The oxylipin signal jasmonic acid is activated by an enzyme that conjugates it to isoleucine in Arabidopsis

Molecular battles between plant and pathogenic bacteria in the phyllosphere

The phyllosphere, i.e., the aerial parts of the plant, provides one of the most important niches for microbial colonization. This niche supports the survival and, often, proliferation of microbes such as fungi and bacteria with diverse lifestyles including epiphytes, saprophytes, and pathogens. Although most microbes may complete the life cycle on the leaf surface, pathogens must enter the leaf and multiply aggressively in the leaf interior. Natural surface openings, such as stomata, are important entry sites for bacteria. Stomata are known for their vital role in water transpiration and gas exchange between the plant and the environment that is essential for plant growth. Recent studies have shown that stomata can also play an active role in limiting bacterial invasion of both human and plant pathogenic bacteria as part of the plant innate immune system. As counter-defense, plant pathogens such as Pseudomonas syringae pv tomato (Pst) DC3000 use the virulence factor coronatine to suppress stomate-based defense. A novel and crucial early battleground in host-pathogen interaction in the phyllosphere has been discovered with broad implications in the study of bacterial pathogenesis, host immunity, and molecular ecology of bacterial diseases.

Nitric oxide participates in the complex interplay of defense-related signaling pathways controlling disease resistance to Sclerotinia sclerotiorum in Arabidopsis thaliana

Studies of the interaction between Arabidopsis thaliana and the necrotrophic fungal pathogen Sclerotinia sclerotiorum have been hampered by the extreme susceptibility of this model plant to the fungus. In addition, analyses of the plant defense response suggested the implication of a complex interplay of hormonal and signaling pathways. To get a deeper insight into this host-pathogen interaction, we first analyzed the natural variation in Arabidopsis for resistance to S. sclerotiorum. The results revealed a large variation of resistance and susceptibility in Arabidopsis, with some ecotypes, such as Ws-4, Col-0, and Rbz-1, being strongly resistant, and others, such as Shahdara, Ita-0, and Cvi-0, exhibiting an extreme susceptibility. The role of different signaling pathways in resistance was then determined by assessing the symptoms of mutants affected in the perception, production, or transduction of hormonal signals after inoculation with S. sclerotiorum. This analysis led to the conclusions that i) signaling of inducible defenses is predominantly mediated by jasmonic acid and abscisic acid, influenced by ethylene, and independent of salicylic acid; and ii) nitric oxide (NO) and reactive oxygen species are important signals required for plant resistance to S. sclerotiorum. Defense gene expression analysis supported the specific role of NO in defense activation.

Towards elucidating the differential regulation of floral and extrafloral nectar secretion

Nectar is a rich source of sugars that serves the attraction of pollinators (floral nectar) or predatory arthropods (extrafloral nectar). We just begin to understand the similarities and differences that underlie the secretory control of these two important types of plant secretions. Jasmonates are phytohormones, which are well documented to be involved in plant developmental processes and plant defence responses against herbivores, including the secretion of extrafloral nectar. Recently, jasmonates have also been implicated in the regulation of floral nectar secretion in Brassica napus. Due to a trade-off between reproduction and defence, however, plants need to functionally separate the regulation of these two secretory processes. In line with this prediction, externally applying jasmonates to leaves did indeed not affect floral nectar secretion. Here we compare the current knowledge on the regulation of floral and extrafloral nectar secretion to understand similarities and dissimilarities between these two secretory processes and highlight future research directions in this context.

Involvement of salicylate and jasmonate signaling pathways in Arabidopsis interaction with Fusarium graminearum

Fusarium graminearum is the principal causative agent of Fusarium head blight (FHB), a devastating disease of wheat and barley. This fungus can also colonize Arabidopsis thaliana. Disease resistance was enhanced in transgenic wheat and Arabidopsis plants that constitutively overexpress the NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1) gene, which regulates salicylic acid (SA) signaling and modulates the activation of jasmonic acid (JA)-dependent defenses. Here, we provide several lines of evidence that reveal an important role for SA and JA signaling in Arabidopsis defense against F. graminearum. SA level was elevated in fungus-inoculated leaves, and SA application and biologically activated systemic acquired resistance enhanced resistance. Furthermore, the disruption of SA accumulation and signaling in the sid2 mutant and NahG transgenic plant, and the npr1 and wrky18 mutants, respectively, resulted in heightened susceptibility to this fungus in leaves and inflorescence. JA signaling was activated in parallel with SA signaling in the fungus-challenged plants. However, the hyperresistance of the JA pathway mutants opr3, coi1, and jar1 indicates that this pathway contributes to susceptibility. Genetic and biochemical experiments indicate that the JA pathway promotes disease by attenuating the activation of SA signaling in fungus-inoculated plants. However, the hypersusceptibility of the jar1 npr1 double mutant compared with the npr1 mutant suggests that JAR1 also contributes to defense, signifying a dichotomous role of JA and a JAR1-dependent mechanism in this interaction.

