Root Development and Abiotic Stress Adaptation

As soon as plants became independent from homogeneous aquatic environments, root-like organs were developed. The interface between land and water bodies was probably the medium for the earliest land plants. Taking into account that those ancestral root-like organs did not face problems of water and nutrient acquisition, they were probably rather simple. As the earliest plants colonized this medium, the sandy substrate was replaced by heterogeneous soil, promoting more sophisticated vegetation and expanding the limits of land plant colonization. Therefore, to increase the efficiency of exploration of heterogeneous soil, during plant evolu‐ tion the ancestral root-like organ was replaced by a complex root system (RS) as the one we now know [1-3]. Land plants nowadays present a wide diversity of root system architectures (RSA; spatial configuration of the root system) among species, from non-branched to highly com‐ plex branching patterns, achieving the most effective performance regarding anchorage and the acquisition of water and nutrients. Each kind of RSA is guided by a genetically controlled postembrionary root developmental program (PERDP). This program is not rigid, and actually permits high phenotypic plasticity in response to stressing environmental conditions. PERDP is essentially driven by two cellular processes, cell division in the apical root meristem and new lateral meristems formed from the pericycle, and cell expansion performed in the root elonga‐ tion area. This particular characteristic permits plants, which are sessile organisms, to change their root architecture to adapt to abiotic stress [4-6]. Soils provide plants with water and nutrients; however, nutrients and water are distributed in a heterogeneous or patchy manner. In order to enhance nutrient capture, plant roots have modified their root architecture to explore those nutrient-rich zones. In the last two decades, progress has been made understanding the physiological, molecular and biochemical basis of how the PERDP could be modified by abiotic environmental cues [5, 7]. The aim of this chapter is to provide a review of how abiotic stress modulates post-embryonic plant root development. We will begin with a discussion of origin, anatomy, morphology and kinds of RS. Then, we will review recent advances in the knowl‐