Abscisic acid was discovered in early 1960s and found that this phytohormone was responsible for breaking seed dormancy (Cornforth et al., 1965). Later on, it has been found that it has potential role in plant development and stress adaptive responses in plants (Guschina et al., 2002; Iqbal, 2015a). For example, ABA is involved in seed maturation and seed dormancy, and regulation of water in plant body through stomatal opening and closing, and adaptations of a plant to environmental stresses. Under water deficit conditions, ABA is biosynthesized in roots and transported to leaves via xylem thereby increase in concentration in the leaves. This increase of the ABA acts as a signal and start signaling cascade in guard cells to regulate cell turgor of guard cells (Danquah et al., 2013; Awan et al., 2017). In the abiotic stress response, the biosynthesis and redistribution of ABA cause stomatal closure and reduce transpiration rate, restricting cell growth (Peleg and Blumwald, 2011). ABA is quite famous for its functions in plant-water relations (Arkhipova et al., 2020).

ABA up-regulates hormone-responsive transcription factors, and in ABA signaling phosphatases and kinases play role in mediating fast responses to various abiotic stresses (Dar et al., 2017).

ABA biosynthesis pathway is derived from carotenoids (Nambara and Marion-Poll, 2005). Stress stimuli rapidly trigger ABA biosynthesis from β-carotene oxidative cleavage using 9-cis-epoxy carotenoid di-oxygenase (NCED) enzyme. The zeaxanthin epoxidase produces trans-violaxanthin, which gets converted to a 15-C compound called neoxanthin. After that, conversion of xanthoxin to abscisic aldehyde occurs, and then its oxidation leads to ABA production. The biosynthesis of ABA by the β-glucosidase homologs from ABA-GE has also been reported as an alternate pathway of ABA synthesis in some plant species exposed to abiotic stresses (Xu et al., 2013; Llanes et al., 2014).

Moreover, ABA not only mediate physiological responses through its own signaling, it also regulate physiological responses through interacting with other PHs in plants exposed to abiotic stresses (He et al., 2018). It is worth mentioning that ABA triggers and mobilizes an array of biochemical defenses such as biosynthesis of proline, antioxidants, ROS detoxifying enzymes, heat shock proteins, and unsaturated fatty acids along with the strengthening of cuticular waxes, which enable plants to ward-off adverse effects of abiotic stress to some extent (Ashraf and Foolad, 2007; Chen et al., 2013; Lee and Suh, 2015; Hoque et al., 2016; Huang et al., 2016; Liu et al., 2018; Yin et al., 2018; Singhal et al., 2021).

Ethylene is in gaseous form and involved in many vital morpho-physiological processes such as triple response in germinating seeds, developing flowers, ripening fruits, and triggering plant responses against environmental stimuli. Besides, ET regulates several stress-related biochemical responses of plants exposed to abiotic stresses such as heat, drought, chilling, salinity, heavy metals, water-logging, flooded, or submerged conditions (Awan et al., 2017). For instance, a close association between high ET level and freezing and cold stress was observed in Arabidopsis (Shi et al., 2012) and Medicago truncatula (Zhao et al., 2014), and regulation of ET homeostasis is crucial for sub optimal temperature stress (chilling and freezing) tolerance. High ET levels are help in salt stress tolerance as in salt tolerant Arabidopsis plants. The ETIO1 (ethylene over producer) act positively in salt stress by maintaining Na⁺/K⁺ homeostasis and ROS production (Yang et al., 2017). Moreover, it plays a crucial role in altering plants' response to pathogen attack, external mechanical wounding, UV radiations, and nutrients deficiency. ET biosynthesis and accumulation were reported in many cases in response to damage or mechanical injury (Abeles et al., 1992; Kendrick and Chang, 2008).

The biosynthesis of ET, as shown in Figure 2, has been quantified in various plant tissues, wilting flowers, and ripening fruits in plants exposed to abiotic stresses (Kendrick and Chang, 2008). The underlying mechanism for ET biosynthesis gets initiated with S-Adenosyl methionine (SAM), which is the precursor of ET and is usually synthesized in large concentrations in various crops and fruit trees. An enzyme called 1- aminocyclopropane-1-carboxylic acid (ACC) synthase catalyzes the first chain reaction to convert SAM to ACC, and methylthioadenosine (MTA), which subsequently gets recycled to L-methionine. Owing to this recycling, L-methionine levels remain unchanged even when ethylene biosynthesis is at its peak. Moreover, the ET biosynthesis pathway is affected by ACC synthase enzyme, which is extremely labile and tends to limit biosynthesis rate and rises proportionally as that of ethylene levels in tissues, flowers, and fruits (Maheshwari et al., 2015).

