Introduction

Abiotic stress factors are the primary cause of crop loss worldwide, reducing yields and quality (Ahmad et al. 2021; El-Mahdy et al. 2021). In major cereal crops, more than 50% of average yield loss has been reported as a consequence of various abiotic stresses (Rao et al. 2016; Agha et al. 2021). Drought stress is one of the significant abiotic stresses responsible for reducing plant growth and development. It occurs when available soil water decreases, and water loss through aerial part enhances due to harsh environmental conditions. Drought frequently occurs in arid and semi-arid regions and regions with adequate but non-uniform seasonal precipitation. The plant exhibits drought avoidance and tolerance mechanisms to improve adaptation and maintain yield under drought stress (Zheng et al. 2017). To optimize yield under drought conditions, the plant maintains balance among different drought resistance mechanisms such as better desiccation tolerance, osmoprotectant, and higher antioxidative capacity (Fang and Xiong 2015; Sofy et al. 2021). Acclimatization processes induced due to drought involve many secondary metabolites (SMs) and phytohormones (PHs) to alter plants’ physiological and biochemical responses. Plant SMs are large organic compounds that usually do not have a direct function, but the endogenous levels of these compounds largely mediate plant growth and development. An enormous type of secondary metabolite (> 100,000) has been described in plants (Edreva et al. 2008; Abu-Shahba et al. 2021). Accumulation of metabolites in plants such as flavonoids, phenolics, diosgenin, digitoxin, glycosides, glycosinolates, colchicines, saikosaponins, etc., subjected to stresses including various elicitors and signalling molecules (Akladious and Mohamed 2017; Dawood et al. 27,28,a, b).

In plants, a complex stress-response mechanism is involved, requiring integrated pathways to be activated in response to stress (Ghonaim et al. 2021; El-Sheshtawy et al. 2022). Phytohormones are the key endogenous factors that can control various physiological processes mediating plant stress response. The major PHs play a role in abiotic stresses are abscisic acid (ABA), ethylene (ET), salicylic acid (SA), jasmonates (JA), auxins, gibberellins (GA), cytokinins (CK), Brassinosteroids (BR) and strigolactones (SLs). Among these, ABA is a major stress hormone that plays a key role in stress tolerance mechanisms (Verma et al. 2016; Wani et al. 2016; Sallam et al. 2019; Fouda and Sofy 2022). Ethylene, SA, JA, BR, and SL are primarily induced during biotic stress; often play significant roles in mediating plant defence response against abiotic stresses (Pavlu et al. 2018; El-Beltagi et al. 2019). Since drought is a complex trait that involves changes manifested at morphological, physiological, and molecular levels; PHs play a protective role such as earliness, higher root-shoot ratio, leaf rolling, reduced leaf area and transpiration, stomata closure, accumulation of osmolytes and activation of stress-responsive genes (Tiwari et al. 2017). Recent studies on the stress-response mechanism have provided substantial evidence for the crosstalk among the PHs regulating plant defence response (Wani et al. 2016; Tiwari et al. 2017). Moreover, the diverse chemical characters and interactions of SMs with groups of PHs participating in defence responses and their signalling pathways are intricately interconnected to facilitate the generation of a sophisticated and efficient stress response (Aly et al. 2017; Abu-Shahba et al. 2022). Therefore, a concise overview of the role of SMs and PHs in drought stress tolerance is provided in the present study.

Drought Induces Plant Responses

Plant response dramatically depends on the severity of drought, plant stage and physiological status (Moursi et al. 2020). Drought-induced physiological, biochemical and molecular changes in plants are presented in Fig. 1. For example, under water stress, plants try to maintain tissue’s water potential via closing stomata and reducing water losses through transpiration. In addition, plant growth and photosynthesis reduce due to decrease CO2 levels and enhanced formation and accumulation of ABA, proline, mannitol and radical scavenging compounds (ascorbate, glutathione, α‑tocopherol etc.), stress proteins and mRNAs (Abobatta 2020).

