated tobacco leaves. Results showed that 90 kD elicitor could induce hypersensitive cell death, phenylpropanoid metabolite pathway and transcription of PR5 gene. Results also indicated that there are some systemic molecules that can transmit rapidly from infiltrated leaves to the non-elicitor-treated neighbor leaves.