Phytohormones as Plant Growth Regulators and Safe Protectors against Biotic and Abiotic Stress

Abstract

Plants are playing important role in the planet by providing food for humans and stability in the environment. Phytohormones are key regulators in various physiological processes and among the most important small signaling molecules affecting plant growth and yield production. These biochemical also initiate adaptive responses caused by external stimuli, such as biotic and abiotic stress. Generally, on the basis of physiology, plant hormones roughly fall into two classes. In class one, phytohormones fall which is responsible for plants growth-promoting activities, such as cell division, cell elongation, seed and fruit development, and pattern of differentiation. On the other hand, the second class of hormone play important role in plants’ response, such as biotic and abiotic stresses. Some other hormones, such as jasmonates, salicylic acid, brassinosteroids, and strigolactones, also play a key role in plants. Their biochemical signaling network and their crosstalk ability make plant hormones excellent candidates to optimize plant growth and/or mediate abiotic and biotic stresses in agriculture. In the end, the future trends of plant hormone analysis are exploring plant hormones and their applications. We believe the perspective may serve as guidance for the research of plant hormones in the analytical, environmental, and botanical fields.

1. Introduction

Plant hormones or phytohormones are naturally occurring organic substances in miniscule concentrations and exert their action either locally or at distant sites. These chemical messengers with varied chemical properties and specific chemical structures directly influence the growth and development of the whole plant via different biochemical processes. These growth regulators substance coordinate the plant’s response simultaneously in abiotic and biotic stresses [1, 2, 3]. The occurrence of plant hormones is ubiquitous; they are present in all higher plants and lower plants as well. Their homeostasis in the plant is regulated by synthesis, metabolism, transport to the targeted tissue, and signal transduction which control its activities in the plant. Bioactive hormones are involved in this special type of regulation however intermediate and conjugated forms also play a pivotal role.

The action of plant hormones at the local and distant sites is mediated through different transport mechanisms. Transport of hormones at a distant target is facilitated by loading from the source into the xylem or phloem. In the last decade, several proteins have been identified that act as transporters of hormones at distant sites while short-distance movement of hormones is mediated by symplast, apoplast, or through transcellular mechanism [4]. At one extreme cytokinin get transported from roots to leaves where they prevent senescence and maintain metabolic activity, while at the other extreme the production of the gas ethylene may bring about changes within the same tissue, or within the same cell, where it is synthesized. Chemically plant hormones have a diversified nature comprising of indole, steroids, terpenes, carotenoids, fatty acids, and derivatives of adenine and such diversity reflect their different biological functions [5].

Generally, phytohormones have been divided into two groups on the basis of their functions; group one hormones including, auxin, gibberellin, cytokinin, brassinosteroids, jasmonic acid, and strigolactones. These endogenous signal molecules play a major role in growth-promoting activities by cell division, cell differentiation, elongation, pattern formation, stomatal movement, flowering, and seed germination and development. Hormones in group two are abscisic acid, salicylic acid, and jasmonic acid; mainly involved in biotic and abiotic stress response under different environmental conditions, such as sunlight, soil conditions, soil water, and nutrients [6, 7, 8].

Phytohormones do not act alone but in concurrence, or in antagonism, to each other such that the final growth or development represents their net effect. These comprise a unique set of compounds, with distinctive metabolism and properties. Their quality for being the natural compounds with the ability to produce the physiological effect in concentrations lower than those where nutrients and vitamins could not affect these processes makes them unique from the other compounds [9]. Most commonly hormones are classified into the following two categories, as shown in Figure 1.

Figure 1.

Classification of phytohormones.