Fig. 1
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Physiological, biochemical and molecular changes in plants in response to drought stress

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Additionally, the primary source of carbohydrates’ metabolism is upset by drought stress, which results in partial stomatal closure at carboxylation sites with lower carbon dioxide availability (Hu et al. 2019) as shown in Fig. 2. Noctor et al. (2014) highlighted the effects of stomata closure on the photosynthetic machinery of plants with the reduction of oxygen by photosystem (I) resulting in the production of superoxide (O2−) and H2O2, which accelerates the water-water cycle (Asada 2006). Excessive reduction in the electron transport causes the production of singlet oxygen (1O2) in Photosystem II (PSII), which elevates H2O2 production in the peroxisome while O2− and H2O2 or 1O2 are produced in the chloroplast due to photorespiration (Zargar and Zargar 2018). Excessive reduction in the photosynthetic electron transport chain as a result of the possible production of 1O2 in PSII inadvertently affects the rate of photosynthesis The level of shoot respiration also rises in response to drought stress in order to maintain metabolic activity. Then, the storage organs of citrus plants’ carbohydrate reserves start to decline (Fahad et al. 2017).

Fig. 2
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Stomata closure restricts the uptake of CO2 in the leaves of a drought-stressed plant leading to the production of (a) H2O2 in the peroxisome by photorespiration, which enhances (b) O2− and H2O2 production, (c) 1O2 production, by the photosynthetic electron transport chain. (PSI and PSII Photosystem 1 and Photosystem II, RuBP Ribulose 1-5 bisphosphate, PGA 3-Phosphoglyceric acid)

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Enhancement of leaf soluble carbohydrate levels, reduction in starch levels, and a low rate of photosynthesis are all effects of mild drought stress. Even though variations in carbon abundance are seen under limited photosynthesis rates, plants may use stored carbohydrates. These plants stored reservoirs to meet plant metabolic demand and endure challenging drought conditions (Liu et al. 2019). But extreme drought stress lowers levels of starch and soluble fraction (Fahad et al. 2017).

Because numerous phytohormones are sole mediators for tolerance, avoidance, and the negative effects of water stress, numerous studies on these mechanisms are still being conducted. Throughout the course of a plant’s life, plant hormones play a critical role in regulating growth and development as well as drought stress responses (Iqbal et al. 2022).

Secondary Metabolites (SMs)

The production of SMs is often low (< 1% of dry weight) and greatly influences on physiological and developmental stages of the plant during stress conditions (Akula and Ravishankar 2011; Ashry et al. 2018). Environmental factors determine the synthesis and accumulation of SMs in response to different signalling molecules (Ashraf et al. 2018; Hussain et al. 2021). Accumulation of SMs considered an adaptive response of the plant to adverse environmental conditions. Some of them, like flavonoids, flavonol, phenolics, isoprene, isoprenoid, and phenylpropanoid, synthesize under drought stress and act as antioxidants (Gosal et al. 2010; Nichols et al. 2015; Naikoo et al. 2019; Tattini et al. 2014). Condensed tannins and anthocyanins, as well as flavonols, are found in the skins of berries like Vitis vinifera L. (Wang et al. 159,160,a, b). SMs typically give plants a particular flavour and aroma to help them attract pollinators and seed dispersers. Additionally, plants release a large number of SMs through their roots, which are made up of volatile organic substances called chemo-attractants. According to reports, plant roots’ volatile organic substances attracted far-off soil bacteria (Du Toit 2018). Fig. 3 depicts the biosynthesis of SMs and how it interacts with primary metabolism inside the plant cell. Terpenes, phenols, alkaloids, and other frequently produced substances are among the diverse range of SMs that are synthesised in plants using different mechanisms (Fig. 3). Mevalonic acid (MVA) and 2‑C-methylerythritol 4-phosphate (MEP) pathways, which are found in the cytosol and plastid, respectively, are two important pathways for the synthesis of terpenes. Isopentenyl pyrophosphate (IPP) and dimethylallyl pyrophosphate (DMAPP), which serve as a universal precursor for all terpenoids localised in multiple cellular compartments, are produced from glycolysis products like pyruvate and glyceraldehyde-3-phosphate (Khare et al. 2020).

Fig. 3
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Types of Secondary metabolite (Divekar et al. 2022)

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Role of Secondary Metabolites Under Drought Stress in Plants

