New molecular probes reveal callose structural diversity in plant cell wall microdomains

Cell walls underpin the mechanics of cell growth, intercellular signalling, and defence against pathogenic organisms. β-(1,3)-glucans (also known as callose) are polysaccharides found in plants, fungi, and some bacterial species. In developing plant organs, callose accumulates around intercellular channels (plasmodesmata) controlling cell-to-cell communication. We developed monoclonal antibodies for the detection of β-(1,3)-glucans and using these identified distinct populations of callose differing in size and secondary structure. Callose sub-populations were in proximal but not overlapping cell wall microdomains surrounding plasmodesmata. Moreover, labelling with an antibody that detects larger more complex β-(1,3)-glucans structures (named LM-BDG2) was absent in Arabidopsis meristems implying distinct spatial and functional microenvironments. Biochemical and studies with mutants suggest that the accumulation of LM-BDG2-labelled callose structures is influenced by interactions with xyloglucans (another plant glycan regulating cell wall properties). This work extends current understanding and demonstrates structural heterogeneity in plant β-(1,3)-glucans. It supports interactions between glycans with roles in the regulation of cell wall properties and functions.