Divergent roles in Arabidopsis thaliana development and defense of two homologous genes, aberrant growth and death2 and AGD2-LIKE DEFENSE RESPONSE PROTEIN1, encoding novel aminotransferases

Signal regulators of systemic acquired resistance

Salicylic acid (SA) is an important phytohormone that plays a vital role in a number of physiological responses, including plant defense. The last two decades have witnessed a number of breakthroughs related to biosynthesis, transport, perception and signaling mediated by SA. These findings demonstrate that SA plays a crictical role in both local and systemic defense responses. Systemic acquired resistance (SAR) is one such SA-dependent response. SAR is a long distance signaling mechanism that provides broad spectrum and long-lasting resistance to secondary infections throughout the plant. This unique feature makes SAR a highly desirable trait in crop production. This review summarizes the recent advances in the role of SA in SAR and discusses its relationship to other SAR inducers.

To die or not to die? Lessons from lesion mimic mutants

Programmed cell death (PCD) is a ubiquitous genetically regulated process consisting in an activation of finely controlled signaling pathways that lead to cellular suicide. Although some aspects of PCD control appear evolutionary conserved between plants, animals and fungi, the extent of conservation remains controversial. Over the last decades, identification and characterization of several lesion mimic mutants (LMM) has been a powerful tool in the quest to unravel PCD pathways in plants. Thanks to progress in molecular genetics, mutations causing the phenotype of a large number of LMM and their related suppressors were mapped, and the identification of the mutated genes shed light on major pathways in the onset of plant PCD such as (i) the involvements of chloroplasts and light energy, (ii) the roles of sphingolipids and fatty acids, (iii) a signal perception at the plasma membrane that requires efficient membrane trafficking, (iv) secondary messengers such as ion fluxes and ROS and (v) the control of gene expression as the last integrator of the signaling pathways.

Insect eggs induce a systemic acquired resistance in Arabidopsis

Although they constitute an inert stage of the insect's life, eggs trigger plant defences that lead to egg mortality or attraction of egg parasitoids. We recently found that salicylic acid (SA) accumulates in response to oviposition by the Large White butterfly Pieris brassicae, both in local and systemic leaves, and that plants activate a response that is similar to the recognition of pathogen-associated molecular patterns (PAMPs), which are involved in PAMP-triggered immunity (PTI). Here we discovered that natural oviposition by P. brassicae or treatment with egg extract inhibit growth of different Pseudomonas syringae strains in Arabidopsis through the activation of a systemic acquired resistance (SAR). This egg-induced SAR involves the metabolic SAR signal pipecolic acid, depends on ALD1 and FMO1, and is accompanied by a stronger induction of defence genes upon secondary infection. Although P. brassicae larvae showed a reduced performance when feeding on Pseudomonas syringae-infected plants, this effect was less pronounced when infected plants had been previously oviposited. Altogether, our results indicate that egg-induced SAR might have evolved as a strategy to prevent the detrimental effect of bacterial pathogens on feeding larvae.

Knockdown of LjALD1, AGD2-like defense response protein 1, influences plant growth and nodulation in Lotus japonicus

The discovery of the enzyme L,L-diaminopimelate aminotransferase (LL-DAP-AT, EC 2.6.1.83) uncovered a unique step in the L-lysine biosynthesis pathway in plants. In Arabidopsis thaliana, LL-DAP-AT has been shown to play a key role in plant-pathogen interactions by regulation of the salicylic acid (SA) signaling pathway. Here, a full-length cDNA of LL-DAP-AT named as LjALD1 from Lotus japonicus (Regel) Larsen was isolated. The deduced amino acid sequence shares 67% identity with the Arabidopsis aminotransferase AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (AtALD1) and is predicted to contain the same key elements: a conserved aminotransferase domain and a pyridoxal-5'-phosphate cofactor binding site. Quantitative real-time PCR analysis showed that LjALD1 was expressed in all L. japonicus tissues tested, being strongest in nodules. Expression was induced in roots that had been infected with the symbiotic rhizobium Mesorhizobium loti or treated with SA agonist benzo-(1, 2, 3)-thiadiazole-7-carbothioic acid. LjALD1 Knockdown exhibited a lower SA content, an increased number of infection threads and nodules, and a slight reduction in nodule size. In addition, compared with wild-type, root growth was increased and shoot growth was suppressed in LjALD1 RNAi plant lines. These results indicate that LjALD1 may play important roles in plant development and nodulation via SA signaling in L. japonicus.

Signaling by small metabolites in systemic acquired resistance

Plants can retain the memory of a prior encounter with a pest. This memory confers upon a plant the ability to subsequently activate defenses more robustly when challenged by a pest. In plants that have retained the memory of a prior, localized, foliar infection by a pathogen, the pathogen-free distal organs develop immunity against subsequent infections by a broad-spectrum of pathogens. The long-term immunity conferred by this mechanism, which is termed systemic acquired resistance (SAR), is inheritable over a few generations. Signaling mediated by the phenolic metabolite salicylic acid (SA) is critical for the manifestation of SAR. Recent studies have described the involvement of additional small metabolites in SAR signaling, including methyl salicylate, the abietane diterpenoid dehydroabietinal, the lysine catabolite pipecolic acid, a glycerol-3-phosphate-dependent factor and the dicarboxylic acid azelaic acid. Many of these metabolites can be systemically transported through the plant and probably facilitate communication by the primary infected tissue with the distal tissues, which is essential for the activation of SAR. Some of these metabolites have been implicated in the SAR-associated rapid activation of defenses in response to subsequent exposure to the pathogen, a mechanism termed priming. Here, we summarize the role of these signaling metabolites in SAR, and the relationship between them and SA signaling in SAR.

Altered growth and improved resistance of Arabidopsis against Pseudomonas syringae by overexpression of the basic amino acid transporter AtCAT1

Amino acid transporters in plants are crucial for distributing amino acids between plant organs and cellular compartments. The H(+)-coupled plasma membrane transporter CAT1 (cationic amino acid transporter 1) facilitates the high-affinity uptake of basic amino acids. The uptake of lysine (Lys) via the roots was not altered in loss-of-function mutants, in accordance with the minor expression of CAT1 in roots, but plants ectopically overexpressing CAT1 incorporated Lys at higher rates. Exogenous Lys inhibited the primary root of Arabidopsis, whereas lateral roots were stimulated. These effects were augmented by the presence or absence of CAT1. Furthermore, the total biomass of soil-grown plants ectopically overexpressing CAT1 was reduced and the time to flowering was accelerated. These effects were accompanied by only minor changes in the overall amino acid profile. Interestingly, CAT1 belongs to a specific small cluster of nitrogen-containing metabolite transporter genes that are rapidly up-regulated upon infection with Pseudomonas syringae and that may participate in the systemic response of plants to pathogen attack. The overexpression of CAT1 indeed enhanced the resistance to the hemibiotrophic bacterial pathogen P. syringae via a constitutively activated salicylic acid (SA) pathway, which is consistent with the developmental defects and the resistance phenotype.

The up-regulation of elongation factors in the barley leaf and the down-regulation of nucleosome assembly genes in the crown are both associated with the expression of frost tolerance

We report a series of microarray-based leaf and crown transcriptome comparisons involving three barley cultivars (cvs. Luxor, Igri and Atlas 68) which express differing degrees of frost tolerance. The transcripts were obtained following the exposure of seedlings to low (above and below zero) temperatures, aiming to identify those genes and signalling/metabolic pathways which are associated with frost tolerance. Both the leaves and the crowns responded to low temperature by the up-regulation of a suite of abscisic acid (ABA)-responsive genes, most of which have already been recognized as components of the plant low temperature response. The inter-cultivar comparison indicated that genes involved in maintaining the leaf's capacity to synthesize protein and to retain chloroplast activity were important for the expression of frost tolerance. In the crown, the repression of genes associated with nucleosome assembly and transposon regulation were the most relevant transcriptional changes associated with frost tolerance, highlighting the role of gene repression in the cold acclimation response.