Phenolic Compounds

Phenolic compounds such as phenolic acids and flavonoids are produced from the shikimate-phenylpropanoid biosynthetic pathway (Jha and Mohamed 2022). They are considered the most widespread substantial groups of plant SMs act as markers of plants’ biotic and abiotic stress tolerance (Quan et al. 2016; Abd El-Rahman et al. 2012). Flavonoids (quercetin glycosides) and cinnamic acid derivatives endowed with high ROS scavenging capacity were suggested as a rationale for drought tolerance in cotton (Aktas et al. 2009). Flavonoids and phenolic acids were synthesized in a large amount in wheat leaves, and cell-damaging oxidants were also generated under drought stress (Ma et al. 2014). In both leaves and flowers of Tridax procumbens, significant increases in total phenolic content were observed under drought stress (Gnanasekaran and Kalavathy 2017). Additionally, SMs were applied to speed up energy production by enhancing the TCA cycle and glycolysis as well as the glutamic acid-mediated proline biosynthesis pathway, which is necessary to improve osmotic regulation in plants (Qu et al. 2019). Sesamum indicum L. underwent a drought stress condition and exhibited a reduction in sesamin, oil, and quercetin content. However, the same plant under drought stress exhibited improved total levels of flavonoids, phenolics, and different polyphenolics as well as enhanced radical scavenging activity (Kermani et al. 2019). Flavonoid accumulation is important to improve drought tolerance in wild-type and Arabidopsis thaliana mutants revealed by transcriptomic and metabolomic approaches (Nakabayashi et al. 2014). Cotton genotype Zhongmian 23 significantly enhanced flavonoids and phenols content via higher gene expression enzymes related to SMs such as PAL, PPO and CAD under drought conditions (Ibrahim et al. 2019). Tea transcript levels for 4‑Coumarate-CoA Ligase (4CL) are decreased by drought treatment. As a result, every piece of evidence points to a connection between tea plants’ increased accumulation of polyphenols like isoflavonoids and catechins and their ability to withstand drought (Rani et al. 2009). In the potato, transcript levels of genes like flavonone-3-hydroxylase, flavonol synthase, and -carotene hydroxylase‑1, which are crucial for the biosynthesis of flavonoids, carotenoids, and other phenolic compounds, were enhanced under drought stress. In potato cultivars, the level of these genes’ expression influences how well they tolerate drought (André et al. 2009). Likewise, under conditions of water deficiency, differential increases in flavonoids and polyphenols or expression of their biosynthesis-related genes were seen in the wheat, peanut, and sesame in cultivars that were resistant to drought as well as those that were sensitive to it (Juliano et al. 2020).

Anthocyanin

Variable accumulation of anthocyanins (purple colour) in chilli suggests that a purple cultivar of chilli resists water stress better than a green cultivar (Akula and Ravishankar 2011). Under water stress conditions, ascorbic acid increases the common bean’s secondary metabolites and antioxidant activity under water stress conditions (Gaafar et al. 2020). The changes in the ratio of chlorophyll “a” and “b” and carotenoids and reduction in chlorophyll content were reported in cotton (Massacci et al. 2008) under drought stress. The mutual relation between regulation and signalling of flavonol and ABA has been reported in higher plants. Under high light conditions, ABA promotes biosynthesis of flavonol involving HY5, ABI5 and PFG genes, while ABA signalling pathways are regulated by flavonol quercetin (Brunetti et al. 2019). Positively charged polyamines exert stabilizing effects via interactions with phosphoric acid residues in DNA, uronic acid residues in cell wall matrix, and negative groups on membrane surfaces, contributing to maintaining their functional and structural integrity (Edreva et al. 2008).

Lignin

In other research, lignification, pathway-related protein caffeoyl-CoA-3-O-methyltransferase (COMT), and other genes involved in lignin biosynthesis are affected by drought in maize, sugarcane, Brassica napus, and grapes (Santos et al. 2015; Giordano et al. 2016; Li et al. 2016; Sharma et al. 2016). According to reports, the lignin-based hydrogel has the potential to improve water content, proline, phosphorus, and biomass in maize plants while reducing electrolyte leakage (Mazloom et al. 2020). Pre-soaking soybean in SA seeds has been found to reduce drought stress by increasing the amount of lignin and pectin in the roots (Al-Hakimi 2006). In a study, it was found that the laccase family of enzymes prevented the accumulation of lignin content. Importantly, Laccase2 was discovered to be down-regulated by miR397b, which stimulates the deposition of lignin as a survival tactic in drought-stressed plants (Sharma et al. 2020). Additionally, under drought stress, syringaldazine peroxidase increased the polyamine process’s breakdown, which promoted lignin deposition in wheat plants (Bala et al. 2016).