2. Types of hormones

2.1 Gibberellin (GA)

Gibberellin (GA) is one of the important plant hormones and it is a tetracyclic di-terpenoid carboxylic acid. It promotes plant growth and development, such as germination of seed, flowering of plants, ripening of fruits, and expansion of leaf while it discourages the growth of trichome. They also play a vital role in the elongation and division of the cells. Its function is also to trigger seed growth and release of seed in dormancy [10]. “To grow or not to grow” is an important verdict for plants to survive. There are requirements of suitable ambiances for plants to grow while in lack of which the growth ceases. So, one of these requirements is the level of growth hormone, GA. That is retained by different synthetic or inactivation of enzymes [11]. GA is also compulsory for the normal growth of roots where its lower concentration is required for maximum root growth other than the shoots. So, GA can be inhibitory for the growth of roots when present in excess quantity [12].

In the 1930s, GA was discovered in Japan while studying a disease related to rice that was with symptoms of excessive growth and yellowing of stems and with lack of production of seeds. GA is with 20 or 19 carbon skeleton (C₂₀GA) or (C₁₉GA), where GA with 19 carbon skeleton was biologically more active. Presently, about 140 different molecules of GA are known and have been isolated from different microorganisms or plants. GA3, GA4, and GA7 are with the maximum biological characteristics and also, they are commercially available [13]. GA3 is also known as gibberellic acid. The biological active GA is commercially used in agriculture. They are sprayed to increase the size of grapes with no seeds, pears, berries as well as to increase the crop yield under salt stress. It is also practiced in beer brewing to increase the process of malting. In 2016, according to some data the international market scope of GA was estimated at USD 548.9 million [14].

2.1.1 Synthetic pathway of gibberellin

Gibberellin is chiefly manufactured via the plastidial methylerythritol pathway (MEP) in plants. The biosynthesis of GA in the initial steps is the same in both plants and fungi and is from geranylgeranyl diphosphate (GGPP) to GA-12 aldehyde. Then there is the cyclization of (GGPP) into ent-copalyl diphosphate and to ent-kaurene in plants that is catalyzed by the copalyl diphosphate synthase (CPSp) and the ent-kaurene synthase (KSp) [15]. It is also manufactured by several bacteria and fungi which are associated with various plants in symbiotic or pathogenic relations. In such circumstances, GAs have no evolving utility in the producing organism but perform on the plant host to relief infection by destroying the immunity or on nitrogen-fixing bacteria to adjust the formation of nodules [16].

2.1.2 Gibberellin against heavy metal stress

Gibberellic acid (GA3) plays an important function for plant growth under salt stress and heavy metal toxicity as well as increases the synthesis of chlorophyll and the action of antioxidant enzymes to prevent lipid peroxidation. GA with calcium (Ca) is added to decrease the salt toxicity like (Ni) toxicity on plant throughput and also to activate different antioxidant enzymes to decrease lipid peroxidation of the cell membrane. They are also involved in regulating the different processes in plants to increase the heavy metal stress [17, 18]. Application of gibberellic acid in Vicia faba L., supported to restore the Cd and Pb-induced reduction in the mitotic index. Due to the application of these growth hormones, GA in plants with heavy metal stress, the ratio of several chromosomal irregularities was expressively reduced. After that, the seeds were harvested from V. faba both from heavy metals and gibberellic acid treatment showed a high level of solubilized sugars, proteins, and nucleic acid. Gibberellic acid in lupin plants with Cd stress improved the activity of amylase along with the CAT enzyme. When Chlorella vulgaris was exposed to heavy metals, such as Pb and Cd, the application of GA3 amplified the number of cells and level of protein to verify that GA can defend life in water polluted areas [19].

2.2 Auxin (IAA)

In the nineteenth century, the idea of a portable substance came into existence that was produced by the leaves and traveled down to encourage the synthesis of the root was capable to control the winding of grass coleoptiles toward the sun. After that these substances were refined, categorized, and were given the name “auxin” from the Greek word “auxin,” which means “to grow or to enhance.” The naturally existing IAA is indole-3-acetic acid IAA [20]. IAA are organic compounds that are small, low in molecular weight, and constitute the most advanced key and diverse group of phytohormones normally present in all plant types. They are indulged in a number of developing practices by regulating the cell division, such as in the control of shoot building, vascular enlargement, and horizontal root construction [21]. IAA can control senescence, also can react with many pathogens and abiotic or heavy metal stresses. Moreover, it can regulate the production of fruit responses in plants [22].

