Recent Advances in Understanding Mechanisms of Plant Tolerance and Response to Aluminum Toxicity

MsWRKY44 regulates Mg-K homeostasis of shoots and promotes alfalfa sensitivities to acid and Al stresses

Mg-K homeostasis is essential for plant response to abiotic stress, but its regulation remains largely unknown. MsWRKY44 cloned from alfalfa was highly expressed in leaves and petioles. Overexpression of it inhibited alfalfa growth, and promoted leaf senescence and alfalfa sensitivities to acid and Al stresses. The leaf tips, margins and interveins of old leaves occurred yellow spots in MsWRKY44-OE plants under pH4.5 and pH4.5 +Al conditions. Meanwhile, Mg-K homeostasis was substantially changed with reduction of K accumulation and increases of Mg as well as Al accumulation in shoots of MsWRKY44-OE plants. Further, MsWRKY44 was found to directly bind to the promoters of MsMGT7 and MsCIPK23, and positively activated their expression. Transiently overexpressed MsMGT7 and MsCIPK23 in tobacco leaves increased the Mg and Al accumulations but decreased K accumulation. These results revealed a novel regulatory module MsWRKY44-MsMGT7/MsCIPK23, which affects the transport and accumulation of Mg and K in shoots, and promotes alfalfa sensitivities to acid and Al stresses.

Effects of aluminum (Al) stress on the isoprenoid metabolism of two Citrus species differing in Al-tolerance

Isoprenoid metabolism and its derivatives took part in photosynthesis, growth regulation, signal transduction, and plant defense to biotic and abiotic stresses. However, how aluminum (Al) stress affects the isoprenoid metabolism and whether isoprenoid metabolism plays a vital role in the Citrus plants in coping with Al stress remain unclear. In this study, we reported that Al-treatment-induced alternation in the volatilization rate of monoterpenes (α-pinene, β-pinene, limonene, α-terpinene, γ-terpinene and 3-carene) and isoprene were different between Citrus sinensis (Al-tolerant) and C. grandis (Al-sensitive) leaves. The Al-induced decrease of CO2 assimilation, maximum quantum yield of primary PSII photochemistry (Fv/Fm), the lower contents of glucose and starch, and the lowered activities of enzymes involved in the mevalonic acid (MVA) pathway and 2-C-methyl-D-erythritol 4-phosphate (MEP) pathway might account for the different volatilization rate of isoprenoids. Furthermore, the altered transcript levels of genes related to isoprenoid precursors and/or derivatives metabolism, such as geranyl diphosphate (GPP) synthase (GPPS) in GPP biosynthesis, geranylgeranyl diphosphate synthase (GGPPS), chlorophyll synthase (CHS) and GGPP reductase (GGPPR) in chlorophyll biosynthesis, limonene synthase (LS) and α-pinene synthase (APS) in limonene and α-pinene synthesis, respectively, might be responsible for the different contents of corresponding products in C. grandis and C. sinensis. Our data suggested that isoprenoid metabolism was involved in Al tolerance response in Citrus, and the alternation of some branches of isoprenoid metabolism could confer different Al-tolerance to Citrus species.

GmMYB183, a R2R3-MYB Transcription Factor in Tamba Black Soybean (Glycine max. cv. Tamba), Conferred Aluminum Tolerance in Arabidopsis and Soybean

Aluminum (Al) toxicity is one of the environmental stress factors that affects crop growth, development, and productivity. MYB transcription factors play crucial roles in responding to biotic or abiotic stresses. However, the roles of MYB transcription factors in Al tolerance have not been clearly elucidated. Here, we found that GmMYB183, a gene encoding a R2R3 MYB transcription factor, is involved in Al tolerance. Subcellular localization studies revealed that GmMYB183 protein is located in the nucleus, cytoplasm and cell membrane. Overexpression of GmMYB183 in Arabidopsis and soybean hairy roots enhanced plant tolerance towards Al stress compared to the wild type, with higher citrate secretion and less Al accumulation. Furthermore, we showed that GmMYB183 binds the GmMATE75 gene promoter encoding for a plasma-membrane-localized citrate transporter. Through a dual-luciferase reporter system and yeast one hybrid, the GmMYB183 protein was shown to directly activate the transcription of GmMATE75. Furthermore, the expression of GmMATE75 may depend on phosphorylation of Ser36 residues in GmMYB183 and two MYB sites in P3 segment of the GmMATE75 promoter. In conclusion, GmMYB183 conferred Al tolerance by promoting the secretion of citrate, which provides a scientific basis for further elucidating the mechanism of plant Al resistance.

