Overexpression of the Aldehyde Dehydrogenase Gene ZmALDH Confers Aluminum Tolerance in Arabidopsis thaliana

Aluminum (Al) toxicity is the main factor limiting plant growth and the yield of cereal crops in acidic soils. Al-induced oxidative stress could lead to the excessive accumulation of reactive oxygen species (ROS) and aldehydes in plants. Aldehyde dehydrogenase (ALDH) genes, which play an important role in detoxification of aldehydes when exposed to abiotic stress, have been identified in most species. However, little is known about the function of this gene family in the response to Al stress. Here, we identified an ALDH gene in maize, ZmALDH, involved in protection against Al-induced oxidative stress. Al stress up-regulated ZmALDH expression in both the roots and leaves. The expression of ZmALDH only responded to Al toxicity but not to other stresses including low pH and other metals. The heterologous overexpression of ZmALDH in Arabidopsis increased Al tolerance by promoting the ascorbate-glutathione cycle, increasing the transcript levels of antioxidant enzyme genes as well as the activities of their products, reducing MDA, and increasing free proline synthesis. The overexpression of ZmALDH also reduced Al accumulation in roots. Taken together, these findings suggest that ZmALDH participates in Al-induced oxidative stress and Al accumulation in roots, conferring Al tolerance in transgenic Arabidopsis.

Expression GWAS of PGIP1 Identifies STOP1-Dependent and STOP1-Independent Regulation of PGIP1 in Aluminum Stress Signaling in Arabidopsis

To elucidate the unknown regulatory mechanisms involved in aluminum (Al)-induced expression of POLYGALACTURONASE-INHIBITING PROTEIN 1 (PGIP1), which is one of the downstream genes of SENSITIVE TO PROTON RHIZOTOXICITY 1 (STOP1) regulating Al-tolerance genes, we conducted a genome-wide association analysis of gene expression levels (eGWAS) of PGIP1 in the shoots under Al stress using 83 Arabidopsis thaliana accessions. The eGWAS, conducted through a mixed linear model, revealed 17 suggestive SNPs across the genome having the association with the expression level variation in PGIP1. The GWAS-detected SNPs were directly located inside transcription factors and other genes involved in stress signaling, which were expressed in response to Al. These candidate genes carried different expression level and amino acid polymorphisms. Among them, three genes encoding NAC domain-containing protein 27 (NAC027), TRX superfamily protein, and R-R-type MYB protein were associated with the suppression of PGIP1 expression in their mutants, and accordingly, the system affected Al tolerance. We also found the involvement of Al-induced endogenous nitric oxide (NO) signaling, which induces NAC027 and R-R-type MYB genes to regulate PGIP1 expression. In this study, we provide genetic evidence that STOP1-independent NO signaling pathway and STOP1-dependent regulation in phosphoinositide (PI) signaling pathway are involved in the regulation of PGIP1 expression under Al stress.

Functional Characterization of Aluminum (Al)-Responsive Membrane-Bound NAC Transcription Factors in Soybean Roots

The membrane-bound NAC transcription (NTL) factors have been demonstrated to participate in the regulation of plant development and the responses to multiple environmental stresses. This study is aimed to functionally characterize soybean NTL transcription factors in response to Al-toxicity, which is largely uncharacterized. The qRT-PCR assays in the present study found that thirteen out of fifteen GmNTL genes in the soybean genome were up-regulated by Al toxicity. However, among the Al-up-regulated GmNTLs selected from six duplicate gene pairs, only overexpressing GmNTL1, GmNTL4, and GmNTL10 could confer Arabidopsis Al resistance. Further comprehensive functional characterization of GmNTL4 showed that the expression of this gene in response to Al stress depended on root tissues, as well as the Al concentration and period of Al treatment. Overexpression of GmNTL4 conferred Al tolerance of transgenic Arabidopsis in long-term (48 and 72 h) Al treatments. Moreover, RNA-seq assay identified 517 DEGs regulated by GmNTL4 in Arabidopsis responsive to Al stress, which included MATEs, ALMTs, PMEs, and XTHs. These results suggest that the function of GmNTLs in Al responses is divergent, and GmNTL4 might confer Al resistance partially by regulating the expression of genes involved in organic acid efflux and cell wall modification.