Jasmonate and Phytochrome A Signaling inArabidopsisWound and Shade Responses Are Integrated through JAZ1 Stability

Jasmonate (JA) activates plant defense, promotes pollen maturation, and suppresses plant growth. An emerging theme in JA biology is its involvement in light responses; here, we examine the interdependence of the JA- and light-signaling pathways in Arabidopsis thaliana. We demonstrate that mutants deficient in JA biosynthesis and signaling are deficient in a subset of high irradiance responses in far-red (FR) light. These mutants display exaggerated shade responses to low, but not high, R/FR ratio light, suggesting a role for JA in phytochrome A (phyA) signaling. Additionally, we demonstrate that the FR light–induced expression of transcription factor genes is dependent on CORONATINE INSENSITIVE1 (COI1), a central component of JA signaling, and is suppressed by JA. phyA mutants had reduced JA-regulated growth inhibition and VSP expression and increased content of cis-(+)-12-oxophytodienoic acid, an intermediate in JA biosynthesis. Significantly, COI1-mediated degradation of JASMONATE ZIM DOMAIN1-β-glucuronidase (JAZ1-GUS) in response to mechanical wounding and JA treatment required phyA, and ectopic expression of JAZ1-GUS resulted in exaggerated shade responses. Together, these results indicate that JA and phyA signaling are integrated through degradation of the JAZ1 protein, and both are required for plant responses to light and stress.

Hormonal and transcriptional profiles highlight common and differential host responses to arbuscular mycorrhizal fungi and the regulation of the oxylipin pathway

Arbuscular mycorrhizal (AM) symbioses are mutualistic associations between soil fungi and most vascular plants. The symbiosis significantly affects the host physiology in terms of nutrition and stress resistance. Despite the lack of host range specificity of the interaction, functional diversity between AM fungal species exists. The interaction is finely regulated according to plant and fungal characters, and plant hormones are believed to orchestrate the modifications in the host plant. Using tomato as a model, an integrative analysis of the host response to different mycorrhizal fungi was performed combining multiple hormone determination and transcriptional profiling. Analysis of ethylene-, abscisic acid-, salicylic acid-, and jasmonate-related compounds evidenced common and divergent responses of tomato roots to Glomus mosseae and Glomus intraradices, two fungi differing in their colonization abilities and impact on the host. Both hormonal and transcriptional analyses revealed, among others, regulation of the oxylipin pathway during the AM symbiosis and point to a key regulatory role for jasmonates. In addition, the results suggest that specific responses to particular fungi underlie the differential impact of individual AM fungi on plant physiology, and particularly on its ability to cope with biotic stresses.

Regulation of Arabidopsis defense responses against Spodoptera littoralis by CPK-mediated calcium signaling

BACKGROUND

Plant Ca2+ signals are involved in a wide array of intracellular signaling pathways after pest invasion. Ca2+-binding sensory proteins such as Ca2+-dependent protein kinases (CPKs) have been predicted to mediate the signaling following Ca2+ influx after insect herbivory. However, until now this prediction was not testable.

RESULTS

To investigate the roles CPKs play in a herbivore response-signaling pathway, we screened the characteristics of Arabidopsis CPK mutants damaged by a feeding generalist herbivore, Spodoptera littoralis. Following insect attack, the cpk3 and cpk13 mutants showed lower transcript levels of plant defensin gene PDF1.2 compared to wild-type plants. The CPK cascade was not directly linked to the herbivory-induced signaling pathways that were mediated by defense-related phytohormones such as jasmonic acid and ethylene. CPK3 was also suggested to be involved in a negative feedback regulation of the cytosolic Ca2+ levels after herbivory and wounding damage. In vitro kinase assays of CPK3 protein with a suite of substrates demonstrated that the protein phosphorylates transcription factors (including ERF1, HsfB2a and CZF1/ZFAR1) in the presence of Ca2+. CPK13 strongly phosphorylated only HsfB2a, irrespective of the presence of Ca2+. Furthermore, in vivo agroinfiltration assays showed that CPK3-or CPK13-derived phosphorylation of a heat shock factor (HsfB2a) promotes PDF1.2 transcriptional activation in the defense response.