Overexpression of ZmSTOP1-A Enhances Aluminum Tolerance in Arabidopsis by Stimulating Organic Acid Secretion and Reactive Oxygen Species Scavenging

Aluminum (Al) toxicity and low pH are major factors limiting plant growth in acidic soils. Sensitive to Proton Rhizotoxicity 1 (STOP1) transcription factors respond to these stresses by regulating the expression of multiple Al- or low pH-responsive genes. ZmSTOP1-A, a STOP1-like protein from maize (Zea mays), was localized to the nucleus and showed transactivation activity. ZmSTOP1-A was expressed moderately in both roots and shoots of maize seedlings, but was not induced by Al stress or low pH. Overexpression of ZmSTOP1-A in Arabidopsis Atstop1 mutant partially restored Al tolerance and improved low pH tolerance with respect to root growth. Regarding Al tolerance, ZmSTOP1-A/Atstop1 plants showed clear upregulation of organic acid transporter genes, leading to increased organic acid secretion and reduced Al accumulation in roots. In addition, the antioxidant enzyme activity in roots and shoots of ZmSTOP1-A/Atstop1 plants was significantly enhanced, ultimately alleviating Al toxicity via scavenging reactive oxygen species. Similarly, ZmSTOP1-A could directly activate ZmMATE1 expression in maize, positively correlated with the number of Al-responsive GGNVS cis-elements in the ZmMATE1 promoter. Our results reveal that ZmSTOP1-A is an important transcription factor conferring Al tolerance by enhancing organic acid secretion and reactive oxygen species scavenging in Arabidopsis.

Weed Species from Tea Gardens as a Source of Novel Aluminum Hyperaccumulators

Increased availability of toxic Al³⁺ is the main constraint limiting plant growth on acid soils. Plants adapted to acid soils, however, tolerate toxic Al³⁺, and some can accumulate Al in their aerial parts to a significant degree. Studies on Al-tolerant and Al-accumulating species have mainly focused on the vegetation of acid soils distributed as two global belts in the northern and southern hemispheres, while acid soils formed outside these regions have been largely neglected. The acid soils (pH 3.4-4.2) of the tea plantations in the south Caspian region of Northern Iran were surveyed over three seasons at two main locations. Aluminum and other mineral elements (including nutrients) were measured in 499 plant specimens representing 86 species from 43 families. Al accumulation exceeding the criterion for accumulator species (>1000 µg g^-1 DW) was found in 36 species belonging to 23 families of herbaceous annual or perennial angiosperms, in addition to three bryophyte species. Besides Al, Fe accumulation (1026-5155 µg g^-1 DW) was also observed in the accumulator species that exceeded the critical toxicity concentration, whereas no such accumulation was observed for Mn. The majority of analyzed accumulator plants (64%) were cosmopolitan or pluriregional species, with a considerable rate of Euro-Siberian elements (37%). Our findings, which may contribute to phylogenetic studies of Al accumulators, also suggest suitable accumulator and excluder species for the rehabilitation of acid-eroded soils and introduce new model species for investigating Al accumulation and exclusion mechanisms.

Biochar-Dual Oxidant Composite Particles Alleviate the Oxidative Stress of Phenolic Acid on Tomato Seed Germination