Identification of differentially expressed proteins and phosphorylated proteins in rice seedlings in response to strigolactone treatment

Strigolactones (SLs) are recently identified plant hormones that inhibit shoot branching and control various aspects of plant growth, development and interaction with parasites. Previous studies have shown that plant D10 protein is a carotenoid cleavage dioxygenase that functions in SL biosynthesis. In this work, we used an allelic SL-deficient d10 mutant XJC of rice (Oryza sativa L. spp. indica) to investigate proteins that were responsive to SL treatment. When grown in darkness, d10 mutant seedlings exhibited elongated mesocotyl that could be rescued by exogenous application of SLs. Soluble protein extracts were prepared from d10 mutant seedlings grown in darkness in the presence of GR24, a synthetic SL analog. Soluble proteins were separated on two-dimensional gels and subjected to proteomic analysis. Proteins that were expressed differentially and phosphoproteins whose phosphorylation status changed in response to GR24 treatment were identified. Eight proteins were found to be induced or down-regulated by GR24, and a different set of 8 phosphoproteins were shown to change their phosphorylation intensities in the dark-grown d10 seedlings in response to GR24 treatment. Analysis of these proteins revealed that they are important enzymes of the carbohydrate and amino acid metabolic pathways and key components of the cellular energy generation machinery. These proteins may represent potential targets of the SL signaling pathway. This study provides new insight into the complex and negative regulatory mechanism by which SLs control shoot branching and plant development.

Chemical inducers of systemic immunity in plants

Systemic acquired resistance (SAR) is a highly desirable form of resistance that protects against a broad-spectrum of related or unrelated pathogens. SAR involves the generation of multiple signals at the site of primary infection, which arms distal portions against subsequent secondary infections. The last decade has witnessed considerable progress, and a number of chemical signals contributing to SAR have been isolated and characterized. The diverse chemical nature of these chemicals had led to the growing belief that SAR might involve interplay of multiple diverse and independent signals. However, recent results suggest that coordinated signalling from diverse signalling components facilitates SAR in plants. This review mainly discusses organized signalling by two such chemicals, glycerol-3-phoshphate and azelaic acid, and the role of basal salicylic acid levels in G3P-conferred SAR.

Mitochondrial 2-hydroxyglutarate metabolism

2-Hydroxyglutarate (2-HG) is a five-carbon dicarboxylic acid with a hydroxyl group at the alpha position, which forms a stereocenter in this molecule. Although the existence of mitochondrial D- and L-2HG metabolisms has long been known in different eukaryotes, the biosynthetic pathways, especially in plants, have not been completely elucidated. While D-2HG is involved in intermediary metabolism, L-2HG may not have a cellular function but it needs to be recycled through a metabolic repair reaction. Independent of their metabolic origin, D- and L-2HG are oxidized in plant mitochondria to 2-ketoglutarate through the action of two stereospecific enzymes, D- and L-2-hydroxyacid dehydrogenases. While plants are to a large extent unaffected by high cellular concentrations of D-2HG, deficiencies in the metabolism of D- and L-2HG result in fatal disorders in humans. We present current data gathered on plant D- and L-2HG metabolisms and relate it to existing knowledge on 2HG metabolism in other organisms. We focus on the metabolic origin of these compounds, the mitochondrial catabolic steps catalyzed by the stereospecific dehydrogenases, and phylogenetic relationships between different studied 2-hydroxyacid dehydrogenases.

Down-regulation of acetolactate synthase compromises Ol-1- mediated resistance to powdery mildew in tomato

BACKGROUND

In a cDNA-AFLP analysis comparing transcript levels between powdery mildew (Oidium neolycopersici)-susceptible tomato cultivar Moneymaker (MM) and near isogenic lines (NILs) carrying resistance gene Ol-1 or Ol-4, a transcript-derived fragment (TDF) M11E69-195 was found to be present in NIL-Ol-1 but absent in MM and NIL-Ol-4. This TDF shows homology to acetolactate synthase (ALS). ALS is a key enzyme in the biosynthesis of branched-chain amino acids valine, leucine and isoleucine, and it is also a target of commercial herbicides.

RESULTS

Three ALS homologs ALS1, ALS2, ALS3 were identified in the tomato genome sequence. ALS1 and ALS2 show high similarity, whereas ALS3 is more divergent. Transient silencing of both ALS1 and ALS2 in NIL-Ol-1 by virus-induced gene silencing (VIGS) resulted in chlorotic leaf areas that showed increased susceptibility to O. neolycopersici (On). VIGS results were confirmed by stable transformation of NIL-Ol-1 using an RNAi construct targeting both ALS1 and ALS2. In contrast, silencing of the three ALS genes individually by RNAi constructs did not compromise the resistance of NIL-Ol-1. Application of the herbicide chlorsulfuron to NIL-Ol-1 mimicked the VIGS phenotype and caused loss of its resistance to On. Susceptible MM and On-resistant line NIL-Ol-4 carrying a nucleotide binding site and leucine rich repeat (NB-LRR) resistance gene were also treated with chlorsulfuron. Neither the susceptibility of MM nor the resistance of NIL-Ol-4 was affected.

CONCLUSIONS

ALS is neither involved in basal defense, nor in resistance conferred by NB-LRR type resistance genes. Instead, it is specifically involved in Ol-1-mediated resistance to tomato powdery mildew, suggesting that ALS-induced change in amino acid homeostasis is important for resistance conferred by Ol-1.

Lysine catabolism, amino acid transport, and systemic acquired resistance: what is the link?

Lysine is an essential amino acid for human nutrition, which is generally low in cereal diets. Its biosynthesis via the aspartate-pathway and catabolism is controlled by complex feedback mechanisms. Recently, aspartate-derived amino acids were found to be elevated during pathogen infection in Arabidopsis and a lysine catabolite, pipecolic acid, was identified as critical regulator of systemic acquired resistance. Pipecolic acid is mobile in plants, functions as an intensifier of defense responses and mediates systemic acquired resistance establishment via signal amplification. The altered pathogen defense in several mutants with altered homeostasis of aspartate-derived amino acids, such as lysine, had already provided a genetic link with amino acid homeostasis. Furthermore, the modification of amino acid transport and distribution within tissues not only affected the plant growth performance, but also the plant-pathogen interaction. The ectopic overexpression of a gene encoding a high affinity importer with preference to basic amino acids, such as lysine, cationic amino acid transporter1 (CAT1), improved the disease resistance to a hemibiotrophic bacterial pathogen in Arabidopsis via a constitutively activated salicylic acid pathway. The importance of Asp-derived amino acid homeostasis for plant systemic acquired resistance and on overall plant growth performance may be relevant to resistance and nutritional quality breeding. Whether nitrogen fertilization has an impact on crop pest control management via amino acid homeostasis is briefly discussed.

