Involvement of the 4-coumarate:coenzyme A ligase 4CL4 in rice phosphorus acquisition and rhizosphere microbe recruitment via root growth enlargement.

MAIN CONCLUSION: The 4-coumarate:coenzyme A ligase 4CL4 is involved in enhancing rice P acquisition and use in acid soil by enlarging root growth and boosting functional rhizosphere microbe recruitment. Rice (Oryza sativa L.) cannot easily acquire phosphorus (P) from acid soil, where root growth is inhibited and soil P is fixed. The combination of roots and rhizosphere microbiota is critical for plant P acquisition and soil P mobilization, but the associated molecular mechanism in rice is unclear. 4CL4/RAL1 encodes a 4-coumarate:coenzyme A ligase related to lignin biosynthesis in rice, and its dysfunction results in a small rice root system. In this study, soil culture and hydroponic experiments were conducted to examine the role of RAL1 in regulating rice P acquisition, fertilizer P use, and rhizosphere microbes in acid soil. Disruption of RAL1 markedly decreased root growth. Mutant rice plants exhibited decreased shoot growth, shoot P accumulation, and fertilizer P use efficiency when grown in soil-but not under hydroponic conditions, where all P is soluble and available for plants. Mutant ral1 and wild-type rice rhizospheres had distinct bacterial and fungal community structures, and wild-type rice recruited some genotype-specific microbial taxa associated with P solubilization. Our results highlight the function of 4CL4/RAL1 in enhancing rice P acquisition and use in acid soil, namely by enlarging root growth and boosting functional rhizosphere microbe recruitment. These findings can inform breeding strategies to improve P use efficiency through host genetic manipulation of root growth and rhizosphere microbiota.

Distinct Patterns of Rhizosphere Microbiota Associated With Rice Genotypes Differing in Aluminum Tolerance in an Acid Sulfate Soil.

Rhizosphere microbes are important for plant tolerance to various soil stresses. Rice is the most aluminum (Al)-tolerant small grain cereal crop species, but the link between rice Al tolerance and rhizosphere microbiota remains unclear. This study aimed to investigate the microbial community structure of aluminum-sensitive and Al-tolerant rice varieties in acid sulfate soil under liming and non-liming conditions. We analyzed the rice biomass and mineral element contents of rice plants as well as the chemical properties and microbial (archaea, bacteria, and fungi) communities of rhizosphere and bulk soil samples. The results showed that the Al-tolerant rice genotype grew better and was able to take up more phosphorus from the acid sulfate soil than the Al-sensitive genotype. Liming was the main factor altering the microbial diversity and community structure, followed by rhizosphere effects. In the absence of liming effects, the rice genotypes shifted the community structure of bacteria and fungi, which accounted for the observed variation in the rice biomass. The Al-tolerant rice genotype recruited specific bacterial and fungal taxa (Bacillus, Pseudomonas, Aspergillus, and Rhizopus) associated with phosphorus solubilization and plant growth promotion. The soil microbial co-occurrence network of the Al-tolerant rice genotype was more complex than that of the Al-sensitive rice genotype. In conclusion, the bacterial and fungal community in the rhizosphere has genotype-dependent effects on rice Al tolerance. Aluminum-tolerant rice genotypes recruit specific microbial taxa, especially phosphorus-solubilizing microorganisms, and are associated with complex microbial co-occurrence networks, which may enhance rice growth in acid sulfate soil.

Microbial keystone taxa drive crop productivity through shifting aboveground-belowground mineral element flows.

