Peripheral? Not Really! The Extracellular Arabinogalactan Proteins Function in Calcium Signaling

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Arabinogalactan proteins (AGPs) are a family of extracellular proteoglycans that exist in land plants and algae (Johnson et al., 2017). AGPs are part of the Hyp-rich glycoprotein superfamily that also includes extensins and Pro-rich proteins. Based on the protein sequence, a glycosylphosphatidylinositol anchor is predicted to be present at the C terminus of many AGPs and is expected to attach the AGPs to the extracellular side of the plasma membrane, thereby forming a narrow AGP-rich region between the cell membrane and the cell wall proper. AGPs are highly decorated with arabinogalactan polysaccharides (AGs) O-linked to the Hyp residues of the protein core. Over 100 proteins are predicted to carry AGs (Borner et al., 2003). Due to the chemical similarity of their carbohydrate moieties and functional redundancy among members of this family, studies of AGPs have been challenging and, consequently, little is known about the general molecular function of AGPs (Tan et al., 2012).

The arabinogalactan polysaccharides of AGPs are composed of a distinctive β-(1→3)-galactan backbone that is further substituted by β-(1→6)-galactan side chains. Glucuronidation is often found at both the backbone and the side chains of AGs. The glucuronic acids of AGPs have been shown in vitro to bind calcium in a pH-dependent manner, leading to the “AGP-Ca²⁺ capacitor” hypothesis (Lamport and Várnai, 2013). This hypothesis proposes that glucuronidated AGPs act as a Ca²⁺ reservoir at the cell-surface apoplast and provide the primary source for cellular Ca²⁺ oscillations, thus coordinating Ca²⁺ signals with plant growth, development, and other physiological events. In a recent study, Lopez-Hernandez et al. (2020) tested aspects of this hypothesis and addressed the role of AGP glucuronidation in vivo using Arabidopsis (Arabidopsis thaliana) mutants (see figure).

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Model for Proposed Roles of Glucuronidated Arabinogalactan Polysaccharides in Calcium Signaling.

[Ca²⁺]_csapo, cell-surface apoplastic free Ca²⁺; [Ca²⁺]_cyt, cytosolic free Ca²⁺; [Me]GlcA, β-glucuronic acids with and without methylation.

(Modified from Lopez-Hernandez et al. [2020], Figure 12.)

With bioinformatics analysis, the authors identified and chose four AG β-glucuronyltransferases (GlcAT14A, -B, -D, and -E) for further mutant studies. AG glucuronidation was significantly reduced in the glcat14a/b double mutant and the glcat14a/b/d and glcat14a/b/e triple mutants. The abundance of extracted glucuronidated oligosaccharides of different lengths varied among these mutants, suggesting varied substrate preferences of different GlcATs. The AGs from glcat14a/b/d mutants bound ∼80% less Ca²⁺ in vitro compared with those from wild-type plants.

The reduction in AG glucuronidation caused pleiotropic growth defects such as shorter inflorescence stems and reduced trichome branching. The glcat14a/b/e mutants displayed a deetiolated phenotype in dark-grown hypocotyls. However, increasing the concentration of calcium in the growth medium suppressed these developmental phenotypes, and this suppression appeared to be specific to calcium and not magnesium. Finally, calcium waves induced by H₂O₂ were perturbed in the roots of AG glucuronidation mutants, indicating altered intracellular calcium signals. Together, these results provide a solid step toward validating the AGP-Ca²⁺ capacitor hypothesis, casting light on the molecular roles of AGPs in plant growth and development.