Isoprenoids

Isoprene is the volatile biogenic compound emitted during drought stress in plants (Dawood 2022). It maintains photosynthesis under stress conditions via strengthening the thylakoid membrane and scavenging ROS species. Photosynthesis rates were higher in isoprene emitting than non-emitting transgenic tobacco plants under mild drought and normal conditions obtained after re-watering. Volatile isoprene may act as a short-term protectant under mild drought conditions, whereas its non-volatile derivative isoprenoid serves a long duration under severe drought conditions (Tattini et al. 2014). Isoprene also had a tissue-specific effect on ABA signalling in transgenic Arabidopsis. It upregulated the RD29B signalling gene and enhanced ABA sensitivity to stomata, while contrastingly, it downregulates ABA sensitivity in seed germination and root. In addition, COR15A and P5 CS genes were downregulated in leaves and enhanced drought tolerance via improving membrane integrity and osmotic stress tolerance (Xu et al. 2020). In Cistus creticus, certain genes are named as 1‑deoxy-D-xylulose-5-phosphate synthase (CcDXS), 3‑hydroxy-3-methyl-glutaryl-CoA reductase (CcHMGR), geranylgeranyl diphosphate synthases CcGGDPS1, 1‑deoxy-D-xylulose-5-phosphate reductoisomerase (CcDXR) and CcGGDPS2 involved in terpenoids biosynthesis were found to be stimulated under drought stress (Ma et al. 2014).

Terpenoids

Drought causes terpinene to undergo biochemical changes in cumin. Additionally, water scarcity converts cumin aldehyde from phenyl‑1,2‑ethanediol, which may be involved in defence mechanisms (Rebey et al. 2012). Different terpenoid responses to drought are seen in the shoot and root. Terpenoid levels rose in the root while falling in the shoot (Kleine and Müller 2014). The main gatekeepers discovered to be involved in the biosynthesis of terpenes are terpene synthases (TPS). Many plants whose terpenes have not yet been characterized contain these genes. Additionally, it has been observed that under drought stress, TPS genes were discovered to be stimulated in some plants, including pearl millet and tea plants (Karunanithi et al. 2020; Zhou et al. 176,175,a, b).

This evidence infers that controlling ROS levels and increasing activities of SMs related enzymes might be an effective mechanism to mitigate the adverse effects of drought stress. Moreover, the composition of secondary metabolites during drought conditions changes at the expense of reduced yield and oil content. For example, Carthamus tinctorius flower extracts had double antioxidant capacity because of a significant decrease in seed yield and oil content (Yeloojeh et al. 2020). Some more examples of induced SMs in different plant species due to drought stress are presented in Tables 1 and 2.

Table 1 Induction of secondary metabolites under drought stress conditions in different plant species
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Table 2 Impact of drought stress on the synthesis of different secondary metabolites
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In-Vitro Production of Secondary Metabolites

The production and productivity of secondary metabolites beneficial to improving drought stress tolerance can be enhanced using in-vitro culture techniques. The production of useful SMs via cell culture and organ culture has been reported in the recent past. Many researchers have tried to enhance various kinds of SMs accumulation by exogenous application of cytokinins (kinetin), Auxins (2,4‑D, IAA, NAA) in culture medium for improvement of flavonoids in Hydrocotyle bonariensis, anthocyanin in Fragaria ananassa, Daucus carota, Ipomoea batatas, Oxalis reclinata (Masoumian et al. 2011; Nakamura et al. 1999; Narayan et al. 2005; Nozue et al. 1995; Makunga et al. 1997). Efforts have been made to improve the productivity of the plant tissue cultures, such as studies on hormone dependency, media composition and light exposure (Tuteja and Sopory 2008; Karuppusamy 2009). Exogenously applied melatonin raises endogenous indole-3-acetic acid (IAA) in roots and stimulates root growth of etiolated seedlings of Brassica juncea (Chen et al. 2009). The major advantages of the in-vitro production of bioactive SMs are that the synthesis is independent of environmental conditions. Enhanced levels of metabolites are further involved in various stress regulation pathways of the plant to increase tolerance levels and maintain yield potential under drought conditions.

Phytohormones (PHs)

In addition to the endogenous developmental process, PHs are the key mediators of plant response to drought stress. Common PHs induces during water stresses are ABA, ET, CKs, JA, Auxins, GAs and SA. Among them, ABA is the major stress-responsive hormone. At this moment, brief overviews of these PHs are given individually, and they cross-talk with each other during drought stress. All phytohormones have different derivatives, including transport, active or inactive storage forms, decomposition byproducts, and—most significantly—sugar or amino acid conjugates. The combined induction of multiple hormones produces the biological effects of many plant growth hormones. A precise concentration is needed for maximum effect estimations because free hormones exhibit biological activity similar to that of these derivatives (Zhao et al., 2019). The estimation of the effect is shown in Fig. 4.

