Inhibition Of Primary Root Elongation Research Articles

Ultraviolet-B Radiation Represses Primary Root Elongation by Inhibiting Cell Proliferation in the Meristematic Zone of Arabidopsis Seedlings.

In Arabidopsis thaliana plants, exposure to UV-B induces an inhibition of primary root elongation. Different mutants have been isolated that are deficient in this response; however, little is known about the cellular and molecular mechanisms that regulate inhibition of root elongation in seedlings exposed to UV-B. In this work, we investigated the effect UV-B irradiation of different organs on primary root elongation. Our results demonstrate that irradiation of the leaves and shoots only induce a partial inhibition of primary root elongation, while when only roots are exposed to this radiation, primary root inhibition is similar as that measured when the complete seedling is irradiated. The consequences of exposure at different root developmental stages and times after the end of the treatment was also studied. We here show that inhibition of primary root elongation is a consequence of a decrease in cell proliferation in the meristematic zone of the primary roots, while the elongation zone size is not affected by the treatment. The decrease in cell number after UV-B exposure is partially compensated by an increase in cell length in the root meristem; however, this compensation is not enough to maintain the meristem size. We also here demonstrate that, similarly as what occurs in developing leaves, GROWTH REGULATING FACTOR 3 (GRF3) transcription factor regulates cell proliferation in UV-B irradiated roots; however, and in contrast to what occurs in the leaves, this response does not depend on the presence of MITOGEN ACTIVATED PROTEIN KINASE 3 (MPK3). Inhibition of primary root elongation by UV-B under our experimental conditions is also independent of the UV-B photoreceptor UV RESISTANT LOCUS 8 (UVR8) or ATAXIA TELANGIECTASIA MUTATED (ATM); but a deficiency in ATM AND RAD3-RELATED (ATR) expression increases UV-B sensitivity in the roots. Finally, our data demonstrate that UV-B affects primary root growth in various Arabidopsis accessions, showing different sensitivities to this radiation.

NITRIC OXIDE ASSOCIATED PROTEIN1 (AtNOA1) is necessary for copper-induced lateral root elongation in Arabidopsis thaliana

Copper (Cu2+) is an essential metal micronutrients required for normal plant growth and development, but is highly toxic when in excess. Inhibition of primary root elongation and stimulation of lateral root (LR) formation are usual reported symptoms when exposure to high concentrations of Cu2+. The mechanism underlying Cu2+-induced LR formation remains to be further elucidated. Here, we reported that Cu2+-induced LR formation is regulated by NITRIC OXIDE ASSOCIATED PROTEIN1 (AtNOA1), mainly through mediating Cu2+-induced LR elongation growth, but not LR primordium initiation. The initiation of LR primordium is inhibited by the mutation of AtNOA1. Transcriptional analysis displayed that the expression of genes involved in Cu2+ homeostasis and tolerance showed significant difference between Atnoa1 and WT. Cu2+-induced auxin redistribution was inhibited in Atnoa1, indicated by auxin response reporter plants (DR5:GUS and DR5:GFP). Furthermore, Cu2+-induced LR formation were more sensitive to exogenous auxin transport inhibitors NOA and NPA in Atnoa1 than in WT. Interestingly, Cu2+-induced H2O2 level was higher in Atnoa1 than WT, but Cu2+-induced LR formation was normal in H2O2-related mutants. These results showed that AtNOA1 regulated the initiation of LR primordium and mediates Cu2+-induced LR elongation in Arabidopsis thaliana. AtNOA1 mediates Cu2+-induced LR elongation maybe through affecting Cu2+ homeostasis and Cu2+-induced auxin redistribution, but not depending on Cu2+-induced H2O2 accumulation.

Coordinated cytokinin signaling and auxin biosynthesis mediates arsenate-induced root growth inhibition.