Among phenolic endogenous growth regulators, salicylic acid is one of the most vital growth regulators and has been characterized in almost all plant species belonging to diversified groups. The SA concentration has been reported to be 1 μg g⁻¹ of fresh biomass of rice, barley, and soybean crops. Its role as a regulator of an array of biochemical and physiological processes in plants has been identified. It helps in induction of systemic acquired resistance to various pathogens has been widely studied in plants (Misra and Saxena, 2009). However, in plants subjected to salinity and osmotic stresses, the SA role remained somewhat ambiguous in various plant species depending on the intensity and duration of osmotic stress. Simultaneously, SA exogenous application alleviated the adverse effects of salinity (Horváth et al., 2007).

During the 1960s, SA was synthesized from cinnamic acid (CA) by two biosynthesis pathways. One pathway involves a side chain of CA, which undergoes decarboxylation to produce benzoic acid, which is then subjected to 2-hydroxylation leading to the synthesis of SA in crops including tobacco (Yalpani et al., 1993) and rice (Silverman et al., 1995). However, it postulated that some other enzymes involved in this pathway are still unknown. The second pathway of SA biosynthesis involves CA, which gets subjected to 2-hydroxylation, leading to o-coumaric acid production, which is subsequently decarboxylated to biosynthesize SA. Trans-cinnamate-4- hydroxylate enzyme is responsible for catalyzing this reaction (Alibert and Ranjeva, 1971; Alibert et al., 1972). This pathway was firstly studied in seedlings of peas (Russell and Conn, 1967). However, the underlying mechanism of this pathway remains unclear.

Jasmonates are a broad group, which are covering various compounds, such as jasmonic acids (JAs), jasmonic acid methyl ester (JAME), precursor of the JAs; octadecanoid cis (+) 12 oxophytodienoic acid (OPDA), amino acid conjugates, and metabolites such as 12-OH-JA and 11-OH-JA, and often these compounds are involved in plant responses to biotic and abiotic stresses. Jasmonates are found throughout the plant body; however, shoot apex, root tips, immature fruits, and young leaves like tender growing parts show remarkably high concentrations. The synthesizing pathway of JAs is said to be via the octadecanoid pathway, starting at linolenic acid and terminating at (+)-7-epi-JAs (Wasternack, 2007). The major plant organs of biosynthesizing Jasmonates are leaves and roots, while chloroplasts and peroxisomes are the sub-cellular primary sites of JAs biosynthesis (Cheong and Choi, 2003). Reports showed that development of the embryo and reproductive organs, determination of sex, seed germination and seedling development, root growth, fruit ripening, leaf movements and senescence, gravitropism, the formation of the trichome, and tubers are mediated by JAs (Wasternack and Hause, 2013; Wasternack, 2014). Further, signaling related to defense mechanisms of insects or pathogen are driven wounding is mediated by jasmonates.

JAs have a crucial role in abiotic stress tolerance; thus, studies were focused on these compounds because of their significant protective capacity on plants against stress (Takeuchi et al., 2011). For example, JAs-mediated plant responses are shown against drought stress, ozone stress (Sasaki-Sekimoto et al., 2005), UV-stress, salinity stress, cold stress (Yoshikawa et al., 2007), and temperature stress. In addition, JAs-mediated secondary metabolites production (Chen et al., 2006) is involved in plant movements related to adapt seasonal and circadian rhythms. Wasternack and Hause (2013) reported that jasmonates regulate microbe-associated symbiotic relationships, such as arbuscular mycorrhizal fungi and plant growth-promoting rhizobacteria (PGPR). The jasmonates involved signaling in abiotic stress regulation in Carica papaya (Mahouachi et al., 2007), citrus, and Arabidopsis thaliana (A. thaliana) (Arbona et al., 2010; Brossa et al., 2011) is well-confirmed. Moreover, the involvement of JAs in salt stress signaling in glycophytes such as tomato, barley, and others have been studied. However, the role of jasmonates in halophytic plants' response to salinity stress is yet to be explored. Certain laboratory experiments showed that changes in JAs profiles in response to different abiotic stressors were imposed using NaCl, mannitol/sorbitol, and water stress.