Transcriptome Analysis of Genes Involved in Aluminum Stress Responses in Peanut (Arachis Hypogaea L.)

Background Aluminum (Al) contamination inhibits plant growth and development, however, mechanisms involved in Al stress tolerance in peanut (Arachis hypogaea L.) were rarely studied. The present study was comprised of four Al levels i.e., 0, 1.25, 2.5 and 5 mmol l−1 AlCl3.18H2O regarded as Al0, Al1, Al2, and Al3. The respective concentrations were added in Hoagland nutrient solution and replaced every three days. Result Results revealed that seeding length low Al concentration (Al1) treatment had no noticeable effect on seeding lenght, while higher Al concentration (Al2 and Al3) treatment significantly inhibited seeding lenght. The differentially expressed genes (DEGs) of plant hormone metabolism pathway were significantly enriched whereas the contents of salicylic acid (SA) and abscisic acid (ABA) were up-regulated, and jasmonic acid (JA) were down-regulated to different levels. Moreover, transcription factors (TFs) and ALMT9 and FRDL1 genes were up-regulated at higher Al concentration and down-regulated at the lowest Al concentration (Al1). Conclusions Overall, Higher Al concentrations up-regulated the expression of transcription factors (TFs), and ALMT9 and FRDL1 genes to resist the stress of high Al concentrations whereas transcriptome analysis revealed that Al stress tolerance is closely related to endogenous hormone contents i.e., salicylic acid (SA), abscisic acid (ABA), and jasmonic acid (JA). This study preliminarily analyzed the molecular mechanism of Al tolerance in peanut and provided a theoretical rationale for developing new Al-tolerant peanut cultivars.

Identification and response analysis of xyloglucan endotransglycosylase/hydrolases (XTH) family to fluoride and aluminum treatment in Camellia sinensis

Background Xyloglucan endotransglycosylase/hydrolases (XTH) can disrupt and reconnect the xyloglucan chains, modify the cellulose-xyloglucan complex structure in the cell wall to reconstruct the cell wall. Previous studies have reported that XTH plays a key role in the aluminum (Al) tolerance of tea plants (Camellia sinensis), which is a typical plant that accumulates Al and fluoride (F), but its role in F resistance has not been reported. Results Here, 14 CsXTH genes were identified from C. sinensis and named as CsXTH1–14. The phylogenetic analysis revealed that CsXTH members were divided into 3 subclasses, and conserved motif analysis showed that all these members included catalytic active region. Furthermore, the expressions of all CsXTH genes showed tissue-specific and were regulated by Al3+ and F− treatments. CsXTH1, CsXTH4, CsXTH6–8 and CsXTH11–14 were up-regulated under Al3+ treatments; CsXTH1–10 and CsXTH12–14 responded to different concentrations of F− treatments. The content of xyloglucan oligosaccharide determined by immunofluorescence labeling increased to the highest level at low concentrations of Al3+ or F− treatments (0.4 mM Al3+ or 8 mg/L F−), accompanying by the activity of XET (Xyloglucan endotransglucosylase) peaked. Conclusion In conclusion, CsXTH activities were regulated by Al or F via controlling the expressions of CsXTH genes and the content of xyloglucan oligosaccharide in C. sinensis roots was affected by Al or F, which might finally influence the elongation of roots and the growth of plants.