Interactions between plant hormones and environmental signals are important for the maintenance of root growth plasticity under ever-changing environmental conditions. Here, we demonstrate that arsenate (AsV), the most prevalent form of arsenic (As) in nature, restrains elongation of the primary root through transcriptional regulation of local auxin biosynthesis genes in the root tips of Arabidopsis (Arabidopsis thaliana) plants. The ANTHRANILATE SYNTHASE ALPHA SUBUNIT 1 (ASA1) and BETA SUBUNIT 1 (ASB1) genes encode enzymes that catalyze the conversion of chorismate to anthranilate (ANT) via the tryptophan-dependent auxin biosynthesis pathway. Our results showed that AsV upregulates ASA1 and ASB1 expression in root tips, and ASA1- and ASB1-mediated auxin biosynthesis is involved in AsV-induced root growth inhibition. Further investigation confirmed that AsV activates cytokinin signaling by stabilizing the type-B ARABIDOPSIS RESPONSE REGULATOR1 (ARR1) protein, which directly promotes the transcription of ASA1 and ASB1 genes by binding to their promoters. Genetic analysis revealed that ASA1 and ASB1 are epistatic to ARR1 in the AsV-induced inhibition of primary root elongation. Overall, the results of this study illustrate a molecular framework that explains AsV-induced root growth inhibition via crosstalk between two major plant growth regulators, auxin and cytokinin.

Interaction of SAUR53 and Its Close Homologs with Calmodulin May Play a Role in Early Development in Arabidopsis

Auxin plays crucial roles in modulating various aspects of plant growth and development throughout the plant life cycle. At the molecular level, auxin rapidly upregulates the expression of many genes, including those belong to three primary auxin-responsive (PAR) gene families. Small auxin-up RNA (SAUR) family is the largest among the three PAR gene families and contains 81 genes; however, molecular and physiological functions of most of them are yet to be discovered. Recently, we showed that the ectopic expression of SAUR53 causes hyper-elongation of cells and organs and alters apical hook development in Arabidopsis. Here we show that SAUR53 and its three close homologs, SAUR42, SAUR52, and SAUR69, physically interact with calmodulin (CaM) through their N-terminal CaM-binding domains. Additionally, seedlings of all four SAUR overexpression (SAUR-OX) transgenic lines are less sensitive to CaM inhibitor mediated inhibition of primary root elongation. We also show that CaM inhibitors accelerate the apical hook opening in dark-grown, wild-type seedlings, similar to dark-grown SAUR53-OX seedlings. Collectively, our present work reveals molecular and physiological relationships between SAURs and CaM proteins.

AtCAF-1 mutants show different DNA damage responses after ultraviolet-B than those activated by other genotoxic agents in leaves.

Chromatin assembly factor-1 (CAF-1) is a histone H3/H4 chaperone that participates in DNA and chromatin interaction processes. In this manuscript, we show that organs from CAF-1 deficient plants respond differently to ultraviolet-B (UV-B) radiation than to other genotoxic stresses. For example, CAF-1 deficient leaves tolerate better UV-B radiation, showing lower cyclobutane pyrimidine dimer (CPD) accumulation, lower inhibition of cell proliferation, increased cell wall thickness, UV-B absorbing compounds, and ploidy levels, whereas previous data from different groups have shown that CAF-1 mutants show shortening of telomeres, loss of 45S rDNA, and increased homologous recombination, phenotypes associated to DNA breaks. Interestingly, CAF-1 deficient roots show increased inhibition of primary root elongation, with decreased meristem size due to a higher inhibition of cell proliferation after UV-B exposure. The decrease in root meristem size in CAF-1 mutants is a consequence of defects in programmed cell death after UV-B exposure. Together, we provide evidence demonstrating that root and shoot meristematic cells may have distinct protection mechanisms against CPD accumulation by UV-B, which may be linked with different functions of the CAF-1 complex in these different organs.

