**4. Auxin-brassinosteroids crosstalk: an important approach for plant salt stress tolerance**

The interplay between auxin and BRs is known to regulate various aspects of plant development and growth, not just at the individual level but also in cross-talk [64]. Despite being a well-investigated concept for over a decade, it is not yet fully understood, particularly in crops. Studies have shown that the root level auxin and BR exhibit opposed actions, with an optimal expression of BZR1 depending on auxin biosynthesis [65]. BR catabolism and BR-mediated signaling lead to the specific spatiotemporal activation of auxin-related genes in the elongation zone, while repressing them in the quiescent center [64]. Indeed, BZR1 directly interacts with ARF proteins to target multiple auxin-related genes, including those involved in transport and signaling, such as AUX/IAA, PINs and TIR/AFB. Therefore, ARFs genes are composed of a carboxy-terminal dimerization domain that facilitates protein-protein interactions, not only within the AUX/IAA family but also between ARF genes. [66]. In addition, it has been established that BR signaling connects with SOB3 (SUPPRESSOR OF PHYTOCHROME B4-3) to control cell elongation and hypocotyl growth through the up-regulation of SAUR19 (SMALL AUXIN UP RNA19) expression [67].

BR plays a crucial role in the transport of auxin by affecting the cellular localization of auxin efflux and influx carriers such as PIN3, PIN4 and AUX1/LAXs [12–74]. Specifically, BR controls accumulation of intracellular auxin flow PIN2 from the root tip towards the shoot by recycling it back to the vasculature via the lateral root cap and epidermis. The accumulation of PIN2 and PIN4 is regulated by BR in a posttranscriptional manner, and BR has a similar effect on PIN21 and PIN4 accumulation in collumella [74]. During plant gravitropism, BR intensifies the accumulation of the PIN2 gene in the root meristem zone and affects the allocation of auxin from the root tip towards the elongation zones, resulting in a difference in IAA levels in the upper and lower sides of roots. It has been demonstrated that during this process, BR activates ROP2 which plays a vital role in modulating the functional localisation of PIN2 through the regulation of F-actins. In contrast, BRX (brevis radix) which regulates cell proliferation and elongation in the roots and shoots is vastly brought by auxin and repressed by BRs [11]. Interestingly, the BR-biosynthetic genes, DWF4 and CPD, have been shown to be activated BRX, highlighting the functional relationship between auxin signaling and BR biosynthesis [75]. The connection between auxin and BR is also evident when roots are treated with exogenous auxin, which increases DWF4 expression, leading to an increase in BR biosynthesis. However, when BR is synthesized, DWF4 is retro-inhibited by BR itself [76]. Several studies have suggested that the effects of BR are also influenced by auxin, either by enhancing sensitivity to this hormone or by altering its levels [77, 78]. According to a study by [79], GH3 genes in soybean and tomato were not promptly activated during BR-induced cell expansion but were activated by BR after cell elongation had begun.

Auxin and BR have a synergistic relationship that is evident in their combined effects on root development. One example of this interaction is demonstrated through the interplay between BIN2 and ARF2 repressors. [76]. BIN2 was found to phosphorylate ARF2, which inhibits its interaction with the AUX/IAA repressor and enhances auxin response [80]. ARF2 is also a target of BZR1 and its expression is decreased by BR treatment [54]. Phosphorylation by BIN2 can reach additional ARFs (ARF7 and ARF19) to induce the transcriptional activity of their target genes LATERAL ORGAN BOUNDARIES-DOMAIN16 (LBD16) and LBD29 acting on lateral root organogenesis [81]. In Arabidopsis, many regulators genes are known to control seed size and endosperm development like SHB1 (SHORT HYPOCOTYL UNDER BLUE 1), IKU1 (HAIKU 1), IKU2 (HAIKU 2), and MINI3 (MINISEED 3) through their interaction in BR-signaling [82]. These proteins are involved in the regulation of BZR1 under the control of the BR-BRI1-BIN2 phosphorylation cascade. Therefore, BR can inhibit the expression of APETALA 2 (AP2), the floral homeotic gene, and AUXIN RESPONSE FACTOR 2 (ARF2), the key negative regulators of seed size and weight [83]. Moreover, the exogenous application of BR induced the expression of auxin-responsive genes implicated in root development of which we can cite IAA7, IAA17 and IAA14. However, BR signaling mutant and biosynthetic mutant det2 and bri1 had significantly decreased gene expression of AXR3/IAA17 as well as several

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

Aux/IAA genes, such as AXR2/IAA7, SLR/IAA14, and IAA28. This finding suggests that BR signaling pathways and auxin signaling pathways are integrated during root development [84]. The interaction between BR and auxin is also involved in regulating plant stress responses. In cucumber plants subjected to different stress conditions, including salt, cold, and PEG, the expression of many YUCCA genes is reduced. However, yucca mutants exhibit higher levels of transcripts of BR-related genes such as BRI1 [85].

Despite the importance of BRs and auxin in regulating salt stress response and root growth, the specific molecular mechanisms involved in this process are still unclear. It has been observed that, under salt stress conditions, transcription factors associated with the BR pathway can affect auxin homeostasis by modulating the expression of genes involved in auxin biosynthesis, conjugation, and degradation. (**Figure 2**; **Table 1**). On the other hand, BZR1/BES1 dephosphorylation causes the rapid induction of genes encoding the auxin biosynthetic enzymes like YUC7/3/8/5 under salt stress. Thus, auxin and BRs signaling participate in regulating a large spectrum of root developmental processes by the formation of an auxin gradient, allowing plant seedlings to cope with salinity. This movement of local auxin concentration was regulated by the expression of CYP79B2, ABCB family, PIN, YUC, GH3 and PAT1 (PHOSPHORIBOSYL ANTHRANILATE TRANSFERASE) especially under abiotic stress by heavy metal [117]. Gene expression studies have revealed that genes involved in tryptophan-dependent IAA biosynthesis pathway like YUC4, NIT1; NIT2, and IAA degradation like DAO were increased by salt stress [118]. Transcriptomic data indicated that some IAA biosynthesis genes such as AAO1 (ARABIDOPSIS ALDEHYDE OXIDASE1); CYP79B2,3 (CYTOCHROME P450 FAMILY 79B2,3) and AMI1 (INDOLE-3-ACETAMIDE) display similar expression pattern under salt stress and control conditions. Nonetheless, the expression of DAO (DIOXYGENASE FOR

#### **Figure 2.**

*A schematic model highlighting the potential molecular-genetic mechanisms involved in the auxin-brassinosteroids crosstalk under salt stress. Black solid arrows show regulation, blue solid arrows salt stress tolerance via IAA and BR signaling pathway. Red solid arrows show crosstalk between IAA and BR signaling pathway.*


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



#### **Table 1.**

*The most important genes involved in the crosstalk between BRs and auxin.*

AUXIN DEGRADATION), which is involved in auxin degradation, did not change significantly in plants grown in the presence of NaCl [118]. Both BR and auxin are recognized as key regulators that exert gradual effects on a range of growth processes, including cell division and cell elongation, particularly under abiotic stress conditions [65]. Recent research has revealed that ARF and BZR collaborate to promote hypocotyl elongation [119]. Similarly, BRs activate the expression of SAUR19 via BZR1 and there is evidence of interaction between ARF6, BZR1 and SAUR genes [67]. More recently, it has been demonstrated that these genes are implicated in enhancing plant tolerance to abiotic stress, particularly drought stress [120].
