Introduction

Variable climatic conditions, limited arable land and decreased water availability are threatening agricultural sustainability in many regions on the planet (Mickelbart et al. 2015). These challenges, coupled with various abiotic stresses, such as drought, salinity, heat, and cold, causes significant crop yield losses and its associated socioeconomic consequences. For example, severe drought resulted in the decline of maize (Zea mays L.) yield in the United States (Boyer et al. 2013); the rapid decline of groundwater at Fertile Crescent caused widespread crop failure which compelled some of the affected farm families to migrate (Kelley et al. 2015); severe heat stress (above 40 °C) led to the failure of a large area of sorghum (Sorghum bicolor L.) in northeast Australia (Lobell et al. 2015). Also, some of the human activities, such as intensive irrigation, poor drainage, and uncontrolled mining led to waterlogging and accelerated the salinization of cultivated lands worldwide (Herbert et al. 2015; Singh 2015). Therefore, the improvement of crop tolerance to abiotic stresses is becoming a prioritized area of research in agricultural science.

Exogenous application of various small molecules or plant growth regulators is a well-known method to enhance the resistance of plants to environmental stresses (Chan and Shi 2015). Among the many identified hormones, regulators or small signaling molecules, 5-aminolevulinic acid (ALA) is known to be effective against the harmful effects caused by various abiotic stresses in plants. It has been reported in several studies that ALA was involved in the regulation of plant growth and development, and has physiological activity as plant hormone; therefore, it can be used as a plant growth regulator in agricultural production (Bindu and Vivekanandan 1998; Akram and Ashraf 2013).

As the common precursor of all tetrapyrroles in biological world, ALA has been reported as a light-sensitive reagent by medical field in fluorescence diagnosis and photodynamic therapy (Guaragna et al. 2015; Hillemanns et al. 2017). ALA and its derivatives have been used in the treatments of actinic keratosis and basal-cell carcinoma of skin, since they release free radicals and singlet oxygen during the transversion from excited state to the ground state when motivated by light, which provide cellular toxicity to target cancer cells (Cosgarea et al. 2013; Morton et al. 2013).

Originally, ALA was discovered in duck blood in 1953, and was identified as the source of protoporphyrin (Shemin and Russell 1953). A few decades later, ALA was found in the culture-medium of Chlorella vulgaris when added with levulinic acid, a competitive inhibitor to ALA dehydratase that hold back the decurrent metabolization of ALA (Beale 1970). ALA has regulating effects towards certain metabolic processes, such as chlorophyll, heme and siroheme biosynthesis (Kim et al. 2014). As a precursor of chlorophyll in higher plants, the early evidence of ALA metabolism was found in corn (Z. mays L.), seedlings of common bean (Phaseolus vulgaris L.) and cotyledons of cucumber (Cucumis sativus L.) (Harel and Klein 1972; Beale and Castelfranco 1974). The biosynthetic pathway of ALA in green plant tissues, namely C5-pathway, requires l-glutamate (Glu) to provide carbon skeleton (Hudson et al. 2011). Glutamate–tRNA reductase enzyme (GluTR), which is encoded by HEMA1, is the key rate-limiting enzyme in this pathway (Apitz et al. 2016).

ALA was considered a critical regulator to plants. Studies towards understanding the regulatory mechanism of ALA in plants have become a key area of research in agricultural science (Czarnecki et al. 2011; Xie et al. 2013; Ali et al. 2014b). Studies have shown the mitigation role of ALA in plants against abiotic stresses when used as an exogenous supplement, but the regulation mechanisms associated with the stress tolerance have not been fully elucidated.

A few excellent reviews have elaborated the physiochemical aspects and regulatory functions of ALA in plants. For example, in a review, the authors have discussed the potential of microbial production of ALA and their application in agricultural crops and medical treatments (Sasaki et al. 2002). In another review article, the authors have highlighted the primary roles of ALA with different modes of action in alleviating abiotic stresses (Akram and Ashraf 2013). However, there is a lack of detailed information with regard to the regulative manner on biosynthesis upstream and metabolism downstream of ALA in plants. Little has been reported in the scientific literature regarding the role of ALA in promoting/regulating plant growth and alleviating damages caused by abiotic stresses through the visual angle from ALA metabolic pathway. The present review starts to fill this knowledge gap.