2.2.1 Synthetic pathway of auxin

It is essential for the development of the plants that the biosynthesis of auxin be localized in specific tissues but IAA is generally produced in fresh leaves and then it is transported to all through the plant. Many practices have been done to quantify it and its metabolites permitting alteration of the prior opinion of auxin dispersal in a minute quantity of plant tissue having a fresh weight of less than one milligram [23]. IAA biosynthesis in plants is still partly clarified; however, with the use of different isotopes, it is shown that IAA can be synthesized by two important pathways. These pathways are tryptophan dependent and tryptophan independent [21]. In tryptophan dependent pathway, four routes have been identified. In the first route, indolacetamide is converted to IAA by amidohydrolase. In the second route, indole-3-pyruvic acid (IPA) is formed from tryptophan with the help of an aminotransferase which is then converted to indole-3-acetaldehyde by IPA decarboxylase, and then indole-3-acetaldehyde is converted to indole acetic acid by indole-3-acetaldehyde oxidase. These routes are not going parallel often they cross each other. Tryptophan is also converted to tryptamine by trp-decarboxylase and then some proteins help the conversion of tryptamine to indole acetic acid after different steps [24].

In the tryptophan independent pathway, IAA was synthesized in the absence of Trp genes. So, several studies showed that in the absence of tryptophan due to the absence of Trp genes or defective Trp genes, there was evidence of IAA in plants. Studies in Arabidopsis showed that trp3-1 and trp2-1 mutants were defective in tryptophan synthase α and β respectively; there was still the production of IAA. It is hypothesized that IAA production might be due to precursors of tryptophan, i.e., indole or indole-3-glycerol phosphate [25].

2.2.2 Auxin against heavy metal stress

The presence of IAA in plants is also helpful in plant growth in changing environments. In the presence of heavy metal toxicity, it plays a crucial role to tolerate this. In this metal stressed plants, auxin can be provided by the inoculation of microbes that can produce IAA in the rhizosphere of these plants to improve plant production. This metal stress is due to the production of different reactive oxygen species in different locations of the plants, such as root cells, containing peroxisome, mitochondria, plastids, and cytoplasm [26]. In different studies, it is elaborated that the level of IAA is altered endogenously due to the heavy metal stress in roots and shoots. There is shown a positive also a negative relation between heavy metal toxicity and level of IAA. It is also under observation that in response to heavy metal stress, regulation of IAA-producing genes may control the locality and accretion of IAA [27]. Several genes are involved in the relation between IAA and reactive oxygen species to attain homeostasis to adjust the level of H₂O₂ by regulating and stimulating the antioxidant enzymes and chlorophyll levels in plants [28]. The abnormal level of metal in the soil causes toxicity in plants and retards the growth and development of the plants due to its accumulation in roots and shoots. The decrease in the rate of growth and development in plants is mainly controlled and maintained by plant hormones like IAA. Indole acetic acid is well known for plant adaptation under heavy metal stress which results in enhanced biomass and production. IAA can be applied to various plants exogenously either directly or through plant growth-promoting rhizobacteria (PGPRs) which produce IAA and in return there is a significant improvement in plant growth under toxic metal concentration [29].

2.3 Cytokinins (CKs)

In plant growth and development, the master regulators are known as CKs a phytohormone or plant hormone. Its main function is in the physiology of the cell-like expansion and division of the cell, P and N₂ metabolism, maintenance of H₂O balance, the integrity of chloroplast, and senescence. N6-substituted adenine derivatives are known to form by CKs. Seed dormancy can be reduced by using CKs [30]. There are several types of CKs counting—thidiazuron, 6-benzyladenine, kinetin, and 2-isopentenyladenine [31]. CKs are present in abundant types which are different in structure, properties like biochemical and biological activities, and the mode of transportation across plant tissues [32]. CKs are produced in roots and apical meristem and then transported to aerial fragments with the help of absorbent material like minerals via the xylem. In xylem exudate, Zeatin riboside is the utmost copious form of CKs [33]. CKs perform an important role in the growth and adaptation of plants all through the life cycle, such as in the initial phases of reproduction; flowering, seedling, and development [34]. CKs are also used to control the N₂ metabolism by increasing the action of nitrate reductase in plants [35].