CONCLUSIONS

These results reveal the involvement of two Arabidopsis CPKs (CPK3 and CPK13) in the herbivory-induced signaling network via HsfB2a-mediated regulation of the defense-related transcriptional machinery. This cascade is not involved in the phytohormone-related signaling pathways, but rather directly impacts transcription factors for defense responses.

The lipid language of plant-fungal interactions

Lipid mediated cross-kingdom communication between hosts and pathogens is a rapidly emerging field in molecular plant-fungal interactions. Amidst our growing understanding of fungal and plant chemical cross-talk lies the distinct, yet little studied, role for a group of oxygenated lipids derived from polyunsaturated fatty acids, termed oxylipins. Endogenous fungal oxylipins are known for their roles in carrying out pathogenic strategies to successfully colonize their host, reproduce, and synthesize toxins. While plant oxylipins also have functions in reproduction and development, they are largely recognized as agents that facilitate resistance to pathogen attack. Here we review the composition and endogenous functions of oxylipins produced by both plants and fungi and introduce evidence which suggests that fungal pathogens exploit host oxylipins to facilitate their own virulence and pathogenic development. Specifically, we describe how fungi induce plant lipid metabolism to utilize plant oxylipins in order to promote G-protein-mediated regulation of sporulation and mycotoxin production in the fungus. The use of host-ligand mimicry (i.e. coronatine) to manipulate plant defense responses that benefit the fungus are also implicated.

Exploring the impact of wounding and jasmonates on ascorbate metabolism

Vitamin C (ascorbate, AsA) is the most abundant water-soluble antioxidant in plants. Ascorbate provides the first line of defense against damaging reactive oxygen species (ROS), and helps protect plant cells from many factors that induce oxidative stress, including wounding, ozone, high salinity, and pathogen attack. Plant defenses against these stresses are also dependent upon jasmonates (JAs), a class of plant hormones that promote ROS accumulation. Here, we review evidence showing that wounding and JAs influence AsA accumulation in various plant species, and we report new data from Arabidopsis and tomato testing the influence of JAs on AsA levels in wounded and unwounded plants. In both species, certain mutations that impair JA metabolism and signaling influence foliar AsA levels, suggesting that endogenous JAs may regulate steady-state AsA. However, the impact of wounding on AsA accumulation was similar in JA mutants and wild type controls, indicating that this wound response does not require JAs. Our findings also indicate that the effects of wounding and JAs on AsA accumulation differ between species; these factors both enhanced AsA accumulation in Arabidopsis, but depressed AsA levels in tomato. These results underscore the importance of obtaining data from more than one model species, and demonstrate the complexity of AsA regulation.

The genuine ligand of a jasmonic acid receptor: improved analysis of jasmonates is now required

Jasmonic acid (JA), its metabolites, such as the methyl ester or amino acid conjugates as well as its precursor 12-oxophytodienoic acid (OPDA) are lipid-derived signals. JA, OPDA and JA-amino acid conjugates are known to function as signals in plant stress responses and development. More recently, formation of JA-amino acid conjugates and high biological activity of JA-Isoleucine (JA-Ile) were found to be essential in JA signaling. A breakthrough was the identification of JAZ proteins which interact with the F-box protein COI1 if JA-Ile is bound. This interaction leads to proteasomal degradation of JAZs being negative regulators of JA-induced transcription. Surprisingly, a distinct stereoisomer of JA-Ile, the (+)-7-iso-JA-Ile [(3R,7S) form] is most active. Coronatine, a bacterial phytotoxine with an identical stereochemistry at the cyclopentanone ring, has a similar bioactivity. This was explained by the recent identification of COI1 as the JA receptor and accords well with molecular modeling studies. Whereas over the last two decades JA was quantified to describe any JA dependent process, now we have to take into account a distinct stereoisomer of JA-Ile. Until recently a quantitative analysis of (+)-7-iso-JA-Ile was missing presumable due to its equilibration to (-)-JA-Ile. Now such an analysis was achieved. These aspects will be discussed based on our new knowledge on JA perception and signaling.

