Unveiling a Microexon Switch: Novel Regulation of the Activities of Sugar Assimilation and Plant-Cell-Wall-Degrading Xylanases and Cellulases by Xlr2 in Trichoderma virens.

Functional microexons have not previously been described in filamentous fungi. Here, we describe a novel mechanism of transcriptional regulation in Trichoderma requiring the inclusion of a microexon from the Xlr2 gene. In low-glucose environments, a long mRNA including the microexon encodes a protein with a GAL4-like DNA-binding domain (Xlr2-α), whereas in high-glucose environments, a short mRNA that is produced encodes a protein lacking this DNA-binding domain (Xlr2-ß). Interestingly, the protein isoforms differ in their impact on cellulase and xylanase activity. Deleting the Xlr2 gene reduced both xylanase and cellulase activity and growth on different carbon sources, such as carboxymethylcellulose, xylan, glucose, and arabinose. The overexpression of either Xlr2-α or Xlr2-ß in T. virens showed that the short isoform (Xlr2-ß) caused higher xylanase activity than the wild types or the long isoform (Xlr2-α). Conversely, cellulase activity did not increase when overexpressing Xlr2-ß but was increased with the overexpression of Xlr2-α. This is the first report of a novel transcriptional regulation mechanism of plant-cell-wall-degrading enzyme activity in T. virens. This involves the differential expression of a microexon from a gene encoding a transcriptional regulator.

Islet autoantibody types mark differential clinical characteristics at diagnosis of pediatric type 1 diabetes.

BACKGROUND: We aimed to study whether islet autoantibody type marks differential characteristics at the time of type 1 diabetes (T1D) diagnosis. METHODS: We studied 711 children with newly diagnosed autoimmune T1D. We compared demographic (sex, age, race/ethnicity), clinical (pubertal development, BMI percentile, diabetic ketoacidosis [DKA]) and laboratory (glucose, hemoglobin A1c [HbA1c], C-peptide, tissue transglutaminase antibodies [tTGA], thyroglobulin antibodies, and thyroid peroxidase antibodies [TPOA]) characteristics by presence/absence of autoantibodies to insulin (IAA), GAD65 (GADA), or IA-2/ICA512 (IA-2A). Islet autoantibody titers were evaluated among the children positive for the relevant autoantibody type. We used multivariable analysis to adjust for potential confounders. RESULTS: IAA+ was statistically associated with younger age (p < 0.0001) and lower HbA1c (p = 0.049) while Tanner stage, GADA status and number of positive islet autoantibodies were not significant in the multivariable model. GADA+ was associated with female sex (OR = 4.0, p = 0.002) and negatively with elevated tTGA titers (>50 U/mL) (OR = 0.21, p = 0.026) but not with age, IAA status, IA-2A status, islet autoantibody number, or thyroid autoimmunity. None of the associations with IA-2A positivity was statistically significant in the multivariable analysis. In multivariable models, IAA titer was significantly associated with younger age (p = 0.006), DKA (p = 0.017) and higher tTGA levels (p = 0.002); GADA titer with female sex (p = 0.028), racial minority (p = 0.046) and TPOA positivity (p = 0.021); and IA-2A titer with older age (p = 0.001) and not being African American (p = 0.024). CONCLUSIONS: Islet autoantibody type is associated with differential characteristics at diagnosis of pediatric T1D. Longitudinal and mechanistic studies are needed to evaluate T1D endotypes by autoantibody type.

Type 2 diabetes in prepubertal children.

OBJECTIVE: Puberty-induced insulin resistance is considered critical in the pathogenesis of type 2 diabetes (T2D) in youth. The development of T2D before puberty suggests distinct risk factors and pathophysiology but, because of its rarity, this has not been well studied. We aimed to describe the clinical characteristics of children with T2D diagnosed before the onset of puberty. RESEARCH DESIGN AND METHODS: We retrospectively studied all children with autoantibody-negative T2D and available pubertal development assessment seen at our center between July 2016 and July 2019, and compared characteristics of those at Tanner stage I (prepubertal, n = 35) versus those at Tanner II-V of pubertal development (n = 341). RESULTS: At T2D diagnosis, prepubertal children compared with those at Tanner II-V had higher body mass index z-score (p = 0.003) and higher C-peptide (p = 0.003) (while glucose levels were not significantly different), with differences retaining significance after adjustment for glucose, race/ethnicity and sex. Dyslipidemia occurred in 100% of prepubertal children versus 89.7% of those diagnosed later (p = 0.036). Of the prepubertal children diagnosed under age 10 (n = 13), 69.2% were female, 100% racial/ethnic minority, 100% had obesity with history of dyslipidemia and none with diabetic ketoacidosis. CONCLUSIONS: T2D, although rarely, can develop before puberty. Children with T2D diagnosed in the prepubertal period have more severe obesity, greater insulin resistance, and more frequent dyslipidemia than older youth. These findings suggest that children with prepubertal T2D are at increased risk for associated morbidity compared with older youth and underscore the significance of interventions to prevent and treat obesity in early childhood.

