3.1. External Exclusion Mechanisms

In the Al tolerance mechanism, plants can secrete OAs, such as citrate, malate, and oxalate, from roots under Al stress [3,26,52]. Accumulating evidence has shown that citrate is more dominant than malate and oxalate in response to Al stress [3]. Al-induced secretion of OAs is mediated through anion channels or transporters [53]. To date, Al-activated malate transporter (ALMT) and multidrug and toxic compound extrusion (MATE) have been identified [5,16,17,27]. Since Sasaki et al. identified TaALMT1 from wheat [54], it has been discovered that ALMT is involved in Al-induced secretion of malate to alleviate Al toxicity in Arabidopsis [55,56], barley (Hordeum vulgare) [57], rape (Brassica napus) [58,59], maize (Zea mays) [60], Yorkshire fog (Holcus lanatus) [61], Camelina (Camelina sativa) [62], rubber tree (Hevea brasiliensis) [63], and cabbage (Brassica oleracea) [31], etc. Interestingly, TaALMT1 also plays a role in alkaline tolerance by promoting exudation of both malate and gamma-aminobutyric acid (GABA) in wheat. In addition, MATEs, which are well characterized as multidrug transporters, can transport various substrates, such as citrate, secondary metabolites, and plant hormones, through electrochemical cation gradients [64]. In recent years, MATE genes have been identified from various plant species, including Arabidopsis [65], cabbage [66], Brachypodium distachyon [32], eucalyptus (Eucalyptus camaldulensis) [67], buckwheat (Fagopyrum esculentum) [68], soybean (Glycine max) [33,69,70], Rubiaceae (Psychotria rubra) [71], sorghum (Sorghum bicolor) [72], rice bean (Vigna umbellata) [73,74,75,76], maize [77,78], wheat [36], wild soybean (Glycine soja) [34], etc. The difference in cell location and Al-induced expression patterns of MATE transporters have been determined. For instance, FeMATE1 localized on the plasma membrane (PM), is specifically expressed in roots, whereas FeMATE2 located on the Golgi membrane, is expressed in both the roots and the leaves [68]. Al stress could induce PtrMATE1 expression in 12 h, in contrast to the expression pattern of PtrMATE2, which occurred 24 h after Al³⁺ treatment [35]. VuMATE1 was expressed at 6 h after Al stress, whereas VuMATE2 was expressed at the early stage of Al stress [73,76]. However, most plants that adopt external exclusion need to protect the root tip from Al toxicity directly on the root surface and ensure the dynamic balance of plant essential nutrients [79]. Al is the most abundant metal element in the earth’s crust; plants cannot alleviate all Al toxicity in acidic soil but can neutralize a portion around the root tip. Therefore, the first reaction of plants to Al toxicity is to prevent the Al³⁺ around the root tip from entering the root cells, which may be the main target of OAs [80].

The transmembrane transport of OAs depends on the driving force formed by the proton electrochemical gradient on both sides of the PM, which provides continuous power for organic acid secretion [81,82]. Al stress increases the activity of PM H⁺-ATPase and inhibits the expression of VHA-a2, VHA-a3, and vacuolar proton pump activity, and therefore increases the proton gradient on both sides of the PM and promotes OAs secretion by AtALMT1 and AtMATE [83]. Al stress reduces the entry of OAs into vacuoles by inhibiting vacuolar proton pump activity and activating the PM transport system, which secretes large amounts of intracellular OAs to the apoplast. While OAs secretion is blocked, vacuolar proton pump activity is activated, indicating that intracellular Al tolerance may be an alternative [83]. Additionally, it is known that OAs secretion depends on the maintenance of internal concentrations of OAs. Zhou et al. identified a cytosolic NADP-malic enzyme, GmME1, which was implicated in the organic acid pool and confers higher Al resistance by increasing internal malate and citrate concentrations and their external efflux [40]. Recently, Sun et al. found that the level of the MsCS transcript was higher in Al tolerant cultivar as compared with the Al sensitive cultivar and the activity of citrate synthase (CS) affected Al resistance through citrate concentration and exudation in alfalfa cultivar [43]. These results suggest that OAs secretion can be regulated through the activation of OA transporters, PM H⁺-ATPase activity, and improvement of the internal OAs pool.