The Arabidopsis P450 protein CYP82C2 modulates jasmonate-induced root growth inhibition, defense gene expression and indole glucosinolate biosynthesis

Jasmonic acid (JA) is a fatty acid-derived signaling molecule that regulates a broad range of plant defense responses against herbivores and some microbial pathogens. Molecular genetic studies have established that JA also performs a critical role in several aspects of plant development. Here, we describe the characterization of the Arabidopsis mutant jasmonic acid-hypersensitive1-1 (jah1-1), which is defective in several aspects of JA responses. Although the mutant exhibits increased sensitivity to JA in root growth inhibition, it shows decreased expression of JA-inducible defense genes and reduced resistance to the necrotrophic fungus Botrytis cinerea . Gene cloning studies indicate that these defects are caused by a mutation in the cytochrome P450 protein CYP82C2. We provide evidence showing that the compromised resistance of the jah1-1 mutant to B . cinerea is accompanied by decreased expression of JA-induced defense genes and reduced accumulation of JA-induced indole glucosinolates (IGs). Conversely, the enhanced resistance to B. cinerea in CYP82C2-overexpressing plants is accompanied by increased expression of JA-induced defense genes and elevated levels of JA-induced IGs. We demonstrate that CYP82C2 affects JA-induced accumulation of the IG biosynthetic precursor tryptophan (Trp), but not the JA-induced IAA or pathogen-induced camalexin. Together, our results support a hypothesis that CYP82C2 may act in the metabolism of Trp-derived secondary metabolites under conditions in which JA levels are elevated. The jah1-1 mutant should thus be important in future studies toward understanding the mechanisms underlying the complexity of JA-mediated differential responses, which are important for plants to adapt their growth to the ever-changing environments.

Salt stress causes peroxisome proliferation, but inducing peroxisome proliferation does not improve NaCl tolerance in Arabidopsis thaliana

The PEX11 family of peroxisome membrane proteins have been shown to be involved in regulation of peroxisome size and number in plant, animals, and yeast cells. We and others have previously suggested that peroxisome proliferation as a result of abiotic stress may be important in plant stress responses, and recently it was reported that several rice PEX11 genes were up regulated in response to abiotic stress. We sought to test the hypothesis that promoting peroxisome proliferation in Arabidopsis thaliana by over expression of one PEX11 family member, PEX11e, would give increased resistance to salt stress. We could demonstrate up regulation of PEX11e by salt stress and increased peroxisome number by both PEX11e over expression and salt stress, however our experiments failed to find a correlation between PEX11e over expression and increased peroxisome metabolic activity or resistance to salt stress. This suggests that although peroxisome proliferation may be a consequence of salt stress, it does not affect the ability of Arabidopsis plants to tolerate saline conditions.

The role of jasmonates in floral nectar secretion

Plants produce nectar in their flowers as a reward for their pollinators and most of our crops depend on insect pollination, but little is known on the physiological control of nectar secretion. Jasmonates are well-known for their effects on senescence, the development and opening of flowers and on plant defences such as extrafloral nectar. Their role in floral nectar secretion has, however, not been explored so far. We investigated whether jasmonates have an influence on floral nectar secretion in oil-seed rape, Brassica napus. The floral tissues of this plant produced jasmonic acid (JA) endogenously, and JA concentrations peaked shortly before nectar secretion was highest. Exogenous application of JA to flowers induced nectar secretion, which was suppressed by treatment with phenidone, an inhibitor of JA synthesis. This effect could be reversed by additional application of JA. Jasmonoyl-isoleucine and its structural mimic coronalon also increased nectar secretion. Herbivory or addition of JA to the leaves did not have an effect on floral nectar secretion, demonstrating a functional separation of systemic defence signalling from reproductive nectar secretion. Jasmonates, which have been intensively studied in the context of herbivore defences and flower development, have a profound effect on floral nectar secretion and, thus, pollination efficiency in B. napus. Our results link floral nectar secretion to jasmonate signalling and thereby integrate the floral nectar secretion into the complex network of oxylipid-mediated developmental processes of plants.

