Continuous stimulation of the plant immune system by the peptide elicitor PIP-1 is required for phytoalexin biosynthesis in tobacco cells

The peptide elicitor PIP-1 (YGIHTH-nh₂) induced various defense responses in tobacco cells. Types of defense responses induced by PIP-1 were different based on its concentration range: oxidative burst (an early response) was induced at low micromolar levels, but phytoalexin production (a late response) required about 10-50-fold higher concentrations than those required for oxidative burst. We assumed that rapid decreases in the PIP-1 concentration due to enzymatic hydrolysis in the culture media could cause this difference. To examine the potential impact of such degradation particularly on induction of phytoalexin biosynthesis, we designed a degradation-resistant analogue, MePIP-1, in which the amide bond between the fifth and sixth residues was N-methylated. MePIP-1 was considerably more stable than PIP-1 and induced significant phytoalexin production upon treatment at low micromolar levels. Further investigation of the mechanism of action of MePIP-1 showed a requirement of continuous elicitor stimulation for 3-6 h for the phytoalexin production, which is likely to be regulated by long-lasting MAP kinase activation.