**1. Introduction**

Abiotic stress responses in plants occur at various organ levels among which the rootspecific processes are of particular importance. Under normal growth condition, root absorbs water and nutrients from the soil and supplies them throughout the plant body, thereby playing pivotal roles in maintaining cellular homeostasis. However, this balanced system is altered during the stress period when roots are forced to adopt several structur‐ al and functional modifications. Examples of these modifications include molecular, cellular, and phenotypic changes such as alteration of metabolism and membrane characteristics, hardening of cell wall, and reduction of root length [1, 2]. The root system has the crucial role of extracting nutrients and water through a complex interplay with soil biogeochemi‐ cal properties and of maintaining these functions under a wide range of stress scenarios to ensure plant survival and reproduction [3].

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Water stress is characterized by a reduction of water content and leaf water potential, closure of stomata, and decreased growth. Severe water stress may result in the arrest of photosyn‐ thesis, disturbance of metabolism, and finally the death of plant [4]. This water loss causes a loss of turgor pressure that may be accompanied by a decrease in cell volume depending on the hardness of the cell wall [5]. The cells of the root must activate processes to limit water loss and mitigate its harmful effects.

Salinity also affects plant growth, activity of major cytosolic enzymes by disturbing intracel‐ lular potassium homeostasis, causing oxidative stress and programmed cell death, reducing nutrient uptake, genetic and epigenetic effects, metabolic toxicity, inhibition of photosynthesis, decreasing CO2 assimilation, and reducing root respiration [6, 7, 8, 9]. Salt stress affects the root in all developmental zones. Cell division decreases in the meristematic zone and cell expansion attenuates in the elongation zone, resulting in reduced overall growth [10]. Cells also expand radially in the elongation zone [11], and root hair outgrowth suppresses in the differentiation zone [12]. Salt stress additionally results in agravitropic growth [13] as well as reduced lateral root number under high-salt conditions and enhanced lateral root number under moderate-salt conditions [14, 15]. Salt stress developes from excessive concentrations of salt, especially sodium chloride (NaCl) in soil. Root is the primary organ of exposure and hence responds rapidly [16]. Salt stress is known to increase Na+ /K+ ratio in the root that leads to cell dehydration and ion imbalance [17, 18, 19].

High temperature increases the permeability of plasma membrane [20], and also reduces water availability [21]. Moreover, low temperature (chilling and frost stress) is also a major limiting factor for productivity of plant indigenous to tropical and subtropical climates [22]. Chilling stress has a direct impact on the photosynthetic apparatus, essentially by disrupting the thylakoid electron transport, carbon reduction cycle, and stomatal control of CO2 supply, together with an increased accumulation of sugars, peroxidation of lipids, and disturbance of water balance [23].

Heavy metal contamination in soil could result in inhibition of plant growth and yield reduction and even poses a great threat to human health via food chain [24]. Among heavy metals, Cadmium (Cd) in particular causes increasingly international concern [25]. Cdcontaminated soil results in considerable accumulation of Cd in edible parts of crops, and then it enters the food chain through the translocation and accumulation by plants [26, 27]. Another metal, chromium (Cr III or VI), is not required by plants for their normal plant metabolic activities. On the contrary, excess of Cr (III or VI) in agricultural soils causes oxidative stress for many crops. Reactive oxygen species (ROS), like hydrogen peroxide (H2O2), hydroxyl radical (OH<sup>⋅</sup> ), and superoxide radical (O2 − ) generated under Cr-stress, are highly reactive and cause oxidative damages to DNA, RNA, proteins, and pigments [28, 29]. Nickel (Ni) is one of the most abundant heavy metal contaminants of the environment due to its release from mining and smelting practices. It is classified as an essential element for plant growth [30]. However, at higher concentrations, nickel is an important environmental pollutant. Ni2+ ions bind to proteins and lipids such as specific subsequences of histones [31] and induce oxidative damage. Copper (Cu) is also an essential micronutrient for most biological organisms. It is a cofactor for a large array of proteins involved in diverse physiological processes, such as photosynthesis, electron transport chain, respiration, cell wall metabolism, and hormone signaling [32, 33]. Cu has emerged as a major environmental pollutant in the past few decades because of its excessive use in manufacturing and agricultural industries [34]. Zinc (Zn) is one of the other essential microelement, the second most abundant transition metal, and plays roles in many metabolic reactions in plants [35, 36]. However, high concentrations of Zn are toxic, induce structural disorders, and cause functional impairment in plants. At organism level, Zn stress reduces rooting capacity, stunted growth, chlorosis, and at cellular level alters mitotic activity [37, 38].

The key to find out abiotic stress tolerance resides in understanding the plant's capacity to accelerate/maintain or repress growth. Most plant hormones play a role in development and have been implicated in abiotic stress responses. One of these hormones, BRs, are a group of steroidal hormones that play significant roles in a wide range of developmental phenomena including cell division and cell elongation in stems and roots, photo-morphogenesis, repro‐ ductive development, leaf senescence, and also in stress responses [39]. Mitchell et al. [40] discovered BRs which were later extracted from the pollen of *Brassica napus* by Grove et al. [41]. To date, more than 70 BR-related phytosteroids have been identified in plants [42].

BRs increase adaptation to various abiotic stresses such as light [43, 44], low or high temper‐ ature [45], drought [46, 47, 48], salt stress [9], and heavy metal stress [49, 50]. BRs may be applied/supplied to plants at different stages of their life cycle such as meiosis stage [51], anthesis stage [52], and root application [9, 53].

In this chapter, the potential role of BRs in alleviating the adverse effects of water, salt, low/ high temperature stresses, and heavy metals on plants, especially roots, were discussed.