Characterization the role of a UFC homolog, AtAuxRP3, in the regulation of Arabidopsis seedling growth and stress response

Auxin is a well-known, crucial regulator of the entire plant lifecycle, not only orchestrating many aspects of plant growth and development, but also playing various roles in biotic and abiotic stress. This study reports the isolation and functional characterization of a DUF-966 domain-containing gene, At3g46110, re-named AtAuxRP3. AtAuxRP3 overexpression in Arabidopsis increased the levels of endogenous indole-3-acetic acid, enhanced expression of the auxin-responsive reporter DR5:GUS near the vegetative shoot apex, and led to ectopic activation of auxin signaling, including dysmorphic (narrow, asymmetric) rosette leaves, abnormal emergence of inflorescence, inhibition of primary root elongation and arrest of dark-grown hypocotyls. AtAuxRP3-OX lines also showed decreased tolerance to NaCl and osmotic stress during Arabidopsis seeds germination and young seedling growth. Genome-wide transcriptomic analysis showed AtAuxRP3-OX seedlings displayed increases in the expression of genes that group in a variety of developmental categories, while other downregulated genes were associated with stress responses. Our results provide evidence for a regulatory role of AtAuxRP3 in endogenous auxin levels, leaf development, and initiation of inflorescence stems early in reproductive development during Arabidopsis seedling growth.

Rice OsDOF15 contributes to ethylene-inhibited primary root elongation under salt stress.

In early seedlings, the primary root adapts rapidly to environmental changes through the modulation of endogenous hormone levels. The phytohormone ethylene inhibits primary root elongation, but the underlying molecular mechanism of how ethylene-reduced root growth is modulated in environmental changes remains poorly understood. Here, we show that a novel rice (Oryza sativa) DOF transcription factor OsDOF15 positively regulates primary root elongation by regulating cell proliferation in the root meristem, via restricting ethylene biosynthesis. Loss-of-function of OsDOF15 impaired primary root elongation and cell proliferation in the root meristem, whereas OsDOF15 overexpression enhanced these processes, indicating that OsDOF15 is a key regulator of primary root elongation. This regulation involves the direct interaction of OsDOF15 with the promoter of OsACS1, resulting in the repression of ethylene biosynthesis. The control of ethylene biosynthesis by OsDOF15 in turn regulates cell proliferation in the root meristem. OsDOF15 transcription is repressed by salt stress, and OsDOF15-mediated ethylene biosynthesis plays a role in inhibition of primary root elongation by salt stress. Thus, our data reveal how the ethylene-inhibited primary root elongation is finely controlled by OsDOF15 in response to environmental signal, a novel mechanism of plants responding to salt stress and transmitting the information to ethylene biosynthesis to restrict root elongation.

HOMEOBOX PROTEIN52 Mediates the Crosstalk between Ethylene and Auxin Signaling during Primary Root Elongation by Modulating Auxin Transport-Related Gene Expression.

The gaseous hormone ethylene participates in many physiological processes in plants. Ethylene-inhibited root elongation involves PIN-FORMED2 (PIN2)-mediated basipetal auxin transport, but the molecular mechanisms underlying the regulation of PIN2 function by ethylene (and therefore auxin distribution) are poorly understood. Here, we report that the plant-specific and ethylene-responsive HD-Zip gene HB52 is involved in ethylene-mediated inhibition of primary root elongation in Arabidopsis thaliana Biochemical and genetic analyses demonstrated that HB52 is ethylene responsive and acts downstream of ETHYLENE-INSENSITIVE3 (EIN3). HB52 knockdown mutants displayed an ethylene-insensitive phenotype during primary root elongation, while its overexpression resulted in short roots, as observed in ethylene-treated plants. In addition, root auxin distribution and gravitropism were impaired in HB52 knockdown and overexpression lines. Consistent with these findings, in vitro and in vivo binding experiments showed that HB52 regulates the expression of auxin transport-related genes, including PIN2, WAVY ROOT GROWTH1 (WAG1), and WAG2 by physically binding to their promoter regions. These findings suggest that HB52 functions in the ethylene-mediated inhibition of root elongation by modulating the expression of auxin transport components downstream of EIN3, revealing a mechanism in which HB52 acts as an important node in the crosstalk between ethylene and auxin signaling during plant growth and development.