Unbalanced fertilization of nutritional elements is a potential threat to environmental quality and agricultural productivity in acid soil. Harnessing keystone taxa in soil microbiome represents a promising strategy to enhance crop productivity as well as reducing the adverse environmental effects of fertilizers, with the goal of agricultural sustainability. However, there is a lack of information on which and how soil microbial keystone taxa contribute to sustainable crop productivity in acid soil. Here, we examined soil microbial communities (including bacteria, fungi, and archaea) and soil nutrients, and the mineral nutrition and yield of maize subjected to different inorganic and organic fertilization treatments over 35 years in acid soil. The application of organic fertilizer alone or in combination with inorganic fertilizers sustained high maize yield when compared with the other fertilization treatments. Microbial abundances and community structures rather than their alpha diversities explained the main variation in maize yield among different treatments. Sixteen soil keystone taxa (a fungal operational taxonomic unit and 15 bacterial operational taxonomic units) were identified from the microbial co-occurrence network. Among them, five keystone taxa (in Hypocreales, Bryobacter, Solirubrobacterales, Thermomicrobiales, and Roseiflexaceae) contributed to high maize yield through increasing phosphorus flow and inhibiting toxic aluminum and manganese flow from soils to plants. However, the remaining eleven keystone taxa (in Conexibacter, Acidothermus, Ktedonobacteraceae, Deltaproteobacteria, Actinobacteria, Elsterales, Ktedonobacterales, and WPS-2) exerted the opposite effects. As a result, maize productivity varied among different fertilization treatments because of the altered maize mineral element flows by microbial keystone taxa. We conclude that microbial keystone taxa drive crop productivity through shifting aboveground-belowground mineral element flows in acid soil. This study highlights the importance of microbial keystone taxa for sustainable crop productivity in acid soil and provides deep insights into the relationships between soil microbial keystone taxa, crop mineral nutrition, and productivity.

Secretion of Gluconic Acid From Nguyenibacter sp. L1 Is Responsible for Solubilization of Aluminum Phosphate.

Phosphorus (P) deficiency is one of the major factors limiting plant growth in acid soils, where most P is fixed by toxic aluminum (Al). Phosphate-solubilizing bacteria (PSBs) are important for the solubilization of fixed P in soils. Many PSBs have been isolated from neutral and calcareous soils, where calcium phosphate is the main P form, whereas PSBs in acid soils have received relatively little attention. In this study, we isolated a PSB strain from the rhizosphere of Lespedeza bicolor, a plant well adapted to acid soils. On the basis of its 16S rRNA gene sequence, this strain was identified as a Nguyenibacter species and named L1. After incubation of Nguyenibacter sp. L1 for 48 h in a culture medium containing AlPO4 as the sole P source, the concentration of available P increased from 10 to 225 mg L-1, and the pH decreased from 5.5 to 2.5. Nguyenibacter sp. L1 exhibited poor FePO4 solubilization ability. When the pH of non-PSB-inoculated medium was manually adjusted from 5.5 to 2.5, the concentration of available P only increased from 6 to 65 mg L-1, which indicates that growth medium acidification was not the main contributor to the solubilization of AlPO4 by Nguyenibacter sp. L1. In the presence of glucose, but not fructose, Nguyenibacter sp. L1 released large amounts of gluconic acid to solubilize AlPO4. Furthermore, external addition of gluconic acid enhanced AlPO4 solubilization and reduced Al toxicity to plants. We conclude that secretion of gluconic acid by Nguyenibacter sp. L1, which is dependent on glucose supply, is responsible for AlPO4 solubilization as well as the alleviation of Al phytotoxicity by this bacterial strain.

Balanced fertilization over four decades has sustained soil microbial communities and improved soil fertility and rice productivity in red paddy soil.

