Plant species-specific recognition of long and short β-1,3-linked glucans is mediated by different receptor systems

Plants survey their environment for the presence of potentially harmful or beneficial microbes. During colonization, cell surface receptors perceive microbe-derived or modified-self ligands and initiate appropriate responses. The recognition of fungal chitin oligomers and the subsequent activation of plant immunity are well described. In contrast, the mechanisms underlying β-glucan recognition and signaling activation remain largely unexplored. Here, we systematically tested immune responses towards different β-glucan structures and show that responses vary between plant species. While leaves of the monocots Hordeum vulgare and Brachypodium distachyon can recognize longer (laminarin) and shorter (laminarihexaose) β-1,3-glucans with responses of varying intensity, duration and timing, leaves of the dicot Nicotiana benthamiana activate immunity in response to long β-1,3-glucans, whereas Arabidopsis thaliana and Capsella rubella perceive short β-1,3-glucans. Hydrolysis of the β-1,6 side-branches of laminarin demonstrated that not the glycosidic decoration but rather the degree of polymerization plays a pivotal role in the recognition of long-chain β-glucans. Moreover, in contrast to the recognition of short β-1,3-glucans in A. thaliana, perception of long β-1,3-glucans in N. benthamiana and rice is independent of CERK1, indicating that β-glucan recognition may be mediated by multiple β-glucan receptor systems.