Phenolic acid is a well-known allelochemical, but also a pollutant in soil and water impeding crop production. Biochar is a multifunctional material widely used to mitigate the phenolic acids allelopathic effect. However, phenolic acid absorbed by biochar can still be released. In order to improve the removal efficiency of phenolic acids by biochar, the biochar-dual oxidant (BDO) composite particles were synthesized in this study, and the underlying mechanism of the BDO particles in ameliorating p-coumaric acid (p-CA) oxidative damage to tomato seed germination was revealed. Upon p-CA treatment, the BDO composite particles application increased the radical length, radical surface area, and germination index by 95.0%, 52.8%, and 114.6%, respectively. Compared to using biochar or oxidants alone, the BDO particles addition resulted in a higher removal rate of p-CA and produced more O₂^•-, HO^•, SO₄^•- and ¹O₂ radicals via autocatalytic action, suggesting that BDO particles removed phenolic acid by both adsorption and free radical oxidation. The addition of BDO particles maintained the levels of the antioxidant enzyme activity close to the control, and reduced the malondialdehyde and H₂O₂ by 49.7% and 49.5%, compared to the p-CA treatment. Integrative metabolomic and transcriptomic analyses revealed that 14 key metabolites and 62 genes were involved in phenylalanine and linoleic acid metabolism, which increased dramatically under p-CA stress but down-regulated with the addition of BDO particles. This study proved that the use of BDO composite particles could alleviate the oxidative stress of phenolic acid on tomato seeds. The findings will provide unprecedented insights into the application and mechanism of such composite particles as continuous cropping soil conditioners.

GOCompare: An R package to compare functional enrichment analysis between two species

Functional enrichment analysis is a cornerstone in bioinformatics as it makes possible to identify functional information by using a gene list as source. Different tools are available to compare gene ontology (GO) terms, based on a directed acyclic graph structure or content-based algorithms which are time-consuming and require a priori information of GO terms. Nevertheless, quantitative procedures to compare GO terms among gene lists and species are not available. Here we present a computational procedure, implemented in R, to infer functional information derived from comparative strategies. GOCompare provides a framework for functional comparative genomics starting from comparable lists from GO terms. The program uses functional enrichment analysis (FEA) results and implement graph theory to identify statistically relevant GO terms for both, GO categories and analyzed species. Thus, GOCompare allows finding new functional information complementing current FEA approaches and extending their use to a comparative perspective. To test our approach GO terms were obtained for a list of aluminum tolerance-associated genes in Oryza sativa subsp. japonica and their orthologues in Arabidopsis thaliana. GOCompare was able to detect functional similarities for reactive oxygen species and ion binding capabilities which are common in plants as molecular mechanisms to tolerate aluminum toxicity. Consequently, the R package exhibited a good performance when implemented in complex datasets, allowing to establish hypothesis that might explain a biological process from a functional perspective, and narrowing down the possible landscapes to design wet lab experiments.

Flavonol-Aluminum Complex Formation: Enhancing Aluminum Accumulation in Tea Plants

Polyphenol-rich tea plants are aluminum (Al) accumulators. Whether an association exists between polyphenols and Al accumulation in tea plants remains unclear. This study revealed that the accumulation of the total Al and bound Al contents were both higher in tea samples with high flavonol content than in low, and Al accumulation in tea plants was significantly and positively correlated with their flavonol content. Furthermore, the capability of flavonols combined with Al was higher than that of epigallocatechin gallate (EGCG) and root proanthocyanidins (PAs) under identical conditions. Flavonol-Al complexes signals (94 ppm) were detected in the tender roots and old leaves of tea plants through solid-state ²⁷Al nuclear magnetic resonance (NMR) imaging, and the strength of the signals in the high flavonol content tea samples was considerably stronger than that in the low flavonol content tea samples. This study provides a new perspective for studying Al accumulation in different tea varieties.

Elevated pH-mediated mitigation of aluminum-toxicity in sweet orange (Citrus sinensis) roots involved the regulation of energy-rich compounds and phytohormones

For the first time, we used targeted metabolome to investigate the effects of pH-aluminum (Al) interactions on energy-rich compounds and their metabolites (ECMs) and phytohormones in sweet orange (Citrus sinensis) roots. The concentration of total ECMs (TECMs) was reduced by Al-toxicity in 4.0-treated roots, but unaffected significantly in pH 3.0-treated roots. However, the concentrations of most ECMs and TECMs were not lower in pH 4.0 + 1.0 mM Al-treated roots (P4AR) than in pH 3.0 + 1.0 mM Al-treated roots (P3AR). Increased pH improved the adaptability of ECMs to Al-toxicity in roots. For example, increased pH improved the utilization efficiency of ECMs and the conversion of organic phosphorus (P) from P-containing ECMs into available phosphate in Al-treated roots. We identified upregulated cytokinins (CKs), downregulated jasmonic acid (JA), methyl jasmonate (MEJA) and jasmonates (JAs), and unaltered indole-3-acetic acid (IAA) and salicylic acid (SA) in P3AR vs pH 3.0 + 0 mM Al-treated roots (P3R); upregulated JA, JAs and IAA, downregulated total CKs, and unaltered MEJA and SA in P4AR vs pH 4.0 + 0 mM Al-treated roots (P4R); and upregulated CKs, downregulated JA, MEJA, JAs and SA, and unaltered IAA in P3AR vs P4AR. Generally viewed, raised pH-mediated increments of JA, MEJA, total JAs, SA and IAA concentrations and reduction of CKs concentration in Al-treated roots might help to maintain nutrient homeostasis, increase Al-toxicity-induced exudation of organic acid anions and the compartmentation of Al in vacuole, and reduce oxidative stress and Al uptake, thereby conferring root Al-tolerance. In short, elevated pH-mediated mitigation of root Al-stress involved the regulation of ECMs and phytohormones.