2.3.1 Synthetic pathway of Cytokinins

Several studies in plants, such as rice (Oryza sativa) and Arabidopsis (Arabidopsis thaliana), stated that there are several means to regulate the de novo synthesis of CKs. Synthesis of CKs is due to the nitrogen status signals; one is nitrate-specific signal and the other is a glutamine-related signal. The availability of nitrogen exogenously represents the nitrate-specific signals while assimilated nitrogen status represents the glutamine-related signals. It is known that the CK synthesis is regulated by the nitrogen that is taken up from the soil and is also a key component of nitrogen integration [36]. CKs synthesis have been recognized to be controlled by vital genes. In the initial step that is catalyzed by isopentenyl transferase (IPT), there is a transfer of the isopentenyl group to an adenine nucleotide (ATP, ADP, AMP) from dimethylallyl diphosphate. Then hydroxylation of the methyl group occurs in the isopentenyl side chain by a cytochrome P450. Then in the last step ribose is released and is catalyzed by a phosphoribohydrolase [37]. The other indirect pathway involves the addition of dimethylallyl pyrophosphate (DMAPP) to adenine A37 on tRNA which results in the discharge of CKs nucleotide through degradation of tRNA and by the elimination of the phosphoribosyl by LOG (LONELY GUY). Prenylation of tRNA is catalyzed by tRNA isopentenyltransferase (tRNA-IPT) [38].

2.3.2 Cytokinins against heavy metal stress

When plants are undergone xenobiotic resistance, there is a key role of CKs with saline resistance, drought, light, and temperature signals [39]. Different signaling pathways are there to regulate the concentration of CKs under heavy metal stress to increase plant resistance. A. thaliana under arsenic stress is investigated when there are reduced endogenous CKs and the reduced CKs signaling while with the mutant plants exhibited CKs synthesis increased the tolerance of plants against arsenic. It is described that exogenous CKs can promote plant resistance against metal stress [40]. CKs controls morphology, division of cells, and several other substantial routes in the plant. Numerous studies described role of CKs for reduction of heavy metal toxicity in crop plants via biosorption of heavy metals. In higher plants, CKs have been verified to restore the heavy metal-induced decrease in mitotic index resulting in an increase in the number of cells. Furthermore, CKs certainly controlled the photosynthetic mechanism and raised the concentration of various monosaccharides and antioxidants which results in the better existence of plants under heavy metal toxicity [41].

One of the contrivances in a high level of heavy metal is the variation in the level of CKs. It is observed that the concentration of CKs decreases when there is an excess of heavy metal to increase the overall efficiency of plants to cope with this toxicity. Most of the studies agree that the shortage of mineral elements reduces the concentration of CKs in plants [42]. It also improves the tolerance of C. vulgaris to Cu, Cd, and Pb due to the activation of the antioxidant defense system, therefore, minimizing the negative values of heavy metal oxidative stress [43]. It is concluded that plant hormones enriched the actions of antioxidant enzymes, i.e., APX, SOD, GR, CAT, and improved the contents of smaller antioxidant molecules, such as glutathione, ascorbate, and proline. CKs protect several proteins and constituents of the process of photosynthesis (carotenes, chlorophylls, and xanthophylls) thus there was a considerable decrease in detrimental effects of heavy metal stress on A. obliquus [43].