The influence of long-term fertilization on soil microbial communities is critical for revealing the association between belowground microbial flora and aboveground crop productivity-a relationship of great importance to food security, environmental protection, and ecosystem functions. Here, we examined shifts in soil chemical properties, microbial communities, and the nutrient uptake and yield of rice subjected to different chemical and organic fertilization treatments over a 40-year period in red paddy soil. Ten different treatments were used: a control without fertilizer, and applications of nitrogen (N), phosphorus (P), potassium (K), NP, NK, PK, NPK, double NPK, or NPK plus manure. Compared with the effects of withholding one or two nutrients (N, P, or K), the balanced application of chemical NPK and organic fertilizers markedly improved soil nutrient status and rice yield. This improvement of soil fertility and rice yield was not associated with bacterial, archaeal, or fungal alpha diversities. The bacterial abundance and community structure and archaeal abundance effectively explained the variation in rice yield, whereas those of fungi did not. The community structure of bacteria and archaea, but not that of fungi, was correlated with soil properties. Among various soil properties, P was the key factor influencing rice yield and soil microbial communities because of the extremely low content of soil available P. Seven keystones at the operational taxonomic unit level were identified: four archaea (belonging to Thermoplasmata, Methanosaeta, Bathyarchaeia, and Nitrososphaeraceae) and three bacteria (in Desulfobacteraceae and Acidobacteriales). These keystones, which were mainly related to soil C and N transformation and pH, may work cooperatively to influence rice yield by regulating soil fertility. Our results collectively suggest that four decades of balanced fertilization has sustained the bacterial and archaeal abundances, bacterial community structure, and keystones, which potentially contribute to soil fertility and rice yield in red paddy soil.

Strategies of cadmium and copper uptake and translocation in different plant species growing near an E-waste dismantling site at Wenling, China.

This study aimed to explore the interactions between cadmium (Cd) and copper (Cu) during uptake and translocation in plants growing in soil polluted with heavy metals derived from electronic waste (E-waste). We collected the roots, stems, leaves, and root-surrounding soils of ten dominant plant species growing in farmland near an E-waste dismantling site, and analyzed their Cd and Cu concentrations. Among the ten plant species, Echinochloa crus-galli (L.) P. Beauv., Cucurbita moschata (Duch. ex Lam.) Duch. ex Poiret, Phragmites australis (Cav.) Trin. ex Steud., and Benincasa hispida (Thunb.) Cogn. accumulated Cd (2.40-4.56 mg kg-1) and Cu (19.60-35.21 mg kg-1) in the roots. In Polygonum hydropiper L. and Sesbania cannabina (Retz.) Poir., the Cd (0.50-0.81 mg kg-1) and Cu (11.04-15.55 mg kg-1) concentrations were similar among the three organs. Glycine max (L.) Merr. accumulated more Cu in the roots (16.42 mg kg-1) than in the stems (5.61 mg kg-1) and leaves (7.75 mg kg-1), and accumulated Cd at similar levels in the three organs (0.65-0.99 mg kg-1). Sesamum indicum L., Bidens pilosa L., and Solidago decurrens Lour. accumulated Cd at similar levels among the three organs (0.16-3.34 mg kg-1) and accumulated less Cu in the stems (6.89-8.28 mg kg-1) than in the roots (12.61-21.63 mg kg-1) and leaves (12.93-22.38 mg kg-1). S. indicum had a stronger capacity to accumulate and translocate Cd and Cu according to transfer coefficient and translocation factor. The concentrations of Cd and Cu in soils were significantly positively correlated with those in the roots (p<0.01) but not those in the stems and leaves. We detected significantly positive correlations between Cd and Cu concentrations in the roots and leaves (p<0.01) but not in the stems. These results suggest that there is a synergetic strategy of Cd and Cu transport from soils to the roots and from the roots to the leaves, while the stems may not be the key organ controlling Cd and Cu transport in plants. These findings have important implications for the phytoremediation of soils contaminated with Cd and Cu, the mechanisms of plant Cd and Cu transport, and the food safety of agricultural products.

Improved Root Growth by Liming Aluminum-Sensitive Rice Cultivar or Cultivating an Aluminum-Tolerant One Does Not Enhance Fertilizer Nitrogen Recovery Efficiency in an Acid Paddy Soil.