Exogenous glycine inhibits root elongation and reduces nitrate-N uptake in pak choi (Brassica campestris ssp. Chinensis L.).

Nitrogen (N) supply, including NO3--N and organic N in the form of amino acids can influence the morphological attributes of plants. For example, amino acids contribute to plant nutrition; however, the effects of exogenous amino acids on NO3--N uptake and root morphology have received little attention. In this study, we evaluated the effects of exogenous glycine (Gly) on root growth and NO3--N uptake in pak choi (Brassica campestris ssp. Chinensis L.). Addition of Gly to NO3--N agar medium or hydroponic solution significantly decreased pak choi seedling root length; these effects of Gly on root morphology were not attributed to the proportion of N supply derived from Gly. When pak choi seedlings were exposed to mixtures of Gly and NO3--N in hydroponic culture, Gly significantly reduced 15NO3--N uptake but significantly increased the number of root tips per unit root length, root activity and 15NO3--N uptake rate per unit root length. In addition, 15N-Gly was taken up into the plants. In contrast to absorbed NO3--N, which was mostly transported to the shoots, a larger proportion of absorbed Gly was retained in the roots. Exogenous Gly enhanced root 1-aminocyclopropane-1-carboxylic acid synthase (ACS) and oxidase (ACO) activities and ethylene production. The ethylene antagonists aminoethoxyvinylglycine (0.5 μM AVG) and silver nitrate (10 μM AgNO3) partly reversed Gly-induced inhibition of primary root elongation on agar plates and increased the NO3--N uptake rate under hydroponic conditions, indicating exogenous Gly exerts these effects at least partly by enhancing ethylene production in roots. These findings suggest Gly substantially affects root morphology and N uptake and provide new information on the specific responses elicited by organic N sources.

Involvement of reactive oxygen species and auxin in serotonin-induced inhibition of primary root elongation

The well-known neurotransmitter 5-hydroxytryptamine (serotonin) not only regulates sleep and mood in humans and animals but may also play important roles in modulating growth, development, and defense responses, such as seed germination, flowering, and abiotic stress tolerance, in plants. Serotonin inhibits primary root (PR) growth; however, the physiological and molecular mechanisms underlying serotonin-mediated PR growth inhibition remain largely unclear. Here, we investigate the effects of serotonin on root growth and development in Arabidopsis. Serotonin inhibits PR elongation by affecting both the meristem and elongation zones. In the meristem zone, serotonin represses both meristem cell division potential and stem cell niche activity. Serotonin induces H2O2 overaccumulation in the elongation zone and reduces O2- accumulation in the meristem zone by a UPB1 pathway, thereby disrupting reactive oxygen species (ROS) equilibrium in root tips, thus resulting in PR growth inhibition. Serotonin also regulates auxin distribution in root tips by decreasing auxin-related gene expression and repressing auxin transport through modulation of AUX1 and PIN2 abundances in root tips. Taken together, our data indicate that high concentrations of serotonin result in stress responses in plants by inhibiting PR elongation through the regulation of H2O2 and O2- distribution in PR tips and through an auxin pathway via the repression of auxin biosynthesis and transport.

Evidence for the Involvement of Auxin, Ethylene and ROS Signaling During Primary Root Inhibition of Arabidopsis by the Allelochemical Benzoic Acid.