New insights into the regulation of plant immunity by amino acid metabolic pathways

Besides defence pathways regulated by classical stress hormones, distinct amino acid metabolic pathways constitute integral parts of the plant immune system. Mutations in several genes involved in Asp-derived amino acid biosynthetic pathways can have profound impact on plant resistance to specific pathogen types. For instance, amino acid imbalances associated with homoserine or threonine accumulation elevate plant immunity to oomycete pathogens but not to pathogenic fungi or bacteria. The catabolism of Lys produces the immune signal pipecolic acid (Pip), a cyclic, non-protein amino acid. Pip amplifies plant defence responses and acts as a critical regulator of plant systemic acquired resistance, defence priming and local resistance to bacterial pathogens. Asp-derived pyridine nucleotides influence both pre- and post-invasion immunity, and the catabolism of branched chain amino acids appears to affect plant resistance to distinct pathogen classes by modulating crosstalk of salicylic acid- and jasmonic acid-regulated defence pathways. It also emerges that, besides polyamine oxidation and NADPH oxidase, Pro metabolism is involved in the oxidative burst and the hypersensitive response associated with avirulent pathogen recognition. Moreover, the acylation of amino acids can control plant resistance to pathogens and pests by the formation of protective plant metabolites or by the modulation of plant hormone activity.

Pipecolic acid enhances resistance to bacterial infection and primes salicylic acid and nicotine accumulation in tobacco

Distinct amino acid metabolic pathways constitute integral parts of the plant immune system. We have recently identified pipecolic acid (Pip), a lysine-derived non-protein amino acid, as a critical regulator of systemic acquired resistance (SAR) and basal immunity to bacterial infection in Arabidopsis thaliana. In Arabidopsis, Pip acts as an endogenous mediator of defense amplification and priming. For instance, Pip conditions plants for effective biosynthesis of the phenolic defense signal salicylic acid (SA), accumulation of the phytoalexin camalexin, and expression of defense-related genes. Here, we show that tobacco plants respond to leaf infection by the compatible bacterial pathogen Pseudomonas syringae pv tabaci (Pstb) with a significant accumulation of several amino acids, including Lys, branched-chain, aromatic, and amide group amino acids. Moreover, Pstb strongly triggers, alongside the biosynthesis of SA and increases in the defensive alkaloid nicotine, the production of the Lys catabolites Pip and α-aminoadipic acid. Exogenous application of Pip to tobacco plants provides significant protection to infection by adapted Pstb or by non-adapted, hypersensitive cell death-inducing P. syringae pv maculicola. Pip thereby primes tobacco for rapid and strong accumulation of SA and nicotine following bacterial infection. Thus, our study indicates that the role of Pip as an amplifier of immune responses is conserved between members of the rosid and asterid groups of eudicot plants and suggests a broad practical applicability for Pip as a natural enhancer of plant disease resistance.

Proteome and metabolome profiling of cytokinin action in Arabidopsis identifying both distinct and similar responses to cytokinin down- and up-regulation

In plants, numerous developmental processes are controlled by cytokinin (CK) levels and their ratios to levels of other hormones. While molecular mechanisms underlying the regulatory roles of CKs have been intensely researched, proteomic and metabolomic responses to CK deficiency are unknown. Transgenic Arabidopsis seedlings carrying inducible barley cytokinin oxidase/dehydrogenase (CaMV35S>GR>HvCKX2) and agrobacterial isopentenyl transferase (CaMV35S>GR>ipt) constructs were profiled to elucidate proteome- and metabolome-wide responses to down- and up-regulation of CK levels, respectively. Proteome profiling identified >1100 proteins, 155 of which responded to HvCKX2 and/or ipt activation, mostly involved in growth, development, and/or hormone and light signalling. The metabolome profiling covered 79 metabolites, 33 of which responded to HvCKX2 and/or ipt activation, mostly amino acids, carbohydrates, and organic acids. Comparison of the data sets obtained from activated CaMV35S>GR>HvCKX2 and CaMV35S>GR>ipt plants revealed unexpectedly extensive overlaps. Integration of the proteomic and metabolomic data sets revealed: (i) novel components of molecular circuits involved in CK action (e.g. ribosomal proteins); (ii) previously unrecognized links to redox regulation and stress hormone signalling networks; and (iii) CK content markers. The striking overlaps in profiles observed in CK-deficient and CK-overproducing seedlings might explain surprising previously reported similarities between plants with down- and up-regulated CK levels.

Successful fertilization requires the presence of at least one major O-acetylserine(thiol)lyase for cysteine synthesis in pollen of Arabidopsis

The synthesis of cysteine (Cys) is a master control switch of plant primary metabolism that coordinates the flux of sulfur with carbon and nitrogen metabolism. In Arabidopsis (Arabidopsis thaliana), nine genes encode for O-acetylserine(thiol)lyase (OAS-TL)-like proteins, of which the major isoforms, OAS-TL A, OAS-TL B, and OAS-TL C, catalyze the formation of Cys by combining O-acetylserine and sulfide in the cytosol, the plastids, and the mitochondria, respectively. So far, the significance of individual OAS-TL-like enzymes is unresolved. Generation of all major OAS-TL double loss-of-function mutants in combination with radiolabeled tracer studies revealed that subcellular localization of OAS-TL proteins is more important for efficient Cys synthesis than total cellular OAS-TL activity in leaves. The absence of oastl triple embryos after targeted crosses indicated the exclusiveness of Cys synthesis by the three major OAS-TLs and ruled out alternative sulfur fixation by other OAS-TL-like proteins. Analyses of oastlABC pollen demonstrated that the presence of at least one functional OAS-TL isoform is essential for the proper function of the male gametophyte, although the synthesis of histidine, lysine, and tryptophan is dispensable in pollen. Comparisons of oastlABC pollen derived from genetically different parent plant combinations allowed us to separate distinct functions of Cys and glutathione in pollen and revealed an additional role of glutathione for pollen germination. In contrast, female gametogenesis was not affected by the absence of major OAS-TLs, indicating significant transport of Cys into the developing ovule from the mother plant.

Molecular cloning of the apoptosis-related calcium-binding protein AsALG-2 in Avena sativa

Victorin, the host-selective toxin produced by the fungus Cochliobolus victoriae, induces programmed cell death (PCD) in victorin-sensitive oat lines with characteristic features of animal apoptosis, such as mitochondrial permeability transition, chromatin condensation, nuclear DNA laddering and rRNA/mRNA degradation. In this study, we characterized a calcium-binding protein, namely AsALG-2, which might have a role in the victorin-induced PCD. AsALG-2 is homologous to the Apoptosis-Linked Gene ALG-2 identified in mammalian cells. Northern blot analysis revealed that the accumulation of AsALG-2 transcripts increased during victorin-induced PCD, but not during necrotic cell death. Salicylic acid, chitosan and chitin strongly activated the expression of general defence response genes, such as PR-10; however, neither induced cell death nor the accumulation of AsALG-2 mRNA. Pharmacological studies indicated that victorin-induced DNA laddering and AsALG-2 expression were regulated through similar pathways. The calcium channel blocker, nifedipine, moderately inhibited the accumulation of AsALG-2 mRNA during cell death. Trifluoperazine (calmodulin antagonist) and K252a (serine-threonine kinase inhibitor) reduced the victorin-induced phytoalexin accumulation, but did not prevent the victorin-induced DNA laddering or accumulation of AsALG-2 mRNA. Taken together, our investigations suggest that there is a calcium-mediated signalling pathway in animal and plant PCD in common.

The plant vascular system: evolution, development and functions

The emergence of the tracheophyte-based vascular system of land plants had major impacts on the evolution of terrestrial biology, in general, through its role in facilitating the development of plants with increased stature, photosynthetic output, and ability to colonize a greatly expanded range of environmental habitats. Recently, considerable progress has been made in terms of our understanding of the developmental and physiological programs involved in the formation and function of the plant vascular system. In this review, we first examine the evolutionary events that gave rise to the tracheophytes, followed by analysis of the genetic and hormonal networks that cooperate to orchestrate vascular development in the gymnosperms and angiosperms. The two essential functions performed by the vascular system, namely the delivery of resources (water, essential mineral nutrients, sugars and amino acids) to the various plant organs and provision of mechanical support are next discussed. Here, we focus on critical questions relating to structural and physiological properties controlling the delivery of material through the xylem and phloem. Recent discoveries into the role of the vascular system as an effective long-distance communication system are next assessed in terms of the coordination of developmental, physiological and defense-related processes, at the whole-plant level. A concerted effort has been made to integrate all these new findings into a comprehensive picture of the state-of-the-art in the area of plant vascular biology. Finally, areas important for future research are highlighted in terms of their likely contribution both to basic knowledge and applications to primary industry.