2.3.3 Cross talk among phytohormones

Phytohormones or plant hormones are small endogenous mediators, such as cytokinin (CK), gibberellin (GA), salicylic acid (SA), brassinosteroid (BR), auxin (IAA), ethylene (ET), jasmonic acid (JA), abscisic acid (ABA), and strigolactone (SL), which coordinate a dual purpose also known as cross talk of plant hormones. Certainly, plant hormones are intermediates that not only direct and organize the progressive practices endogenously but also deliver the environmental incitements to initiate adaptive reactions to biotic & abiotic stress [44].

It was observed that there was an improvement in the differentiation of callus and the number of cells was also increased when CKs were applied with IAA. There was also the contribution of both to uplift the strength of the plant, localization of nutrients, and to increase the grain yield in numerous plants. CKs was found in excessive amount in emerging tissue, such as cambium, root, and shoot tips [45]. Normally, GA and CKs are observed to be antagonists, because both of them opposed the effect of one another on shoot apex, root tips, and elongation of the plant. DELLA proteins are thought to be responsible for such mechanisms where these antagonistic hormones cross talk to function as negative stimulators of GA signals [46]. ABA is known to be a plant stress hormone that accumulates promptly in plants when they are under dehydration or drought stress. A relation exists between ABA and CKs activity for the maintenance of seed development, pre and post-seed development, and stress stimuli. Cross talk concerning ABA and CK is often opposing. When there is an increased concentration of CK, it suppresses the ABA reactions. These antagonistic functional relations are observed for the maintenance of the Arabidopsis under drought conditions although ABA decreases seed sprouting [47]. ET is a gaseous hormone that controls proliferation and expansion of cells, development of fruit, senescence, and various reactions toward biotic and abiotic pressures. The behavior of CKs and ET is generally antagonistic in shoots, where CKs are involved in greening and cell multiplying while ethylene with aging processes, such as maturing, senescence, and the inhibition of cell propagation. Moreover, both of them work cooperatively in different routes, such as maintenance of roots by inhibiting root development. Considerably, CKs positively encourage the production of ET by activating ACC synthase. The production of ET facilitates the CKs to impede hypocotyl elongation in seeds and also to constrain root growth [48]. SA is a hormone of plant origin and performs a vital role against biotrophic pathogens for plant defense in contrast with JA which performs against necrotrophic pathogens. CKs cross talk with SA signals and help in the protected responses via their interactions. CKs increase the SA reactions that help to increase transcription of genes relevant to defense like SID2 and PR1, for the biosynthesis of SA and an indicator or marker gene for SA response respectively [49].

The GA synthesis in an ovule is increased by IAA during fertilization. There is a synergistic effect between auxin and GA signals that regulate fruit development. Their concentrations can be applied externally to regulate the fruit growth and also prompt parthenocarpy [50]. The interpretation of GA and IAA indication cascade has significantly simplified how these hormones manage with each other to control the development of fruit. IAA performs to upstream the level of GA throughout the fruit growth. In Arabidopsis, the fertilization-induced IAA responses or IAA applications activate GA biosynthesis; however, GA applications do not encourage the IAA response [51]. Recently, it is investigated that a high concentration of IAA and the activity of IAA signals enhance ABA-mediated dormancy [52]. Both of them are involved in the maintenance of water status in plants, with opposing performance in the shoots and roots. Indications for water status in the plant have to be integrated to adjust water conductivity in roots and permit modifications in stomatal opening. As a constraint, ABA and IAA signals never exist in a linear fashion but essentially form a system that interconnects through cross talk [53]. IAA and ET cooperatively control numerous developing routes in plants. To date, a whole heap of evidence is existed at the molecular level to promote cross talk between IAA and ET at synthetic, transporter, and signaling levels. That comprises transcriptome profiling datasets to define new entrants for the molecular cross talk. When there is any disturbance in IAA synthesis and transportation, ET helps to promote/deplete the IAA level, or to redistribute it in plants, thereby activating morph-genic reactions. Though, the function of ET in the cross talk is not limited to IAA redistribution [54]. SA and IAA cross talk is fairly obvious from both experimental confirmation and RNA-seq. Data exploration [55]. IAA and SA not only share a common precursor but also play important functions in the maintenance of fruit development and ripening. A metabolic and functional cross talk between them and with other plant hormones occurs in a spatiotemporal fashion to magnificently control the growth of seasonal and non-seasonal fruits [56].