The biosynthesis of ALA in higher plants

All tetrapyrroles in vivo, including chlorophyll, heme, siroheme, vitamin B12 and phytochromobilin are derived from a common precursor, that is, ALA (Senge et al. 2014). The biosynthesis of ALA is a momentous bioprocess in both heterotroph organisms and photosynthetic species. In heterotroph organisms, the biosynthetic pathway of ALA is called Shemin pathway or C4-pathway (Bradshaw et al. 1993; Neidle and Kaplan 1993), whereas in photosynthetic species, such as plants, algae and most photosynthetic bacteria and archaea, it is called Beale pathway, or also known as the C5-pathway or the monovinyl/divinyl monocarboxylic acid cycle. The C5-pathway serves as the dominant process in photosynthetic species (Fig. 1) (Kořený et al. 2013; Akram and Ashraf 2013). l-Glutamate is the source of ALA synthesis in the Beale Pathway, it ligates tRNAGlu and generates l-glutamy–tRNA ultimately; this reaction is simultaneously catalyzed by glutamyl–tRNA synthetase (GluTS) (Czarnecki and Grimm 2012). Then, GluTR plays a catalyzing role where the carboxyl group of Glu–tRNA is reduced to formyl group; this process enables the conversion of l-Glu–tRNA into l-glutamic acid 1-semialdehyde (GSA) (Tanaka and Tanaka 2007). At the last step, ALA is created through transamination, which catalyzed by glutamate-1-semialdehyde aminotransferase (GSAT) (Akram and Ashraf 2013). These reactions are located in stroma of chloroplast (Wang and Grimm 2015). GluTR plays a key role during the synthesis pathway of ALA, to some extent; it adjusts content of ALA and has rate-limiting effect to ALA biosynthesis (Zhao et al. 2014). In higher plants, this reductase is encoded by HEMA1 (Nagahatenna et al. 2015). A study has shown that the regulating response of GluTR gene may be controlled by various stimulus, like plant hormone, light and circadian rhythms (Apitz et al. 2016). In the transgenic Arabidopsis thaliana expressed antisense HEMA1 mRNA, the protein content of GluTR decreased significantly, with the lowest protein content being only 1% of the non-transgenic plants (Kumar and Söll 2000). The protein content of GSAT in the transgenic A. thaliana was not significantly different compared with the control, whereas ALA content was 21–56% of the control plants. These results indicate that expression level of HEMA1 inevitably influence the catalytic action of GluTR on Beale pathway. Sustained high light (1500–1600 µE/m2/s) gravely restrained protein content of GluTR in cucumber cotyledons and ALA biosynthesis was declined with no suppression to HEMA1 gene expression, but the protein content of GSAT remained unchanged (Aarti et al. 2007). This indicates that high light has a negative impact toward GluTR mainly on the transciriptional level. Moreover, during de-etiolation, the HEMA1 and Lhcb are executed like co-ordinated regulation under parallel light by shared phytochrome- and cryptochrome-signalling pathways (McCormac and Terry 2002). In addition, GluTR can be impacted according to content of metabolic products on the downstream of ALA. Heme is an end-product in one of metabolic fluxes of ALA, and it is described as a feedback inhibitor to ALA formation, since it depresses the activity of GluTR (Zhang et al. 2015c). Similar phenomenon of feedback regulation also emerged in another metabolic branch of ALA, the Mg-branch. In a study with barley (Hordeum vulgare L.), the protochlorophyllide (Pchlide) performed a rapid accumulation after transition from light to dark and ALA formation whittled down immediately in the leaves (Richter et al. 2010).

Fig. 1
figure 1

The biosynthetic pathway of ALA in higher plants. The main biosynthetic pathway of ALA in higher plants was called Beal pathway or C5-pathway. This pathway starts from glutamic acid, which is produced by TCA cycle. Glu ligates tRNAGlu and generates Glu–tRNA are catalyzed by GluTS. Then, GluTR acts a catalyzing role that converts Glu–tRNA into GSA. At last, catalyzed by GSAT, ALA is created in stroma of chloroplast

The metabolism and regulation at downstream pathway of ALA

The common steps

Tetrapyrroles, like chlorophyll, heme, siroheme, vitamin B12, and phytochromobilin, are ring structured intermediates; they participate in many biochemical processes and have vital roles in vivo. Within the downstream metabolic flux of ALA, they own a stretch of common steps, from ALA to uroporphyrinogen III (Uro III). After ALA biosynthesis, two ALA molecules are coalesced to form a pyrrol ring, called porphobilinogen (PBG); this reaction is catalyzed by ALA dehydratase (ALAD), and can be inhibited by aluminum and mercury (Pereira et al. 2006; Gupta et al. 2013). Then, four molecules of PBG catalyzed by PBG deaminase are polymerized to produce a linear tetrapyrrole, 1-hydroxymethylbillane (HMB); this is the essential linear tetrapyrrole ring of all tetrapyrroles. Therefore, under the catalytic condition of uroporphyrinogen III synthase (UROS), HMB forms the unsymmetrical closed macrocycle, Uro III (Fig. 2) (Tanaka and Tanaka 2007).