The GAs are one of the longest and well-known groups of regulatory hormones integrated into numerous developmental processes of plants such as seed germination, inter-nodal elongation, induced flowering and fruit development (Islam et al., 2021). Recent studies have explored genetic basis and genes encoding GAs biosynthesis and deactivating enzymes using novel biochemical and genetic approaches (Yamaguchi, 2008; Pearce et al., 2015). More than 250 members in the GAs group are reported, though only a few of them are biologically active and play multiple roles in plant development. The cellular level regulation of the GAs is said to be complex. For example, the GAs metabolic pathway phases are regulated by enzymes in small multi-genic families; of those, each member plays a specific pattern of expression. However, two gene families encoding GAs, 20-oxidases (GA20ox) and GA3-oxidases (GA3ox) that catalyze the final steps in the synthesis of bioactive GAs, are strongly associated with GA biosynthesis (Hedden and Thomas, 2012).

Receptor protein of GA is nuclear localized GID1 and binding of GA with receptor protein help to interact with repressor DELLA protein. Protein-protein interacting domain of DELLA protein interact with F-Box protein which recruit SLY1 complex for its ubiquitination via SCF-E3 ligase complex. Upon ubiquitination, DELLA repressor protein is degraded by 26S proteasome and thus help in binding of transcription factor with promoter region of GA-inducible genes. Thus, GA mediate signaling by degrading repressor protein that bring about various biochemical and physiological responses. Thus, accumulation of GA in plant tissues either due to endogenous biosynthesis or due to exogenous application counteract negative effects of stresses and thus induce stress tolerance in plants. For instance, under water deficit conditions, decline in endogenous GA level by GA2ox6 ectopic expression improved the seed yield and drought stress tolerance in rice (Lo et al., 2017). Moreover, GA regulate redox homeostasis through stimulating electron mobilization in H. vulgare (Mark et al., 2016). The involvement of GA in redox equilibrium helps plants to acclimatize in suboptimal growth conditions. GRAS transcription factors play crucial role in plant development and signaling including GA biosynthesis and signal transduction pathways. Liu et al. (2017) demonstrated that abiotic stress treatments like NaCl, H₂O₂, etc. enhanced the GRAS40 expression in tomato. SlGRAS40 interacted with auxin and GA pathways during vegetative and reproductive phases of tomato and transgenic expression of SlGRAS40 plant and its expression induced the drouth and salt tolerance.

Auxins are endogenous plant growth regulators, similar to CKs, and are mainly involved in root/shoot formation and relative growth (Sachs, 2005). Studies have shown that auxins work together with CKs in various cellular or physiological processes such as cell cycle progression, cell expansion, apical dominance, leaf development, and embryonic development during seed maturation (Tromas et al., 2009; Jurado et al., 2010). Indeed, auxins under environmental stresses influence the plant growth responses. In contrast, subsequent alterations in auxin homeostasis due to such environmental changes can result in distorted growth and development in plants, causing altered morphogenesis. Such stress-induced morphogenic responses are an acclimation strategy, which helps prevent or reduce the damaging effects of environmental stresses (Potters et al., 2009; Tognetti et al., 2012).

The genetic studies and in vitro assays claimed that Auxin biosynthetic pathways have one tryptophan (Trp)-independent and four Trp-dependent pathways, namely indole-3-acetamide (IAM) pathway, indole-3-acetaldoxime (IAOx) pathway, tryptamine (TAM) pathway, and indole-3-pyruvic acid (IPA) pathway (Woodward and Bartel, 2005; Taiz et al., 2015). Amongst them, the TAM and IPA pathways said to be contributed to plant development.

Similar to GA, receptor protein of auxin is also nuclear localized. Interestingly, receptor protein of auxin is a repressor protein. Binding of auxin with receptor protein or repressor protein recruits SCF^TIR1-E3 ligase which ubiquitinate repressor protein subsequently degraded by 26S proteasome. Degradation of repressor protein allowed to auxin response factors to bind with promoter region of auxin-inducible gene and thus mediate gene expression. Auxin response factor (ARF) family has been identified as transcription factors that mediated auxin's actions; for example, certain ARFs displayed crucial involvements in lateral root development (Wilmoth et al., 2005). A recent study reported that micro RNAs' collective function determined the quantity of lateral root growth, miR390 and TAS3-derived trans-acting short interfering RNAs and Auxin Responsive Factors forming auxin-responsive regulatory network (Marin et al., 2010). This complex auxin regulatory network has fine-tuned the vigor and elasticity of lateral root growth. However, the involvement of genes that mediate interactive environmental stresses toward growth responses is yet to be discovered.