In addition to the secretion of OAs, the release of phenols has been verified to have the capacity to detoxify Al through chelating Al³⁺ ions. For example, Al and silicon (Si) trigger the release of catechol, catechins, and quercetin by root tips, which could potentially detoxify Al [84]. Ma et al. also found that phenolic compounds were involved in coping with Al toxicity in the Chinese fir by comparative transcriptome [85]. Chen et al. reported the enhancement of polyphenolic metabolism as an adaptive response to Al stress in lettuce roots [86]. Recently, the findings of Fu et al. indicated that Al-polyphenol complexes improved Al resistance in tea plants [87]. Therefore, a comprehensive exploration of the roles of phenolic compounds under Al stress might be an ideal target for genetic engineering in the future.

The CW is the first barrier for plants to resist external stress and has been recognized as the major target of Al toxicity; plants can alleviate Al stress by modifying the CW [88]. For example, OsSTAR1 interacts with OsSTAR2 to form an ATP-binding cassette (ABC) transporter complex, which is specifically responsible for transporting uridine diphosphate (UDP)-glucose and modifying CWs in rice [89]. Similarly, FeSTAR1 and FeSTAR2 also form an ABC transporter complex, which participates in Al tolerance through the CW matrix polysaccharide metabolism in buckwheat [37,90]. Recently, Fan et al. showed that abscisic acid (ABA) alleviation of Al toxicity in rice beans depends on ABI5-mediated CW modification and osmoregulation [91]. More recently, Liu et al. identified a 4-coumarate, which influences Al resistance through the modification the CW [46]. Moreover, pectin methylesterase (PME) plays an important role in Al tolerance by regulating the degree of pectin esterification in the CW, which converts highly methylated pectin into a negatively charged demethylation form, resulting in more Al³⁺ binding to pectin [92]. Overexpression of OsPME14 accumulated the content of Al in the root tip CW and increased its sensitivity to Al in rice [93]. Furthermore, it was found that Al-tolerant varieties showed higher methylated pectin ratio and lower PME activity in rice, corn, and buckwheat [13,94,95]. Therefore, Al-resistant crop varieties can be cultivated by reducing the pectin content of plant root tips or increasing the degree of methylation of pectin.

Additionally, rhizosphere alkalization is one of Al tolerance mechanisms by reducing the solubility of Al [96]. For example, Yang et al. showed that elevated pH from 2.5 to 4.0, alleviates the Al-toxicity of Citrus by increasing root accumulation of malate and citrate [97,98]. Further experiment showed that raised pH alleviates Al-induced a decrease in total soluble protein level and an increase in electrolyte leakage in citrus leaves and roots by conferring the ability to maintain a balance between production and detoxification of ROS and methylglyoxal [92]. Moreover, Liu et al.’s study on wild barley showed that low pH and higher Al tolerance in XZ16 were associated with a higher ability of H⁺ uptake and rhizospheric alkalization [99]. Magnesium promotes the elevated root surface pH regulation in Populus, resulting in root surface alkalization in the transition zone, which alleviates the toxicity of high concentration Al [100].

3.2. Internal Tolerance Mechanisms

Internal tolerance mechanisms are mainly involved in Al detoxification, uptake, translocation, and accumulation of non-phytotoxic complexes of Al in the various organs [101,102]. For instance, buckwheat, hydrangea, and tea can accumulate a large amount of Al in the aboveground parts via transporting nontoxic Al complexes [103,104,105]. Several transporters have been reported to be involved in the absorption, sequestration, and transportation of Al from roots to aboveground parts in plants. In rice, OsNrat1, a PM-located transporter, belongs to the natural resistance-associated macrophage protein (Nramp) and has low similarity with other Nramp members, which specifically transports Al³⁺ rather than bivalent metals (Mn²⁺, Fe²⁺, and Cd²⁺) [102]. Bioinformatics suggest that the Ala-Ile-Ile-Thr element is the key determinant of Nrat1 for Al selectivity [106]. In Arabidopsis, AtALS3 acts as an Al transporter to redistribute Al outside sensitive tissues [107]. Moreover, OsALS1, an ABC transporter located in the tonoplast, sequestrates Al³⁺ in the vacuole. The knockout of OsALS1 leads the high sensitivity of rice to Al stress [26]. FeALS1.1 and FeALS1.2, OsALS1 homologs, can also sequestrate Al³⁺ in the vacuoles and detoxify Al in the roots and leaves in buckwheat [108]. Likewise, the AvABCG1 transporter confers Al tolerance by accumulating Al³⁺ in specific areas of Andropogon virginicus [109]. HvABCB25 transports Al from the cytoplasm to the vacuoles for sequestration in barley [42]. In addition, HmPALT1 and HmVALT1 are involved in transporting Al in hydrangea (Hydrangea macrophylla) [105]. The mechanism of Al detoxification in plants is achieved by forming non-phytotoxic complexes of small molecular organic compounds with Al³⁺ [10,27,52]. NIP1;2, the closest homolog to HmPALT1, facilitates the transport of Al-malate from the CW to the symplast in Arabidopsis [110]. NIP1;2-mediated transport of Al-malate complex depends on Al-induced malate secretion mediated by AtALMT1. Therefore, the coordinated operation of the Al detoxification mechanism between external and internal parts of the plants is linked by NIP1;2 and AtALMT1.