The root is the main site of nitrogen (N) acquisition and aluminum (Al) toxicity. The objective of this study is to investigate whether liming and cultivation of an Al-tolerant rice (Oryza sativa L.) cultivar can improve root growth, thereby increasing N acquisition by rice plants in acid paddy soil. Two rice cultivars ('B690', Al-sensitive, and 'Yugeng5', Al-tolerant) were cultivated with 15N-labeled urea, and with or without lime in an acid paddy soil (pH 4.9) in pots. We examined root and shoot growth, soil pH, soil exchangeable Al, N uptake, 15N distribution in plant-soil system, and fertilizer N recovery efficiency. Results showed that liming improved the root growth of 'B690' by decreasing soil exchangeable Al concentrations, in both N-limited and N-fertilized soils. Liming enhanced the N uptake of 'B690' only in the absence of N fertilizer. The root weight of 'Yugeng5' was greater than that of 'B690' without lime, but the two cultivars showed similar N uptake. The fertilizer N recovery efficiency and N loss did not differ significantly between limed and non-limed conditions, or between the two rice cultivars. Thus, liming an Al-sensitive rice cultivar and cultivating an Al-tolerant one improves root growth, but does not enhance fertilizer N recovery efficiency in the present acid paddy soil.

Role of Silicon in Mediating Phosphorus Imbalance in Plants.

The soil bioavailability of phosphorus (P) is often low because of its poor solubility, strong sorption and slow diffusion in most soils; however, stress due to excess soil P can occur in greenhouse production systems subjected to high levels of P fertilizer. Silicon (Si) is a beneficial element that can alleviate multiple biotic and abiotic stresses. Although numerous studies have investigated the effects of Si on P nutrition, a comprehensive review has not been published. Accordingly, here we review: (1) the Si uptake, transport and accumulation in various plant species; (2) the roles of phosphate transporters in P acquisition, mobilization, re-utilization and homeostasis; (3) the beneficial role of Si in improving P nutrition under P deficiency; and (4) the regulatory function of Si in decreasing P uptake under excess P. The results of the reviewed studies suggest the important role of Si in mediating P imbalance in plants. We also present a schematic model to explain underlying mechanisms responsible for the beneficial impact of Si on plant adaption to P-imbalance stress. Finally, we highlight the importance of future investigations aimed at revealing the role of Si in regulating P imbalance in plants, both at deeper molecular and broader field levels.

Ammonium Alleviates Manganese Toxicity and Accumulation in Rice by Down-Regulating the Transporter Gene OsNramp5 Through Rhizosphere Acidification.

Ammonium ( N H 4 + ) alleviates manganese (Mn) toxicity in various plant species, but the underlying mechanisms are still unclear. In this study, we compared the effects of N H 4 + and nitrate ( N O 3 - ) on rice (Oryza sativa L.) growth, accumulation and distribution of Mn, accumulation of iron (Fe), zinc (Zn) and copper (Cu), root cell wall components, and expression of Mn and Fe transporter genes. After rice seedlings were grown in non-pH-buffered nutrient solution for 2 days, the pH of growth medium changed from an initial value of 4.5 to 3.5 and to 5.5 in the presence of N H 4 + and in the presence of N O 3 - , respectively. Compared with N O 3 - , ammonium decreased nutrient-solution pH and alleviated Mn toxicity and accumulation in rice under non-pH-buffered conditions. This alleviation disappeared when 5 mM Homo-PIPES pH buffer was added. Regardless of N form, roots, shoots, root cell sap, and xylem sap accumulated much lower Mn at pH 3.5 than at pH 5.5, whereas Mn distribution in different leaves and Mn accumulation in root cell walls was affected by neither N form nor pH. Ammonium decreased the expression of the Mn influx transporter gene OsNramp5 in roots under non-pH-buffered conditions, but not under pH-buffered ones. OsNramp5 expression was down-regulated at pH 3.5 compared with pH 5.5. Another efflux Mn transporter gene, OsMTP9, was not regulated by either N form or pH. High pH (5.5) enhanced the expression of the Fe transporter gene OsIRT1 and increased the accumulation of Zn but not Fe or Cu in shoots compared with pH 3.5. Taken together, our results indicate that N H 4 + alleviates Mn toxicity and accumulation in rice through the down-regulatory effects of rhizosphere acidification on the Mn influx transporter gene OsNramp5. In addition, the up-regulation of OsIRT1 expression may contribute to the increased Zn uptake by rice at high pH of nutrient solution.

