CUTIN SYNTHASE 2 Maintains Progressively Developing Cuticular Ridges in Arabidopsis Sepals

The cuticle is a crucial barrier on the aerial surfaces of land plants. In many plants, including Arabidopsis, the sepals and petals form distinctive nanoridges in their cuticles. However, little is known about how the formation and maintenance of these nanostructures is coordinated with the growth and development of the underlying cells. Here we report the characterization of the Arabidopsis cutin synthase 2 (cus2) mutant, which causes a great reduction in cuticular ridges on the mature sepal epidermis, but only a moderate effect on petal cone cell ridges. Using scanning electron microscopy and confocal live imaging combined with quantification of cellular growth, we find that cuticular ridge formation progresses down the sepal from tip to base as the sepal grows. pCUS2::GFP-GUS reporter expression coincides with cuticular ridge formation, descending the sepal from tip to base. Ridge formation also coincides with the reduction in growth rate and termination of cell division of the underlying epidermal cells. Surprisingly, cuticular ridges at first form normally in the cus2 mutant, but are lost progressively at later stages of sepal development, indicating that CUS2 is crucial for the maintenance of cuticular ridges after they are formed. Our results reveal the dynamics of both ridge formation and maintenance as the sepal grows.