Fig. 4
figure 4

Different phytohormones’ function in plants is to defend against various stressful situations by either raising or lowering their levels. Unstable levels of these hormones enable plants to defend themselves and maintain healthy, normal growth (Iqbal et al. 2022)

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Role of Phytohormones Under Drought Stress in Plants

Abscisic Acid (ABA)

Biosynthesis of ABA occurs in plastids and cytoplasm via the terpenoid pathway from zeaxanthin in higher plants. It is found in every living organ from the apical bud to the root cap but produce predominantly in roots and then transported to the leaves for the regulation of the stomata movement, channel activities and expression of ABA-responsive genes (Kazan 2015; Kuromori et al. 2010). Water losses through stomata transpiration account for 90 per cent of total transpiration losses (Wang et al. 2009). During water stress conditions, ABA-enhanced stomata closure helps in osmoregulation and maintaining cell turgidity (Montillet et al. 2013). ABA triggers efflux of potassium (K+) and anions from guard cells and stimulates an increase in cytoplasmic calcium (second messenger), resulting in lower turgor pressure and subsequent closure of stomata (Misra et al. 2015). In addition, ROS also play a significant role in guard cell signalling; H2O2 plays a central role in ABA-induced stomatal closure (Jha and Mohamed 2022). In addition to stomata closure due to drought stress, CO2 intake from stomata reduces, adversely affecting the photosynthetic process and limiting plant growth (Mo et al. 2016). Detached leave assay of the senescence-induced receptor-like kinase (OsSRLK) may degrade chlorophyll by participating in a phytohormone-mediated pathway has been reported in rice (Shin et al. 2020). Deeper root systems are the characteristic feature of drought tolerance. ABA enhances the root-shoot ratio by restricting the plant’s aerial growth and promoting root growth through diverting photosynthetic assimilates to the root zone (Rowe et al. 2016). Cotton transgenic, characterized by elongated roots, showed significant drought resistance (Liu et al. 2014).

In addition, ABA regulates the expression of numerous stress-responsive genes and promotes the synthesis of LEA proteins, dehydrins, and other protective proteins (Sreenivasulu et al. 2012; Wani et al. 2016). ABA-dependent and ABA-independent are two important pathways mediated during ABA perception and signal transduction. During osmotic stress, ABA-dependent signalling pathways play a critical role in stress-responsive gene expression (Mehrotra et al. 2014). A basic ABA signalling pathway of stress conditions is presented in Fig. 3. In normal conditions, ABA quantity is low, inhibiting SnRK2 protein kinase by PP2C phosphatases. Under stress condition, the level of ABA increases, which then binds to PYR/PYL/RCARs and inactivate PP2Cs. Dissociation of SnRK2s from PP2Cs leads to auto-activation and downstream phosphorylation targets, triggering ABA-induced molecular and physiological drought responses (Ullah et al. 2018). Protein like WDR55 responsible for developing the reproductive and vegetative stages, is positively involved in the ABA-mediated drought tolerance response in Arabidopsis (Park et al. 2020). The soybean GmNAC019 transcription factor is a positive regulator of ABA-mediated plant response to drought in transgenic Arabidopsis (Hoang et al. 2020). Moreover, genetically engineered plants are characterized by additional regulatory mechanisms which may further be utilized to enhance and increase yield under drought conditions. In addition, some of the PHs-mediated enhanced yields of transgenic plants under drought stress have been listed in Table 3.

Table 3 Phytohormones mediated enhanced yield of transgenic plants under drought stress
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Jasmonic Acid (JA)

It is an endogenous plant growth-regulating substance that plays a role in biotic and abiotic stress in higher plants. Jasmonates (JAs) is a collective term that includes JA and its methyl ester (MeJA) and isoleucine conjugate (JA-Ile), which are derivatives of a class of fatty acids (Ruan et al. 2019). JAs produces in flowers, and synthesis occurs in the plastid, peroxisome and cytosol at the cellular level (Ullah et al. 2018). Like ABA, JA induces drought tolerance in plants through various mechanisms, including stomata closure, scavenging of ROS, root development, secondary metabolism, and direct and indirect responses (Riemann et al. 2015; Fang et al. 2016; Wani et al. 2016). JA modulates root hydraulic conductivity to uptake water from the soil in moisture deficit conditions (Sánchez-Romera et al. 2014). Exogenous application of JA act as an antioxidant and significantly enhances glutathione reductase (GR), ascorbate peroxidase (APX), dehydroascorbate reductase (DHAR) and monodehydroascorbate reductase (MDHAR) in wheat seedlings under drought stress (Shan et al. 2015). Foliar application of JA enhances the antioxidant enzyme activities under water deficit conditions and promotes growth in sugar beet (Ghaffari et al. 2019). Moreover, the VaNAC17 transcription factor from grapevine (Vitis amurensis) plays a positive role in drought tolerance by modulating endogenous JA biosynthesis and ROS scavenging in transgenic Arabidopsis (Su et al. 2020). In addition, De Ollas et al. (2013); Sánchez-Romera et al. (2014) observed that the transient presence of jasmonic acid in roots is required under drought stress to enhance the abscisic acid content. JA also plays a significant role in water conductivity from soil under restricted moisture conditions. Additionally, the interaction of JA signalling pathways with ABA signalling pathways suggests their function in the response to drought stress. It was recently discovered that JA interacted with calcium and ABA-dependent and independent signalling pathways to increase the hydraulic conductivity of plant roots under drought stress (De Ollas and Dodd 2016).