Bacillus pumilus increases boron uptake and inhibits rapeseed growth under boron supply irrespective of phosphorus fertilization.

The present study was carried out to investigate how plant growth-promoting bacteria (PGPB) influence plant growth and uptake of boron (B) and phosphorus (P) in rapeseed (Brassica napus). Rapeseed was subjected to control, B, P and B + P treatments, either with or without B. pumilus (PGPB) inoculation, and grown in pot culture for 6 weeks. In the absence of B. pumilus, the addition of B, P or both elements improved the growth of rapeseed compared with the control. Interestingly, B. pumilus inoculation inhibited plant growth and enhanced B uptake under B and B + P but not under control and P conditions. In addition, B. pumilus inoculation decreased the pH of soil under B and B + P supplies. Bacillus pumilus inoculation thus increased rapeseed B uptake and inhibited growth under B supply, which suggests that the effects of PGPB on rapeseed growth depend on the addition of B to soil. Bacillus pumilus inoculation may therefore be recommended for the enhancement of rapeseed B levels in B-deficient soils but not in B-sufficient ones.

Effects of cadmium exposure on expression of glutathione synthetase system genes in Acidithiobacillus ferrooxidans.

The glutathione synthetase system (GSS) is an important pathway of glutathione synthesis and plays a key role in heavy metal resistance. In this work, the response of Acidithiobacillus ferrooxidans to extracellular Cd2+ was investigated, and the interplay between Cd2+ resistance and the expression of GSS related-genes was analyzed by reverse-transcription quantitative PCR (RT-PCR). During growth in the presence of 5, 15 and 30 mM Cd2+, the transcript levels of eight GSS pathway genes were affected between 0.81- and 7.12-fold. Increased transcription was also reflected in increased enzyme activities: with those of glutathione reductase (GR) increased by 1.10-, 2.26- and 1.54-fold in the presence of 5, 15 and 30 mM Cd2+, respectively. In contrast, the activities of catalase (CAT) and superoxide dismutase (SOD) were decreased in the presence of Cd2+. At the metabolite level, intracellular methane dicarboxylic aldehyde (MDA) content was increased 1.97-, 3.31- and 1.92-fold in the presence of 5, 15 and 30 mM Cd2+, respectively. These results suggest that Cd2+ directly inhibits the activities of CAT and SOD, breaks the redox balance of the cells, which leads to the activation of the other antioxidant pathway of GSS. Resistance of A. ferrooxidans to Cd2+ may involve modulation of expression levels of glutathione S-transferase (GST), GR, and glutathione synthetase, which may protect against oxidative damage.

Utilization of a Wheat660K SNP array-derived high-density genetic map for high-resolution mapping of a major QTL for kernel number.

In crop plants, a high-density genetic linkage map is essential for both genetic and genomic researches. The complexity and the large size of wheat genome have hampered the acquisition of a high-resolution genetic map. In this study, we report a high-density genetic map based on an individual mapping population using the Affymetrix Wheat660K single-nucleotide polymorphism (SNP) array as a probe in hexaploid wheat. The resultant genetic map consisted of 119 566 loci spanning 4424.4 cM, and 119 001 of those loci were SNP markers. This genetic map showed good collinearity with the 90 K and 820 K consensus genetic maps and was also in accordance with the recently released wheat whole genome assembly. The high-density wheat genetic map will provide a major resource for future genetic and genomic research in wheat. Moreover, a comparative genomics analysis among gramineous plant genomes was conducted based on the high-density wheat genetic map, providing an overview of the structural relationships among theses gramineous plant genomes. A major stable quantitative trait locus (QTL) for kernel number per spike was characterized, providing a solid foundation for the future high-resolution mapping and map-based cloning of the targeted QTL.