NFκB activation by modified vaccinia virus as a novel strategy to enhance neutrophil migration and HIV-specific T-cell responses.

Neutrophils are antigen-transporting cells that generate vaccinia virus (VACV)-specific T-cell responses, yet how VACV modulates neutrophil recruitment and its significance in the immune response are unknown. We generated an attenuated VACV strain that expresses HIV-1 clade C antigens but lacks three specific viral genes (A52R, K7R, and B15R). We found that these genes act together to inhibit the NFκB signaling pathway. Triple ablation in modified virus restored NFκB function in macrophages. After virus infection of mice, NFκB pathway activation led to expression of several cytokines/chemokines that increased the migration of neutrophil populations (Nα and Nß) to the infection site. Nß cells displayed features of antigen-presenting cells and activated virus-specific CD8 T cells. Enhanced neutrophil trafficking to the infection site correlated with an increased T-cell response to HIV vector-delivered antigens. These results identify a mechanism for poxvirus-induced immune response and alternatives for vaccine vector design.

Yield and cold storage of Trichoderma conidia is influenced by substrate pH and storage temperature.

In this study we examined the influence of the ambient pH during morphogenesis on conidial yield of Trichoderma sp. "atroviride B" LU132 and T. hamatum LU593 and storage at low temperatures. The ambient pH of the growth media had a dramatic influence on the level of Trichoderma conidiation and this was dependent on the strain and growth media. On malt-extract agar, LU593 yield decreased with increasing pH (3-6), whereas yield increased with increasing pH for LU132. During solid substrate production the reverse was true for LU132 whereby yield decreased with increasing pH. The germination potential of the conidia decreased significantly over time in cold storage and the rate of decline was a factor of the strain, pH during morphogenesis, growth media, and storage temperature.

Identification of growth stage molecular markers in Trichoderma sp. 'atroviride type B' and their potential application in monitoring fungal growth and development in soil.

Several members of the genus Trichoderma are biocontrol agents of soil-borne fungal plant pathogens. The effectiveness of biocontrol agents depends heavily on how they perform in the complex field environment. Therefore, the ability to monitor and track Trichoderma within the environment is essential to understanding biocontrol efficacy. The objectives of this work were to: (a) identify key genes involved in Trichoderma sp. 'atroviride type B' morphogenesis; (b) develop a robust RNA isolation method from soil; and (c) develop molecular marker assays for characterizing morphogenesis whilst in the soil environment. Four cDNA libraries corresponding to conidia, germination, vegetative growth and conidiogenesis were created, and the genes identified by sequencing. Stage specificity of the different genes was confirmed by either Northern blot or quantitative reverse-transcriptase PCR (qRT-PCR) analysis using RNA from the four stages. con10, a conidial-specific gene, was observed in conidia, as well as one gene also involved in subsequent stages of germination (L-lactate/malate dehydrogenase encoding gene). The germination stage revealed high expression rates of genes involved in amino acid and protein biosynthesis, while in the vegetative-growth stage, genes involved in differentiation, including the mitogen-activated protein kinase kinase similar to Kpp7 from Ustilago maydis and the orthologue to stuA from Aspergillus nidulans, were preferentially expressed. Genes involved in cell-wall synthesis were expressed during conidiogenesis. We standardized total RNA isolation from Trichoderma sp. 'atroviride type B' growing in soil and then examined the expression profiles of selected genes using qRT-PCR. The results suggested that the relative expression patterns were cyclic and not accumulative.