Acclimation responses of Arabidopsis thaliana to sustained phosphite treatments

Phosphite (H₂PO⁻₃) induces a range of physiological and developmental responses in plants by disturbing the homeostasis of the macronutrient phosphate. Because of its close structural resemblance to phosphate, phosphite impairs the sensing, membrane transport, and subcellular compartmentation of phosphate. In addition, phosphite induces plant defence responses by an as yet unknown mode of action. In this study, the acclimation of Arabidopsis thaliana plants to a sustained phosphite supply in the growth medium was investigated and compared with plants growing under varying phosphate supplies. Unlike phosphate, phosphite did not suppress the formation of lateral roots in several Arabidopsis accessions. In addition, the expression of well-documented phosphate-starvation-induced genes, such as miRNA399d and At4, was not repressed by phosphite accumulation, whilst the induction of PHT1;1 and PAP1 was accentuated. Thus, a mimicking of phosphate by phosphite was not observed for these classical phosphate-starvation responses. Metabolomic analysis of phosphite-treated plants showed changes in several metabolite pools, most prominently those of aspartate, asparagine, glutamate, and serine. These alterations in amino acid pools provide novel insights for the understanding of phosphite-induced pathogen resistance.

The AvrB_AvrC domain of AvrXccC of Xanthomonas campestris pv. campestris is required to elicit plant defense responses and manipulate ABA homeostasis

Plant disease induced by Xanthomonas campestris pv. campestris depends on type III effectors but the molecular basis is poorly understood. Here, AvrXccC8004 was characterized, and it was found that the AvrB_AvrC domain was essential and sufficient to elicit defense responses in an Arabidopsis-resistant ecotype (Col-0). An upregulation of genes in responding to the AvrB_AvrC domain of AvrXccC8004 was shown in a profile of host gene expression. The molecular changes were correlated with morphological changes observed in phenotypic and ultrastructural characterizations. Interestingly, the abscisic acid (ABA)-signaling pathway was also a prominent target for the AvrB_AvrC domain of AvrXccC8004. The highly elicited NCED5, encoding a key enzyme of ABA biosynthesis, was increased in parallel with ABA levels in AvrXccC8004 transgenic plants. Consistently, the X. campestris pv. campestris 8004 ΔavrXccC mutant was severely impaired in the ability to manipulate the accumulation of ABA and induction of ABA-related genes in challenged leaves. Moreover, exogenous application of ABA also enhanced the susceptibility of Arabidopsis to the X. campestris pv. campestris strains. These results indicate that the AvrB_AvrC domain of AvrXccC8004 alone has the activity to manipulate ABA homeostasis, which plays an important role in regulating the interactions of X. campestris pv. campestris and Arabidopsis.

The Arabidopsis TUMOR PRONE5 gene encodes an acetylornithine aminotransferase required for arginine biosynthesis and root meristem maintenance in blue light

Arginine is an essential amino acid necessary for protein synthesis and is also a nitrogen storage compound. The genes encoding the enzymes of arginine biosynthesis in plants are not well characterized and have mainly been predicted from homologies to bacterial and fungal genes. We report the cloning and characterization of the TUMOR PRONE5 (TUP5) gene of Arabidopsis (Arabidopsis thaliana) encoding an acetylornithine aminotransferase (ACOAT), catalyzing the fourth step of arginine biosynthesis. The free arginine content was strongly reduced in the chemically induced recessive mutant tup5-1, root growth was restored by supplementation with arginine and its metabolic precursors, and a yeast (Saccharomyces cerevisiae) ACOAT mutant was complemented by TUP5. Two null alleles of TUP5 caused a reduced viability of gametes and embryo lethality, possibly caused by insufficient Arg supply from maternal tissue. TUP5 expression is positively regulated by light, and a TUP5-green fluorescent protein was localized in chloroplasts. tup5-1 has a unique light-dependent short root phenotype. Roots of light-grown tup5-1 seedlings switch from indeterminate growth to determinate growth with arresting cell production and an exhausted root apical meristem. The inhibitory activity was specific for blue light, and the inhibiting light was perceived by the root. Thus, tup5-1 reveals a novel role of amino acids and blue light in regulating root meristem function.

Medicago truncatula dihydrodipicolinate synthase (DHDPS) enzymes display novel regulatory properties

Lysine biosynthesis in plants is tightly regulated by feedback inhibition of the end product on the first enzyme of the lysine-specific branch, dihydrodipicolinate synthase (DHDPS). Three complete DHDPS coding sequences and one partial sequence were obtained in Medicago truncatula via inverse PCR. Analysis of the MtDHDPS sequences indicated the presence of isozymes (MtDHDPS2 and MtDHDPS3) with multiple amino acid substitutions on positions previously shown to be involved in feedback inhibition and of residues important for catalytic activity, possibly affecting the enzymatic properties of these isoforms. Sequences similar to MtDHDPS2 and 3 are present in Lotus japonicus and Glycine max, suggesting the existence of a specific conserved class of DHDPS genes within the Fabaceae family. The MtDHDPS genes were found by quantitative RT-PCR analysis to be expressed in an organ-specific manner in M. truncatula. All four MtDHDPS enzymes were expressed separately in Escherichia coli, revealing a strongly reduced sensitivity of the MtDHDPS2 protein to lysine feedback inhibition and a severely reduced activity of the MtDHDPS3 protein. Remarkably, MtDHDPS3 expression in Arabidopsis thaliana produced transgenic plants with a significantly increased threonine level, suggesting a dominant DHDPS inhibiting role of this isoform. This is supported by co-expression experiments in E. coli which indicate that AtDHDPS and MtDHDPS3 interact and may form hetero-oligomers with strongly reduced enzymatic activity. In conclusion, analysis of DHDPS in M. truncatula revealed the presence of unique isozymes displaying novel regulatory properties.

The Arabidopsis elongator complex subunit2 epigenetically regulates plant immune responses

The Arabidopsis thaliana Elongator complex subunit2 (ELP2) genetically interacts with NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1), a key transcription coactivator of plant immunity, and regulates the induction kinetics of defense genes. However, the mechanistic relationship between ELP2 and NPR1 and how ELP2 regulates the kinetics of defense gene induction are unclear. Here, we demonstrate that ELP2 is an epigenetic regulator required for pathogen-induced rapid transcriptome reprogramming. We show that ELP2 functions in a transcriptional feed-forward loop regulating both NPR1 and its target genes. An elp2 mutation increases the total methylcytosine number, reduces the average methylation levels of methylcytosines, and alters (increases or decreases) methylation levels of specific methylcytosines. Interestingly, infection of plants with the avirulent bacterial pathogen Pseudomonas syringae pv tomato DC3000/avrRpt2 induces biphasic changes in DNA methylation levels of NPR1 and PHYTOALEXIN DEFICIENT4 (PAD4), which encodes another key regulator of plant immunity. These dynamic changes are blocked by the elp2 mutation, which is correlated with delayed induction of NPR1 and PAD4. The elp2 mutation also reduces basal histone acetylation levels in the coding regions of several defense genes. Together, our data demonstrate a new role for Elongator in somatic DNA demethylation/methylation and suggest a function for Elongator-mediated chromatin regulation in pathogen-induced transcriptome reprogramming.

