Located between the epidermis and endodermis,cortex is a major constituent of the ground tissue in plants.Cortex is initially derived from the same initial cell that gives rise to the endodermis,another cell type of ground tissue.Subsequently,the endodermis can divide repeatedly,producing multiple layers of cortex while maintaining a single layer of endodermis.This is the case in most plant species,but in the root of Arabidopsis thaliana there are generally only one or two cortex cell layers (Baum et al.,2002).The second layer,called the middle cortex (MC),is not produced until the seedling is 1 week old (Figure 1).Although MC formation seems to be a very simple process of cell proliferation,studies in the past decade have shown that it is regulated by multiple genes and signaling pathways.Some signaling pathways even act in an unexpected manner.Thus,it is essential to understand the mechanisms underlying MC formation for a comprehensive understanding of the roles of various signaling pathways in plant growth and development.