The mitochondrial Dnm1-like fission component is required for lgA2-induced mitophagy but dispensable for starvation-induced mitophagy in Ustilago maydis.

Selective elimination of mitochondria by autophagy (mitophagy) is a crucial developmental process to dispose of disintegrated or superflous organelles. However, little is known about underlying regulatory mechanisms. We have investigated mitophagy in response to conditional overexpression of the a2 mating-type locus gene lga2, which encodes a small mitochondrial protein critically involved in uniparental mitochondrial DNA inheritance during sexual development of Ustilago maydis. In this study, we show that conditional overexpression of lga2 efficiently triggers mitophagy that is dependent on atg8 and atg11, consistent with selective autophagy. lga2-triggered mitophagy is preceded by mitochondrial dysfunction, including depletion of mitochondrial RNA transcripts, and is mechanistically distinct from starvation-induced mitophagy despite a common requirement for atg11. In particular, lga2-triggered mitophagy strongly depends on the mitochondrial fission factor Dnm1, but it is only slightly affected by N-acetylcysteine, which is an inhibitor of starvation-induced mitophagy. To further delineate the role of mitochondrial fission, we analyzed lga2 effects in Δfis1 mutants. This revealed that mitochondrial fragmentation was only attenuated and mitophagy was largely unaffected. In further support of a Fis1-independent role for Dnm1, mitochondrial association of green fluorescent protein-tagged Dnm1 as well as Dnm1-opposed mitochondrial fusion during sexual development were fis1 independent. In conclusion, our results specify the role of the mitochondrial fission factor Dnm1 in mitophagy and uncover differences between mitophagy pathways in the same cellular system.

Demographic and diagnostic markers in new onset pediatric type 1 and type 2 diabetes: differences and overlaps.

PURPOSE: Type 1 diabetes (T1D) is the most common type of diabetes in children, but the frequency of type 2 diabetes (T2D) is increasing rapidly. Classification of diabetes is based on a constellation of features that vary by type. We aimed to compare demographic, clinical, and laboratory characteristics at diagnosis of pediatric T1D and T2D. METHODS: We studied children who visited a large academic hospital in Houston, Texas (USA) with a new diagnosis of T2D (n=753) or T1D (n=758). We compared age, sex, race/ethnicity, presence of obesity, glucose, hemoglobin A1c, islet autoantibody positivity, C-peptide, and presence of diabetic ketoacidosis (DKA) at diabetes diagnosis. RESULTS: At diagnosis, children with T2D, compared with those with T1D, were older (13.6 years vs. 9.7 years), more likely female (63.2% vs. 47.8%), of racial/ethnic minority (91.1% vs. 42.3%), and obese (90.9% vs. 19.4%) and were less likely to have DKA (7.8% vs. 35.0%) and diabetes autoantibodies (5.5% vs. 95.4%). Children with T2D also had significantly lower glucose, lower hemoglobin A1c and lower C-peptide level (all comparisons, p<0.0001). In multiple logistic regression analysis, older age, racial/ethnic minority, obesity, higher C-peptide, and negative islet autoantibodies were independently associated with T2D (all, p<0.05), while sex, glucose, hemoglobin A1c, and DKA were not (model p<0.0001). CONCLUSION: There are important demographic, clinical, and laboratory differences between T1D and T2D in children. However, none of the characteristics were unique to either diabetes type, which poses challenges to diabetes classification at diagnosis.

Histidine kinase two-component response regulators Ssk1, Skn7 and Rim15 differentially control growth, developmental and volatile organic compounds emissions as stress responses in Trichoderma atroviride.