Variety of abiotic stresses involved in auxin homeostasis, distribution, and their metabolism in the cellular environment. Two molecular mechanisms have been suggested for such alterations in auxin distribution under stress environments; changers in PIN gene expression, which is necessary for polar auxin transport, and inhibitory action of polar auxin transport generated via phenolic compounds accumulated during stress exposure (Kovtun et al., 2000; Potters et al., 2009). Moreover, stress-induced auxin metabolism is regulated by IAA degradation, which is catalyzed by peroxidases (Jain and Khurana, 2009). Thus, auxins can be considered as stress hormones having direct or indirect mediation, modifying the expression of certain stress-responsive genes; among the multi-functions of auxin in plants, the formation of lateral roots considered as of special significance as lateral roots play a key role in plant development regulating the architecture of the root system, stability of the plant and efficient nutrient and water uptake for the whole organism.

The CKs are key PHs often considered ABA antagonists and auxin antagonists/synergists in various processes in plants responsible for plant growth, development, and tolerance against different abiotic stresses (Pospíšilová, 2003; Danilova et al., 2016). Besides, other hormonal pathways (e.g., ABA), CKs are activated when a plant is exposed to salt stress. By interacting with other plant hormones like auxins and ABA, CKs can considerably increase salt stress tolerance (Iqbal et al., 2006).

Plant endogenous CKs are adenine derivatives with either isoprenoid or aromatic side chains found in plants at a lower abundance (Sakakibara, 2006). The isoprenoid CKs can be distinguished as isopentenyl adenine (iP), trans-zeatin (tZ), cis-zeatin (cZ), or dihydrozeatin-type derivatives according to the hydroxylation and reduction of the side chain. The rate-limiting step of isoprenoid CK biosynthesis is catalyzed by isopentenyl transferase (IPTs). Rivero et al. (2010) generated transgenic tobacco carrying an Agrobacterium tumefaciens isopentenyl transferase gene (IPT). Similarly, the promoter of a senescence-associated receptor protein kinase (SARK) gene, from Phaseolus vulgaris, was constructed in front of the IPT gene. These transgenic plants (PSARK::IPT) are more tolerant of drought-induced leaf senescence, which results in a remarkable level of water stress tolerance. During water stress, a greater reduction in photosynthesis occurs in susceptible plants than the stress-tolerant transgenic plants due to the photosynthetic apparatus being degraded in suspectable plants under low photosynthesis conditions. Therefore, CKs may protect photosynthetic processes contributing to the stress tolerance in the transgenic plants (Rivero et al., 2010). In addition, recent studies on CK signaling have suggested that this hormone is indeed involved in plant salt stress responses shown in Figure 1. Synthetic CKs reverse the plants' drought-induced alterations and allow normal growth and developmental activities (Figure 3). Plenty of research findings suggest that CKs support normal growth and development under osmotic stresses and improve plants' drought tolerance ability. However, the precise molecular mechanism of CK-mediated drought tolerance is yet to be discovered.

In 1995, two groups of researchers simultaneously identified the presence of melatonin (N-acetyl-5-methox-ytrytamine) in vascular plants for the first time. Melatonin is a pleiotropic molecule with many diverse actions in plants (Wang et al., 2017). It is considered an antioxidant with important actions in the control of ROS (Raza et al., 2020) (Figure 4) and reactive nitrogen species (RNS), among other free radicals, and harmful oxidative molecules present in plant cells. In addition, plant melatonin is involved in multiple physiological actions, such as growth, rooting, seed germination, photosynthesis, and protection against abiotic and biotic stressors (Arnao and Hernández-Ruiz, 2015). The recent identification of the first plant melatonin receptor opened the door to this regulatory molecule being considered a new plant hormone. However, due to the diversity of its actions, melatonin has also been proposed as a plant master regulator.