Structure of ALD1, a plant-specific homologue of the universal diaminopimelate aminotransferase enzyme of lysine biosynthesis

Diaminopimelate aminotransferase (DAP-AT) is an enzyme in the lysine-biosynthesis pathway. Conversely, ALD1, a close homologue of DAP-AT in plants, uses lysine as a substrate in vitro. Both proteins require pyridoxal-5'-phosphate (PLP) for their activity. The structure of ALD1 from the flowering plant Arabidopsis thaliana (AtALD1) was solved at a resolution of 2.3 Å. Comparison of AtALD1 with the previously solved structure of A. thaliana DAP-AT (AtDAP-AT) revealed similar interactions with PLP despite sequence differences within the PLP-binding site. However, sequence differences between the binding site of AtDAP-AT for malate, a purported mimic of substrate binding, and the corresponding site in AtALD1 led to different interactions. This suggests that either the substrate itself, or the substrate-binding mode, differs in the two proteins, supporting the known in vitro findings.

Long-distance communication and signal amplification in systemic acquired resistance

Systemic acquired resistance (SAR) is an inducible defense mechanism in plants that confers enhanced resistance against a variety of pathogens. SAR is activated in the uninfected systemic (distal) organs in response to a prior (primary) infection elsewhere in the plant. SAR is associated with the activation of salicylic acid (SA) signaling and the priming of defense responses for robust activation in response to subsequent infections. The activation of SAR requires communication by the primary infected tissues with the distal organs. The vasculature functions as a conduit for the translocation of factors that facilitate long-distance intra-plant communication. In recent years, several metabolites putatively involved in long-distance signaling have been identified. These include the methyl ester of SA (MeSA), the abietane diterpenoid dehydroabietinal (DA), the dicarboxylic acid azelaic acid (AzA), and a glycerol-3-phosphate (G3P)-dependent factor. Long-distance signaling by some of these metabolites also requires the lipid-transfer protein DIR1 (DEFECTIVE IN INDUCED RESISTANCE 1). The relative contribution of these factors in long-distance signaling is likely influenced by environmental conditions, for example light. In the systemic leaves, the AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1)-dependent production of the lysine catabolite pipecolic acid (Pip), FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1) signaling, as well as SA synthesis and downstream signaling are required for the activation of SAR. This review summarizes the involvement and interaction between long-distance SAR signals and details the recently discovered role of Pip in defense amplification and priming that allows plants to acquire immunity at the systemic level. Recent advances in SA signaling and perception are also highlighted.

Long-distance communication and signal amplification in systemic acquired resistance

Systemic acquired resistance (SAR) is an inducible defense mechanism in plants that confers enhanced resistance against a variety of pathogens. SAR is activated in the uninfected systemic (distal) organs in response to a prior (primary) infection elsewhere in the plant. SAR is associated with the activation of salicylic acid (SA) signaling and the priming of defense responses for robust activation in response to subsequent infections. The activation of SAR requires communication by the primary infected tissues with the distal organs. The vasculature functions as a conduit for the translocation of factors that facilitate long-distance intra-plant communication. In recent years, several metabolites putatively involved in long-distance signaling have been identified. These include the methyl ester of SA (MeSA), the abietane diterpenoid dehydroabietinal (DA), the dicarboxylic acid azelaic acid (AzA), and a glycerol-3-phosphate (G3P)-dependent factor. Long-distance signaling by some of these metabolites also requires the lipid-transfer protein DIR1 (DEFECTIVE IN INDUCED RESISTANCE 1). The relative contribution of these factors in long-distance signaling is likely influenced by environmental conditions, for example light. In the systemic leaves, the AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1)-dependent production of the lysine catabolite pipecolic acid (Pip), FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1) signaling, as well as SA synthesis and downstream signaling are required for the activation of SAR. This review summarizes the involvement and interaction between long-distance SAR signals and details the recently discovered role of Pip in defense amplification and priming that allows plants to acquire immunity at the systemic level. Recent advances in SA signaling and perception are also highlighted.

Reprogramming of plants during systemic acquired resistance

Genome-wide microarray analyses revealed that during biological activation of systemic acquired resistance (SAR) in Arabidopsis, the transcript levels of several hundred plant genes were consistently up- (SAR(+) genes) or down-regulated (SAR(-) genes) in systemic, non-inoculated leaf tissue. This transcriptional reprogramming fully depended on the SAR regulator FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1). Functional gene categorization showed that genes associated with salicylic acid (SA)-associated defenses, signal transduction, transport, and the secretory machinery are overrepresented in the group of SAR(+) genes, and that the group of SAR(-) genes is enriched in genes activated via the jasmonate (JA)/ethylene (ET)-defense pathway, as well as in genes associated with cell wall remodeling and biosynthesis of constitutively produced secondary metabolites. This suggests that SAR-induced plants reallocate part of their physiological activity from vegetative growth towards SA-related defense activation. Alignment of the SAR expression data with other microarray information allowed us to define three clusters of SAR(+) genes. Cluster I consists of genes tightly regulated by SA. Cluster II genes can be expressed independently of SA, and this group is moderately enriched in H2O2- and abscisic acid (ABA)-responsive genes. The expression of the cluster III SAR(+) genes is partly SA-dependent. We propose that SA-independent signaling events in early stages of SAR activation enable the biosynthesis of SA and thus initiate SA-dependent SAR signaling. Both SA-independent and SA-dependent events tightly co-operate to realize SAR. SAR(+) genes function in the establishment of diverse resistance layers, in the direct execution of resistance against different (hemi-)biotrophic pathogen types, in suppression of the JA- and ABA-signaling pathways, in redox homeostasis, and in the containment of defense response activation. Our data further indicated that SAR-associated defense priming can be realized by partial pre-activation of particular defense pathways.

Two Arabidopsis thaliana dihydrodipicolinate synthases, DHDPS1 and DHDPS2, are unequally redundant

In Arabidopsis thalinana (L.) Heynh., DHDPS1 and DHDPS2 encode orthologous dihydrodipicolinate synthases (DHDPS), the first enzyme of the lysine (Lys) biosynthesis pathway. A TDNA insertion mutant of dhdps2 was previously reported to be viable and to accumulate free threonine (Thr). Analysis of additional TDNA insertion lines showed that dhdps1 and dhdps2 mutants are both viable and that whereas dhdps2 mutants accumulate Thr, dhdps1 plants do not. Thr-accumulation was complemented by heterologous expression of Escherichia coli DapA, indicating that the phenotype is due to reduced DHDPS activity in dhdps2. DHDPS1 contributes ~30% towards the total DHDPS activity in leaves of young plants and DHDPS2 contributes 70%; therefore, the threshold of activity resulting in Thr accumulation lies within this narrow range. dhdps1-dhdps2 double mutants could not be isolated, even after exogenous feeding with Lys. Segregation analysis indicated that gametes lacking functional DHDPS genes are defective, as are embryos. Plants carrying only a single DHDPS2 gene do not accumulate Thr, but they show a gametophytic defect that is partially rescued by Lys application. Despite the accumulation of Thr, dhdps2 seedlings are no more sensitive than wild-type plants to growth inhibition by Lys or the Lys precursor diaminopimelate. They also are not rescued by methionine at growth-inhibitory Lys concentrations. Exogenous application of Lys and methionine to dhdps2 mutants did not reduce the accumulation of Thr.