The Skn7, Ssk1 and Rim15 proteins are response regulators involved in osmotic, oxidative and nutritional stress in fungi. In order to verify the involvement of these genes in Trichoderma atroviride IMI206040's growth, conidiation, direct antagonism against plant pathogens (Rhizoctonia solani and Sclerotinia sclerotiorum), production of volatile organic compounds (VOCs) with fungistatic effect, and interaction with plants (growth promotion), single mutants were generated, and the phenotypic patterns were analysed in comparison to the wild-type (wt) strain. The mutants were submitted to osmotic, oxidative, membrane and cell wall stress conditions in vitro. The Δskn7 and Δrim15 mutants did not show either significant differences at morphological level, or marked decreases in mycelial growth and conidiation in relation to wt, whereas Δssk1 had altered phenotypes in most conditions tested. The plant-growth promotion of Arabidopsis thaliana seedlings induced by VOCs was not quantitatively modified by any of the mutants in relation to the wt strain, although possible differences in secondary root hairs was noticed for Δrim15. The fungistatic activity was significantly altered for Δssk1 and Δrim15. Overall, the Δssk1 strain showed remarkable morphological differences, with decrease in mycelial growth and conidiation, being also affected in the antagonistic capacity against plant pathogens. The impacts demonstrated by the deletion of ssk1 suggest this gene has a relevant participation in the signalling response to different stresses in T. atroviride and in the interactive metabolism with phytopathogens and plants. On the other hand, unlike other fungal models, Skn7 did not appear to have a critical participation in the above-mentioned processes; Rim15 seemed to confirm its involvement in modulating cellular responses to nutritional status, although with a possible cross-talk with other cellular processes. Our results suggest that Ssk1 likely plays a key regulatory role, not only in basic metabolisms of T. atroviride, but also in biocontrol-related characteristics.

The a2 mating-type locus genes lga2 and rga2 direct uniparental mitochondrial DNA (mtDNA) inheritance and constrain mtDNA recombination during sexual development of Ustilago maydis.

Uniparental inheritance of mitochondria dominates among sexual eukaryotes. However, little is known about the mechanisms and genetic determinants. We have investigated the role of the plant pathogen Ustilago maydis genes lga2 and rga2 in uniparental mitochondrial DNA (mtDNA) inheritance during sexual development. The lga2 and rga2 genes are specific to the a2 mating-type locus and encode small mitochondrial proteins. On the basis of identified sequence polymorphisms due to variable intron numbers in mitochondrial genotypes, we could demonstrate that lga2 and rga2 decisively influence mtDNA inheritance in matings between a1 and a2 strains. Deletion of lga2 favored biparental inheritance and generation of recombinant mtDNA molecules in combinations in which inheritance of mtDNA of the a2 partner dominated. Conversely, deletion of rga2 resulted in predominant loss of a2-specific mtDNA and favored inheritance of the a1 mtDNA. Furthermore, expression of rga2 in the a1 partner protected the associated mtDNA from elimination. Our results indicate that Lga2 in conjunction with Rga2 directs uniparental mtDNA inheritance by mediating loss of the a1-associated mtDNA. This study shows for the first time an interplay of mitochondrial proteins in regulating uniparental mtDNA inheritance.

TrichoGate: An Improved Vector System for a Large Scale of Functional Analysis of Trichoderma Genes.

Species of the genus Trichoderma are ubiquitous in the environment and are widely used in agriculture, as biopesticides, and in the industry for the production of plant cell wall-degrading enzymes. Trichoderma represents an important genus of endophytes, and several Trichoderma species have become excellent models for the study of fungal biology and plant-microbe interactions; moreover, are exceptional biotechnological factories for the production of bioactive molecules useful in agriculture and medicine. Next-generation sequencing technology coupled with systematic construction of recombinant DNA molecules provides powerful tools that contribute to the functional analysis of Trichoderma genetics, thus allowing for a better understanding of the underlying factors determining its biology. Here, we present the creation of diverse vectors containing (i) promoter-specific vectors for Trichoderma, (ii) gene deletions (using hygromycin phosphotransferase as selection marker), (iii) protein localization (mCherry and eGFP, which were codon-optimized for Trichoderma), (iv) gene complementation (neomycin phosphotransferase) and (v) overexpression of encoding gene proteins fused to fluorescent markers, by using the Golden Gate cloning technology. Furthermore, we present the design and implementation of a binary vector for Agrobacterium-mediated transformation in Trichoderma to increase the homologous recombination rate and the generation of a novel selection marker based on carboxin resistance.

The NADPH Oxidases Nox1 and Nox2 Differentially Regulate Volatile Organic Compounds, Fungistatic Activity, Plant Growth Promotion and Nutrient Assimilation in Trichoderma atroviride.

