A cyclic lipopeptide produced by an antagonistic bacterium relies on its tail and transient receptor potential‐type Ca2+ channels to immobilize a green alga

Summary The antagonistic bacterium Pseudomonas protegens secretes the cyclic lipopeptide (CLiP) orfamide A, which triggers a Ca2+ signal causing rapid deflagellation of the microalga Chlamydomonas reinhardtii. We performed chemical synthesis of orfamide A derivatives and used an aequorin reporter line to measure their Ca2+ responses. Immobilization of algae was studied using a modulator and mutants of transient receptor potential (TRP)‐type channels. By investigating targeted synthetic orfamide A derivatives, we found that N‐terminal amino acids of the linear part and the terminal fatty acid region are important for the specificity of the Ca2+‐signal causing deflagellation. Molecular editing indicates that at least two distinct Ca2+‐signaling pathways are triggered. One is involved in deflagellation (Thr3 change, fatty acid tail shortened by 4C), whereas the other still causes an increase in cytosolic Ca2+ in the algal cells, but does not cause substantial deflagellation (Leu1 change, fatty acid hydroxylation, fatty acid changes by 2C). Using mutants, we define four TRP‐type channels that are involved in orfamide A signaling; only one (ADF1) responds additionally to low pH. These results suggest that the linear part of the CLiP plays one major role in Ca2+ signaling, and that orfamide A uses a network of algal TRP‐type channels for deflagellation.