Fig. 2
figure 2

The downstream metabolism of ALA and regulatory factors among metabolic pathway. ALA is the common precursor of chlorophyll, heme and siroheme. Moreover, feedback inhibition effect plays an important regulative role in the pathway, where the pathway is associated with the positive regulators, like GUN4 and FHY3/FAR1 proteins; and negative regulator, like FLU protein

Siroheme

The starting point to the first branch of ALA metabolic flux is siroheme biosynthesis (Fig. 2). Transmethylation occurs to Uro III by a S-adenosyl-methionine: uroporphyrinogen III methyltransferase (SUMT), which forms dihydrosirohydrochlorin (also known as precorrin-2) (Storbeck et al. 2011). It gives sirohydrochlorin when precorrin-2 is subsequently catalyzed by an oxidase (precorrin-2 oxidase, PCOX); finally, sirohydrochlorin ferrochelatase (SCFC) combines Fe2+ with sirohydrochlorin forms siroheme (Bali et al. 2014). Disorganization in biosynthesis pathway of siroheme will induce the accumulation of some light-sensitive intermediates from chlorophyll pathway and then lead to reactive oxygen species (ROS) synthesis (Tripathy et al. 2010). Siroheme plays a crucial part in the reduction of nitrate and sulfate as a kind of accessorial factor. Since plants can utilize ammonium nitrogen and sulfur amino acid, instead of nitrate and sulfate directly from soil, the Fe2+ that chelated in the center of siroheme is capable of assisting the electronation of reduction of nitrate and sulfate (Hu et al. 2015; Garai et al. 2016).

Heme

It is worth mentioning that heme and chlorophyll share common synthesizers on the pathway from Uro III to protoporphyrin IX (Proto IX) (Fig. 2) (Akram and Ashraf 2013). Uro III casts off carboxyl group and turns to coproporphyrinogen III (CPG III), which catalyzed by uroporphyrinogen III decarboxylase (UROD). Coproporhyrinogen III oxidase (CPOX) converts CPG III into protoporphyrinogen IX (Protogen IX), and then protoporphyrinogen IX oxidase (PPOX) extracts six electrons of Protogen IX to form Proto IX (Nagahatenna et al. 2015).

Ferrochelatase (FECH), encoded by CsFeC1, CsFeC2 genes, is the key enzyme in heme biosynthetic branch, and these genes are correlated with photosynthetic and nonphotosynthetic tissues in plants (Fig. 2) (Suzuki et al. 2002). FECH chelates Fe2+ into the porphyrin ring of Proto IX molecule to create heme (Dailey and Meissner 2013). The appearance of heme is correlated to the main post-translational feedback regulatory of GluTR as mentioned earlier, but the particular mechanism of GluTR inhibition by heme have not been evaluated yet (Apitz et al. 2014). Heme is an essential functional molecule which participates in many physiological reactions. In human and animal, heme is responsible for oxygen transfer and metabolism. Simultaneously, heme takes part in electron transfer and secondary metabolism within higher plants (Espinas et al. 2012). Heme can also be oxidized by heme oxygenase (HO), which will transform heme into CO, free iron (Fe2+), and biliverdin (BV) (Kwon et al. 2011). Among the offspring, BV will turn to bilirubin (BR), an intracellular potent antioxidant, under the catalyzing effect of BV reductase. Moreover, CO plays a critical role as signaling molecule and participates in regulating against various abiotic threats to plants (Wang and Liao 2016).

Chlorophyll

Chlorophyll is created by another branch which starts at Proto IX (Fig. 2). What activates the chelation reaction is the key enzyme in chlorophyll biosynthesis known as Mg-chelatase (MCH), which can install Mg2+ into Proto IX, and give Mg-protoporphyrin IX (Mg-Proto IX) (Sobotka 2014). MCH consists of three subunits, ChlH, ChlI and ChlD in higher plants (Richter and Grimm 2013). Among these, it is ChlH that is primarily responsible for catalytic action of MCH. Proved by research in the chlorophyll-deficient mutant of Chlamydomonas reinhardtii, the levels of mRNA and protein output of ChlH are both increased while ChlI and ChlD remained unalterable from dark condition to light (Chekounova et al. 2001). Then, Mg-protoporphyrin IX methyltransferase (MgMT) devolves a methyl group from S-adenosyl-l-methionine to Mg-Proto IX, giving Mg-protoporphyrin IX monomethyl ester (Mg-Proto IX ME) (Nguyen et al. 2016). Followed by Mg-protoporphyrin IX monomethyl ester cyclase (MgCy), the reaction merges atomic oxygen (O) to Mg-Proto IX ME and creates 3,8-divinyl protochlorophyllide. In the next step, divinyl protochlorophyllide is deoxidized to form Pchlide, which is catalyzed by divinyl chlorophyllide 4-vinyl reductase (DVR) (Chen 2014). After that, under the existance of protochlorophyllide oxidoreductase (POR), chlorophyllide is formed. The last procedure of chlorophyll branch is promoted by chlorophyll synthase (CS) and thus, eventually creating chlorophyll a (Chl a) (Akram and Ashraf 2013). In addition, Chl a can be converted into chlorophyll b (Chl b) with the catalytic condition provided by chlorophyllide a oxygenase (CAO) (Kunugi et al. 2013).