In eukaryotic systems, membrane-bound NADPH oxidases (Nox) generate reactive oxygen species (ROS) as a part of normal physiological functions. In the soil-borne mycoparasitic and plant facultative symbiont Trichoderma atroviride, Nox1 and the regulator NoxR are involved in differentiation induced by mechanical damage, while the role of Nox2 has not been determined. The knock-out strains Δnox1, ΔnoxR and Δnox2 were compared to the parental strain (WT) in their ability to grow and conidiate under a series of stress conditions (osmotic, oxidative, membrane, and cell-wall stresses). All three genes were differentially involved in the stress-response phenotypes. In addition, several interactive experiments with biotic factors (plant seedlings and other fungi) were performed comparing the mutant phenotypes with the WT, which was used as the reference strain. Δnox1 and ΔnoxR significantly reduced the antagonistic activity of T. atroviride against Rhizoctonia solani and Sclerotinia sclerotiorum in direct confrontation assays, but Δnox2 showed similar activity to the WT. The Δnox1, ΔnoxR, and Δnox2 mutants showed quantitative differences in the emission of several volatile organic compounds (VOCs). The effects of a blend of these volatiles on plant-growth promotion of Arabidopsis thaliana seedlings were determined in closed-chamber experiments. The increase in root and shoot biomass induced by T. atroviride VOCs was significantly lowered by ΔnoxR and Δnox1, but not by Δnox2. In terms of fungistatic activity at a distance, Δnox2 had a significant reduction in this trait against R. solani and S. sclerotiorum, while fungistasis was highly increased by ΔnoxR and Δnox1. Identification and quantification of individual VOCs in the blends emitted by the strains was performed by GC-MS and the patterns of variation observed for individual volatiles, such as 6-Pentyl-2H-pyran-2-one (6PP-1) and (E)-6-Pent-1-enylpyran-2-one (6PP-2) were consistent with their negative effects in plant-growth promotion and positive effects in fungistasis at a distance. Nox1 and NoxR appear to have a ubiquitous regulatory role of in a variety of developmental and interactive processes in T. atroviride either as positive or negative modulators. Nox2 may also have a role in regulating production of VOCs with fungistatic activity.

Environmental Growth Conditions of Trichoderma spp. Affects Indole Acetic Acid Derivatives, Volatile Organic Compounds, and Plant Growth Promotion.

Trichoderma species are soil-borne filamentous fungi widely utilized for their many plant health benefits, such as conferring improved growth, disease resistance and abiotic stress tolerance to their hosts. Many Trichoderma species are able to produce the auxin phytohormone indole-3-acetic acid (IAA), and its production has been suggested to promote root growth. Here we show that the production of IAA is strain dependent and diverse external stimuli are associated with its production. In in vitro assays, Arabidopsis primary root length was negatively affected by the interaction with some Trichoderma strains. In soil experiments, a continuum effect on plant growth was shown and this was also strain dependent. In plate assays, some strains of Trichoderma spp. inhibited the expression of the auxin reporter gene DR5 in Arabidopsis primary roots but not secondary roots. When Trichoderma spp. and A. thaliana were physically separated, enhancement of both shoot and root biomass, increased root production and chlorophyll content were observed, which strongly suggested that volatile production by the fungus influenced the parameters analyzed. Trichoderma strains T. virens Gv29.8, T. atroviride IMI206040, T. sp. "atroviride B" LU132, and T. asperellum LU1370 were demonstrated to promote plant growth through volatile production. However, contrasting differences were observed with LU1370 which had a negative effect on plant growth in soil but a positive effect in plate assays. Altogether our results suggest that the mechanisms and molecules involved in plant growth promotion by Trichoderma spp. are multivariable and are affected by the environmental conditions.

Isolation and Mass Production of Trichoderma.

Members of the genus Trichoderma comprise the majority of commercial fungal biocontrol agents of plant diseases. As such, there is a wealth of information available on the analysis of their biocontrol potential and the mechanisms behind their superior abilities. This chapter aims to summarize the most common methods utilized within a Trichoderma biocontrol program for the isolation, identification, and mass propagation of individual strains.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species.

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

Arabidopsis thaliana polyamine content is modified by the interaction with different Trichoderma species.

