Brassinosteroids control gravitropism by changing cellulose orientation and mannan content in Arabidopsis hypocotyls

Shoot gravitropism is essential for plants to direct the growth of above-ground tissues towards the soil surface after germination. Brassinosteroids influence shoot gravitropism and we used this as a tool to untangle the function of cell wall polymers during etiolated shoot growth. The ability of etiolated Arabidopsis seedlings to grow upwards was suppressed in the presence of 24-epibrassinolide (EBL) but enhanced in the presence of Brassinazole (BRZ), an inhibitor of brassinosteroid biosynthesis. These effects were accompanied by changes in cell wall mechanics and composition. Cell wall biochemical analyses and confocal microscopy of the cellulose-specific pontamine S4B dye revealed that the EBL and BRZ treatments correlated with changes in cellulose fibre organization and mannan content. Indeed, a longitudinal re-orientation of cellulose fibres supported upright growth whereas the presence of mannans reduced gravitropic bending. The negative effect of mannans on gravitropic bending is a new function for this class of hemicelluloses, and provides insight into evolutionary adaptations by which aquatic ancestors of terrestrial plants colonized land.