Pipecolic acid, an endogenous mediator of defense amplification and priming, is a critical regulator of inducible plant immunity

Metabolic signals orchestrate plant defenses against microbial pathogen invasion. Here, we report the identification of the non-protein amino acid pipecolic acid (Pip), a common Lys catabolite in plants and animals, as a critical regulator of inducible plant immunity. Following pathogen recognition, Pip accumulates in inoculated Arabidopsis thaliana leaves, in leaves distal from the site of inoculation, and, most specifically, in petiole exudates from inoculated leaves. Defects of mutants in AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1) in systemic acquired resistance (SAR) and in basal, specific, and β-aminobutyric acid-induced resistance to bacterial infection are associated with a lack of Pip production. Exogenous Pip complements these resistance defects and increases pathogen resistance of wild-type plants. We conclude that Pip accumulation is critical for SAR and local resistance to bacterial pathogens. Our data indicate that biologically induced SAR conditions plants to more effectively synthesize the phytoalexin camalexin, Pip, and salicylic acid and primes plants for early defense gene expression. Biological priming is absent in the pipecolate-deficient ald1 mutants. Exogenous pipecolate induces SAR-related defense priming and partly restores priming responses in ald1. We conclude that Pip orchestrates defense amplification, positive regulation of salicylic acid biosynthesis, and priming to guarantee effective local resistance induction and the establishment of SAR.

SOS - too many signals for systemic acquired resistance?

Following pathogen infection, activation of systemic acquired resistance (SAR) in uninfected tissues requires transmission of a signal(s) from the infected tissue via the vasculature. Several candidates for this long-distance signal have been identified, including methyl salicylate (MeSA), an SFD1/GLY1-derived glycerol-3-phosphate (G3P)-dependent signal, the lipid-transfer protein DIR1, the dicarboxylic acid azelaic acid (AzA), the abietane diterpenoid dehydroabietinal (DA), jasmonic acid (JA), and the amino acid-derivative pipecolic acid (Pip). Some of these signals work cooperatively to activate SAR and/or regulate MeSA metabolism. However, Pip appears to activate SAR via an independent pathway that may impinge on these other signaling pathway(s) during de novo salicylic acid (SA) biosynthesis in the systemic tissue. Thus, a complex web of cross-interacting signals appears to activate SAR.

Functional characterization of aromatic amino acid aminotransferase involved in 2-phenylethanol biosynthesis in isolated rose petal protoplasts

In rose flowers, 2-phenylethanol (2PE) is biosynthesized from l-phenylalanine (l-Phe) via phenylacetaldehyde (PAld) by the actions of two enzymes, pyridoxal-5'-phosphate (PLP)-dependent aromatic amino acid decarboxylase (AADC) and phenylacetaldehyde reductase (PAR). We here report that Rosa 'Yves Piaget' aromatic amino acid aminotransferase produced phenylpyruvic acid (PPA) from l-Phe in isolated petal protoplasts. We have cloned three full length cDNAs (RyAAAT1-3) of aromatic amino acid aminotransferase families based on rose EST database and homology regions. The RyAAATs enzymes were heterogeneously expressed in Escherichia coli and characterized biochemically. The recombinant RyAAAT3 showed the highest activity toward l-Phe in comparison with l-tryptophan, l-tyrosine, d-Phe, glycine, and l-alanine, and showed 9.7-fold higher activity with l-Phe rather than PPA as a substrate. RyAAAT3 had an optimal activity at pH 9 and at 45-55°C with α-ketoglutaric acid, and was found to be a PLP dependent enzyme based on the inhibition test using Carbidopa, an inhibitor of PLP-dependent enzymes. The transcript of RyAAAT3 was expressed in flowers as well as other organs of R. 'Yves Piaget'. RNAi suppression of RyAAAT3 decreased 2PE production, revealing the involvement of RyAAAT3 in 2PE biosynthesis in rose protoplasts and indicating that rose protoplasts have potentially two different 2PE biosynthetic pathways, the AADC route and the new route via PPA from l-Phe.

The Arabidopsis glucosyltransferase UGT76B1 conjugates isoleucic acid and modulates plant defense and senescence

Plants coordinate and tightly regulate pathogen defense by the mostly antagonistic salicylate (SA)- and jasmonate (JA)-mediated signaling pathways. Here, we show that the previously uncharacterized glucosyltransferase UGT76B1 is a novel player in this SA-JA signaling crosstalk. UGT76B1 was selected as the top stress-induced isoform among all 122 members of the Arabidopsis thaliana UGT family. Loss of UGT76B1 function leads to enhanced resistance to the biotrophic pathogen Pseudomonas syringae and accelerated senescence but increased susceptibility toward necrotrophic Alternaria brassicicola. This is accompanied by constitutively elevated SA levels and SA-related marker gene expression, whereas JA-dependent markers are repressed. Conversely, UGT76B1 overexpression has the opposite effect. Thus, UGT76B1 attenuates SA-dependent plant defense in the absence of infection, promotes the JA response, and delays senescence. The ugt76b1 phenotypes were SA dependent, whereas UGT76B1 overexpression indicated that this gene possibly also has a direct effect on the JA pathway. Nontargeted metabolomic analysis of UGT76B1 knockout and overexpression lines using ultra-high-resolution mass spectrometry and activity assays with the recombinant enzyme led to the ab initio identification of isoleucic acid (2-hydroxy-3-methyl-pentanoic acid) as a substrate of UGT76B1. Exogenously applied isoleucic acid increased resistance against P. syringae infection. These findings indicate a novel link between amino acid-related molecules and plant defense that is mediated by small-molecule glucosylation.

Genetic dissection of salicylic acid-mediated defense signaling networks in Arabidopsis

Properly coordinated defense signaling networks are critical for the fitness of plants. One hub of the defense networks is centered on salicylic acid (SA), which plays a key role in activating disease resistance in plants. However, while a number of genes are known to affect SA-mediated defense, relatively little is known about how these gene interact genetically with each other. Here we exploited the unique defense-sensitized Arabidopsis mutant accelerated cell death (acd) 6-1 to dissect functional relationships among key components in the SA hub. We show that while enhanced disease susceptibility (eds) 1-2 and phytoalexin deficient (pad) 4-1 suppressed acd6-1-conferred small size, cell death, and defense phenotypes, a combination of these two mutations did not incur additive suppression. This suggests that EDS1 and PAD4 act in the same signaling pathway. To further evaluate genetic interactions among SA regulators, we constructed 10 pairwise crosses in the acd6-1 background among mutants defective in: SA INDUCTION-DEFICIENT 2 for SA biosynthesis; AGD2-LIKE DEFENSE 1, EDS5, and PAD4 for SA accumulation; and NONEXPRESSOR OF PR GENES 1 for SA signaling. Systematic analysis of the triple mutants based on their suppression of acd6-1-conferred phenotypes revealed complex and interactive genetic relationships among the tested SA genes. Our results suggest a more comprehensive view of the gene networks governing SA function and provide a framework for further interrogation of the important roles of SA and possibly other signaling molecules in regulating plant disease resistance.

Indispensable Roles of Plastids in Arabidopsis thaliana Embryogenesis

The plastid is an organelle vital to all photosynthetic and some non-photosynthetic eukaryotes. In the model plant Arabidopsis thaliana, a number of nuclear genes encoding plastid proteins have been found to be necessary for embryo development. However, the exact roles of plastids in this process remain largely unknown. Here we use publicly available datasets to obtain insights into the relevance of plastid activities to A. thaliana embryogenesis. By searching the SeedGenes database (http://www.seedgenes.org) and recent literature, we found that, of the 339 non-redundant genes required for proper embryo formation, 108 genes likely encode plastid-targeted proteins. Nineteen of these genes are necessary for development of preglobular embryos and/or their conversion to globular embryos, of which 13 genes encode proteins involved in non-photosynthetic metabolism. By contrast, among 38 genes which are dispensable for globular embryo formation but necessary for further development, only one codes for a protein involved in metabolism. Products of 21 of the 38 genes play roles in plastid gene expression and maintenance. Examination of RNA profiles of embryos at distinct growth stages obtained in laser-capture microdissection coupled with DNA microarray experiments revealed that most of the identified genes are expressed throughout embryo morphogenesis and maturation. These findings suggest that metabolic activities are required at preglobular and throughout all stages of embryo development, whereas plastid gene expression becomes necessary during and/or after the globular stage to sustain various activities of the organelle including photosynthetic electron transport.