Comparative Inter- and IntraSpecies Transcriptomics Revealed Key Differential Pathways Associated With Aluminium Stress Tolerance in Lentil

Aluminium stress causes plant growth retardation and engenders productivity loss under acidic soil conditions. This study accentuates morpho-physiological and molecular bases of aluminium (Al) tolerance within and between wild (ILWL-15) and cultivated (L-4602 and BM-4) lentil species. Morpho-physiological studies revealed better cyto-morphology of tolerant genotypes over sensitive under Al3+ stress conditions. Mitotic lesions were observed in root cells under these conditions. Transcriptome analysis under Al3+ stress revealed 30,158 specifically up-regulated genes in different comparison groups showing contigs between 15,305 and 18,861 bp. In tolerant genotypes, top up-regulated differentially expressed genes (DEGs) were found to be involved in organic acid synthesis and exudation, production of antioxidants, callose synthesis, protein degradation, and phytohormone- and calcium-mediated signalling under stress conditions. DEGs associated with epigenetic regulation and Al3+ sequestration inside vacuole were specifically upregulated in wild and cultivars, respectively. Based on assembled unigenes, an average of 6,645.7 simple sequence repeats (SSRs) and 14,953.7 high-quality single nucleotide polymorphisms (SNPs) were spotted. By quantitative real-time polymerase chain reaction (qRT-PCR), 12 selected genes were validated. Gene ontology (GO) annotation revealed a total of 8,757 GO terms in three categories, viz., molecular, biological, and cellular processes. Kyoto Encyclopaedia of Genes and Genomes pathway scanning also revealed another probable pathway pertaining to metacaspase-1,−4, and −9 for programmed cell death under Al-stress conditions. This investigation reveals key inter- and intraspecies metabolic pathways associated with Al-stress tolerance in lentil species that can be utilised in designing future breeding programmes to improve lentil and related species towards Al3+ stress.

The Aluminum Distribution and Translocation in Two Citrus Species Differing in Aluminum Tolerance

Background: Many citrus orchards of south China suffer from soil acidification, which induced aluminum (Al) toxicity. The Al-immobilization in vivo is crucial for Al detoxification. However, the distribution and translocation of excess Al in citrus species were not well illustrated.Results: The seedlings of ‘Xuegan’ [Citrus sinensis (L.) Osbeck] and ‘Shatianyou’ [Citrus grandis (L.) Osbeck] that differed in Al tolerance were hydroponically treated with nutrient solution (Control) or supplemented by 1.0 mM Al3+ (Al toxicity) for 21 days after three months of pre-culture. The Al distribution at the tissue level of citrus species following the order: lateral roots > primary roots > leaves > stems. The fragmentation of fresh lateral roots revealed the ratio of Al distribution at the cell wall, cell organelle and cytoplasmic supernatant was about 8:2:1 of two citrus species under Al stress. Besides, the Al distribution at the lateral root cell wall components suggested the pectin is the most Al-accumulating site in citrus species. Compared to C. grandis, C. sinensis had a significantly higher Al concentration on the cell wall of lateral roots whereas remarkably lower Al levels on the leaves and stems. Furthermore, the Al translocation revealed by the absorption kinetics of the cell wall demonstrated that C. sinensis had a higher Al retention and stronger Al affinity on the root cell wall than C. grandis. According to the FTIR (Fourier transform infrared spectroscopy) analysis, the Al distribution and translocation might be affected by modifying the structure and components of the citrus lateral root cell wall. Conclusions: A higher Al-retention, mainly targeted by the pectin of the root cell wall, and a lower Al translocation efficiency from roots to shoots contributed to a higher Al tolerance of C. sinensis than C. grandis.

Zinc finger protein ZFP36 and pyruvate dehydrogenase kinase PDK1 are key elements in the ABA-mediated aluminum tolerance in rice

Aluminum (Al) toxicity is the major limiting factor for plant production and crop yield on acid soils. Previous works revealed that ZFP36 played a key role in plant adaptation to water stress and rice blast. Here, we report another function of ZFP36, which is associated with detoxification of Al toxicity through abscisic acid (ABA)- dependent mechanism, in rice. Al exposure significantly increased ZFP36 expression level in an ABA-dependent manner. Knocking down ZFP36 triggered Al sensitivity, while overexpressing ZFP36 resulted in an increased Al tolerance. Chromatin immunoprecipitation-sequencing (ChIP-seq), yeast one-hybrid assay, and dual luciferase test revealed that a tonoplast-localized Al transporter, OsALS1, and antioxidant defense enzymes were targeted by ZFP36. Moreover, by combining yeast two-hybrid assay, bimolecular fluorescence complement (BiFC), GST pull-down and in vitro phosphorylation assay, we showed that ZFP36 could interact with and be phosphorylated by the pyruvate dehydrogenase kinase OsPDK1. A strong association was found between the expression, protein content and kinase activity of OsPDK1 with the responses to ABA and Al. Using transgenic plants, we have showed that OsPDK1 could inhibit OsALS1 expression via regulating ZFP36. Overall, our data demonstrates a new ZFP36-controlled mechanism of Al defense in rice plants, which is mediated by OsPDK1.

