**5. Auxin-Brassinosteroids pathways: molecular and genetic perspectives for improved salt tolerance in cereal crops**

Hormone signaling and metabolism pathways are widely regarded as promising targets for improving abiotic stress tolerance in plants, particularly in cereal crops [5]. As a result, maintaining the balance of phytohormones is critical for promoting optimal growth and development in plants [121]. Auxin and brassinosteroid are among the most commonly investigated phytohormones for improving abiotic stress tolerance in crops. These hormones are known to play a crucial role in mitigating salt stress, and they exhibit a diverse range of functions in this regard [122]. Consequently, several key enzymes involved in auxin and brassinosteroid signaling pathways have been genetically engineered to enhance abiotic stress tolerance in plants [121]. However, limited information is available on the mechanisms underlying the crosstalk between brassinosteroids and auxin in cereal crops. Nonetheless, transferring knowledge from model plant species such as Arabidopsis to cereal crops like wheat and barley is advantageous, as the signaling mechanisms in plants are evolutionarily conserved across [49]. Regarding Br signaling [123], discovered a negative correlation between the brassinosteroid pathway and abiotic stress tolerance in *Brachypodium distachyon*. In particular, when the mutant form of the BRI1 gene (bri1) was present, the plant exhibited an improvement in abiotic stress tolerance,

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

particularly under drought stress conditions. Furthermore, in rice BZR1, a dependent BR-gene, functions as a positive regulator of the BR signaling. The RNAi-mediated silencing of the OsBZR1 gene expression results in the BR insensitivity, semi-dwarfism and erect phenotype [124].

In rice, Hwang et al. [125] have demonstrated that a complex composed of BR-OsBRI1-OsBAK1 inactivates the OsGSK2 which, in turn, inactivates the BR signaling output regulators, namely OsBZR1, LIC (TILLER ANGLE INCREASED CONTROLLER), OsGRF4 (Growth-Regulating Factor), and CYC-U2 (cyclin U-type) in rice. De-phosphorylated OsBZR1 regulates the target components (CYC-U4;1, LIC, ILI (lilliputian1), and DLT (LOW-TILLERING)) involved in primary BR response in rice. Another BR-signaling component, SERK2, was identified in rice cultivars. The generation of mutant alleles of SERK2 by CRISPR/Cas9 editing showed a higher sensitivity to salt stress with an increase of grain size. In contrast, the overexpression of SERK2 enhances resistance of plants to salt stress without affecting plant architecture [126].

Therefore, plants subjected to abiotic stress especially salinity and exogenous BR exhibit two main patterns of gene regulation: (i) BR rescue expression of developmental proteins that are suppressed under salt stress and (ii) BR induce higher levels of protective proteins than salt stress alone. Transcriptomic analyses reveal that salt stress causes the downregulation of many genes critical to cell wall synthesis, photosynthesis carbon assimilatory process, starch transport and accumulation, as well as many metabolic pathways [127]. Contrariwise, BR up-regulated genes are associated with plant growth and development processes, targeting genes encoding cell elongation and cell wall modification enzymes, auxin responsive factors, and TFs, among others, indicating the mechanisms by which BR act to alleviate abiotic stress especially salt stress [47]. A recent study identified a specific interaction between BR and auxin pathways RLA1/SMOS1 (REDUCED LEAF ANGLE 1/SMALL ORGAN SIZE 1), a transcriptional regulator of BR signaling pathway, which form a complex with OsBZR1 and activates the BR signal transduction [125]. In rice, it has been shown that the RLA1/SMOS1 gene can be activated by auxin, indicating a potential crosstalk between the auxin and brassinosteroid pathways [128]. In addition to the previously mentioned RLA1/SMOS1 genes, LPA1 (Loose Plant Architecture1) has also been found to play a role in regulating plant architecture and auxin homeostasis in rice [129]. Two BR-mediated pathways are two BR-mediated pathways that interact with auxin to regulate the leaf inclination in rice: the BR biosynthesis-dependent pathway and the OsBRI1-mediated pathway. LPA1 has been shown to inhibit auxin signaling by interacting with C-22-hydroxylated and 6-deoxo BRs, independently of the OsBRI1 mediated pathway. However, there is no evidence of a direct interaction between OsBRI1 and LPA1 proteins [130].