Regulation mechanism on the pathway

POR is one of the key reductases in this pathway, and it is a light-dependent enzyme in higher plants (Nickelsen et al. 2011). Inhibition of POR under dark condition causes the accumulation of Pchlide instantaneously, and then ALA synthesis is down-regulated, since there is a feedback regulation mechanism between ALA and Pchlide synthesis (Richter et al. 2010). Another negative feedback regulator is the FLU protein in plastid membranes; it has been proposed to have a synergetic role for chlorophyll branch, similar to the function of heme in Fe-branch (Kauss et al. 2012). GluTR is the effecting target of protein FLU (Zhang et al. 2015b). In barley, the flu ortholog mutant tigrina d12, which unmakes ALA synthesis from dark-suppression, accumulated Pchlide under dark condition (Richter et al. 2010; Lee et al. 2003). The excessive accumulation of Pchlide led to the death of the plants after being illuminated, since substantial Pchlide produced massive active oxygen by light-motivated and seriously damaged chloroplast. This feedback regulation mechanism in higher plants is conducted to adapt to dark environments and protect plant tissue from peroxidative damage. Synergistically, lowering GluTR content by embedding HEMA-RNA-interference (RNAi) gene into tobacco (Nicotiana tabacum L.), resulted in the decline of MCH and FECH activities, causing the diminution of chlorophyll and heme content, respectively, as the transcript levels of these remained unchanged (Hedtke et al. 2007).

One more regulator to intermediates in this pathway is the GUN4 protein (Fig. 2). GUN4 has positive regulation roles in chlorophyll biosynthesis as it binds with intermediates (e.g. Proto IX, Mg-Proto IX and Mg-Proto IX ME) and enhances MCH activity (Fig. 2) (Yurina et al. 2012). Overexpression of Arabidopsis GUN4 protein in tobacco revealed general stimulation of tetrapyrrole biosynthesis, including the levels of chlorophyll, heme, Proto IX, and Mg-porphyrins and the activity of MCH, compared with the wild-type tobacco (de Menezes Daloso et al. 2014). Besides, transcription factors like Far-red Elongated Hypocotyl 3 (FHY3) and Far-red Impaired Response 1 (FAR1) have positive regulative role to chlorophyll biosynthesis since they can bind and activate the expression of HEMB (encodes ALAD) (Tang et al. 2012).

Effects of ALA on plant physiology and growth process

At present, ALA is not only a metabolic intermediate in botany, but also a growth regulator in plant cultivation. ALA is regarded as a plant growth promoting hormone since it was found to regulate growth and development of higher plants by many researchers. ALA regulates plant growth and development in many ways and shows a concentration-dependent manner which will be discussed in detail below (Table 1).

Table 1 Overview of ALA—regulated physiology and growth process in plants

Seed germination

The germination of seeds can be boosted by plant hormone (e.g. gibberellin) and signaling molecule (e.g. CO, H2S), resulting in the enhancement of germination percentage (Oracz et al. 2011; Amooaghaie et al. 2015; Wang et al. 2012). It is analogical that nearly all those stimulative factors are provided with dose-dependent manner. Equally, as a potential plant growth regulator, ALA also accelerates seed germination in a dose-dependent manner. Research showed that 1 mg/L ALA greatly promoted the final germination percentage of Elymus nutans seeds and enhanced the respiration under cold condition (5 °C), however, high level of ALA (25 mg/L) inhibited germination (Fu et al. 2014). Meanwhile, the different concentrations of ALA affected distinct germinating indices. Under low-temperature stress (15 °C), the final germination percentage of Capsicum annuum seeds reached the maxim under 25 mg/L ALA condition while 10 mg/L ALA treatment made the germination rate reach the highest value (Korkmaz and Korkmaz 2009). On the contrary, 0.5 mM ALA in germination medium prevented seed germination of Chinese cabbage (Brassica rapa L.) (Chon 2003).

Vegetative growth

Except for the promotive role it plays in seed germination, ALA also play effective role in plant growth. For example, ALA (30 mg/L) ameliorated the osmotic potential and relative water content (RWC) of oilseed rape (B. napus L.) by foliar applying to seedlings (Naeem et al. 2011). Exogenous ALA produced by Rhodopseudomonas palustris strains, one kind of purple nonsulfur bacteria which could secrete ALA (2.67 µM), distinctly increased the relative root growth and dry weight of rice under NaCl stress (Nunkaew et al. 2014). Moreover, 2 mg/L ALA applied in the germination stage of B. napus as a pretreatment, resulted in obviously enlarging on leaf length, leaf width, radical and hypocotyls length and root biomass under 100 mM Cd condition (Ali et al. 2013a). Besides, plant tissue culture is an indispensable way in plant science and it is not only for callus induction, rapid propagation (micropropagation) and chemical production, but it can also be applied in selecting and breeding of crops, in order to choose elite cultivars or improve resistance (Yukimune et al. 2000; Taghizadeh et al. 2015). The use of ALA, as a growth promoter, promoted callus induction and micropropagation of Vigna unguiculata L., callus propagation of Laminaria japonica and paclitaxel histological production of Taxus cuspidata (Bindu and Vivekanandan 1998; Tabuchi et al. 2009; Yamamoto et al. 2015). Moreover, seed potato breeding and the growth of tissue culture-derived Phoenix dactylifera L. seedling were promoted by ALA (Zhang et al. 2006; Awad 2008; Awad and Al-Qurashi 2011).