Salicylic Acid (SA)

It’s a seven-carbon compound, derivatives of SMs phenolic acid, synthesized predominantly in the chloroplast (Naeem et al. 2020). It shows a regulatory role in various physiological processes like transpiration, flowering, photosynthesis, and chlorophyll synthesis. It is considered a potential enzymatic antioxidant agent and enhances the activity of ROS scavenger enzymes under water stress conditions (Wani et al. 2016; Arif et al. 2020). SA generally promotes drought tolerance at low concentrations, while higher concentrations may cause oxidative stress. Both drought tolerance and plant growth were suppressed when a high concentration of SA was applied to wheat seedlings (Kang et al. 2012). Exogenous application of SA reduces adverse effects and stimulates physiological traits, productivity and plant resistance to drought stress in various plants such as maize, rice and Arabidopsis (Farooq et al. 2009; Hunter et al. 2013). Exogenous applications of methyl jasmonate (MeJA) and salicylic acid (SA) to seeds and leaves have positive effects on drought responses in wheat, Brassica rapa and maize seedlings (Tayyab et al. 2020; Kareem et al. 2019; Lee et al. 2019). SA-mediated induced expression of Pathogen Related (PR) genes also enhances drought tolerance through stomatal closure in Arabidopsis (Liu et al. 84,85,a, b; Miura et al. 98,99,a, b).

Ethylene (ET)

ET is a gaseous hormone produced from methionine (Met) via S‑adenosyl methionine (AdoMet) by the action of ACC synthase (ACS) and ACC oxidase (ACO) (Tiwari et al. 2017). It performs multiple actions, including fruit ripening, senescence and response to various biotic and abiotic stresses. It has also been shown to act as a positive regulator for root and root hair formation in several species. Overexpression of the AtERF019 gene in transgenic Arabidopsis show delayed senescence and flowering (Scarpeci et al. 2016). In cotton, AP2/EREBP (APETELA2/ethylene-responsive element-binding protein) genes were identified that respond to drought and heat, respectively (Liu and Zhang 2017). The expression of the MAT (Methionine Adenosyl Transferase) gene, the first enzyme in the biosynthesis pathway responsible for the production of AdoMet, was enhanced under water stress in soybean (Arraes et al. 2015). In chickpea and soybean, expression of ACO and ACS were enhanced (Tiwari et al. 2016; Arraes et al. 2015), and a reduction in the level of expression of ETR (ET receptors) and CTR genes was seen under drought stress in soybean (Arraes et al. 2015; Eppel and Rachmilevitch 2016).

Previous research demonstrated that abscisic acid (ABA) and jasmonic acid (JA) endogenous levels in Brassica napus enhanced during drought, causing a rise in ABA/SA and (ABA C JA)/SA (Lee et al. 2019). In plants under drought stress, salicylic acid levels also rose and may have reached levels five times higher than those typically observed in Phillyrea augustifolia evergreen shrubby plants (Munné-Bosch and Peñuelas 2003). Salicylic acid is responsible for the improved drought tolerance and disorder resistance seen in mutants of Arabidopsis spp. such as adr1, acd6, myb961, and cpr5 (Miura et al. 2013). Since the SA-regulated induction of PR gene expression resulted in drought tolerance by closing the stomatal openings in Arabidopsis (Liu et al. 2013; Miura et al. 2013), stomatal closure was also observed there due to salicylic acid accumulation under stressed conditions. This significantly increased drought tolerance. In Arabidopsis, stomatal closure occurred through the accumulation of SA under the influence of SIZI.

