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.

Integration of Small RNA and Degradome Sequencing Reveals the Regulatory Network of Al-Induced Programmed Cell Death in Peanut

Peanut is one of the most important oil crops in the world. In China, the peanut is highly produced in its southern part, in which the arable land is dominated by acid soil. At present, miRNAs have been identified in stress response, but their roles and mechanisms are not clear, and no miRNA studies have been found related to aluminum (Al)-induced programmed cell death (PCD). In the present study, transcriptomics, sRNAs, and degradome analysis in the root tips of two peanut cultivars ZH2 (Al-sensitive, S) and 99-1507 (Al-tolerant, T) were carried out. Here, we generated a comprehensive resource focused on identifying key regulatory miRNA-target circuits that regulate PCD under Al stress. Through deep sequencing, 2284 miRNAs were identified and 147 miRNAs were differentially expressed under Al stress. Furthermore, 19237 target genes of 749 miRNAs were validated by degradome sequencing. GO and KEGG analyses of differential miRNA targets showed that the pathways of synthesis and degradation of ketone bodies, citrate cycle (TCA cycle), and peroxisome were responded to Al stress. The combined analysis of the degradome data sets revealed 89 miRNA-mRNA interactions that may regulate PCD under Al stress. Ubiquitination may be involved in Al-induced PCD in peanut. The regulatory networks were constructed based on the differentially expressed miRNAs and their targets related to PCD. Our results will provide a useful platform to research on PCD induced by Al and new insights into the genetic engineering for plant stress response.

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.

Correction: Fernandes et al. Stress Granule Assembly Can Facilitate but Is Not Required for TDP-43 Cytoplasmic Aggregation. Biomolecules 2020, 10, 1367

In the original article [...]

Genome-Wide Identification and Gene Expression Analysis of Acyl-Activating Enzymes Superfamily in Tomato (Solanum lycopersicum) Under Aluminum Stress

In response to changing environments, plants regulate gene expression and subsequent metabolism to acclimate and survive. A superfamily of acyl-activating enzymes (AAEs) has been observed in every class of creatures on planet. Some of plant AAE genes have been identified and functionally characterized to be involved in growth, development, biotic, and abiotic stresses via mediating diverse metabolic pathways. However, less information is available about AAEs superfamily in tomato (Solanum lycopersicum), the highest value fruit and vegetable crop globally. In this study, we aimed to identify tomato AAEs superfamily and investigate potential functions with respect to aluminum (Al) stress that represents one of the major factors limiting crop productivity on acid soils worldwide. Fifty-three AAE genes of tomato were identified and named on the basis of phylogenetic relationships between Arabidopsis and tomato. The phylogenetic analysis showed that AAEs could be classified into six clades; however, clade III contains no AAE genes of tomato. Synteny analyses revealed tomato vegetable paralogs and Arabidopsis orthologs. The RNA-seq and quantitative reverse-transcriptase PCR (qRT-PCR) analysis indicated that 9 out of 53 AAEs genes were significantly up- or downregulated by Al stress. Numerous cis-acting elements implicated in biotic and abiotic stresses were detected in the promoter regions of SlAAEs. As the most abundantly expressed gene in root apex and highly induced by Al, there are many potential STOP1 cis-acting elements present in the promoter of SlAAE3-1, and its expression in root apex was specific to Al. Finally, transgenic tobacco lines overexpressing SlAAE3-1 displayed increased tolerance to Al. Altogether, our results pave the way for further studies on the functional characterization of SlAAE genes in tomato with a wish of improvement in tomato crop in the future.

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.

Uncovering the transcriptional response of popcorn (Zea mays L. var. everta) under long-term aluminum toxicity

To date, the investigation of genes involved in Al resistance has focused mainly on microarrays and short periods of Al exposure. We investigated genes involved in the global response under Al stress by tracking the expression profile of two inbred popcorn lines with different Al sensitivity during 72 h of Al stress. A total of 1003 differentially expressed genes were identified in the Al-sensitive line, and 1751 were identified in the Al-resistant line, of which 273 were shared in both lines. Genes in the category of “response to abiotic stress” were present in both lines, but there was a higher number in the Al-resistant line. Transcription factors, genes involved in fatty acid biosynthesis, and genes involved in cell wall modifications were also detected. In the Al-resistant line, GST6 was identified as one of the key hub genes by co-expression network analysis, and ABC6 may play a role in the downstream regulation of CASP-like 5. In addition, we suggest a class of SWEET transporters that might be involved in the regulation of vacuolar sugar storage and may serve as mechanisms for Al resistance. The results and conclusions expand our understanding of the complex mechanisms involved in Al toxicity and provide a platform for future functional analyses and genomic studies of Al stress in popcorn.

Cultivar Differences in the Biochemical and Physiological Responses of Common Beans to Aluminum Stress

Soil conditions leading to high levels of available aluminum are detrimental to plant growth, but data are limited on genotypic differences in tolerance to aluminum stress in some crops. The aim of this study was to examine the morphological, biochemical, and physiological changes in roots and shoots of 25 common bean (Phaseolus vulgaris L.) cultivars (Pinto market class) under aluminum (Al) treatment. Additionally, this study aimed to assess the range of responses amongst the common bean cultivars relative to their Al toxicity tolerance and sensitivity. Plants were grown hydroponically using a simplified nutrient solution with or without 20 µM AlCl3. Reactive oxygen species (ROS), activities of the antioxidant enzymes superoxide dismutase (SOD) and guaiacol peroxidase (POD), and malondialdehyde (MDA) concentration, an indicator of lipid peroxidation, were measured to establish the effects of Al treatment on the plants. In addition, growth parameters such as shoot and root dry weight, root-to-shoot ratio, root elongation, and root volume changes were also investigated. The cultivar effect was significant for all the measured parameters, except for shoot dry weight. Inhibition of the root and shoot dry weight for selected common bean cultivars shows that the response of common bean to Al stress is genotype-specific. Additionally, Al-induced root elongation inhibition and root volume changes varied among the cultivars. Most cultivars had significantly higher SOD activity (20 of 25 cultivars) and POD activity (12 cultivars) under AlCl3 treatment compared to the controls. A positive significant correlation was observed between MDA and ROS, showing that Al stress induced the accumulation of ROS along with an increase in lipid peroxidation. According to the results of this study, Arapaho and AC Island cultivars could potentially be used in the future production of common beans under Al stress. Therefore, these two cultivars could also be included in Al tolerance breeding programs.

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.