The application of exogenous ALA to higher plants revealed dose-effect, that is, relative high concentration of ALA usually caused damage to plants. For example, low concentration of ALA (0.05–0.5 mM) increased the growth of five cultivars of barley (H. vulgare L.) in different degrees, however, it retarded the growth of barley at a high level (5, 10 mM) (Kuk et al. 2003). Moreover, 8 mM ALA solution used for seed treatment inhibited plant height of seedling of alfalfa (Medicago sativa L.) (Chon 2003). White necrosis occurred on rice seedling leaves when the plants were sprayed with 5 mM ALA, and MDA content was increased (Phung and Jung 2014). The reason for high ALA concentration causing plant damage is that increase of endogenous ALA level will lead to up-regulation of photosensitive intermediates downstream of ALA metabolism, and over-accumulation of these will lead to photo-oxidation damage in plant tissue. This mechanism makes ALA function as a nontoxic herbicide for practical application in agriculture (Papenbrock and Grimm 2001; Sasaki et al. 2002; Dayan and Duke 2014). The growth of a major weed of rape, crickweed (Malachium aquaticum L.), was suppressed by 100 mM ALA which caused oxidative stress and chloroplast ultrastructure disorder (Xu et al. 2015). In addition, monocotyledon weed Setaria viridis and dicotyledon weed Ixeris dentate were sensitive to exogenous ALA, and the shoot fresh weight of these plants were significantly inhibited (Kuk et al. 2003).

Fruit coloring

The study of ALA in regulating the color of fruits is a relative new research area. According to studies, the application of ALA is beneficial to fruit coloration and maturity of higher plants. For example, the fresh weight, fruit color and °Brix value of berries were enhanced by foliage application of 100 mg/L ALA, at flowering period in a 2-year-old grapevines plants (Vitis vinifera L.) (Watanabe et al. 2006). During fruit growth and maturation, the coloring of pannexterna is due to anthocyanin biosynthesis and accumulation, which can be affected by light and plant hormone (including abscisic acid and naphthaleneacetic acid) (Jeong et al. 2004; Vimolmangkang et al. 2014; Li et al. 2016). Genes in ‘Fuji’ apple skin, including enzyme genes (Pal, Chs and Ufgt) and transcription factors (Myb, bHLH and Wd40) related to anthocyanin biosynthesis, were all up-regulated by ALA solution sprayed on the surface of fruits (Xie et al. 2013; Chagné et al. 2016). Moreover, application of exogenous ALA on developing fruit also improved the anthocyanin accumulation of apple (Malus × domestica Borkh.) and Litchi chinensis Sonn. peel (Chen et al. 2015; Feng et al. 2015). The molecular mechanism of ALA enhances anthocyanin accumulation in fruit skin might be an ALA-induced up-regulation of MdMADS1, a developmental transcription regulator of fruit ripening; because over-expressed MdMADS1 in apple (Malus × domestica Borkh.) calli resulted in increasing anthocyanin content (Feng et al. 2016). Moreover, in fruit of Prunus persica L. Batsch, six structural genes (CHS, CHI, F3H, DFR, LDOX and UFGT) and two transcription factors (MYB10 and WD40) involved in anthocyanin biosynthesis were all evidently upregulated by ALA treatment (200, 400 mg/L) (Ye et al. 2017).

Role of ALA in plants under abiotic stresses

The application of ALA against various abiotic stresses has been extensively reported. Details of the role of ALA against the adverse effects of herbicide, shade, cold, drought, salt and heavy metals is summarized in Table 2. However, the physiological mechanisms of ALA in stress tolerance have not been adequately reported.