Allelopathy is mediated by plant-derived secondary metabolites (allelochemicals) which are released by donor plants and affect the growth and development of receptor plants. The plant root is the first organ which senses soil allelochemicals this results in the production of a shorter primary root. However, the mechanisms underlying this process remain elusive. Here, we report that a model allelochemical benzoic acid (BA) inhibited primary root elongation of Arabidopsis seedlings by reducing the sizes of both the meristem and elongation zones, and that auxin signaling affected this process. An increase in auxin level in the root tips was associated with increased expression of auxin biosynthesis genes and auxin polar transporter AUX1 and PIN2 genes under BA stress. Mutant analyses demonstrated that AUX1 and PIN2 rather than PIN1 were required for the inhibition of primary root elongation during BA exposure. Furthermore, BA stimulated ethylene evolution, whereas blocking BA-induced ethylene signaling with an ethylene biosynthesis inhibitor (Co2+), an ethylene perception antagonist (1-methylcyclopropene) or ethylene signaling mutant lines etr1-3 and ein3eil1 compromised BA-mediated inhibition of root elongation and up-regulation of auxin biosynthesis-related genes together with AUX1 and PIN2, indicating that ethylene signal was involved in auxin-mediated inhibition of primary root elongation during BA stress. Further analysis revealed that the BA-induced reactive oxygen species (ROS) burst contributed to BA-mediated root growth inhibition without affecting auxin and ethylene signals. Taken together, our results reveal that the allelochemical BA inhibits root elongation by increasing auxin accumulation via stimulation of auxin biosynthesis and AUX1/PIN2-mediated auxin transport via stimulation of ethylene production and an auxin/ethylene-independent ROS burst.

Ethylene mediates dichromate-induced inhibition of primary root growth by altering AUX1 expression and auxin accumulation in Arabidopsis thaliana.

The hexavalent form of chromium [Cr(VI)] causes a major reduction in yield and quality of crops worldwide. The root is the first plant organ that interacts with Cr(VI) toxicity, which inhibits primary root elongation, but the underlying mechanisms of this inhibition remain elusive. In this study, we investigate the possibility that Cr(VI) reduces primary root growth of Arabidopsis by modulating the cell cycle-related genes and that ethylene signalling contributes to this process. We show that Cr(VI)-mediated inhibition of primary root elongation was alleviated by the ethylene perception and biosynthesis antagonists silver and cobalt, respectively. Furthermore, the ethylene signalling defective mutants (ein2-1 and etr1-3) were insensitive, whereas the overproducer mutant (eto1-1) was hypersensitive to Cr(VI). We also report that high levels of Cr(VI) significantly induce the distribution and accumulation of auxin in the primary root tips, but this increase was significantly suppressed in seedlings exposed to silver or cobalt. In addition, genetic and physiological investigations show that AUXIN-RESISTANT1 (AUX1) participates in Cr(VI)-induced inhibition of primary root growth. Taken together, our results indicate that ethylene mediates Cr(VI)-induced inhibition of primary root elongation by increasing auxin accumulation and polar transport by stimulating the expression of AUX1.

GaMYB85, an R2R3 MYB gene, in transgenic Arabidopsis plays an important role in drought tolerance

BackgroundMYB transcription factors (TFs) are one of the largest families of TFs in higher plants and are involved in diverse biological, functional, and structural processes. Previously, very few functional validation studies on R2R3 MYB have been conducted in cotton in response to abiotic stresses. In the current study, GaMYB85, a cotton R2R3 MYB TF, was ectopically expressed in Arabidopsis thaliana (Col-0) and was functionally characterized by overexpression in transgenic plants.ResultsThe in-silico analysis of GaMYB85 shows the presence of a SANT domain with a conserved R2R3 MYB imperfect repeat. The GaMYB85 protein has a 257-amino acid sequence, a molecular weight of 24.91 kD, and an isoelectric point of 5.58. Arabidopsis plants overexpressing GaMYB85 exhibited a higher seed germination rate in response to mannitol and salt stress, and higher drought avoidance efficiency than wild-type plants upon water deprivation. These plants had notably higher levels of free proline and chlorophyll with subsequent lower water loss rates and higher relative water content. Germination of GaMYB85 transgenics was more sensitive to abscisic acid (ABA) and extremely liable to ABA-induced inhibition of primary root elongation. Moreover, when subjected to treatment with different concentrations of ABA, transgenic plants with ectopically expressed GaMYB85 showed reduced stomatal density, with greater stomatal size and lower stomatal opening rates than those in wild-type plants. Ectopic expression of GaMYB85 led to enhanced transcript levels of stress-related marker genes such as RD22, ADH1, RD29A, P5CS, and ABI5.ConclusionsOur results indicate previously unknown roles of GaMYB85, showing that it confers good drought, salt, and freezing tolerance, most probably via an ABA-induced pathway. These findings can potentially be exploited to develop improved abiotic stress tolerance in cotton plants.