Syncytium gene expression in Glycine max([PI 88788]) roots undergoing a resistant reaction to the parasitic nematode Heterodera glycines

The plant parasitic nematode, Heterodera glycines is the major pathogen of Glycine max (soybean). H. glycines accomplish parasitism by creating a nurse cell known as the syncytium from which it feeds. The syncytium undergoes two developmental phases. The first is a parasitism phase where feeding sites are selected, initiating the development of the syncytium. During this earlier phase (1-4 days post infection), syncytia undergoing resistant and susceptible reactions appear the same. The second phase is when the resistance response becomes evident (between 4 and 6dpi) and is completed by 9dpi. Analysis of the resistant reaction of G. max genotype PI 88788 (G. max([PI 88788])) to H. glycines population NL1-RHg/HG-type 7 (H. glycines([NL1-RHg/HG-type 7])) is accomplished by laser microdissection of syncytia at 3, 6 and 9dpi. Comparative analyses are made to pericycle and their neighboring cells isolated from mock-inoculated roots. These analyses reveal induced levels of the jasmonic acid biosynthesis and 13-lipoxygenase pathways. Direct comparative analyses were also made of syncytia at 6 days post infection to those at 3dpi (base line). The comparative analyses were done to identify localized gene expression that characterizes the resistance phase of the resistant reaction. The most highly induced pathways include components of jasmonic acid biosynthesis, 13-lipoxygenase pathway, S-adenosyl methionine pathway, phenylpropanoid biosynthesis, suberin biosynthesis, adenosylmethionine biosynthesis, ethylene biosynthesis from methionine, flavonoid biosynthesis and the methionine salvage pathway. In comparative analyses of 9dpi to 6dpi (base line), these pathways, along with coumarin biosynthesis, cellulose biosynthesis and homogalacturonan degradation are induced. The experiments presented here strongly implicate the jasmonic acid defense pathway as a factor involved in the localized resistant reaction of G. max([PI 88788]) to H. glycines([NL1-RHg/HG-type 7]).

A tomato enzyme synthesizes (+)-7-iso-jasmonoyl-L-isoleucine in wounded leaves

Jasmonoyl-L-isoleucine (JA-Ile) is a key jasmonate signal that probably functions in all plant species. The JASMONATE RESISTANT 1 (JAR1) enzyme synthesizes JA-Ile in Arabidopsis [Arabidopsis thaliana (L.) Heynh.], but a similar enzyme from tomato [Solanum lycopersicum (L.)] was not previously described. Tomato SlJAR1 has 66% sequence identity with Arabidopsis JAR1 and the SlJAR1-GST fusion protein purified from Escherichia coli catalyzed the formation of JA-amino acid conjugates in vitro. Kinetic analysis showed the enzyme has a strong preference for Ile over Leu and Val and it was about 10-fold more active with (+)-7-iso-JA than with its epimer (-)-JA. Leaf wounding rapidly increased JA-Ile 50-fold to about 450 pmol g(-1) FW at 30 min after wounding, while conjugates with Leu, Phe, Val and Met were only marginally increased or not detected. Nearly all of the endogenous JA-Ile was the bioactive epimer (+)-7-iso-JA-Ile and there was no evidence for its conversion to (-)-JA-Ile up to 6 h after wounding. A transgenic RNAi approach was used to suppress SlJAR1 transcript that reduced JA-Ile accumulation by 50-75%, suggesting that other JA conjugating enzymes may be present. These results show that SlJAR1 synthesizes the bioactive conjugate (+)-7-iso-JA-Ile and this is the predominant isomer accumulated in wounded tomato leaves.

Jasmonate-dependent and -independent pathways mediate specific effects of solar ultraviolet B radiation on leaf phenolics and antiherbivore defense

Ultraviolet B (UV-B) radiation, a very small fraction of the daylight spectrum, elicits changes in plant secondary metabolism that have large effects on plant-insect interactions. The signal transduction pathways that mediate these specific effects of solar UV-B are not known. We examined the role of jasmonate signaling by measuring responses to UV-B in wild-type and transgenic jasmonate-deficient Nicotiana attenuata plants in which a lipoxygenase gene (NaLOX3) was silenced (as-lox). In wild-type plants, UV-B failed to elicit the accumulation of jasmonic acid (JA) or the bioactive JA-isoleucine conjugate but amplified the response of jasmonate-inducible genes, such as trypsin proteinase inhibitor (TPI), to wounding and methyl jasmonate, and increased the accumulation of several phenylpropanoid derivatives. Some of these phenolic responses (accumulation of caffeoyl-polyamine conjugates) were completely lacking in as-lox plants, whereas others (accumulation of rutin and chlorogenic acid) were similar in both genotypes. In open field conditions, as-lox plants received more insect damage than wild-type plants, as expected, but the dramatic increase in resistance to herbivory elicited by UV-B exposure, which was highly significant in wild-type plants, did not occur in as-lox plants. We conclude that solar UV-B (1) uses jasmonate-dependent and -independent pathways in the elicitation of phenolic compounds, and (2) increases sensitivity to jasmonates, leading to enhanced expression of wound-response genes (TPI). The lack of UV-B-induced antiherbivore protection in as-lox plants suggests that jasmonate signaling plays a central role in the mechanisms by which solar UV-B increases resistance to insect herbivores in the field.