Perturbation of Arabidopsis amino acid metabolism causes incompatibility with the adapted biotrophic pathogen Hyaloperonospora arabidopsidis

Reliance of biotrophic pathogens on living plant tissues to propagate implies strong interdependence between host metabolism and nutrient uptake by the pathogen. However, factors determining host suitability and establishment of infection are largely unknown. We describe a loss-of-inhibition allele of ASPARTATE KINASE2 and a loss-of-function allele of DIHYDRODIPICOLINATE SYNTHASE2 identified in a screen for Arabidopsis thaliana mutants with increased resistance to the obligate biotrophic oomycete Hyaloperonospora arabidopsidis (Hpa). Through different molecular mechanisms, these mutations perturb amino acid homeostasis leading to overaccumulation of the Asp-derived amino acids Met, Thr, and Ile. Although detrimental for the plant, the mutations do not cause defense activation, and both mutants retain full susceptibility to the adapted obligate biotrophic fungus Golovinomyces orontii (Go). Chemical treatments mimicking the mutants' metabolic state identified Thr as the amino acid suppressing Hpa but not Go colonization. We conclude that perturbations in amino acid homeostasis render the mutant plants unsuitable as an infection substrate for Hpa. This may be explained by deployment of the same amino acid biosynthetic pathways by oomycetes and plants. Our data show that the plant host metabolic state can, in specific ways, influence the ability of adapted biotrophic strains to cause disease.

Salicylic acid beyond defence: its role in plant growth and development

In recent years salicylic acid (SA) has been the focus of intensive research due to its function as an endogenous signal mediating local and systemic plant defence responses against pathogens. It has also been found that SA plays a role during the plant response to abiotic stresses such as drought, chilling, heavy metal toxicity, heat, and osmotic stress. In this sense, SA appears to be, just like in mammals, an 'effective therapeutic agent' for plants. Besides this function during biotic and abiotic stress, SA plays a crucial role in the regulation of physiological and biochemical processes during the entire lifespan of the plant. The discovery of its targets and the understanding of its molecular modes of action in physiological processes could help in the dissection of the complex SA signalling network, confirming its important role in both plant health and disease. Here, the evidence that supports the role of SA during plant growth and development is reviewed by comparing experiments performed by exogenous application of SA with analysis of genotypes affected by SA levels and/or perception.

L,L-diaminopimelate aminotransferase from Chlamydomonas reinhardtii: a target for algaecide development

In some bacterial species and photosynthetic cohorts, including algae, the enzyme l,l-diaminopimelate aminotransferase (DapL) (E.C. 2.6.1.83) is involved in the anabolism of the essential amino acid L-lysine. DapL catalyzes the conversion of tetrahydrodipicolinate (THDPA) to l,l-diaminopimelate (l,l-DAP), in one step bypassing the DapD, DapC and DapE enzymatic reactions present in the acyl DAP pathways. Here we present an in vivo and in vitro characterization of the DapL ortholog from the alga Chlamydomonas reinhardtii (Cr-DapL). The in vivo analysis illustrated that the enzyme is able to functionally complement the E. coli dap auxotrophs and was essential for plant development in Arabidopsis. In vitro, the enzyme was able to inter-convert THDPA and l,l-DAP, showing strong substrate specificity. Cr-DapL was dimeric in both solution and when crystallized. The structure of Cr-DapL was solved in its apo form, showing an overall architecture of a α/β protein with each monomer in the dimer adopting a pyridoxal phosphate-dependent transferase-like fold in a V-shaped conformation. The active site comprises residues from both monomers in the dimer and shows some rearrangement when compared to the apo-DapL structure from Arabidopsis. Since animals do not possess the enzymatic machinery necessary for the de novo synthesis of the amino acid l-lysine, enzymes involved in this pathway are attractive targets for the development of antibiotics, herbicides and algaecides.

Identification and Partial Characterization of an L-Tyrosine Aminotransferase (TAT) from Arabidopsis thaliana

The aminotransferase gene family in the model plant Arabidopsis thaliana consists of 44 genes. Twenty six of these enzymes are classified as characterized meaning that the reaction(s) that the enzyme catalyzes are documented using experimental means. The remaining 18 enzymes are uncharacterized and are therefore deemed putative. Our laboratory is interested in elucidating the function(s) of the remaining putative aminotransferase enzymes. To this end, we have identified and partially characterized an aminotransferase (TAT) enzyme from Arabidopsis annotated by the locus tag At5g36160. The full-length cDNA was cloned and the purified recombinant enzyme was characterized using in vitro and in vivo experiments. In vitro analysis showed that the enzyme is capable of interconverting L-Tyrosine and 4-hydroxyphenylpyruvate, and L-Phenylalanine and phenylpyruvate. In vivo analysis by functional complementation showed that the gene was able to complement an E. coli with a background of aminotransferase mutations that confers auxotrophy for L-Tyrosine and L-Phenylalanine.

Deciphering transcriptional and metabolic networks associated with lysine metabolism during Arabidopsis seed development

In order to elucidate transcriptional and metabolic networks associated with lysine (Lys) metabolism, we utilized developing Arabidopsis (Arabidopsis thaliana) seeds as a system in which Lys synthesis could be stimulated developmentally without application of chemicals and coupled this to a T-DNA insertion knockout mutation impaired in Lys catabolism. This seed-specific metabolic perturbation stimulated Lys accumulation starting from the initiation of storage reserve accumulation. Our results revealed that the response of seed metabolism to the inducible alteration of Lys metabolism was relatively minor; however, that which was observable operated in a modular manner. They also demonstrated that Lys metabolism is strongly associated with the operation of the tricarboxylic acid cycle while largely disconnected from other metabolic networks. In contrast, the inducible alteration of Lys metabolism was strongly associated with gene networks, stimulating the expression of hundreds of genes controlling anabolic processes that are associated with plant performance and vigor while suppressing a small number of genes associated with plant stress interactions. The most pronounced effect of the developmentally inducible alteration of Lys metabolism was an induction of expression of a large set of genes encoding ribosomal proteins as well as genes encoding translation initiation and elongation factors, all of which are associated with protein synthesis. With respect to metabolic regulation, the inducible alteration of Lys metabolism was primarily associated with altered expression of genes belonging to networks of amino acids and sugar metabolism. The combined data are discussed within the context of network interactions both between and within metabolic and transcriptional control systems.

Salicylic Acid, a multifaceted hormone to combat disease

For more than 200 years, the plant hormone salicylic acid (SA) has been studied for its medicinal use in humans. However, its extensive signaling role in plants, particularly in defense against pathogens, has only become evident during the past 20 years. This review surveys how SA in plants regulates both local disease resistance mechanisms, including host cell death and defense gene expression, and systemic acquired resistance (SAR). Genetic studies reveal an increasingly complex network of proteins required for SA-mediated defense signaling, and this process is amplified by several regulatory feedback loops. The interaction between the SA signaling pathway and those regulated by other plant hormones and/or defense signals is also discussed.

