Control of Plant Responses to Salt Stress: Significance of Auxin and Brassinosteroids

Abstract

Salinity of soils represents a significant abiotic stress factor that not only reduces productivity of most crops but also poses a threat to the global food security. Understanding the mechanisms underpinning plant stress responses as a whole is essential for enhancing crop productivity in salt-affected soils. To improve crop production on salt-affected lands, it is crucial to have a comprehensive understanding of the mechanisms underlying plant stress responses. Phytohormones are key players in these processes, regulating plant growth, development and germination. Among phytohormones, auxin and brassinosteroids (BRs) have been found to overlap to lessen salt stress in plants. In order to help plants balance growth and salt stress tolerance, auxin, BRs, and their interactions are currently known to play a number of important roles. This chapter gives a summary of these findings and discusses how molecular and genetic approaches can be used to engineer auxin, BRs, and thereby develop more salt-resistant cereal crops in the future.

1. Introduction

Plants cannot avoid the multiple abiotic stresses to which they are constantly exposed because of their sessile nature. Stresses like salinity, drought, cold and heat greatly hinder the growth and productivity of plants. Among those, soil salinity significantly affects crop output and growth all over the world [1]. It is estimated that by 2050, roughly 20% of agricultural land will be unproductive due to soil salinization, which will affect nearly 50% of agricultural land [2]. Salt stress occurs when there is too much Na⁺ in the soil solution, preventing plants from receiving water and nutrients from the soil. Na⁺ accumulation is harmful because it causes osmotic and ionic stresses that promote the formation of ROS, alter plant metabolism, and upset the balance of ions in the environment [3].

Based on their capacity to flourish in salty conditions, plants are divided into two main groups called glycophytes and halophytes. Since most cultivated plants are glycophytes, they cannot withstand salty environments with concentrations of more than 100 mM NaCl. In the case of cereals, rice (Oryza sativa) is classified as the least tolerant species, followed by durum wheat (Triticum durum), common wheat (Triticum aestivum), maize (Zea mays) and barley (Hordeum vulgare) which is considered as the most tolerant species [4].

A proper hormonal balance is required in plants in order to limit the potential negative impacts of environmental variables. Most phytohormones are know to play a major role in controlling plant growth and development as well as stress responses [5, 6, 7, 8]. Thanks to their extensive range of functions and complex interplay, auxin and brassinosteroids (BRs) are considered as two of the phytohormones that hold the most promise for tailoring abiotic stress tolerance in crop plants [9, 10]. Many auxin-responsive genes have been demonstrated to be synergistically regulated by the interaction between BRs and auxin pathways [11]. Furthermore, through separate processes, auxin and BRs can stimulate root development and increase cell expansion [12]. This chapter attempts to offer new insights into our understanding of how auxin, BR and their interactions can support plants’ ability to balance growth and salt stress tolerance (Figure 1).

Figure 1.

Scheme representing the major structure of auxin and brassinosteroids, and their connection with salt stress.

2. Auxins: major roles in plants under salinity stress

Many physiological and developmental processes, including the formation of lateral and adventitious roots, flowering, senescence, and morphogenesis, are regulated by auxins, primarily indole-3 acetic acid (IAA) [13]. Auxin is one of the most significant phytohormones involved in the regulation of lateral root growth, main root elongation, and halotropism, the special capacity of plants to avoid salty circumstances when they are under salt stress. Auxin-mediated lateral root formation and the cessation of their growth in response to excessive salt have been shown to be antagonistic [14]. The gradient, concentration, and spatiotemporal expression of receptor genes tightly govern auxin’s regulatory mechanisms [15]. The intense regulation of this phytohormone at several levels, as well as its manufacture and signaling, led to a drop in endogenous auxin levels being identified under salt stress [16]. Three key auxin sources exist in plants, namely synthetic auxin, endogenous auxin and microbial auxin sources from the rhizosphere [17].

Auxin signal transduction pathway has been broadly examined [18, 19] and TIR1 encodes a nuclear auxin receptor belonging to the F-box protein [20] that interacts with a group of AUX/IAA (Auxin/Indole-3-Acetic Acid) proteins. Through their interaction with the transcriptional regulators of ARF (auxin response factors), AUX/IAA operates as negative regulators by impeding transcriptional auxin output [21]. ARFs, with their 23 members, can therefore activate or repress target genes and mediate the auxin responses, and they are destroyed when auxin binds to TIR1 [22].