Proofs for protein physiology as a connecting pivot between GA and ABA signaling systems have been progressively evolving both on the functional and on the chromosomal level, mostly in the period of early plant growth. In Arabidopsis, substantial influences of GA and ABA in the signals from hormones have been recognized after the light-reversibility in kernel development. Enhanced production of GA in ABA deficient gene initiated better light-dependent development and concludes an antagonistic association of GA and ABA production in growing and rising seed [57]. ET has a key role in maintaining the developing processes in plants under stress. It is shown to be positively involved with ET when oxygen is deficient. For the elongation of internodes of deepwater rice, GA and ET activity are required. During rice immersing into the water, a decreased level of O₂ is recommended to prompt ET synthesis which in turn impede the ABA production. This altered balance between GA and ABA causes elongation of stem induced by GA [58]. GA and SA contribute to the maintenance of many plant reactions. They are concerned to stimulate the expression of proteins involved in pathogenesis. Furthermore, they cooperatively develop plant defenses under biotic and abiotic stresses as SA enhances resistance to abiotic stress in plants. It also has the capability to raise antioxidants and decline the process of lipid peroxidation. In Arabidopsis, seeds were exposed to SA with salt toxicity resulted in the increase of SA because of the activation of two superoxide dismutase, which recovers seed development and upsurges antioxidant capabilities, to enhance the salt tolerance in plants under salt stress [59]. The list of various hormones is given in Figure 2.

Figure 2.

List of various kinds of phytohormones.

2.4 Abscisic acid (ABA)

Abscisic acid (ABA) is an isoprenoid compound associated with seed dormancy, drought responses, and other growth processes. ABA plays vital roles in plant responses to a range of abiotic stresses such as drought, salinity, high light, nutrient deficiency, and heavy metals. ABA has also been found to be associated with color change in fruits during ripening. ABA at higher concentrations inhibits root growth but in stress conditions, it also plays a vital role in the elongation of the root. Various environmental factors regulate the levels of ABA, including seed maturation, the genotype of plant, water and soil conditions. ABA concentrations are generally increased in nutrient deficiency and decreased at higher temperatures 40°C. Root tissue generally contains lower concentrations of ABA than leaves, dehydration of detached roots from various species, ages, and branching orders also stimulate ABA synthesis [60].

The biosynthesis, catabolism, transport, downstream response, and modulation of ABA have been extensively investigated in angiosperms. ABA is primarily synthesized from carotenoids under the catalytic action of various enzymes such as b-carotene hydroxylases, zeaxanthin epoxidase (ZEP, ABA1), 9-cis-epoxycarotenoid dioxygenase (NCEDs), short-chain alcohol dehydrogenase/reductases (SDRs, such as ABA2), abscisic aldehyde oxidases (AAOs), molybdenum cofactor sulfurase (MOCO, ABA3), and ABA4. ABA levels are regulated by two major pathways—hydroxylation and esterification mediated by four CYP707As and eight glucosyltransferases (UGTs). The inactivated ABA-glucosyl ester (ABA-GE) conjugation is a storage form of ABA and the site can be cleaved by b-glucosidases (BGLUs) [61].

2.4.1 Abscisic acid against heavy metal stress

ABA is one of the foremost phytohormones driving plant resistance to toxic metals and metalloids, such as Cd, and Pb. Mechanisms of ABA in response to heavy metals and metalloids stresses in non-angiosperm plant lineages is still limited and not completely understood [62], however, ABA act in different ways in response to heavy metal stress, including by alleviating toxic metal and metalloid stress via ABCGs, PSE1, and WRKY13, limiting their uptake, altering the distribution between roots and shoot and promoting chelation and vacuolar sequestration [7, 8, 9].

