**3. Brassinosteroids: major roles in plants under salt stress**

Brassinosteroids (BRs) are a group of plant steroid hormones, firstly isolated from Brassica pollen. There are about 60 compounds [11, 44, 45] of which the most bioactive BRs are brassinolide, 24-epibrassinolide, and 28-homobrassinolide. The identification of BR signaling components through molecular and genetic studies has provided a significant breakthrough in biotechnological modification for enhancing crop yield and stress tolerance in the context of global climate change. This pathway is considered as the most crucial target for these modifications [46–49]. Recent research has confirmed the significant role of BR in the regulation of various physiological processes in plants, such as the regulation of metabolic reactions in response to different biotic and abiotic conditions. This includes responses to pathogen-triggered reactions, as well as the management of salt and drought stress, scavenging of Reactive Oxygen Species (ROS), and reactions to herbicides and pesticides [49]. In addition to its regulatory role in plant physiology, BR also plays a crucial role in morphogenetic processes during plant growth and development. However, this regulation is complex and involves a sophisticated interplay between the components of the BR signaling pathway and the signal transduction pathways of other phytohormones [50].

The perception of BRs begins with the binding of the hormone to a transmembrane polypeptide BRI1 (Brassinosteroid-Insensitive 1) as well as its co-receptor BAK1

#### *Control of Plant Responses to Salt Stress: Significance of Auxin and Brassinosteroids DOI: http://dx.doi.org/10.5772/intechopen.111449*

(BRASSINOSTEROID INSENSITIVE 1-associated receptor kinase 1) both of which belong to Leucine-Rich repeat Receptor-like kinases (LRR-RLK 1) family [51]. After the perception of BR by BRI1, the signal is transduced by several events of reversible phosphorylation leading to the activation of a BRASSINAZOLE RESISTANT 1 (BZR1), and BZR2, also known as BRI1-EMS SUPPRESSOR1 (BES1) transcription factors [52]. After being perceived by BRI1 and BAK1, unphosphorylated BZR1 and BES1 move into the nucleus and regulate the expression of their target genes [53–56]. This pathway is highly complex and involves the interplay of numerous proteins, including PP2A, which regulates both BRI1 and BES1 in the pathway. Furthermore, recent studies have highlighted the contribution of a type 1 protein phosphatase (TdPP1) in the dephosphorylation of BES1 and its subsequent activation upon BR treatment. These findings have shed new light on the intricacies of the BR signaling pathway and its potential for further biotechnological advancements in crop yield and stress tolerance.

Decades of research have explored the relationship between BR and plant stress response. One study [57] reported that treating barley with BR improved salt tolerance, potentially through the regulation of water loss via reduced stomatal conductance and density [58]. In many species, BR application through the root-growing media has been shown to promote seed germination under salt stress. In *Brassica napus*, the inhibitory effect of salt stress was reduced by the addition of BRs to the germination medium [59]. Another study [60] has demonstrated that pre-soaking rice seeds with BRs and NaCl alleviates the inhibitory effect of salt on seed germination and seedling growth associated with increased levels of nucleic acids and soluble proteins in the kernel of rice.

Maize plants treated with BRs have been shown to alleviate oxidative stress in salt, leading to improved seedling growth and reduced lipid peroxidation, likely through the induction of antioxidant enzyme activities such as CAT, SOD and POD [61]. Similarly, in wheat, the addition of exogenous BRs has been shown to enhance plant growth under saline conditions [62]. In Arabidopsis, endogenous BR is positively involved in the plant response to salt stress as confirmed by the hypersensitivity of BR-deficient mutant det2-1 and BR-insensitive mutant bin2-1 to salt stress during seedling growth and seed germination. This hypersensitivity is correlated with the inhibited induction of stress-related genes, namely P5CS1, COR78 and proline accumulation under salt stress conditions [63]. Furthermore, the addition of exogenous BR has been shown to improve NaCl-induced proline accumulation and eliminate the inhibition of root elongation in WT plants [63]. These findings provide insight into the potential of BR-based biotechnological interventions for enhancing plant stress response and improving crop yield under adverse conditions.