Aspartate-Derived Amino Acid Biosynthesis in Arabidopsis thaliana

The aspartate-derived amino acid pathway in plants leads to the biosynthesis of lysine, methionine, threonine, and isoleucine. These four amino acids are essential in the diets of humans and other animals, but are present in growth-limiting quantities in some of the world's major food crops. Genetic and biochemical approaches have been used for the functional analysis of almost all Arabidopsis thaliana enzymes involved in aspartate-derived amino acid biosynthesis. The branch-point enzymes aspartate kinase, dihydrodipicolinate synthase, homoserine dehydrogenase, cystathionine gamma synthase, threonine synthase, and threonine deaminase contain well-studied sites for allosteric regulation by pathway products and other plant metabolites. In contrast, relatively little is known about the transcriptional regulation of amino acid biosynthesis and the mechanisms that are used to balance aspartate-derived amino acid biosynthesis with other plant metabolic needs. The aspartate-derived amino acid pathway provides excellent examples of basic research conducted with A. thaliana that has been used to improve the nutritional quality of crop plants, in particular to increase the accumulation of lysine in maize and methionine in potatoes.

Arabidopsis proteins important for modulating defense responses to Pseudomonas syringae that secrete HopW1-1

Plant infection responses result from the interaction of pathogen-derived molecules with host components. For the bacterial pathogen Pseudomonas syringae, these molecules are often effector proteins (Hops) that are injected into plant cells. P. syringae carrying hopW1-1 have restricted host range on some Arabidopsis thaliana accessions. At least two Arabidopsis genomic regions are important for the natural variation that conditions resistance to P. syringae/hopW1-1. HopW1-1 elicits a resistance response, and consequently the accumulation of the signal molecule salicylic acid (SA) and transcripts of HWI1 (HopW1-1-Induced Gene1). This work identified three HopW1-1-interacting (WIN) plant proteins: a putative acetylornithine transaminase (WIN1), a protein phosphatase (WIN2) and a firefly luciferase superfamily protein (WIN3). Importantly, WIN2 and WIN3 are partially required for HopW1-1-induced disease resistance, SA production and HWI1 expression. The requirement for WIN2 is specific for HopW1-1-induced resistance, whereas WIN3 is important for responses to several effectors. Overexpression of WIN2 or WIN3 confers resistance to virulent P. syringae, which is consistent with these proteins being defense components. Several known genes important for SA production or signaling are also partially (EDS1, NIM1/NPR1, ACD6 and ALD1) or strongly (PAD4) required for the robust resistance induced by HopW1-1, suggesting a key role for SA in the HopW1-1-induced resistance response. Finally, WIN1 is an essential protein, the overexpression of which over-rides the resistance response to HopW1-1 (and several other defense-inducing effectors), and delays SA and HWI1 induction. Thus, the WIN proteins have different roles in modulating plant defense.

A lesion-mimic syntaxin double mutant in Arabidopsis reveals novel complexity of pathogen defense signaling

The lesion-mimic Arabidopsis mutant, syp121 syp122, constitutively expresses the salicylic acid (SA) signaling pathway and has low penetration resistance to powdery mildew fungi. Genetic analyses of the lesion-mimic phenotype have expanded our understanding of programmed cell death (PCD) in plants. Inactivation of SA signaling genes in syp121 syp122 only partially rescues the lesion-mimic phenotype, indicating that additional defenses contribute to the PCD. Whole genome transcriptome analysis confirmed that SA-induced transcripts, as well as numerous other known pathogen-response transcripts, are up-regulated after inactivation of the syntaxin genes. A suppressor mutant analysis of syp121 syp122 revealed that FMO1, ALD1, and PAD4 are important for lesion development. Mutant alleles of EDS1, NDR1, RAR1, and SGT1b also partially rescued the lesion-mimic phenotype, suggesting that mutating syntaxin genes stimulates TIR-NB-LRR and CC-NB-LRR-type resistances. The syntaxin double knockout potentiated a powdery mildew-induced HR-like response. This required functional PAD4 but not functional SA signaling. However, SA signaling potentiated the PAD4-dependent HR-like response. Analyses of quadruple mutants suggest that EDS5 and SID2 confer separate SA-independent signaling functions, and that FMO1 and ALD1 mediate SA-independent signals that are NPR1-dependent. These studies highlight the contribution of multiple pathways to defense and point to the complexity of their interactions.

Finding novel metabolic genes through plant-prokaryote phylogenomics

Plants and prokaryotes share thousands of genes. Those with known functions mostly encode enzymes of primary metabolism or other key biochemical components, and the same is almost surely true of those whose function is still obscure. The availability of hundreds of sequenced genomes and of rich postgenomic resources now makes possible the use of comparative genomics ('phylogenomics') of plants and prokaryotes to infer, and then verify, functions for such unknown genes. In this type of analysis, plant and prokaryote data each inform the search for function, and do so synergistically. This breaks with the past pattern of gene discovery, in which the information flow was most often unidirectional from prokaryotes to plants.

A key role for the Arabidopsis WIN3 protein in disease resistance triggered by Pseudomonas syringae that secrete AvrRpt2

Effector proteins injected by the pathogenic bacteria Pseudomonas syringae into plants can have profound effects on the pathogen-host interaction due to their efficient recognition by plants and the subsequent triggering of defenses. The AvrRpt2 effector triggers strong local and systemic defense (called systemic acquired resistance [SAR]) responses in Arabidopsis thaliana plants that harbor a functional RPS2 gene that encodes an R protein in the coiled-coil, nucleotide-binding domain, leucine-rich repeat class. The newly identified win3-T mutant shows greatly reduced resistance to P syringae carrying avrRpt2. In win3-T plants, RIN4 cleavage, an early AvrRpt2-induced event, is normal. However, salicylic acid accumulation is compromised, as is SAR induction and the local hypersensitive cell death response after infection by P syringae carrying avrRpt2. WIN3 encodes a member of the firefly luciferase protein superfamily. Expression of WIN3 at an infection site partially requires PAD4, a protein known to play a quantitative role in RPS2-mediated signaling. WIN3 expression in tissue distal to an infection site requires multiple salicylic acid regulatory genes. Finally, win3-T plants show modestly increased susceptibility to virulent P syringae and modestly reduced SAR in response to P. syringae carrying avrRpm1. Thus, WIN3 is a key element of the RPS2 defense response pathway and a basal and systemic defense component.

Genetic dissection of histidine biosynthesis in Arabidopsis

The biosynthesis of histidine (His) in microorganisms, long studied through the isolation and characterization of auxotrophic mutants, has emerged as a paradigm for the regulation of metabolism and gene expression. Much less is known about His biosynthesis in flowering plants. One limiting factor has been the absence of large collections of informative auxotrophs. We describe here the results of a systematic screen for His auxotrophs of Arabidopsis (Arabidopsis thaliana). Ten insertion mutants disrupted in four different biosynthetic genes (HISN2, HISN3, HISN4, HISN6A) were identified through a combination of forward and reverse genetics and were shown to exhibit an embryo-defective phenotype that could be rescued by watering heterozygous plants with His. Male transmission of the mutant allele was in several cases reduced. Knockouts of two redundant genes (HISN1B and HISN5A) had no visible phenotype. Another mutant blocked in the final step of His biosynthesis (hisn8) and a double mutant altered in the redundant first step of the pathway (hisn1a hisn1b) exhibited a combination of gametophytic and embryonic lethality in heterozygotes. Homozygous mutant seedlings and callus tissue produced from rescued seeds appeared normal when grown in the presence of His but typically senesced after continued growth in the absence of His. These knockout mutants document the importance of His biosynthesis for plant growth and development, provide valuable insights into amino acid transport and source-sink relationships during seed development, and represent a significant addition to the limited collection of well-characterized auxotrophs in flowering plants.