Expression levels of genes involved in metal homeostasis, physiological adaptation, and growth characteristics of rice (Oryza sativa L.) genotypes under Fe and/or Al toxicity

Acid sulphate soil contains high amounts of iron (Fe) and aluminum (Al), and their contamination has been reported as major problems, especially in rainfed and irrigated lowland paddy fields. Rice is sensitive to Fe and Al grown in acid soil (pH < 5.5), leading to growth inhibition and grain yield loss. The objective of this study was to evaluate Fe and/or Al uptake, translocation, physiological adaptation, metal toxicity, and growth inhibition in rice genotypes grown in acid soil. Fe and Al in the root tissues of all rice genotypes were enriched depending on the exogenous application of either Fe or Al in the soil solution, leading to root growth inhibition, especially in the KDML105 genotype. Expression level of OsYSL1 in KDML105 was increased in relation to metal uptake into root tissues, whereas OsVIT2 was downregulated, leading to Fe (50.3 mg g^-1 DW or 13.1 folds over the control) and Al (4.8 mg g^-1 DW or 2.2 folds over the control) translocation to leaf tissues. Consequently, leaf greenness (SPAD), net photosynthetic rate (P_n), stomatal conductance (g_s), and transpiration rate (E) in the leaf tissues of genotype KDML105 under Fe + Al toxicity significantly declined by 28.4%, 35.3%, 55.6%, and 51.6% over the control, respectively. In Azucena (AZU; Fe/Al tolerant), there was a rapid uptake of Fe and Al by OsYSL1 expression in the root tissues, but a limited secretion into vacuole organelles by OsVIT2, leading to a maintenance of low level of toxicity driven by an enhanced accumulation of glutathione together with downregulation of OsGR expression level. In addition, Fe and Al restrictions in the root tissues of genotype RD35 were evident; therefore, crop stress index (CSI) of Fe + Al-treated plants was the maximum, leading to an inhibition of g_s (53.6% over the control) and E (49.0% over the control). Consequently, free proline, total phenolic compounds, and ascorbic acid in the leaf tissues of rice under Fe + Al toxicity significantly increased by 3.2, 1.2, and 1.5 folds over the control, respectively, indicating their functions in non-enzymatic antioxidant defense. Moreover, physiological parameters including leaf temperature (T_leaf) increment, high level of CSI (>0.6), SPAD reduction, photon yield of PSII (Φ_PSII) diminution, P_n, g_s, and E inhibition in rice genotype IR64 (Fe/Al-sensitive) under Fe + Al treatment were clearly demonstrated as good indicators of metal-induced toxicity. Our results on Fe- and/or Al-tolerant screening to find out the candidate genotypes will contribute to present screening and breeding efforts, which in turn help increase rice production in the Fe/Al-contaminated acid soil under lowland conditions.

Low Concentration of Aluminum-Stimulated Pollen Tube Growth of Apples (Malus domestica)

Aluminum (Al) is an important element in soil constitution. Previous studies have shown that high concentration of Al affects the normal growth of crops, resulting in crop yield reduction and inferior quality. Nevertheless, Al has also been referred to as a beneficial element, especially when used at low concentrations, but the cytological mechanism is not clear. Influences of low concentration AlCl₃ on the pollen tube growth of apple (Malus domestica) and its possible cytological mechanism were investigated in this study. The results showed that 20 μM AlCl₃ promoted pollen germination and tube elongation; 20 μM AlCl₃ enhanced Ca²⁺ influx but did not affect [Ca²⁺]c of the pollen tube tip; and 20 μM AlCl₃ decreased acid pectins in pollen tubes but increased esterified pectins and arabinan pectins in pollen tubes. According to the information provided in this research, 20 μM AlCl₃ stimulated growth of pollen tubes by enhancing Ca²⁺ influx and changing cell wall components.