Physiological, nutritional, and molecular responses of Brazilian sugarcane cultivars under stress by aluminum

Background Sugarcane is a crop of global importance and has been expanding to areas with soils containing high levels of exchangeable aluminum (Al), which is a limiting factor for crop development in acidic soils. The study of the sugarcane physiological and nutritional behavior together with patterns of gene expression in response to Al stress may provide a basis for effective strategies to increase crop productivity in acidic soils. Methods Sugarcane cultivars were evaluated for physiological parameters (photosynthesis, stomatal conductance, and transpiration), nutrient (N, P, K, Ca, Mg, and S) and Al contents in leaves and roots and gene expression, of the genes MDH, SDH by qPCR, both related to the production of organic acids, and SOD, related to oxidative stress. Results Brazilian sugarcane RB867515, RB928064, and RB935744 cultivars exhibited very different responses to induced stress by Al. Exposure to Al caused up-regulation (SOD and MDH) or down-regulation (SDH, MDH, and SOD), depending on the cultivar, Al level, and plant tissue. The RB867515 cultivar was the most Al-tolerant, showing no decline of nutrient content in plant tissue, photosynthesis, transpiration, and stomatal conductance after exposure to Al; it exhibited the highest Al content in the roots, and showed important MDH and SOD gene expression in the roots. RB928064 only showed low expression of SOD in roots and leaves, while RB935744 showed important expression of the SOD gene only in the leaves. Sugarcane cultivars were classified in the following descending Al-tolerance order: RB867515 > RB928064 = RB935744. These results may contribute to the obtention of Al-tolerant cultivars that can play their genetic potential in soils of low fertility and with low demand for agricultural inputs; the selection of potential plants for breeding programs; the elucidation of Al detoxification mechanisms employed by sugarcane cultivars.

Identification and characterization of novel QTL conferring internal detoxification of aluminum in soybean

Aluminum (Al) toxicity inhibits soybean root growth, leading to insufficient water and nutrient uptake. In this research, two soybean lines (Magellan and PI 567731) were identified differing in Al tolerance as determined by primary root length ratio (PRL_Ratio), total root length ratio (TRL_Ratio), and root tip number ratio (RTN_Ratio) under Al stress compared to unstressed controlled conditions. Serious root necrosis was observed in PI 567731, but not in Magellan under Al stress. An F8 recombinant inbred line population derived from a cross between Magellan and PI 567731 was used to map the quantitative trait loci (QTL) for Al-tolerance. Three QTL on chromosomes 3, 13, and 20, with tolerant-alleles from Magellan, were identified. qAl_Gm13 and qAl_Gm20, explained large phenotypic variations (13-27%) and played roles in maintaining root elongation. qAl_Gm03 was involved in maintaining root initiation under Al stress. These results suggested the importance of using the parameters of root elongation and root initiation in Al tolerance studies. In addition, qAl_Gm13 and qAl_Gm20 were confirmed in near-isogenic backgrounds and were identified to epistatically regulate Al tolerance in internal detoxification instead of Al 3+ exclusion. The candidate genes for qAl_Gm13 and qAl_Gm20 were suggested by analyzing a previous RNA-seq study. Phylogenetic and pedigree analysis identified the tolerant alleles of both loci derived from the US ancestor line, A.K.[FC30761], originally from China. Our results provide novel genetic resources for breeding Al-tolerant soybeans and suggest that the internal detoxification contributes to soybean tolerance to excessive soil Al.