Brassinosteroids (BRs)

BRs are polyhydroxy steroidal plant hormones present in almost every part of plants. It plays a critical role in numerous developmental processes such as stem and root growth, floral initiation and development of flowers and fruits (Wani et al. 2016). The increased resistance in BR-treated potato tubers was associated with enhanced levels of ABA, ethylene, phenolics and terpenoid compounds (Akula and Ravishankar 2011). The role of BRs in drought tolerance has also been studied in various plants, such as wheat and transgenic Arabidopsis (Hayes 2019; Zhou et al. 2020). Arabidopsis thaliana has recently been found to contain a variety of WRKY transcription factors, and it is believed that these transcription factors play a role in plant growth and drought stress response. In response to drought stress, BRs engage in extensive interactions with both of these transcription factors and GA to control plant growth (Sánchez-Romera et al. 2014; Sánchez-Martín et al. 2015).

Cytokinins (CKs)

It’s a multifunctional plant hormone that plays a major role in plant growth and development. Mechanisms contributing to the cytokinin-mediated enhancement of drought tolerance may include protecting the photosynthetic machinery, promoting antioxidant systems, regulating water balance, influencing plant growth, differentiation and crosstalk with other stress PHs (Pavlu et al. 2018). CKs level reduces during drought stress in plants, and negative regulators of cytokinin signalling AHP6 and ARR5 also probably participate in this process (Jang et al. 2017; Huang et al. 2018). A decreased CK level reduces CO2 assimilation rates, which were accompanied by lower stomata conductance under water stress in transgenic Barley (Vojta et al. 2016). CKs are indirectly involved in the priming of antioxidant systems and could protect the cell from excessive ROS accumulation and maintain chloroplast integrity (Ma et al. 2018; Pavlu et al. 2018). Ectopic expression of CKs activates genes involved in flavonoid biosynthesis, which involves drought tolerance in Barley (Nakabayashi et al. 2014; Vojta et al. 2016). The CKs signalling is an inter-cellular communication network essential to crosstalk with ABA and their regulating pathways in mediating plant stress response under drought stress (Hai et al. 2020).

Auxins

Auxins are a group of multifunctional phytohormones which play a significant role in plant growth, development, and response to various stresses (Singh et al. 2017b). Studies reported that auxins, especially IAA (indole-3-acetic acid) involved in drought tolerance mechanisms in many crops like white clover, maize, and poplar (Zhang et al. 2020; Quiroga et al. 2020; Salehin et al.2019). Overexpression of IAA encourages stay green properties in potatoes and promotes branching of roots, potentially enhancing drought tolerance (Ullah et al. 2018). Additionally, auxins encourage root branching and may be involved in mechanisms that help tobacco seedlings tolerate drought (Wang et al. 2018). Auxin response factors (ARFs) bind directly to the promoters of auxin-responsive genes, enabling them to be transcriptionally activated or repressed and improving tomato stress tolerance (Bouzroud et al. 2018). TLD1/OsGH3.13, which encodes the indole-3-acetic acid (IAA)-amido synthase, has been used to study the role of auxin in drought stress. This gene enhances the expression of late embryogenesis abundant (LEA) genes, which boosts the resistance of plants to drought stress. Furthermore, Aux/IAA genes were found in rice, and the majority of these genes were expressed in response to drought stress (Iqbal et al. 2022). Additionally, auxin improved drought resistance through interactions with other phytohormones. Auxin, for instance, controls several members of the gene family for the rate-limiting enzyme in the production of ethylene, 1‑aminocyclopropane-1-carboxylate synthase (ACS). This interaction strengthens plants’ resistance to drought stress (Colebrook et al. 2014).

Gibberellins (GAs)

The GAs is a group of tetracyclic diterpenoid carboxylic acids which significantly influence seed germination, leaf expansion, stem elongation, and flower and fruit development in plants (Yamaguchi 2008). Reduced GAs level enhances drought tolerance in higher plants such as transgenic tomato and rice (Nir et al. 2014; Lo et al. 2017). The GA interactions with ET include both positive and negative mutual regulation in various developmental and stimulus-response processes (Munteanu et al. 2014). Lowered GA levels in plants are said to improve drought tolerance. Arabidopsis thaliana GA Methyl Transferase‑1 (AtGAMT1) gene overexpression results in transgenic tomatoes. An enzyme that breaks down the methylation of active GA to produce inactive GA methyl esters is encoded by the gene AtGAMT1. Through improved drought tolerance, the transgenic tomato demonstrated a reduction in gibberellins. Transgenic tomatoes under drought stress were found to have more water in their leaves due to plant transpiration (Nir et al. 2014). In drought conditions, the DELLAs proteins are the main inducers of GA responses in plants. These nuclear regulators have the ability to prevent plants from being stimulated by gibberellin. When gibberellins bind to the GID1 (GA insensitive dwarf 1) receptor, DELLAs are degraded by the 26S proteasome and gibberellin responses are stimulated (Li et al. 2012).