Melatonin has a range of possible cellular and physiological effects, such as changes in intracellular Ca²⁺ and the permeability of membranes mediated by ion transporters, changes in the opening and/or closing of stomata carbohydrate, lipid, and nitrogen metabolisms, and also in osmoprotectant metabolites. Melatonin affects processes such as growth promotion, rooting induction, tropism, seed germination promotion, photosynthesis, optimizing efficiency, and leaf water/CO₂ exchange (Sharif et al., 2018). It also regulates other processes, such as ripening or senescence, the internal biological clock, and parthenocarpy. Finally, melatonin also acts as an endogenous plant bio-stimulator against abiotic or biotic stressors (Sun et al., 2014).

One of the basic principles of any animal or plant hormonal function is a receptor responsible for interaction with the hormone and triggering downstream signaling chain elements. One of the most important limitations in studies on melatonin in plants is the lack of an identified receptor. This has also been a hurdle to considering melatonin as a plant hormone. However, similar to other plant hormones, identifying the receptor only came after obtaining many biochemical and physiological data relevant to melatonin (Zhang and Chang, 2016). The turning point came with the recent identification of CAND2/PMTR1, a phytomelatonin receptor in A. thaliana. Localized in the plasma membrane with a receptor-like topology, it interacts with the G-protein a subunit (GPA1), while its expression in different tissues is induced by melatonin. Phytomelatonin-receptor binding triggers the dissociation of Ggb and Ga, which activates NADPH oxidase-dependent H₂O₂ production (RBOH), enhancing Ca²⁺ influx and promoting K⁺ efflux, finally resulting in stomatal closure. Like others, phytomelatonin is a plant hormone that controls stomatal closure through the CAND2/PMTR1-mediated signaling pathway, regulating H₂O₂ production. ABA application induced stomatal closure in CAND2 (a melatonin-insensitive phenotype in mutants lacking ATCAND2), suggesting that melatonin-induced stomatal closure has a receptor that is different from that for ABA while sharing some components with ABA signaling (e.g., Ga subunit, H₂O₂, and Ca²⁺ signals) (Guo et al., 2017; Arnao and Hernández-Ruiz, 2019).

Strigolactones (SLs) are carotenoid derived PHs, which exuded from 80% of the plants that propose a symbiotic relationship with arbuscular soil mycorrhiza (Akiyama and Hayashi, 2006). These PHs were originally discovered as an “ecological signal” for parasitic seed germination and a symbiotic relationship among plants and beneficial microbes. The first natural SL (called strigol) was discovered as a germination stimulant of Striga lutea. This obligate hemiparasite plant requires a living host for germination and initial development, and subsequently, these compounds were collectively termed as strigolactones (Cook et al., 1972).

Several strigolactone analogs, including GR5, GR7, and GR24 (a compound with the highest activity), have been chemically synthesized in many plants, and succeeding characterizations have described their functional roles in various developmental processes, including root development, shoot branching, leaf senescence, reproductive development, and controlling the architecture of plant organs (Gomez-Roldan et al., 2008; Kapulnik et al., 2011; Kohlen et al., 2011; Ruyter-Spira et al., 2011). Like other plant hormones, the major functions of SLs are the development, and its interaction with auxin dominates in the SL-regulated developmental processes (Hayward et al., 2009). SLs are apocarotenoids derived from carotenoid cleavage mediated by Carotenoid cleavage dioxygenase (CCDs) enzymes (Booker et al., 2005). The first step of biosynthesis occurs in the plastids with the help of three plastids localized enzyme D27, CCD7, and CCD8 utilizing-trans-β-carotene in plastids producing caprolactone (CL) (Alder et al., 2012). Carlactone is then further oxidized by cytochrome P450 monooxygenase MAX1 or other homologous genes into different forms of SLs by few other unidentified steps catalyzed by novel, unknown enzymes. SLs are involved in the symbiosis relationship of plants and microbes in the rhizosphere. They activate hyphal branching and enhanced the growth and energy metabolism of symbiotic arbuscular mycorrhiza fungi after being exuded from the root.