Several kinds of efflux/influx carriers control the auxin transport in the plant root, resulting in the auxin gradient. The auxin-resistant 1/like aux1 (AUX/LAX) family of influx carriers mediates an active polar transport [23]. On the other hand, auxin efflux transporters include ABC transporter family [24], NRT1/PRT family of nitrate transporter 1/peptide transporter [25], and PIN-FORMED carriers [26, 27]. PINs constitute a family of 7 proteins that are found in the plasma membrane and are thought to be involved in the control of auxin transport [17]. Salt stress lowers auxin levels, which in turn drops the expression of auxin transporters [16], linking auxin distribution and biosynthesis [28]. PIN abundance is primarily responsible for the disruption in auxin transport. The authors in [29] have discovered that the PIN2 auxin efflux carrier is the most specific to salt stress since it is seen to actively redistribute auxin in the root tip when exposed to a salt gradient. Auxin redistribution and directional bending of the root away from high salt levels are mediated by PIN2 internalization, which is stimulated by salt-induced phospholipase-D at the side of the root facing the higher salt concentration. Under salt stress, the downregulation of PIN1, PIN3 and PIN7 is also an important part of the asymmetric distribution of auxin, which affects root bending from the salt [15]. Furthermore, primary root size is abridged under salt stress alongside with a decrease in lateral root density due to decreased levels of PIN1, PIN3 and PIN7 [16].

Auxin carriers’ function can be regulated through post-translational changes, subcellular localization, and regulation of their expression in addition to the regulation of their expression. In order to allow auxin redistribution and for the directional bending of the root away from the higher salt concentration, PIN2 and AUX1 alter their subcellular location in endosomes [29, 30]. AUX1 and PIN2 are required for the establishment of gravity-inducing asymmetric auxin response. Auxin transport in the elongation zone requires AUX1, but its transport back to the root tip is mostly mediated by PIN2 [31]. According to the model of [32] that was predicted to occur during halotropism, auxin asymmetry is caused by an imbalance in the PIN2 and AUX1 pathways in the root tip, with PIN2 decreasing on the side of the root that is exposed to salt and changing auxin levels on the opposite side. This asymmetry of auxin was amplified by the AUX1 auxin transporter. The AUX1 auxin transporter increased this auxin asymmetry.

What is trusty to mention is that PIN polarity and intracellular auxin polar fluxes are both required for PIN phosphorylation [33]. For instance, during phototropic and gravitropic reactions mediates PIN3 phosphorylation to establish an auxin gradient during phototropic and gravitropic responses [34]. The balanced actions of PID kinase and protein phosphatases do in fact regulate the status of phosphorylation in PINs. RCN1 (ROOTS CURL IN NPA 1) encodes a regulatory subunit of protein phosphatase 2A (PP2A). It has been reported that the mutant rcn1 shows an elevation in gravitropic root bending curvature [33, 34, 35]. Auxin biosynthesis is carried out by different reactions of the indole-3-acetaldoxime pathway mediated by enzymes like tryptophan aminotransferase (TAA)/YUCCA (YUC) [36]. The processes of IAA production, conjugation, and degradation determine the amounts of IAA in cells. Firstly, the YUCCA (YUC) family of enzymes, which are present throughout the root system, controls IAA production [37]. Auxin redistribution caused by the movement of auxin production from columella cells to the root epidermis during salt stress is partially connected with a reduction in the growth of primary and lateral roots [38]. Interestingly, YUC gene expression is calibrated in response to salt stress [39]. In fact, transcriptomic data show that YUC5 is up-regulated immediately after salt exposure [40] demonstrating that this gene plays a major role in auxin-mediated salt stress response.

Secondly, IAA levels are impacted by both conjugation and degradation processes, as observed through the positive correlation between free IAA levels and the levels of IAA conjugates and catabolites in roots and shoots. The degradation of IAA is the primary factor responsible for its rapid turnover and is mainly catalyzed by DAO1 and 2. A study has shown that DAO1/2 plays a crucial role in the root response to salt stress induced by lateral root density. Indole-3-butyric acid (IBA) contributes significantly to lateral root growth, root hair elongation, and adventitious root formation during root development. However, IBA can undergo oxidation and transform into IAA, which can negatively impact a plant’s ability to tolerate salinity [41]. The irreversible conjugation of aspartic acid to IAA tags it for oxidation as well as for catabolism and IAA-asp conjugates are known to be involved in IAA detoxification [42].