Table 2 Overview of the promotive role of ALA under abiotic stress in plants

Heightened photosynthesis

Photosynthesis can be disorganized by environmental stresses which can lead to degradation of photosynthetic pigments, retardation of chlorophyll biosynthesis, reduction of light, change of gas-exchange characteristics, or inactivation of photosynthetic enzymes (Ashraf and Harris 2013). As a key precursor in the biosynthesis pathway of chlorophyll, ALA was reported to have promotive role in photosynthesis under stresses. For example, chlorophyll content was evidently increased by foliar application of ALA in leaves of pakchoi (Brassica campestris ssp. chinensis) and Ginkgo biloba plants (Memon et al. 2009; Feng et al. 2011). Abiotic stress caused damages to the configuration of chloroplastid in plants, including diminished chloroplast, swollen grana, dilations of the thylakoids, decreased starch and increased plastoglobules, etc (Paramonova et al. 2004; Ali et al. 2014a). Nevertheless, these damages were reversed through the application of ALA in B. napus, and chloroplast ultrastructures were recovered (Naeem et al. 2012; Gill et al. 2015). In addition, gas-exchange characteristics are important indexes for measuring plant photosynthetic capacity, which can be disordered under external stress generally (Piao et al. 2008). Under 100 mM NaCl stress, 200 mg/L ALA treated peach (P. persica L.) seedlings showed significantly ameliorated in net photosynthetic rate (Pn), stomatal conductance (Gs), transpiration rate (Tr) and intercellular CO2 concentration (Ci), the salt tolerance of peach plants was enhanced (Ye et al. 2016). Regardless of the application method used, ALA enhanced gas exchange capacity severally in date palm (P. dactylifera L.) and oilseed rape (B. napus L.) under salinity and cadmium stress (Youssef and Awad 2008; Ali et al. 2013b). Moreover, the application of ALA improved chlorophyll fluorescence parameters (including photochemical efficiency of photosystem II (Fv/Fm), photochemical efficiency (Fv′/Fm′), PSII actual photochemical efficiency (ΦPSII), and photochemical quench coefficient (qP)) in Cassia obtusifolia L. under salt stress were, but the non-photochemical quenching coefficient (NPQ) was decreased, indicating that photochemical activity of PSII can be repaired by ALA (Zhang et al. 2013). In addition, under Cd stress, the photosynthetic fluorescence parameters were improved by ALA application (Ali et al. 2015). In the presence of competitor (methyl viologen) and inhibitor (3-(3,4-dichlorophenyl)-1,1-dimethyl urea) for electron transport, 150 µM ALA treatment on strawberry (Fragaria ananassa Duch.) root could help leaves to keep relative high electron transfer efficiency in PSI (Sun et al. 2016). Indicating that the improvement of photosynthesis by ALA in plants was not only related to PSII, but also to PSI and electron transfer chain. In addition, the application of ALA results in increases in the content of endogenous ALA. As elucidated in the research of oilseed rape, exogenous ALA enhanced the tetrapyrrol biosynthesis pathway and the content of chlorophyll (Liu et al. 2016). The relative expression of upstream gene of ALA, GluTS, was up-regulated by exogenous ALA (30 mg/L) and contents of intermediates (Proto IX, Mg-Proto IX and Pchlide) were increased in B. napus L. under 200 mM NaCl stress (Xiong et al. 2018). Moreover, under 50 mM NaCl stress, spraying ALA on cucumber leaves significantly reversed the depression of chlorophyll biosynthesis, up-regulated the expression level of genes related to Mg-branch, including CHLH, POR and CAO (Wu et al. 2018). It indicated that the primary fluorescence of chlorophyll and the electron transfer rate of light harvesting pigment will be enhanced, and ultimately result in the promotion of photosynthesis in photosynthetical system. Besides, exogenous ALA can also benefit for carbon assimilation stage. For example, activities of ribulose-1,5-bisphosphate carboxylase (RuBPCase) and phosphoenolpyruvate carboxylase (PEPCase) in low-temperature (14/5 °C, day/night) treated maize (Z. mays L.) seedlings were enhanced through spraying 0.15 mM ALA (Wang et al. 2018).

Amended ions and nutrients uptake

Abiotic stresses lead to ion and nutrient imbalance in plants in general, under higher saline condition, there must have lower osmotic potential in rhizosphere environment, which breaks the original ionic equilibrium in plants (Zhu 2001). Salinity increases the concentration of Na+ and Cl in plant tissue but the content of K+ and Ca2+ reduces. However, the application of ALA could restore the ionic balance, in sunflower (Helianthus annuus L.) and B. napus (Akram and Ashraf 2011; Naeem et al. 2012). Under normal growth condition, plants usually keep a relatively higher K+/Na+, while high extracellular Na+ concentration will bring a large Na+ electrochemical potential gradient and then cause ion stress (Su et al. 2015). Exogenesis ALA alleviated the salinity stress of creeping bentgrass and promoted its growth and organic acids accumulation, which was mainly due to the suppression of ion toxicity caused by Na+ (Yang et al. 2014). Application of ALA increased P content in seeds and leaves under NaCl stress in date palm (P. dactylifera L.) and sunflower (Awad and Al-Qurashi 2011; Akram et al. 2011). Moreover, under sodium and lead toxicity, absorption of other macro- and micro-elements including Ca, Mg, Mn and Cu was significantly improved by ALA, and the stress-induced damages were ameliorated (Naeem et al. 2010; Liu et al. 2014; Ali et al. 2014b). Furthermore, nutritive material uptake, such as sulfate or nitrate, could be strengthened by application of ALA. Nitrogen metabolism of watermelon seedling, which was affected by salinity, could be regulated through by ALA which significantly increased the activities of nitrate reductase (NR), glutamine synthetase (GS), glutamate synthase (GOGAT), and glutamate dehydrogenase (GDH) (Chen et al. 2017). In plants under nutrient deficiency condition, applied-ALA increased the transcription and translation level of NR or SULTR (a gene related to sulfur transport assimilation) (Maruyama-Nakashita et al. 2010; Wei et al. 2012; Beyzaei et al. 2014).

Coupled with the biosynthesis of siroheme discussed above, a hypothesis can be set here that nitrogen and sulfur uptake might be strengthened by increasing level of siroheme through application of ALA, since siroheme is a crucial part of accessorial factor in reduction actions of nitrate and sulfate in plants. This can be a potential justification for studying the regulative mechanism of ALA towards nutrition uptake of plants.