Furthermore, SLs impact the quantitative development of root nodule symbiosis with symbiotic nitrogen-fixing bacteria (Rochange et al., 2019). The extent of SLs production in plants is strictly regulated and dependent on the stresses that plants confront at various stages of their development. Several studies confirm the importance of SLs in regulating multiple physiological and molecular processes during the adaptation of plants to different abiotic stresses such as salinity, drought, nutrient starvation, temperature, and pathogenic assail (Marzec et al., 2013). Evidence for crosstalk between SLs and other PHs in responses to abiotic stresses suggests that SLs actively participate within regulatory networks of plant stresses.

Recently, Ling et al. (2020) reported that exogenous application of GR24 in seedling of rice resulted in better growth, increased POD and SOD activity, and improved net photosynthetic rate, stomatal conductance, and intercellular CO₂ concentrations. These results show that GR24 may alleviate the damage of rice caused by salt stress, improve the adaptability to high-saline environments, and provide a relatively stable yield. The plant produces high amounts of SLs under nutrient deficiency conditions that lead to suppressing shoot branching and stimulating symbiosis (Gomez-Roldan et al., 2008; Umehara et al., 2008). SLs also showed to play a crucial role in nitrogen and phosphorous deficiency, inducing modification of root and shoot architecture and promoting the symbiosis of rhizobial bacteria and AM fungi (Marzec, 2016). As reported above, the significance of SLs has been well-recognized in several stress conditions as salinity, drought, temperature, and nutrient deficiency; however, their establishments as stress-related PHs demand extensive research.

Brassinosteroids (BRs) are polyhydroxylated steroid PHs. They regulate several physiological and biochemical processes in the plant, such as cell elongation, cell division, photomorphogenesis, xylem differentiation, growth, and reproduction (Nolan et al., 2020). The BRs exist in free and conjugated, and nearly 69 and 5 conjugated and free BRs have been identified (Bajguz, 2011). The BRs are diverse in nature and biological activity. Among the BRs, brassinolide (BL) has been documented as the most active BR, and it was isolated and purified from Brassica napus pollen (Grove et al., 1979). The BRs are closely related to auxins, through the modulation of its transport, coordinating the tropic responses of plant organs (Li et al., 2005), and promoting lateral root primordial initiation during lateral root development (Bao et al., 2004). The endoplasmic reticulum most likely served as the site for BRs synthesis. The formation of a protein complex comprising enzymes (metabolon) to efficiently route the substrate to specified enzymes in a single biosynthetic pathway has been anticipated in plants only, and BRs biosynthesis occurs through cyto-chromeP450 (CYP), a triterpenoid pathway (Ralston and Yu, 2006; Choe, 2007; Chung et al., 2011). Hydroxylation of membrane-induced campesterol initiates the BR synthetic pathway (Choe et al., 1998). The intermediates gradually become more polar with hydroxyl group addition on side chain on C-22, C-23, and C-2, C-3, C-6 on steroid rings, respectively (Vukašinović and Russinova, 2018).

Unlike other PHs, BRs are used in contiguity to synthesizing cells rather than long-distance transport. Nevertheless, BRs crosstalk with PHs such as auxin exerts a long-distance effect (Symons et al., 2008; Vriet et al., 2013). The BRs metabolism involves various processes, including acylation, glycosylation, and sulphonation, to maintain the desired levels of bioactive BR in the cells (Saini et al., 2015). The BRs signaling is initiated by leucine rich repeat receptor like kinase (LRR-RLKs) BRI1 which is membrane bound. BRs signaling used two strategies initiation of phosphorylation cascade by kinases and degradation of repressor protein BIN2 and thus allowing transcription factors to bind with DNA binding elements for gene expression. Thus, BRs signal is also amplified during signal transductions. That's why a small amount of BRs is sufficient to initiate developmental programs under normal or stress conditions (Nolan et al., 2017). Numerous studies have documented the abiotic stress tolerance in plants with exogenous application of BRs (Kagale et al., 2007; Bajguz and Hayat, 2009; Yuan et al., 2010; Anjum et al., 2011; Divi et al., 2016). Nevertheless, the BRs need in minimal quantity like other PHs. Therefore, plant responses to exogenous BRs treatment are concentration-dependent. A high BR application rate is found to inhibit the plant growth, while the opposite is observed at lower concentrations (Chaiwanon and Wang, 2015; Belda-Palazon et al., 2018). The abiotic stresses enhanced ROS generation leading to oxidative stress, while BRs help regulates the cellular ROS level under stressful environments. For instance, 28-homobrassinolide application to Brassica juncea L. plants subjected to combined temperature and salt stress enhanced enzymatic antioxidant activities (SOD, CAT, APOX, DHAR, and MDHAR) and ROS homeostasis (Kaur et al., 2018). Likewise, in Lycopersicum esculentum, BRs application ameliorated the supra optimal temperature-induced photosynthesis inhibition and augmented the carboxylation and activities of the antioxidant system (Ogweno et al., 2007). Recently, Fàbregas et al. (2018) demonstrated that overexpression of BRL3 (vascular BR receptor) promotes drought responses without penalizing growth in Arabidopsis. In another study, Serna et al. (2015) documented the alleviation effects of BRs on lettuce salt-stressed plants. They found that BRs were involved in the partial reversion of NaCl accumulation in cells. Moreover, some critical agronomic traits in crops are potentially regulated by BRs, such as plant height, leaf angle, and inflorescence structure (Yamamuro et al., 2000; Hong et al., 2003; Sakamoto et al., 2006; Yang et al., 2018). Altogether, these studies demonstrate that BRs are potent PHs due to their versatile functions, and support the important role of BRs as anti-stress agents. Besides the great progress in the last decade in BRs research, many questions remained unanswered. The complete understanding of the dynamics of BRs homeostasis and its interactions with other PHs will add key knowledge that will allow the modulation of various useful traits in plants and address current challenges in agriculture.