It has been established that abiotic stress, particularly salt, has an impact on these processes. The GH3 (Gretchen Hagen) family of enzymes can catalyze the addition of some groups and generate diverse auxin conjugate genes (i.e: ILR/IAR (IAA-amido hydrolase) and ILL (ILR1 likes)). Salt stress tolerance is mediated in part by auxin homeostasis, which is controlled by a collection of GH3 enzymes through a negative feedback regulation. WES1 (a kind of GH3) gene regulation is essential for salt stress tolerance [43]. Indeed, GH3 gene family is a desirable option for salt stress breeding since it is extensively expressed in roots and is increased when exposed to salt stress [39].

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, 47, 48, 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 (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, 54, 55, 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.

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, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 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 post-transcriptional 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 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 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].

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.

Genes involved in the crosstalk BR-auxin	Gene accession number	Description	Ref.
YUC (YUCCA)	AT4G32540	Catalyzes the conversion of IPA (indole-3-pyruvic acid) to IAA	[86]
NIT1 (NITRILASE1)	AT3G44310	Participates in the conversion of IAN (indole-3-acetonitrile) to IAA	[86]
NIT2(NITRILASE2)	AT3G44300	Participates in the conversion of IAN (indole-3-acetonitrile) to IAA	[86]
TAA1 (TRYPTOPHAN AMINOTRANSFERASE)	AT1G70560	Encodes an aminotransferase that converts Trp (Tryptophane) to IPA (, indole-3-pyruvic acid)	[87, 88]
SAUR (SMALL AUXIN UPREGULATED RNA)	AT1G11803	Encode Small Auxin Up RNAs, that regulate leaf growth through controlling cell expansion or division, contributing to auxin-regulated leaf growth and development.	[89, 90]
AXR1 (AUXIN RESISTANT1)	AT1G05180	Encode a protein related to the ubiquitin-activating enzyme, and its function in the ubiquitin conjugation pathway	[91]
GH3 (GRETCHEN HAGEN3)	AT1G48660	Encodes an auxin-conjugating enzyme; and contributes to the auxin-mediated modulation of plant growth in response to environmental stresses.	[92]
PIN1 (PIN-FORMED 1)	AT1G73590	Encodes an auxin efflux carrier involved in shoot and root development. It is involved in the maintenance of embryonic auxin gradients.	[16, 93]
PIN2 (PIN-FORMED 2)	AT5G57090	PIN2 functions as an auxin efflux facilitator mediating proximal shootward auxin transport in the Arabidopsis root	[94]
PIN3 (PIN-FORMED 3)	AT1G70940	PIN3 is an auxin efflux carrier which is expressed in the cortical cells situated in front of the LRP(Lateral root primordium) and it is induced by auxin in this tissue.	[16, 95]
PIN4 (PIN-FORMED 4)	AT2G01420	PIN4 is involved in the regulation of auxin homeostasis and patterning through sink-mediated auxin distrubition in root tips.	[16, 96]
PIN5 (PIN-FORMED 5)	AT5G16530	PIN5 regulates intracellular auxin homeostasis and metabolism.	[16, 96]
PIN6 (PIN-FORMED 6)	AT1G77110	PIN6 is an important component of auxin transport and auxin homeostasis and contributes to auxin-dependant growth and development processes such as root and shoot.	[16, [97]
PIN7 (PIN-FORMED 7)	AT1G23080	PIN7 involved in the redistribution of auxin from the maximum in the collumella initials to the epidermis and lateral root cap. Gravity stimulation of roots induces rapid polarization of PIN7 towards the lateral plasmamembrane.	[16, 98]
SAUR32 (SMALL AUXIN UPREGULATED RNA 32)	AT2G46690	SAUR32 is implicated in the reduction of hypocotyl growth and abolished apical hook formation in the dark.	[39, 99]
SAUR19 (SMALL AUXIN UPREGULATED RNA 19)	AT5G18010	SAUR19 is considered as a positive effector of cell expansion. The regulation of this gene is achived through the modulation of auxin transport.	[39, 100]