Plants are associated with a wide range of microorganisms throughout their life cycle, and some interactions result on plant benefits. Trichoderma species are plant beneficial fungi that enhance plant growth and development, contribute to plant nutrition and induce defense responses. Nevertheless, the molecules involved in these beneficial effects still need to be identify. Polyamines are ubiquitous molecules implicated in plant growth and development, and in the establishment of plant microbe interactions. In this study, we assessed the polyamine profile in Arabidopsis plants during the interaction with Trichoderma virens and Trichoderma atroviride, using a system that allows direct plant-fungal contact or avoids their physical interaction (split system). The plantlets that grew in the split system exhibited higher biomass than the ones in direct contact with Trichoderma species. After 3 days of interaction, a significant decrease in Arabidopsis polyamine levels was observed in both systems (direct contact and split). After 5 days of interaction polyamine levels were increased. The highest levels were observed with T. atroviride (split system), and with T. virens (direct contact). The expression levels of Arabidopsis ADC1 and ADC2 genes during the interaction with the fungi were also assessed. We observed a time dependent regulation of ADC1 and ADC2 genes, which correlates with polyamine levels. Our data show an evident change in polyamine profile during Arabidopsis - Trichoderma interaction, accompanied by evident alterations in plant root architecture. Polyamines could be involved in the changes undergone by plant during the interaction with this beneficial fungus.

Phosphate starvation induces a determinate developmental program in the roots of Arabidopsis thaliana.

When growing under limiting phosphate (P) conditions, Arabidopsis thaliana plants show dramatic changes in root architecture, including a reduction in primary root length, increased formation of lateral roots and greater formation of root hairs. Here we report that primary root growth inhibition by low P is caused by a shift from an indeterminate to a determinate developmental program. In the primary root, the low P-induced determinate growth program initiates with a reduction of cell elongation followed by the progressive loss of meristematic cells. At later stages, cell proliferation ceases and cell differentiation takes place at the former cell elongation and meristematic regions of the primary root. In low P, not only the primary but also almost all mature lateral roots enter the determinate developmental program. Kinetic studies of expression of the cell cycle marker CycB1;1:uidA and the quiescent center (QC) identity marker QC46:GUS showed that in low P conditions, reduction in proliferation in the primary root was preceded by alterations in the QC. These results suggest that in Arabidopsis, P limitation can induce a determinate root developmental program that plays an important role in altering root system architecture and that the QC could act as a sensor of environmental signals.

Organic acid metabolism in plants: from adaptive physiology to transgenic varieties for cultivation in extreme soils.

During the last 20 years increasing experimental evidence has associated organic acid metabolism with plant tolerance to environmental stress. Current knowledge shows that organic acids not only act as intermediates in carbon metabolism but also as key components in mechanisms that some plants use to cope with nutrient deficiencies, metal tolerance and plant-microbe interactions operating at the root-soil interphase. In this review we summarize recent knowledge on the physiology and occurrence of organic acids in plants and their special relevance concerning nitrate reduction, phosphorus and iron acquisition, aluminum tolerance and soil ecology. We also discuss novel findings in relation to the biotechnological manipulation of organic acids in transgenic models ranging from cell cultures to whole plants. This novel perspective of organic acid metabolism and its potential manipulation may represent a way to understand fundamental aspects of plant physiology and lead to new strategies to obtain crop varieties better adapted to environmental and mineral stress.

Phosphate availability alters architecture and causes changes in hormone sensitivity in the Arabidopsis root system.

The postembryonic developmental program of the plant root system is plastic and allows changes in root architecture to adapt to environmental conditions such as water and nutrient availability. Among essential nutrients, phosphorus (P) often limits plant productivity because of its low mobility in soil. Therefore, the architecture of the root system may determine the capacity of the plant to acquire this nutrient. We studied the effect of P availability on the development of the root system in Arabidopsis. We found that at P-limiting conditions (<50 microM), the Arabidopsis root system undergoes major architectural changes in terms of lateral root number, lateral root density, and primary root length. Treatment with auxins and auxin antagonists indicate that these changes are related to an increase in auxin sensitivity in the roots of P-deprived Arabidopsis seedlings. It was also found that the axr1-3, axr2-1, and axr4-1 Arabidopsis mutants have normal responses to low P availability conditions, whereas the iaa28-1 mutant shows resistance to the stimulatory effects of low P on root hair and lateral root formation. Analysis of ethylene signaling mutants and treatments with 1-aminocyclopropane-1-carboxylic acid showed that ethylene does not promote lateral root formation under P deprivation. These results suggest that in Arabidopsis, auxin sensitivity may play a fundamental role in the modifications of root architecture by P availability.