Polyamines (PAs) are small aliphatic nitrogenous bases produced as a result of cellular metabolism. The PAs have not planted hormones, but due to their involvement in regulating several growth and development processes and responses to abiotic stress in plants, they have been proposed as a new category of plant growth regulators (Liu et al., 2007; Chen et al., 2019). Initially, the ability of PAs to bind with anionic macromolecules was supposed to link with their biological functions, which lead to the consideration of PAs as polycations having distinctive structural roles. However, later may study demonstrated that PAs act as regulatory molecules in key cellular processes such as cell division, cell differentiation, DNA and protein synthesis, and gene expression (Seiler and Raul, 2005; Alcázar et al., 2010; Igarashi and Kashiwagi, 2010; Childs et al., 2017). Furthermore, PAs are involved in various physiological processes in plants including embryogenesis, organogenesis, reproductive development, leaf senescence, and fruit maturity.

Moreover, the several studies have reported the protective role of PAs against environmental stresses (see reviews Alcázar et al., 2010; Gill and Tuteja, 2010; Minocha et al., 2014; Chen et al., 2019). Spermidine (Spd), spermine (Spm), and Putrescine (Put) are the major PAs in plants, while cadaverine (Cad) and diamino propane (Dap) are less studied in plants. The PAs are present in conjugated (covalent and non-covalent bounded) or free form (Gholami et al., 2013). The main product of PAs biosynthetic pathway is Put, which serves as the precursor for Spm and Spd (Xu et al., 2009). In plants, Put biosynthesis occur in three different routes: via Arginine by arginine decarboxylase (ADC) (the most frequent route), via ornithine (Orn) by ornithine decarboxylase (ODC), and citrulline (Cit) by citrulline decarboxylase (CDC) (Chen et al., 2019). The PAs catabolism is contingent on the action of diamine oxidase, and PA oxidase and PAs metabolism is closely associated with several other metabolic pathways in plants. The H₂O₂ produced due to PA oxidation is involved in signal transduction and plant responses to biotic and environmental stresses (Freitas et al., 2017; Mellidou et al., 2017; Sariyev et al., 2020). The PA biosynthetic pathway is linked to ethylene synthesis, sharing the same precursor (S-adenosylmethionine) and competing. Further, PAs metabolism is closely associated with nitric oxide generation (Pál et al., 2015), which triggers a signal transduction process related to plant growth. The distribution of PAs is organ and tissue-specific in plants. For instance, Put is the most abundant PA found in leaves, while the higher level of Spd is present in other plant organs (Reginato et al., 2012; Takahashi et al., 2017).

Polyamines are implied in response to different abiotic stresses. Generally, transgenic plants overexpressing PA biosynthetic enzymes, such as spermidine synthase, arginine decarboxylase, and S-adenosylmethionine synthetase, showed the protective roles of polyamines under abiotic stress conditions. Moreover, exogenous application of PAs showed increased stress tolerance in several plant species (Alcázar et al., 2010; Qi et al., 2010; Minocha et al., 2014).