Enhanced antioxidant defense system

The production of ROS in plants at typical growth condition is a normal physiological phenomenon, and ROS, to some extent, have connection with cell proliferation and differentiation (Gechev and Hille 2012). But in the process of stress, more ROS will appear and accumulate in chloroplast, mitochondria, and peroxisome, then cause damage to carbon–carbon double bond (s) of polyunsaturated fatty acids (PUFAs) in membrane lipid, and bring a secondary product, malondialdehyde (MDA), which is also an important index to determine degree of peroxidation (Choudhury et al. 2016; Dietz et al. 2016; Huang et al. 2016; Rodríguez-Serrano et al. 2016; Ayala et al. 2014). Treatments with ALA suppressed the increasing of H2O2 and MDA (or TBARS) under abiotic stress by enhancing enzymes activities of antioxidant defense system (Balestrasse et al. 2010; Ahmad et al. 2012). Under shade stress and Cd toxicity, the activities of superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) were increased in watermelon seedlings and oilseed rape by ALA (Sun et al. 2009; Ali et al. 2013a). In the research of rice seedlings under cold stress (10 °C), the activities of SOD, POD, ascorbate peroxidase (APX) and glutathione peroxidase (GPX) were boosted by soaking ALA (8.5 mM) during seed germination (Sheteiwy et al. 2017). Studies toward genome level suggested that 1 mg/L ALA up-regulated the expression of POD, CAT, APX of oilseed rape (B. napus L.) under water-deficit condition and cucumber (C. sativus L.) seedlings under NaCl condition (Liu et al. 2011; Zhen et al. 2012). Another antioxidant mechanism which mainly scavenges H2O2 in plants is known as ascorbate–glutathione (AsA–GSH) cycle, consists of enzymes and non-enzymatic antioxidants (Li et al. 2010). The application of ALA could enhance reduced/oxidized glutathione ratio (GSH/GSSG) and reduced/oxidized ascorbic acid ratio (AsA/DHA) via strengthening the activities of glutathione reductase (GR) and dehydroascorbate reductase (DHAR) (Nishihara et al. 2003; Li et al. 2011; Liu et al. 2018).

Besides, it is feasible to relate these results with one of the branches of ALA metabolic pathway, which is the Fe-branch. For the oxidation resisting role of BR, a decomposition product of heme, it could inhibit protein oxidation in vitro in the presence of a variety of oxidants including superoxide and hydroxyl radicals (Wegiel et al. 2014; Xie et al. 2015). The heme content was increased significantly by ALA in maize (Z. mays L.) under non-stressful condition (Yonezawa et al. 2015). Moreover, in a research of transgenic rice, which overexpressed the FECH gene of Bradyrhizobium japonicum, resulted in increasing activity of FECH, raising content of heme and enhancing tolerance of oxidative stress (Kim et al. 2014). Therefore, this may provide a kind of new thought to explain the promotive roles of ALA towards oxidative stress resistance.

Promoted osmoregulation

ALA pretreatment before waterlogging stress promoted RWC in Ficus carica Linn. leaves (An et al. 2016). In spring wheat (Triticum aestivum L.) under drought stress, content of an osmotic adjustment substance, glycine betaine (GB), increased by foliar application of ALA (Kosar et al. 2015). Also, contents of proline, soluble sugar and soluble protein increased in plants treated with ALA and improved tolerance against salinity or heat induced osmotic stress (Zhang et al. 2012; Akram et al. 2012). Furthermore, amylase activity and expression of RsBAMY1 protein were up-regulated by ALA in radish taproot (Raphanus sativus L.); it might suggest that ALA increased osmotic adjustment substances by strengthening starch-degrading enzymes (Hara et al. 2011). What was interesting was the considerably accumulation of proline and inhibition of endogenous ALA biosynthesis under severe NaCl stress. Since proline and ALA have a common precursor, glutamate, in both of their synthetic routes, the researchers conjectured that the metabolic pathway of glutamic acid may converted from ALA-synthesizing to the proline synthesis pathway, which enhanced the proline accumulation against osmotic disturbance (Averina et al. 2010).

Beyond the biosynthesis of proline, enhanced photosynthesis will produce more carbohydrate in plant, and the decomposition of carbohydrates will provide numerous osmotic adjustment substances (such as soluble sugar) and energy to react against abiotic stresses.

Manipulation of relative genes in ALA pathway

In recent years, molecular breeding has become more popular in plant research to promote and modify crops. In the light of the regulative role of exogenous ALA in plants response to various environmental stresses, ALA-related genes manipulation could theoretically regulate the tetrapyrrol biosynthesis or enhance plant stress tolerance.

