Hornwort stomata walls are not built for movement

Guard cell walls are built to resist bending and deformation to open and close the pore. Pectins provide flexibility and resilience to walls; in particular arabinans and unesterified homo-galacturonans are required for stomata function. Merced and Renzaglia use immunolabelling to investigate how wall architecture and pectin composition of Arabidopsis stomata compare to the unresponsive stomata of the hornwort Phaeoceros (Notothyladaceae, Anthocerotophyta).

Hornwort stomata

Walls of the angiosperm with active stomata contain arabinans and homogalacturonans that are completely unesterified, while these components are not present in hornworts. Structure and composition of guard cell walls reflect the divergent function of stomata in hornworts and angiosperms, and support gradual evolution of active movement.

Further reading

Merced, A., & Renzaglia, K. S. (2018). Contrasting pectin polymers in guard cell walls of Arabidopsis and the hornwort Phaeoceros reflect physiological differences. Annals of Botany, 123(4), 579–585. https://doi.org/10.1093/aob/mcy168