Jasmonates: structural requirements for lipid-derived signals active in plant stress responses and development

Jasmonates are lipid-derived signals that mediate plant stress responses and development processes. Enzymes participating in biosynthesis of jasmonic acid (JA) (1, 2) and components of JA signaling have been extensively characterized by biochemical and molecular-genetic tools. Mutants of Arabidopsis and tomato have helped to define the pathway for synthesis of jasmonoyl-isoleucine (JA-Ile), the active form of JA, and to identify the F-box protein COI1 as central regulatory unit. However, details of the molecular mechanism of JA signaling have only recently been unraveled by the discovery of JAZ proteins that function in transcriptional repression. The emerging picture of JA perception and signaling cascade implies the SCF(COI1) complex operating as E3 ubiquitin ligase that upon binding of JA-Ile targets JAZ repressors for degradation by the 26S-proteasome pathway, thereby allowing the transcription factor MYC2 to activate gene expression. The fact that only one particular stereoisomer, (+)-7-iso-JA-l-Ile (4), shows high biological activity suggests that epimerization between active and inactive diastereomers could be a mechanism for turning JA signaling on or off. The recent demonstration that COI1 directly binds (+)-7-iso-JA-l-Ile (4) and thus functions as JA receptor revealed that formation of the ternary complex COI1-JA-Ile-JAZ is an ordered process. The pronounced differences in biological activity of JA stereoisomers also imply strict stereospecific control of product formation along the JA biosynthetic pathway. The pathway of JA biosynthesis has been unraveled, and most of the participating enzymes are well-characterized. For key enzymes of JA biosynthesis the crystal structures have been established, allowing insight into the mechanisms of catalysis and modes of substrate binding that lead to formation of stereospecific products.

Mass spectrometry-based metabolomics oriented by correlation analysis for wound-induced molecule discovery: identification of a novel jasmonate glucoside

INTRODUCTION

Jasmonates are members of the oxylipin family involved in various plant regulatory processes, particularly in the response to biotic and abiotic stresses. They are present in very low amounts in wounded plants, which complicates their detection within complex plant extracts. Therefore, advanced analytical methods are needed for the profiling and characterisation of novel jasmonate derivatives.

OBJECTIVE

To use metabolomics to search for original wound-induced metabolites belonging to the jasmonate family.

METHODOLOGY

Numerous Arabidopsis specimens harvested at various time points after wounding were analysed by ultra-high pressure liquid chromatography coupled to time-of-flight mass spectrometry (UHPLC/TOFMS). A correlation analysis based on kinetic profile similarities with known jasmonates was applied to find wound-induced molecules having a potential role in defence signalling. Their characterisation was performed by tandem mass spectrometry and capillary nuclear magnetic resonance spectroscopy.

RESULTS

The statistical data treatment highlighted several previously reported jasmonates as well as a new glucoside derivative of the jasmonate 3-oxo-2-(2Z-pentenyl) cyclopentane-1-butyric acid (OPC-4). The monitoring of its kinetics in response to wounding revealed a delayed accumulation compared to the profile of OPC-4. This suggests an inactivation or elimination of OPC-4 through the formation of a polar glucosylated metabolite.

CONCLUSION

The metabolomic approach developed has proved useful in the discovery of original jasmonates synthesised in response to plant wounding.

Jasmonates

ARABIDOPSIS IS A SUPERB MODEL FOR THE STUDY OF AN IMPORTANT SUBGROUP OF OXYLIPINS: the jasmonates. Jasmonates control many responses to cell damage and invasion and are essential for reproduction. Jasmonic acid (JA) is a prohormone and is conjugated to hydrophobic amino acids to produce regulatory ligands. The major receptor for active jasmonate ligands is closely related to auxin receptors and, as in auxin signaling, jasmonate signaling requires the destruction of repressor proteins. This chapter uses a frequently asked question (FAQ) approach and concludes with a practical section.

