**Abstract**

Agricultural productivity world over is threatened by abiotic stress, intensifying food security issues. The plant hormones play a significant role in mitigating abiotic stresses, including drought stress, salinity stress, heat stress, and heavy metal stress, faced by the plants. Considerable research has been conducted to understand hormone-mediated abiotic stress responses in plants and the underlying biosynthetic and regulatory pathways. Deciphering these pathways would allow their manipulation in the laboratory and possible extension to the field. In the present chapter, an overview of the role plant hormones play in mitigating abiotic stress, the underlying mechanisms of their action, and the cross-talk between their signaling pathways to mitigate abiotic stress is presented.

**Keywords:** abiotic stress, plant hormones, stress response, stress mitigation, plant productivity

### **1. Introduction**

Plant hormones or phytohormones are biochemicals required for the normal growth and development of plants [1–3]. Plant hormones include auxins (IAA), gibberellins (GAs) cytokinins (CK), abscisic acid (ABA), ethylene (ET), besides jasmonates (JA), salicylic acid (SA), brassinosteroids (BR), strigolactones (SL), and nitric oxide (NO). Apart from their role in plant growth and development, hormones also mediate response to biotic (disease, pathogens, herbivores, etc.) and abiotic (drought, heat, salinity, heavy metals, etc.) stress [3–6]. Hormones act at the site of their biosynthesis or some distance away from it [3, 6–8]. Hormone biosynthesis, distribution, and patterns of their signal transduction change under stress conditions [8, 9]. Ethylene and ABA play remarkable roles in regulating the abiotic stress response [8]. The exogenous supply of phytohormones also increases stress endurance in plants [10, 11]. Abiotic stress factors rarely occur individually, and many stresses produce the same effects at the cellular level with an overlap in the expression pattern of stress response genes [12]. In the current chapter, the hormone-mediated response of plants to the abiotic stress, including drought, heat, and salinity, is discussed.

### **1.1 What is abiotic stress?**

Ecological factors favor plant growth at optimum levels and constitute stress at sub- or supra-optimal levels. Abiotic stress reduces crop productivity by about 50% (**Table 1**) [8, 29]. High temperatures lead to 20% decrease in the yield, low temperatures 7%, salinity 10%, drought 9% and other forms of stress cause 4% yield loss [30]. In grain crops, grain size, number, and dry weight are influenced by abiotic stress, especially if present during the reproductive phase [30]. Various aspects of plant growth as affected by abiotic stress are presented in **Figure 1**. Crop productivity may be reduced by 2.5–16% by a 1°C rise in seasonal temperature in tropical and subtropical regions [31]. The stress response depends on the genetic constitution and adaptive response of a plant [32].