Comprehensive Genome-Wide Analysis of the Aux/IAA Gene Family in Eucalyptus: Evidence for the Role of EgrIAA4 in Wood Formation

Auxin plays a pivotal role in various plant growth and development processes, including vascular differentiation. The modulation of auxin responsiveness through the auxin perception and signaling machinery is believed to be a major regulatory mechanism controlling cambium activity and wood formation. To gain more insights into the roles of key Aux/IAA gene regulators of the auxin response in these processes, we identified and characterized members of the Aux/IAA family in the genome of Eucalyptus grandis, a tree of worldwide economic importance. We found that the gene family in Eucalyptus is slightly smaller than that in Populus and Arabidopsis, but all phylogenetic groups are represented. High-throughput expression profiling of different organs and tissues highlighted several Aux/IAA genes expressed in vascular cambium and/or developing xylem, some showing differential expression in response to developmental (juvenile vs. mature) and/or to environmental (tension stress) cues. Based on the expression profiles, we selected a promising candidate gene, EgrIAA4, for functional characterization. We showed that EgrIAA4 protein is localized in the nucleus and functions as an auxin-responsive repressor. Overexpressing a stabilized version of EgrIAA4 in Arabidopsis dramatically impeded plant growth and fertility and induced auxin-insensitive phenotypes such as inhibition of primary root elongation, lateral root emergence and agravitropism. Interestingly, the lignified secondary walls of the interfascicular fibers appeared very late, whereas those of the xylary fibers were virtually undetectable, suggesting that EgrIAA4 may play crucial roles in fiber development and secondary cell wall deposition.

The nitrate transporter MtNPF6.8 (MtNRT1.3) transports abscisic acid and mediates nitrate regulation of primary root growth in Medicago truncatula.

Elongation of the primary root during postgermination of Medicago truncatula seedlings is a multigenic trait that is responsive to exogenous nitrate. A quantitative genetic approach suggested the involvement of the nitrate transporter MtNPF6.8 (for Medicago truncatula NITRATE TRANSPORTER1/PEPTIDE TRANSPORTER Family6.8) in the inhibition of primary root elongation by high exogenous nitrate. In this study, the inhibitory effect of nitrate on primary root elongation, via inhibition of elongation of root cortical cells, was abolished in npf6.8 knockdown lines. Accordingly, we propose that MtNPF6.8 mediates nitrate inhibitory effects on primary root growth in M. truncatula. pMtNPF6.8:GUS promoter-reporter gene fusion in Agrobacterium rhizogenes-generated transgenic roots showed the expression of MtNPF6.8 in the pericycle region of primary roots and lateral roots, and in lateral root primordia and tips. MtNPF6.8 expression was insensitive to auxin and was stimulated by abscisic acid (ABA), which restored the inhibitory effect of nitrate in npf6.8 knockdown lines. It is then proposed that ABA acts downstream of MtNPF6.8 in this nitrate signaling pathway. Furthermore, MtNPF6.8 was shown to transport ABA in Xenopus spp. oocytes, suggesting an additional role of MtNPF6.8 in ABA root-to-shoot translocation. (15)NO3(-)-influx experiments showed that only the inducible component of the low-affinity transport system was affected in npf6.8 knockdown lines. This indicates that MtNPF6.8 is a major contributor to the inducible component of the low-affinity transport system. The short-term induction by nitrate of the expression of Nitrate Reductase1 (NR1) and NR2 (genes that encode two nitrate reductase isoforms) was greatly reduced in the npf6.8 knockdown lines, supporting a role of MtNPF6.8 in the primary nitrate response in M. truncatula.

