Defense responses in female gametophytes of Saccharina japonica (Phaeophyta) induced by flg22-derived peptides

Abstract

Plants and algae can detect the presence of bacteria via sensing of proteins or peptides of bacterial origin. Flg22, a fragment of bacterial flagellin, is one of these peptides and has been shown to be an effective elicitor in both plants and algae. Here, we investigated the elicitor activity of flg22-derived peptides in the brown alga, Saccharina japonica. By monitoring luminol-dependent fluorescence, we could observe that the release of H2O2 induced by flg22-derived peptides is maximal at 2 h after induction. The elicitor activity was depending on the length of the peptides in the order of flg22 > flg15 > flg14. Cytological observations regarding the presence of reactive oxygen species (ROS) after induction were consistent with quantitative measurements of H2O2 generation using a 2′,7′-dichlorofluorescein diacetate (DCFH-DA) fluorescent probe. Addition of 1 μM each of flg22 and flg15 was sufficient to inhibit growth of female gametophytes. Furthermore, the elicitor activity of C-terminally shortened flg15-derived peptides suggests that flg15 apparently is the smallest peptide with elicitor activity. Amino acid position D43 at the N-terminus of a flagellin was demonstrated to be involved in the elicitor activity. Finally, H2O2 was localized in the plasma membranes of female gametophytes by an NADPH oxidase inhibitor and electron-dense deposits of cerium perhydroxide by transmission electron microscope.