To grow or not to grow: the enigma of plant root growth dynamism

Root growth and modulation in plants is a highly dynamic yet strictly regulated process. Primary root development in Arabidopsis for example, is an intricate balance between cell division in the meristematic zone and cell elongation in the elongation zone, followed by subsequent differentiation. This process involves an orchestrated series of events that depend on environmental and developmental cues. Regulation is imparted by a complex network of hormone signaling primarily involving auxin, in crosstalk with cytokinin, abscisic acid, jasmonic acid, ethylene and others. In course of evolution, plants have developed an incredible array of mechanisms to address water scarcity, including modulation of root system architecture. During low to moderate water deficiency plants, adopt a "searching-for-water" strategy evoking growth-promoting responses. This is marked by elongation of the primary root for water exploration in deeper soil layers. However, during severe drought stress, plants resort to a "stop growth" strategy and restrict root growth, to conserve resources and energy for survival. The balance between these adaptive responses is critically regulated by key phytohormones which interact synergistically or antagonistically, to control root growth under varying levels of water shortage. Understanding these adaptation strategies as to how plants integrate environmental cues and associated hormone signaling to influence root growth generates a wide range of possibilities for agricultural innovation. This article aims to provide an overview of the mechanisms acquired by the root system at the morphological, physiological, and molecular levels, under optimum growth conditions and in response to varying degrees of water paucity.