López-Hernández et al. reveal that arabinogalactans function in calcium signaling from outside to inside the cell.
Plant Cell https://doi.org/10.1105/tpc.20.00027
Federico López Hernández and Paul Dupree
University of Cambridge, Department of Biochemistry, Tennis Court Road, Cambridge, CB2 1QW UK
Background: Arabinogalactan (AG) is a complex carbohydrate, made of sugars, that surrounds all plant cells. No one quite knows how and why it is there, despite many years of research. AG is located on so-called arabinogalactan proteins that are anchored to the cell membrane. In addition to the sugars arabinose and galactose, AG usually contains a charged sugar, called glucuronic acid (GlcA), that helps it bind to calcium ions.
Question: We wanted to know whether, and how, AG is important for plants. In particular, we wanted to know if the GlcA in AG is essential for any part of plant cell biology, especially any aspect that might use calcium outside cells. To answer this question, we used Arabidopsis mutants deficient in the enzymes that transfer GlcA to the AG.
Findings: The AG in the Arabidopsis mutant plants was able to bind less calcium. We found that the plants with this defective AG were smaller, with deformed leaves, and their leaf hairs were less branched. When we grew them on growth medium with excess calcium, the mutant plants grew more normally. In wild-type plants, calcium concentrations within root cells change in response to stimulation from outside the cells. Interestingly, these normal waves in intracellular calcium concentration were missing in the roots of the mutant plants. Altogether, these observations provide evidence that AG is important for calcium signaling from outside the cell to inside the cell.
Next steps: There are many questions remaining about how binding of AG to calcium affects calcium signaling. Is AG a calcium capacitor? Future studies may also uncover the relationship between specific types of arabinogalactan proteins, calcium signaling and plant development.
Federico Lopez-Hernandez, Theodora Tryfona, Annalisa Rizza, Xiaolan L. Yu, Matthew O.B. Harris, Alex A.R. Webb, Toshihisa Kotake, Paul Dupree. (2020). Calcium Binding by Arabinogalactan Polysaccharides Is Important for Normal Plant Development. Plant Cell; DOI: https://doi.org/10.1105/tpc.20.00027