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.