The structure and sequence of HEMA, which encodes 5-aminolevulinic acid synthase (ALAS), was first studied in B. japonicum in 1987 (McClung et al. 1987). However, few reports are available on transgenic plants with manipulative genes of ALA (Table 3). ALAS gene from B. japonicum overexpressed in Oryza sativa showed an increase of contents of ALA, Proto IX, and protochlorophyllide, and then caused photodynamic damage (Jung et al. 2004b, 2008a). Similarly, A. thaliana mutant with overexpressed HEMB1, which encodes ALAD, revealed dysgenesis, and when knocked down the endogenous HEMB1 expression in wild type, the seedling cotyledons turned white or pale since they could not synthetize chlorophyll effectively (Tang et al. 2012). Human mitochondrial PPOX gene (PPO) overexpressed in transgenic rice plants revealed severe necrotic spots on leaves and growth retardation since tetrapyrrole overaccumulated in the plant (Jung et al. 2008b). Activity of PPOX in transgenic tobacco (N. tabacum L.) overexpressed plastid PPOX of Arabidopsis was enhanced and was not affected by 300 nM acifluorfen stress, this indicated that toxic tetrapyrrole was metabolized efficiently under herbicide stress (Lermontova and Grimm 2000). Seeds of transgenic rice with overexpressing PPOX gene from Myxococcus xanthus protox germinated normally under 500 µM oxyfluorfen stress but the wildtype seeds could not germinate under 1 µM oxyfluorfen (Jung et al. 2004a). Transgenic rice plants with overexpressed M. xanthus PPOX gene were characterized by more stable Fv/Fm, lower concentration of H2O2 and less MDA compared with the wild-type under incubated oxyfluorfen stress (0.1–10 µmol/L), and this indicated that the transgenic lines of rice had enhanced resistance (Jung and Back 2005). Transgenic Arabidopsis plants with yeast ALAS gene (YHem1) showed higher resistance when exposed to NaCl stress, their germination, growth, chlorophyll and heme contents were much higher than the wild type (Zhang et al. 2010). Besides, in the study of transgenic rice, which overexpressed B. japonicum FECH gene, resulted in increasing activity of FECH, then raising content of heme and enhancing the tolerance of oxidative stress (Kim et al. 2014).

Table 3 Genetic manipulation towards ALA-related genes in plants

Besides, in microbial production field, ALA extracellular accumulation of Escherichia coli was actualized by inserting HEMA gene from B. japonicum (Choi et al. 1999). In addition, the up-regulation of HEMD and HEMF (encoding UROS and CPOX respectively) were propitious to ALA accumulation in E. coli (Zhang et al. 2015a).

Conclusion and future perspective

The yield reduction of crops and damage caused to plants as a result of climate change, which makes amelioration and improvement of stress tolerance in agricultural plants more important and an urgent issue in the twenty-first century. As an intermediate in vivo, ALA can be used in medical and agricultural fields (Fig. 3). Exogenous application of ALA is a relatively new among plenty of hormones, regulators or small signaling molecules. As a nontoxic compound, ALA can be synthesized in the metabolic pathway of plants, and as a common precursor of all tetrapyrrole, including chlorophyll, heme and siroheme (Fig. 3). Moreover, as an intermediary substance, the content of ALA affect metabolic pathway downstream and finally impact the outcomes. Simultaneously, feed-back regulation of downstream products regulates the biosynthesis of ALA in an opposite manner. There are regulatory factors on the pathway, including positive factors, like GUN4 protein (acts on MCH) and FHY3/FAR1 protein (acts on ALAD); then negative factor, like FLU protein (acts on GluTR). However, the mechanism of feed-back regulation of heme towards ALA has not been elucidated. Thus, it is a viable area for research. Additionally, the control effect or balance regulation between branches of ALA downstream metabolism, like Fe-branch and Mg-branch, has not been adequately studied.

Fig. 3
figure 3

Overview for biosynthesis, metabolism and application of ALA in medical and agricultural fields. The biosynthetic ways of ALA and its metabolic productions are shown (orange boxes). Moreover, beneath ALA are the potential roles of the metabolic productions could involve in physiological process of plants. In addition, the application aspects of ALA in medical and agricultural fields are shown (blue boxes). (Color figure online)

ALA does not only regulate plant growth and development (for example seed germination, vegetative growth and fruit coloring of crops) under non-stressful condition, but also helps improves plants resistance against abiotic stresses by regulating their photosynthesis, capacity of nutrients uptake, antioxidant defense system and osmoregulation directly or indirectly. The great majority of existing researches on regulative role of ALA in plants focused on the physiological effects rather than the molecular mechanism. Further studies are recommended to investigate the mechanism at molecular level and to interpret the ameliorative role of ALA to various plant physiological and growth process in depth. In addition, there is another perspective that studies of the regulative mechanism of ALA could associated with its biosynthesis and metabolism pathway, which has barely received some research attention (Fig. 3).

As a plant growth regulator, there is a little research about interaction between ALA and other plant hormone. Therefore, this is a promising research point.

Transgenic breeding using genetic engineering methods is a powerful way to reform plant genome and modified crops for specific purposes. Therefore, application of genetic manipulation toward ALA biosynthesis and metabolic pathways to regulate and control the production of endogenous ALA or other intermediate materials can be a feasible way to enhance plant growth under stressful or non-stressful environment. Nevertheless, there are few researches in this area, and fewer of these studies succeeded in strengthening plant resistance to abiotic stresses. Therefore, studies in genetic engineering of ALA role in enhancing the stress tolerance of plants and in promoting plant growth are promising and require research attention.