Strigolactones (SLs)

These are carotenoid-derived phytohormones, mainly induced during host-pathogen interaction in the plant. Recently, its role in drought and salt stress tolerance has been reported in maintaining shoot, root architecture, and stress signalling with ABA (Pandey et al. 2016). In Arabidopsis, SLs potentially modulate stomata behaviour, and its foliar spray reduces drought stress in grapevine (Min et al. 2019; Visentin et al. 2020).

Crosstalk Among Phytohormones

Drought stress regulation is a complex phenomenon, highly influenced by plant genotype and prevalent environmental factors. Various morpho-physiological and biochemical processes respond together during stress conditions. The defence responses in plants depend upon the mode of crosstalk between the hormone signalling pathways rather than individual contributions of PHs as shown in Fig. 6 (Verma et al. 2016). A general outline of the crosstalk of PHs is presented in Fig. 5 (Tiwari et al. 2017), which indicates that PHs share common elements and participate in crosstalk with one another to maintain cellular homeostasis (Danquah et al. 2014). Many researchers have reported that ABA interacts with JA and ET signalling pathways at several points to stimulate stomata closure (Singh et al. 2017b; Nazareno and Hernandez 2017). In addition to JA, other stress hormones like ET and CKs also interact with ABA to regulate root growth under osmotic stress conditions (Rowe et al. 2016). ET regulates the expression of various genes associated with IAA synthesis, and auxin also affects ET biosynthesis in multiple pathways. IAA involve in the regulation of the ET biosynthesis enzyme, ACS (1-aminocyclopropane-1-carboxylate synthase), which is a rate-limiting factor (Tsuchisaka and Theologis 2004). CKs were a positive regulator for IAA biosynthesis and maintained an appropriate level of IAA in developing root and shoot tissues (Jones et al. 2010). The GAs induced the expression level of gene NPR1, responsible for the biosynthesis of SA and showed improved oxidative stress in Arabidopsis (Alonso-Ramírez et al. 2009).

Fig. 5
figure 5

Hormonal crosstalk and various gene interactions under drought stress conditions are depicted here. Arrow and dashed lines indicate positive regulation, while blocked lines indicate negative regulation

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Fig. 6
figure 6

Basic signaling pathway of ABA inducing during drought stress. ABA quantity under normal conditions is low. Free PP2C phosphatases then bind and deactivate SnRK2 protein kinase activity. Under stress condition, ABA level increases. ABA then complexes with PYR/PYL/PCARs and inactivates PP2Cs activity. SnRK2s triggers ABA induced physiological and molecular responses via phosphorylation of downstream targets

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In some cases, negative regulation of ABA is also reported in the biosynthesis of ET, IAA and CKs during drought stress signalling. ABA decreases CKs level by downregulating the expression of IPT (adenosine phosphate-iso-pentenyl transferase) genes to enhance drought tolerance (Nishiyama et al. 2011). Thus, coordination between PHs collectively led to better adaptation and tolerance of plants under drought conditions.

Conclusion

Reduced yields and quality are mostly caused by abiotic stress factors globally. The most common and distinctive characteristic of arid and semi-arid regions is drought stress. The effort of lessening the effects of drought on field crops is greatly hampered by concerns related to climate change and global warming. However, the plant itself makes an effort to mitigate the negative impact by creating systems at the molecular, physiological, and morphological levels. Major field crops are being developed for drought resistance by researchers. Understanding various drought-induced secondary metabolites (SMs) and phytohormones (PHs) are prerequisites for a scientist to develop a drought-tolerant variety. SMs and PHs such as flavonoid, polyphenol, abscisic acid (ABA), ET, IAA, CKs, GA, SA, JA, BRs etc., play a vital role in drought stress signalling pathways of plants. Crosstalk between PHs involving SMs and synthesis can be manipulated using the in-vitro culture technique to a preparedly enhanced level for drought tolerance in plants. Studies of transcriptomic, functional genomics, and metabolomics have shown that drought tolerance reactions greatly influence several stresses responsive genes whose expression is controlled directly or indirectly by SMs and PHs. The application of genome editing technology, such as CRISPR-Cas and the most recent metabolic markers techniques, is one of the thrust areas to eliminate and specify the impact of drought responses caused at the sub-cellular level, though, for the development of elite SMs and PHs and widening the scope.