Hydroxy-Al and cell-surface negativity are responsible for the enhanced sensitivity of Rhodotorula taiwanensis to aluminum by increased medium pH.

Aluminum (Al) is ubiquitous and toxic to microbes. High Al3+ concentration and low pH are two key factors responsible for Al toxicity, but our present results contradict this idea. Here, an Al-tolerant yeast strain Rhodotorula taiwanensis RS1 was incubated in glucose media containing Al with a continuous pH gradient from pH 3.1-4.2. The cells became more sensitive to Al and accumulated more Al when pH increased. Calculations using an electrostatic model Speciation Gouy Chapman Stern indicated that, the increased Al sensitivity of cells was associated with AlOH2+ and Al(OH) 2+ rather than Al3+. The alcian blue (a positively charged dye) adsorption and zeta potential determination of cell surface indicated that, higher pH than 3.1 increased the negative charge and Al adsorption at the cell surface. Taken together, the enhanced sensitivity of R. taiwanensis RS1 to Al from pH 3.1-4.2 was associated with increased hydroxy-Al and cell-surface negativity.

Genome-wide Identification, Characterization, and Expression Analysis of PHT1 Phosphate Transporters in Wheat.

The PHT1 family of phosphate (Pi) transporters mediates phosphorus (P) uptake and re-mobilization in plants. A genome-wide sequence analysis of PHT1 genes in wheat (Triticum aestivum) was conducted, and their expression locations and responses to P availability were further investigated. We cloned 21 TaPHT1 genes from the homologous alleles at TaPHT1.1 to 1.10 through screening a BAC library and amplifying genomic sequences. The TaPHT1 transporters were clustered into five branches in the phylogenetic tree of PHT1 proteins, and the TaPHT1 genes from a given branch shared high similarities in sequences, expression locations, and responses to P availability. The seven tested PHT1 genes all showed Pi-transport activity in yeast (Saccharomyces cerevisiae) cells grown under both low Pi and high Pi conditions. The expression of TaPHT1.1/1.9, 1.2, and 1.10 were root specific. The expression of these TaPHT1 genes at flowering positively correlated with P uptake after stem elongation across three P application rates and two wheat varieties in a field experiment. Therefore, modification of PHT1 expression may improve P use efficiency in a broad regime of P availability.

A potential contribution of the less negatively charged cell wall to the high aluminum tolerance of Rhodotorula taiwanensis RS1.

Rhodotorula taiwanensis RS1 (Rt) is a high-aluminum (Al)-tolerant yeast that can survive Al at concentrations up to 200 mM. In this study, we compared Rt with an Al-sensitive congeneric strain, R. mucilaginosa AKU 4812 (Rm) and Al sensitive mutant 1 (alsm1) of Rt, to explore the Al tolerance mechanisms of Rt. The growth of Rm was completely inhibited by 1 mM Al, but that of Rt was not inhibited until Al concentration was more than 70 mM. The growth of alsm1 was inhibited much more by 70 mM and 100 mM Al than that of Rt. Compared with Rm cells, Rt cells accumulated less Al in the cell wall and cytoplasm. A time-course analysis showed that Al was absorbed by Rm cells much more rapidly than by Rt cells when exposed to the same Al concentration. Meanwhile, the Al content of alsm1 was higher than that of Rt. Although the cell wall of Rt was thicker than that of alsm1 and Rm under control and 0.1 mM Al, that of Rt was thinner than that of alsm1 under 70 mM Al despite that their cell walls were thickened. The alcian blue adsorption was lower and cell wall zeta-potential was higher in Rt and alsm1 than in Rm, indicating a less negative charge of cell wall of Rt and alsm1 than that of Rm. Taken together, the less negatively charged cell wall of Rt may restrict the adsorption of cationic Al in cells, potentially contributing to its high Al tolerance. Copyright © 2016 John Wiley & Sons, Ltd.