AtWNK9 is regulated by ABA and dehydration and is involved in drought tolerance in Arabidopsis

WNK (with no lysine [K]) kinases play important regulatory roles in flowering, as well as salt and osmotic stress tolerance in plants. Here, we report that AtWNK9, a member of the Arabidopsis WNK gene family, was induced by exogenous abscisic acid (ABA) treatment and dehydration stress. Overexpression of AtWNK9 from the cauliflower mosaic virus 35S promoter in Arabidopsis resulted in increased sensitivity to ABA, strong inhibition of primary root elongation, increased proline accumulation, reduced stomatal aperture, and a reduced rate of water loss. In addition, plant survival under drought stress was improved compared to wild type. In contrast, a mutant with a T-DNA insertion in AtWNK9 showed reduced ABA sensitivity and an increased rate of water loss; further, it showed increased susceptibility to drought stress. The transcription of a number of ABA signaling components, including ABI1, ERA1, ABI3, and ABF3, was up-regulated in AtWNK9 transgenic plants and down-regulated in the wnk9 mutant in response to ABA. Some ABA-responsive and biosynthetic genes, as well as other drought-related genes, were altered at various levels in AtWNK9 transgenic plants and wnk9 mutants under dehydration stress. Overall, these findings suggest that AtWNK9 plays a positive role in ABA signaling and improves drought tolerance in transgenic Arabidopsis.

Copper Regulates Primary Root Elongation Through PIN1-Mediated Auxin Redistribution

The heavy metal copper (Cu) is an essential microelement required for normal plant growth and development, but it inhibits primary root growth when in excess. The mechanism underlying how excess Cu functions in this process remains to be further elucidated. Here, we report that a higher concentration of CuSO4 inhibited primary root elongation of Arabidopsis seedlings by affecting both the elongation and meristem zones. In the meristem zone, meristematic cell division potential was reduced by excess Cu. Further experiments showed that Cu can modulate auxin distribution, resulting in higher auxin activities in both the elongation and meristem zones of Cu-treated roots based on DR5::GUS expression patterns. This Cu-mediated auxin redistribution was shown to be responsible for Cu-mediated inhibition of primary root elongation. Additional genetic and physiological data demonstrated that it was PINFORMED1 (PIN1), but not PIN2 or AUXIN1 (AUX1), that regulated this process. However, Cu-induced hydrogen peroxide accumulation did not contribute to Cu-induced auxin redistribution for inhibition of root elongation. When the possible role of ethylene in this process was analyzed, Cu had a similar impact on the root elongation of both the wild type and the ein2-1 mutant, implying that Cu-mediated inhibition of primary root elongation was not due to the ethylene signaling pathway.

Succinate dehydrogenase assembly factor 2 is needed for assembly and activity of mitochondrial complex II and for normal root elongation in Arabidopsis

