Genetic and Hormonal Blueprint of Shoot-Borne Adventitious Root Development in Rice and Maize

The evolution of root architecture in plants was a prerequisite for the absorption of water and minerals from the soil, and thus a major determinant of terrestrial plant colonization. Cereals have a remarkably complex root system consisting of embryonic primary roots and post-embryonic lateral roots and shoot-borne adventitious roots. Among grass species, rice adventitious roots (also called crown roots) are developed from compressed nodes at the stem base, whereas in maize, besides crown roots, several aboveground brace roots are also formed, thus adventitious root types display species-specific diversity. Despite being the backbone for the adult root system in monocots, adventitious roots are the least studied of all the plant organs. In recent times, molecular genetics, genomics and proteomics-based approaches have been utilized to dissect the mechanism of post-embryonic meristem formation and tissue patterning. Adventitious root development is a cumulative effect of the actions and interactions of crucial genetic and hormonal regulators. In this review, we provide a comprehensive view of the key regulators involved during the different stages of adventitious root development in two important crop plants, rice and maize. We have reviewed the roles of major phytohormones, microRNAs and transcription factors and their crosstalk during adventitious root development in these cereal crops.