E3 ubiquitin ligase Cbl-b negatively regulates C-type lectin receptor-mediated antifungal innate immunity.

Activation of various C-type lectin receptors (CLRs) initiates potent proinflammatory responses against various microbial infections. However, how activated CLRs are negatively regulated remains unknown. In this study, we report that activation of CLRs Dectin-2 and Dectin-3 by fungi infections triggers them for ubiquitination and degradation in a Syk-dependent manner. Furthermore, we found that E3 ubiquitin ligase Casitas B-lineage lymphoma protein b (Cbl-b) mediates the ubiquitination of these activated CLRs through associating with each other via adapter protein FcR-γ and tyrosine kinase Syk, and then the ubiquitinated CLRs are sorted into lysosomes for degradation by an endosomal sorting complex required for transport (ESCRT) system. Therefore, the deficiency of either Cbl-b or ESCRT subunits significantly decreases the degradation of activated CLRs, thereby resulting in the higher expression of proinflammatory cytokines and inflammation. Consistently, Cbl-b-deficient mice are more resistant to fungi infections compared with wild-type controls. Together, our study indicates that Cbl-b negatively regulates CLR-mediated antifungal innate immunity, which provides molecular insight for designing antifungal therapeutic agents.

Aluminium alleviates manganese toxicity to rice by decreasing root symplastic Mn uptake and reducing availability to shoots of Mn stored in roots.

BACKGROUND AND AIMS: Manganese (Mn) and aluminium (Al) phytotoxicities occur mainly in acid soils. In some plant species, Al alleviates Mn toxicity, but the mechanisms underlying this effect are obscure. METHODS: Rice (Oryza sativa) seedlings (11 d old) were grown in nutrient solution containing different concentrations of Mn(2+) and Al(3+) in short-term (24 h) and long-term (3 weeks) treatments. Measurements were taken of root symplastic sap, root Mn plaques, cell membrane electrical surface potential and Mn activity, root morphology and plant growth. KEY RESULTS: In the 3-week treatment, addition of Al resulted in increased root and shoot dry weight for plants under toxic levels of Mn. This was associated with decreased Mn concentration in the shoots and increased Mn concentration in the roots. In the 24-h treatment, addition of Al resulted in decreased Mn accumulation in the root symplasts and in the shoots. This was attributed to higher cell membrane surface electrical potential and lower Mn(2+) activity at the cell membrane surface. The increased Mn accumulation in roots from the 3-week treatment was attributed to the formation of Mn plaques, which were probably related to the Al-induced increase in root aerenchyma. CONCLUSIONS: The results show that Al alleviated Mn toxicity in rice, and this could be attributed to decreased shoot Mn accumulation resulting from an Al-induced decrease in root symplastic Mn uptake. The decrease in root symplastic Mn uptake resulted from an Al-induced change in cell membrane potential. In addition, Al increased Mn plaques in the roots and changed the binding properties of the cell wall, resulting in accumulation of non-available Mn in roots.

Altered cell wall properties are responsible for ammonium-reduced aluminium accumulation in rice roots.

