Lipopolysaccharide-induced plasma membrane hyperpolarization in Chara corallina and Arabidopsis thaliana: evolutionary conservation of bacterial elicitor perception across the streptophyte lineage

Abstract

The green alga Chara corallina (Charophyceae) is a long-standing model for plant electrophysiology and, owing to its phylogenetic proximity to the ancestors of land plants, an informative system for probing the evolutionary depth of plant-microbe interactions. Bacterial lipopolysaccharide (LPS) is a potent microbe-associated molecular pattern (MAMP) that triggers innate immune signaling in both animals and land plants, but nothing was known about how a charophyte alga responds to this elicitor at the level of the plasma membrane. Here we measured resting membrane potential (Vm) changes in C. corallina internodal cells exposed to purified LPS from three Gram-negative bacteria: Escherichia coli, Pseudomonas aeruginosa, and Pectobacterium carotovorum subsp. carotovorum. All three LPS preparations triggered hyperpolarization of the plasma membrane, but with strikingly different kinetics: E. coli LPS produced a transient hyperpolarization that decayed within minutes, whereas LPS from P. aeruginosa and P. carotovorum produced a sustained, non-transient hyperpolarization that persisted for the duration of recording. Pretreatment with vanadate, a specific inhibitor of P-type H+-ATPases, abolished the electrogenic component of the resting potential and strongly reduced the LPS-induced hyperpolarization, suggesting that the plasma membrane proton pump participates in this response. Strikingly, membrane potential recordings performed in Arabidopsis thaliana cell suspension cultures reproduced chemotype specific kinetic dichotomy: LPS from the non-phytopathogen E. coli and P. aeruginosa, both induced a transient hyperpolarization, whereas LPS from the phytopathogen P. carotovorum subsp. carotovorum induced a non-transient, sustained hyperpolarization of A. thaliana cells. These results demonstrate that Chara possesses an electrophysiologically detectable LPS-sensitive perception system that pre-dates the divergence of land plants, that this system can discriminate between LPS chemotypes with distinct kinetic signatures, and that the same transient/non-transient signature is conserved in A. thaliana. These findings argue that the transient/non-transient dichotomy documented here may reflect an ancestral, broadly conserved capacity to decode structural variation in bacterial glycolipids at the level of the plasma membrane.