Cell experiment:

One milliliter of the female gametophytes is added to 100 mL sterilized seawater containing 1 μM of Flagelin 22, flg15, flg14, and flg22D43A, respectively. Controls are sterilized seawater and 0.1% BSA in sterilized seawater. Gametophytes are grown at 10℃ with a 24-h photoperiod at 50 μmol photons m-2 s-1. Sterilized seawater medium is provided with 0.2 mM KNO3, 0.02mM KH2PO4, and 1 μM of the four respective peptides and refreshed every week. The gametophytes are briefly blotted dry, and the fresh weight is measured after 40 days[1].

Flagelin 22 (Flagellin 22), a fragment of bacterial flagellin, is an effective elicitor in both plants and algae.

Flagelin 22 (flg22) is a 22-amino-acid peptide, which corresponds to the highly conserved N-terminal region of flagellin, can induce immunity reaction in various plants such as tomato (Solanum lycopersicum), potato (Solanum tuberosum), tobacco (Nicotiana tabacum), and Arabidopsis thaliana. Flagelin 22 can induce oxidative bursts and hypersensitive responses (HR) in both female gametophytes and sporophytes of Saccharina japonica, indicating that algae and plants may share similar mechanisms for recognizing pathogens. After culturing the female gametophytes of S. japonica in the presence of Flagelin 22, flg15, flg14, and flg22D43A for 40 days, both Flagelin 22 and flg15 significantly induce growth inhibition of the algae at a concentration of 1 μM. The fresh weights of Flagelin 22- and flg15-challenged female gametophytes are less than one half of the control[1].

[1]. Bojun Lu, et al. Defense responses in female gametophytes of Saccharina japonica (Phaeophyta) induced by flg22-derived peptides. Journal of Applied Phycology (2016), 28(3), 1793-1801.