AGP-Ca2+ binding is essential for pollen development and pollen tube growth in Arabidopsis thaliana

Arabinogalactan-proteins (AGPs) are highly glycosylated cell wall proteins essential for plant reproduction, although their mode of action remains unclear. AGPs have been proposed to bind and store calcium (Ca2+) in the apoplast via glucuronic acid (GlcA) residues added by glucuronosyltransferases (GLCATs), potentially acting as Ca2+ capacitors. Here we report that Ca2+ binding by AGPs is required for successful double fertilisation in Arabidopsis. Analysis of glcat14a glcat14b glcat14d triple mutants revealed reduced seed set due to the abortion of pollen grains, which lacked cytoplasmic content and an intine layer, as confirmed by the absence of cellulose, resembling the phenotype of AGP-deficient mutants. When grown under Ca2+-deficient conditions, GLCAT mutants showed exacerbated and conditional reproductive defects that were attenuated under standard Ca2+ conditions. Our findings establish a functional requirement for GlcA-mediated Ca2+ binding by AGPs and support their role as apoplastic Ca2+ stores essential for male fertility and reproductive success in plants.