Research Advances in the Mutual Mechanisms Regulating Response of Plant Roots to Phosphate Deficiency and Aluminum Toxicity

Low phosphate (Pi) availability and high aluminum (Al) toxicity constitute two major plant mineral nutritional stressors that limit plant productivity on acidic soils. Advances toward the identification of genes and signaling networks that are involved in both stresses in model plants such as Arabidopsis thaliana and rice (Oryza sativa), and in other plants as well have revealed that some factors such as organic acids (OAs), cell wall properties, phytohormones, and iron (Fe) homeostasis are interconnected with each other. Moreover, OAs are involved in recruiting of many plant-growth-promoting bacteria that are able to secrete both OAs and phosphatases to increase Pi availability and decrease Al toxicity. In this review paper, we summarize these mutual mechanisms by which plants deal with both Al toxicity and P starvation, with emphasis on OA secretion regulation, plant-growth-promoting bacteria, transcription factors, transporters, hormones, and cell wall-related kinases in the context of root development and root system architecture remodeling that plays a determinant role in improving P use efficiency and Al resistance on acidic soils.

Adaptive Mechanisms Make Lupin a Choice Crop for Acidic Soils Affected by Aluminum Toxicity

Almost half of the world's agricultural soils are acidic, and most of them present significant levels of aluminum (Al) contamination, with Al³⁺ as the prevailing phytotoxic species. Lupin is a protein crop that is considered as an optimal alternative to soybean cultivation in cold climates. Lupins establish symbiosis with certain soil bacteria, collectively known as rhizobia, which are capable of fixing atmospheric nitrogen. Moreover, some lupin species, especially white lupin, form cluster roots, bottlebrush-like structures specialized in the mobilization and uptake of nutrients in poor soils. Cluster roots are also induced by Al toxicity. They exude phenolic compounds and organic acids that chelate Al to form non-phytotoxic complexes in the rhizosphere and inside the root cells, where Al complexes are accumulated in the vacuole. Lupins flourish in highly acidic soils where most crops, including other legumes, are unable to grow. Some lupin response mechanisms to Al toxicity are common to other plants, but lupin presents specific tolerance mechanisms, partly as a result of the formation of cluster roots. Al-induced lupin organic acid secretion differs from P-induced secretion, and organic acid transporters functions differ from those in other legumes. Additionally, symbiotic rhizobia can contribute to Al detoxification. After revising the existing knowledge on lupin distinct Al tolerance mechanisms, we conclude that further research is required to elucidate the specific organic acid secretion and Al accumulation mechanisms in this unique legume, but definitely, white lupin arises as a choice crop for cultivation in Al-rich acidic soils in temperate climate regions.

Enhanced aluminum tolerance in sugarcane: evaluation of SbMATE overexpression and genome-wide identification of ALMTs in Saccharum spp

BACKGROUND

A major limiting factor for plant growth is the aluminum (Al) toxicity in acidic soils, especially in tropical regions. The exclusion of Al from the root apex through root exudation of organic acids such as malate and citrate is one of the most ubiquitous tolerance mechanisms in the plant kingdom. Two families of anion channels that confer Al tolerance are well described in the literature, ALMT and MATE family.

RESULTS

In this study, sugarcane plants constitutively overexpressing the Sorghum bicolor MATE gene (SbMATE) showed improved tolerance to Al when compared to non-transgenic (NT) plants, characterized by sustained root growth and exclusion of aluminum from the root apex based on the result obtained with hematoxylin staining. In addition, genome-wide analysis of the recently released sugarcane genome identified 11 ALMT genes and molecular studies showed potential new targets for aluminum tolerance.

CONCLUSIONS

Our results indicate that the transgenic plants overexpressing the Sorghum bicolor MATE has an improved tolerance to Al. The expression profile of ALMT genes revels potential candidate genes to be used has an alternative for agricultural expansion in Brazil and other areas with aluminum toxicity in poor and acid soils.