2.5 Jasmonic acid (JA)

Jasmonic acid is a signaling chemical that mediates the number of biotic and abiotic stress process in plants, such as fruit ripening and seed germination, wounding, and ultraviolet radiation. It is produced from linolenic acid but activated after conjugation with isoleucine which permits it to join with COI1and act as a JA receptor. The first role of JA as a senescence-promoting was observed from a compound isolated from wormwood cause rapid loss of chlorophyll in oat [63]. JA also stimulates the secretion of volatile oil in plants which have antimicrobial properties. In a current study remarkable role of JA acid was observed in the regulation of the life cycle in plants [64]. JA is a naturally grown regulator and is extensively found in plants. Normally, JA does not work in isolated form, and extensively cross talk behavior was studied with other hormones. JA’s role in a hot climate as a water conserver was also observed through stomatal closure. In addition, JA also helps to cope with drought stress by promoting some enzymes, such as superoxide dismutase (SOD), peroxidase (POD), catalase (CAT), proline. The role of JA against fungal pathogens and some other plants pathogen was also observed [65]. JA not only works as a plant’s growth regulator but also stimulates the immune system in plants. The list of hormones with their origin and functions is mentioned in Table 1.

Phytohormones	Origin/precursor of Synthesis	Functions							References
		Germination	Growth	Flowering	Fruit development	Abscission	Seed dormancy	Defense mechanism
Gibberellin	Young shoots, growing seeds/Glyceraldehyde 3-phosphate	Yes	Yes	Yes	Yes	No	Yes	No	[10]
Auxin	Young leaves, developing Fruit/Tryptophan or IAA	No	Yes	Yes	Yes	No	No	No	[22]
Cytokinins	Root tip, young leaves, growing seeds/Adenine	No	Yes	Yes	Yes	No	No	Yes	[35]
Abscisic acid	Roots & Leaves/Carotenoids	No	No	No	No	Yes	Yes	No	[60]
Jasmonic acid	Various tissues/Polyunsaturated Fats/ Linolenic acid	Yes	No	No	Yes	No	No	Yes	[65]
Salicyclic acid	Various tissues/Phenylalanine	No	Yes	No	No	No	No	Yes	[66]

Table 1.

List of phytohormones with their origin and functions.

2.6 Salicyclic acid (SA)

Salicyclic acid is one of the most important plant hormones naturally produced in plants within the cytoplasm of the cell. SA is a key player in the regulation of very important functions, such as photosynthesis, growth, and even in defense of plants [67]. Different past studies demonstrated the role of SA in plants against biotic and abiotic stress. SA stimulates the SAR mechanism in plants which activates pathogenesis-related proteins which work against different kinds of phytopathogens, such as fungus and bacteria. It also induces a variety of metabolic processes in plants and also regulates plant and water relations [68]. The role of SA was also observed in different kinds of signaling which leads to gene expression and protein synthesis. Similarly, SA also works with other hormones in cross talk. The SA treatment in cucumber or tobacco plants induced heavy metal-like cupper tolerance ability. It also induced cd tolerance ability in plants but the exact mechanism is still unknown [66]. The hormones of various names with their functions are given in Figure 3.

Figure 3.

Phytohormones and their role in plants.

3. Conclusion

Growth is an essential property for every living organism and is usually regulated by various external and internal factors. Generally, plants attain this property via synthesizing a small amount of chemical substance known as phytohormones. These chemical substances trigger biochemical changes that ultimately initiate several growth changes in plants, such as the formation of flowers, roots, stems, and fruits. As a result, these processes increase the yield. Some phytohormones also play important role in a plant’s life from dormancy to senescence. Consequently, perform a vital role in agriculture and horticulture, etc. Conclusively, phytohormone regulates the physiology of plants but the information about the molecular mechanisms still remains unclear.