Phytohormonal control of cortical layer development in plant roots.

Root radial patterning determines the organization of root tissues and is essential for plant growth, adaptation to diverse abiotic stresses, and biotic interactions with rhizobia and symbiotic fungi. In Arabidopsis (Arabidopsis thaliana), two GRAS family transcription factors, SHORT-ROOT (SHR) and SCARECROW (SCR), control formative cell divisions of cortex/endodermis initial daughter (CEID) cells, giving rise to the single layers of cortex and endodermis. At later developmental stages, additional formative divisions of endodermal cells generate the middle cortex, located between the endodermis and the inner cortex. Phytohormones, including auxin, gibberellic acids, abscisic acid, and ethylene, play key roles in regulating these formative divisions. Although increasing evidence indicates that the SHR-SCR module also plays a central role in regulating formative divisions of CEID (and endodermal) cells in plant species with multiple cortical cell layers, including many important crops, the roles of phytohormones in regulating these additional formative divisions remain largely unknown. Here, I summarize the phytohormonal control of root radial patterning in Arabidopsis, and recent advances in understanding the mechanisms underlying root anatomical responses to abiotic stress in cereals and biotic interactions of legumes, and discuss the potential roles of phytohormones in regulating the development of multiple cortical cell layers.