Mitochondria complex II (succinate dehydrogenase, SDH) plays a central role in respiratory metabolism as a component of both the electron transport chain and the tricarboxylic acid cycle. We report the identification of an SDH assembly factor by analysis of T-DNA insertions in At5g51040, a protein with unknown function that was identified by mass spectrometry analysis as a low abundance mitochondrial protein. This gene is co-expressed with a number of genes encoding mitochondrial proteins, including SDH1-1, and has low partial sequence similarity to human SDHAF2, a protein required for flavin-adenine dinucleotide (FAD) insertion into SDH. In contrast to observations of other SDH deficient lines in Arabidopsis, the sdhaf2 line did not affect photosynthetic rate or stomatal conductance, but instead showed inhibition of primary root elongation with early lateral root emergence, presumably due to the low SDH activity caused by the reduced abundance of SDHAF2. Both roots and leaves showed succinate accumulation but different responses in the abundance of other organic acids and amino acids assayed. Isolated mitochondria showed lowered SDH1 protein abundance, lowered maximal SDH activity and less protein-bound flavin-adenine dinucleotide (FAD) at the molecular mass of SDH1 in the gel separation. The short root phenotype and SDH function of sdhaf2 was fully complemented by transformation with SDHAF2. Application of the SDH inhibitor, malonate, phenocopied the sdhaf2 root architecture in WT. Whole root respiratory assays showed no difference between WT and sdhaf2, but micro-respirometry of the tips of roots clearly showed low oxygen consumption in sdhaf2 which could explain a metabolic deficit responsible for root tip growth.

Arabidopsis mutants lacking asparaginases develop normally but exhibit enhanced root inhibition by exogenous asparagine

Asparaginase catalyzes the degradation of L-asparagine to L-aspartic acid and ammonia, and is implicated in the catabolism of transported asparagine in sink tissues of higher plants. The Arabidopsis genome includes two genes, ASPGA1 and ASPGB1, belonging to distinct asparaginase subfamilies. Conditions of severe nitrogen limitation resulted in a slight decrease in seed size in wild-type Arabidopsis. However, this response was not observed in a homozygous T-DNA insertion mutant where ASPG genes had been inactivated. Under nitrogen-sufficient conditions, the ASPG mutant had elevated levels of free asparagine in mature seed. This phenotype was observed exclusively under conditions of low illumination, when a low ratio of carbon to nitrogen was translocated to the seed. Mutants deficient in one or both asparaginases were more sensitive than wild-type to inhibition of primary root elongation and root hair emergence by L-asparagine as a single nitrogen source. This enhanced inhibition was associated with increased accumulation of asparagine in the root of the double aspga1-1/-b1-1 mutant. This indicates that inhibition of root growth is likely elicited by asparagine itself or an asparagine-derived metabolite, other than the products of asparaginase, aspartic acid or ammonia. During germination, a fusion between the ASPGA1 promoter and beta-glucuronidase was expressed in endosperm cells starting at the micropylar end. Expression was initially high throughout the root and hypocotyl, but became restricted to the root tip after three days, which may indicate a transition to nitrogen-heterotrophic growth.

Auxin and ethylene are involved in the responses of root system architecture to low boron supply in Arabidopsis seedlings

Changes in root architecture are one of the adaptive strategies used by plants to compensate for nutrient deficiencies in soils. In this work, the temporal responses of Arabidopsis (Arabidopsis thaliana) root system architecture to low boron (B) supply were investigated. Arabidopsis Col-0 seedlings were grown in 10 µM B for 5 days and then transferred to a low B medium (0.4 µM) or control medium (10 µM) for a 4-day period. Low B supply caused an inhibition of primary root (PR) growth without altering either the growth or number of lateral roots (LRs). In addition, low B supply induced root hair formation and elongation in positions close to the PR meristem not observed under control conditions. The possible role of auxin and ethylene in the alteration of root system architecture elicited by low B supply was also studied by using two Arabidopsis reporter lines (DR5:GUS and EBS:GUS) and two Arabidopsis mutants with impaired auxin and ethylene signaling (aux1-22 and ein2-1). Low B supply increased auxin reporter DR5:GUS activity in PR tip, suggesting that low B alters the pattern of auxin distribution in PR tip. Moreover, PR elongation in aux1-22 mutant was less sensitive to low B treatment than in wild-type plants, which suggests that auxin resistant 1 (AUX1) participates in the inhibition of PR elongation under low B supply. From all these results, a hypothetical model to explain the effect of low B treatment on PR growth is proposed. We also show that ethylene, via ethylene-insensitive 2 (EIN2) protein, is involved in the induction of root hair formation and elongation under low B treatment.