Studies have showed that the transportation of other ions could also alleviate Al toxicity [19]. For example, Ca²⁺ serves as an essential second messenger to modulate developmental plasticity in plants, which reduces the concentration of active Al and fixation of P under Al stress [111,112]. Treatment with higher Ca²⁺ concentration alleviates Al-induced inhibition of root growth, which is attributed to higher cytosolic Ca²⁺ concentrations through specific Ca²⁺ signatures triggering downstream responses [113]. Moreover, Mg²⁺ is involved in metabolism-activating enzymes such as CS and malate synthase by functioning as a cofactor for enzymes, thereby activating OA synthesis to alleviate Al toxicity [114]. Overexpression of AtMGT1 and OsMGT1 confers Al tolerance in plants by increasing the absorption of Mg and inhibiting potential targets of Al [115]. Li et al. showed that Mg promoted root growth and increased Al tolerance by modulating the production of nitric oxide in Arabidopsis [116]. Similarly, Kong et al. showed that the addition of Mg to the Al treatment solution alleviated Al-induced inhibition of root growth, suppressed Al uptake, and reduced hydrogen peroxide (H₂O₂) concentration in maize [99]. Furthermore, K⁺ efflux was related to Al tolerance by accompanying OA secretion [99]. Recently, Li et al. showed that ectopic expression of either HtNHX1 or HtNHX2, from Jerusalem artichoke (Helianthus tuberosus), could enhance rice tolerance to Al stress and soil acidity by altering K⁺ and H⁺ fluxes and the CW structure [41]. In addition, the application of zinc has been shown to alleviate Al-induced damage via competing with Al and increasing the IAA content in alfalfa [46]. Sulfate supplementation activates short-term tolerance to Al toxicity in perennial ryegrass (Lolium perenne) roots by upregulating total superoxide dismutase (SOD) activity [117]. Further studies are required to investigate how the adjustment of the formula of fertilizer may alleviate Al toxicity.

Analogous to other abiotic stimuli, Al stress induces the overproduction of ROS and lipid peroxidation, resulting in serious cell damage and even cell death [118]. To protect plants from Al-triggered oxidative stress, plant tolerance to Al toxicity is enhanced by improving the activity of ROS-scavenging enzymes, reducing the production of ROS, and weakening lipid peroxidation [119]. For example, overexpression of WMnSOD1, an Al-induced SOD, increases oxidative resistance, and Al tolerance [120]. Overexpression of AtBCB and NtGDI1 ameliorates oxidative stress and confers a degree of resistance to Al stress [121]. Overexpression of AtPrx64 reduces the accumulation of ROS and Al, thereby promoting root growth [29]. Recently, ZmAT6, a chloroplast-located protein, has been shown to increase the expression level of the ZmSOD gene and improve the activity of antioxidant enzymes SOD in the antioxidant enzymatic system. In addition, the overexpression of ZmAT6 in maize and Arabidopsis increased the activity of several enzymes within the antioxidant system, thereby enhancing Al toxicity tolerance [50]. Moreover, methyltransferase could reduce ROS, lipid peroxidation, and ion leakage, and overexpression of the methyltransferase gene can improve stress resistance of plants [122]. For example, overexpression of ZjOMT enhanced Al tolerance of Escherichia coli by increasing the content of melatonin [49]. In addition, a metabolic change is an internal tolerance mechanistic in response to Al stress [48]. Overexpression of VuFDH increased Al tolerance, which is likely due to their decreased Al-induced formate production in tobacco (Nicotiana tabacum) [48]. Likewise, VuAAE3 played a critical role in Al tolerance mechanisms via function as oxalyl-CoA synthetase [47].