The phytotoxicity of aluminium (Al) ions can be alleviated by ammonium (NH4(+)) in rice and this effect has been attributed to the decreased Al accumulation in the roots. Here, the effects of different nitrogen forms on cell wall properties were compared in two rice cultivars differing in Al tolerance. An in vitro Al-binding assay revealed that neither NH4(+) nor NO3(-) altered the Al-binding capacity of cell walls, which were extracted from plants not previously exposed to N sources. However, cell walls extracted from NH4(+)-supplied roots displayed lower Al-binding capacity than those from NO3(-)-supplied roots when grown in non-buffered solutions. Fourier-transform infrared microspectroscopy analysis revealed that, compared with NO3(-)-supplied roots, NH4(+)-supplied roots possessed fewer Al-binding groups (-OH and COO-) and lower contents of pectin and hemicellulose. However, when grown in pH-buffered solutions, these differences in the cell wall properties were not observed. Further analysis showed that the Al-binding capacity and properties of cell walls were also altered by pHs alone. Taken together, our results indicate that the NH4(+)-reduced Al accumulation was attributed to the altered cell wall properties triggered by pH decrease due to NH4(+) uptake rather than direct competition for the cell wall binding sites between Al(3+) and NH4(+).

CARD9 mediates Dectin-1-induced ERK activation by linking Ras-GRF1 to H-Ras for antifungal immunity.

Dectin-1 functions as a pattern recognition receptor for sensing fungal infection. It has been well-established that Dectin-1 induces innate immune responses through caspase recruitment domain-containing protein 9 (CARD9)-mediated NF-κB activation. In this study, we find that CARD9 is dispensable for NF-κB activation induced by Dectin-1 ligands, such as curdlan or Candida albicans yeast. In contrast, we find that CARD9 regulates H-Ras activation by linking Ras-GRF1 to H-Ras, which mediates Dectin-1-induced extracellular signal-regulated protein kinase (ERK) activation and proinflammatory responses when stimulated by their ligands. Mechanistically, Dectin-1 engagement initiates spleen tyrosine kinase (Syk)-dependent Ras-GRF1 phosphorylation, and the phosphorylated Ras-GRF1 recruits and activates H-Ras through forming a complex with CARD9, which leads to activation of ERK downstream. Finally, we show that inhibiting ERK activation significantly accelerates the death of C. albicans-infected mice, and this inhibitory effect is dependent on CARD9. Together, our studies reveal a molecular mechanism by which Dectin-1 induces H-Ras activation that leads to ERK activation for host innate immune responses against fungal infection.

C-type lectin receptor dectin-3 mediates trehalose 6,6'-dimycolate (TDM)-induced Mincle expression through CARD9/Bcl10/MALT1-dependent nuclear factor (NF)-κB activation.

Previous studies indicate that both Dectin-3 (also called MCL or Clec4d) and Mincle (also called Clec4e), two C-type lectin receptors, can recognize trehalose 6,6'-dimycolate (TDM), a cell wall component from mycobacteria, and induce potent innate immune responses. Interestingly, stimulation of Dectin-3 by TDM can also induce Mincle expression, which may enhance the host innate immune system to sense Mycobacterium infection. However, the mechanism by which Dectin-3 induces Mincle expression is not fully defined. Here, we show that TDM-induced Mincle expression is dependent on Dectin-3-mediated NF-κB, but not nuclear factor of activated T-cells (NFAT), activation, and Dectin-3 induces NF-κB activation through the CARD9-BCL10-MALT1 complex. We found that bone marrow-derived macrophages from Dectin-3-deficient mice were severely defective in the induction of Mincle expression in response to TDM stimulation. This defect is correlated with the failure of TDM-induced NF-κB activation in Dectin-3-deficient bone marrow-derived macrophages. Consistently, inhibition of NF-κB, but not NFAT, impaired TDM-induced Mincle expression, whereas NF-κB, but not NFAT, binds to the Mincle promoter. Dectin-3-mediated NF-κB activation is dependent on the CARD9-Bcl10-MALT1 complex. Finally, mice deficient for Dectin-3 or CARD9 produced much less proinflammatory cytokines and keyhole limpet hemocyanin (KLH)-specific antibodies after immunization with an adjuvant containing TDM. Overall, this study provides the mechanism by which Dectin-3 induces Mincle expression in response to Mycobacterium infection, which will have significant impact to improve adjuvant and design vaccine for antimicrobial infection.