DAO1(DIOXYGENASE FOR AUXIN OXIDATION 1)	AT1G14130	DAO1 catalyzes the formation of oxIAA and regulate auxin homeostasis and plant growth. DAO1 plays a crucial role in plant morphogenesis.	[101]
DAO2 (DIOXYGENASE FOR AUXIN OXIDATION 2)	AT1G14120	DAO2 catalyzes the oxidation of IAA into oxIAA. DAO2 is expressed in root tip.	[101]
ARF7 (AUXIN RESPONSE FACTOR 7)	AT5G20730	ARF7 is considered as a transcriptional activators of auxin-response genes. ARF7 Regulates Lateral Root Formation via Direct Activation of LBD/ASL Genes in Arabidopsis	[102]
ARF19 (AUXIN RESPONSE FACTOR 19)	AT1G19220	ARF19 Regulate Lateral Root Formation via Direct Activation of LBD/ASL Genes in Arabidopsis.	[102]
ARF2 (AUXIN RESPONSE FACTOR 2)	AT5G62000	The main function of ARF2 is in the auxin-mediated control of Arabidopsis leaf longivety.	[103]
ARF9 (AUXIN RESPONSE FACTOR 9)	AT4G23980	ARF9 reveals en inhibitory effect on auxin-responsive root hair growth.	[104, 105]
AXR2/IAA7 (AUXIN RESISTANT2/INDOLE-3-ACETIC ACID 7)	AT3G23050	AXR2/IAA7 is one of the core components of the auxin signaling pathway and it is involved in stem development.	[106, 107]
AXR3/IAA17 (AUXIN RESISTANT3/INDOLE-3-ACETIC ACID 17)	AT1G04250	The involvement of AXR3/IAA17 gene expression in brassinosteroid (BR)-regulated root development. It is involved in the reduction of root elongation and cause and increased adventitious root.	[16, 84]
IAA14 (INDOLE-3-ACETIC ACID 14)	AT4G14550	IAA14 is a transcriptional repressor of auxin signaling.	[108, 109]
AUX1/LAX1 (AUXIN RESISTANT 1/LIKE AUX1)	AT5G01240	AUX1/LAX is an auxin influx carriers, AUX1/LAX genes are implicated in the regulation of key plant processing including root, lateral root development, root gravitropism, root hair development and leaf morphogenesis.	[110, 111]
ABCB (ATP-BINDING CASETTESUBFAMILY B)	AT1G02520	ABCB participates in polar movement of auxin by exclusion from and prevention of uptake at the plasma membrane.	[110, 111]
PILS1 (PIN-LIKES1)	AT1G20925	PILS proteins are putative auxin carriers that regulate the auxin transport from the cytosol into the lumen of the ER and also it mediate intracellular auxin accumulation	[112]
DWF4 (DWARF4)	AT3G50660	DWF4 is involved in BR Biosynthesis	[113]
CPD (l-(2’-CARBOXYPHENYL)-3-PHENYLPROPANE-1,3-DIONE)	AT5G05690	CPD is a BR-specific biosynthesis genes	[114]
BZR1/BES1 (BRASSINAZOLE-RESISTANT 1/BRI1-EMS-SUPPRESSOR)	AT1G75080	BES1 and BZR1 can directly or indirectly regulate the expression of thousands of BR-responsive genes and ultimately affect plant growth, development, and stress adaptation	[115]
BIN2 (BRASSINOSTEROID INSENSITIVE2)	AT4G18710	BIN2 encodes a negative regulator of BR-signaling in plant growth.	[67]
SOB3 (SUPPRESSOR OF PHYB)	AT1G76500	SOB3 was present in BR-signaling and it is implicated in the transcription of genes envolved in cell elongation and hypocotyl growth.	[67]
BSU1 (BRI1 SUPPRESSOR 1)	AT1G08420	BSU1 dephosphorylates and inactivates downstream BRASSINOSTEROID INSENSITIVE2	[116]

Table 1.

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

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, 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].

6. Conclusions and prospects

Recently, advanced research has focused on unraveling the genetic and molecular mechanisms of BR and auxin pathways in plants responses to salt stress. BR plays a crucial role in promoting responses to salt stress, by activating a family of transcription factors BZR1/BES1 which can directly or indirectly regulate the expression of BR-responsive genes. It can ultimately affect not only development and growth of plants but also auxin biosynthesis and signaling. The interplay between auxin and BR pathways has been shown to be crucial in multiple plant development processes under salt stress including hypocotyl elongation, root development, halotropism and gravitropism. Future thorough studies are required to fully understand the interdependency between auxin and BR in improving salt stress tolerance before implementing new approaches to engineer stress resilient crops.