Non-specific phytohormonal induction of AtMYB44 and suppression of jasmonate-responsive gene activation in Arabidopsis thaliana

The Arabidopsis thaliana transcription factor gene AtMYB44 was induced within 10 min by treatment with methyl jasmonate (MeJA). Wound-induced expression of the gene was observed in local leaves, but not in distal leaves, illustrating jasmonate-independent induction at wound sites. AtMYB44 expression was not abolished in Arabidopsis mutants insensitive to jasmonate (coi1), ethylene (etr1), or abscisic acid (abi3-1) when treated with the corresponding hormones. Moreover, various growth hormones and sugars also induced rapid AtMYB44 transcript accumulation. Thus, AtMYB44 gene activation appears to not be induced by any specific hormone. MeJA-induced activation of jasmonate-responsive genes such as JR2, VSP, LOXII, and AOS was attenuated in transgenic Arabidopsis plants overexpressing the gene (35S:AtMYB44), but significantly enhanced in atmyb44 knockout mutants. The 35S:MYB44 and atmyb44 plants did not show defectiveness in MeJA-induced primary root growth inhibition, indicating that the differences in jasmonate-responsive gene expression observed was not due to alterations in the jasmonate signaling pathway. 35S:AtMYB44 seedlings exhibited slightly elevated chlorophyll levels and less jasmonate- induced anthocyanin accumulation, demonstrating suppression of jasmonate-mediated responses and enhancement of ABA-mediated responses. These observations support the hypothesis of mutual antagonistic actions between jasmonate- and abscisic acid-mediated signaling pathways.

A virus-induced gene silencing screen identifies a role for Thylakoid Formation1 in Pseudomonas syringae pv tomato symptom development in tomato and Arabidopsis

Pseudomonas syringae pv tomato DC3000 (Pst DC3000), which causes disease in tomato (Solanum lycopersicum) and Arabidopsis (Arabidopsis thaliana), produces coronatine (COR), a non-host-specific phytotoxin. COR, which functions as a jasmonate mimic, is required for full virulence of Pst DC3000 and for the induction of chlorosis in host plants. Previous genetic screens based on insensitivity to COR and/or methyl jasmonate identified several potential targets for COR and methyl jasmonate. In this study, we utilized Nicotiana benthamiana and virus-induced gene silencing to individually reduce the expression of over 4,000 genes. The silenced lines of N. benthamiana were then screened for altered responses to purified COR. Using this forward genetics approach, several genes were identified with altered responses to COR. These were designated as ALC (for altered COR response) genes. When silenced, one of the identified genes, ALC1, produced a hypersensitive/necrosis-like phenotype upon COR application in a Coronatine-Insensitive1 (COI1)-dependent manner. To understand the involvement of ALC1 during the Pst DC3000-host interaction, we used the nucleotide sequence of ALC1 and identified its ortholog in Arabidopsis (Thylakoid Formation1 [THF1]) and tomato (SlALC1). In pathogenicity assays performed on Arabidopsis thf1 mutant and SlALC1-silenced tomato plants, Pst DC3000 induced accelerated coalescing necrotic lesions. Furthermore, we showed that COR affects ALC1 localization in chloroplasts in a COI1-dependent manner. In conclusion, our results show that the virus-induced gene silencing-based forward genetic screen has the potential to identify new players in COR signaling and disease-associated necrotic cell death.

Proteome of Gluconacetobacter diazotrophicus co-cultivated with sugarcane plantlets

Gluconacetobacter diazotrophicus is a micro-aerobic bacterium able to fix atmospheric nitrogen in endophytic mode. A proteomic approach was used to analyze proteins differentially expressed in the presence and absence of sugarcane plantlets. Two-dimensional gel electrophoresis (2-DE) showed 42 spots with altered levels of expression. Analysis of these spots by matrix-assisted laser desorption ionization time-of-flight in tandem (MALDI-TOF-TOF) identified 38 proteins. Differentially expressed proteins were associated with carbohydrate and energy metabolism, folding, sorting and degradation processes, and transcription and translation. Among proteins expressed in co-cultivated bacteria, four belong to membrane systems; others, like a transcription elongation factor (GreA), a 60 kDa chaperonin (GroEL), and an outer membrane lipoprotein (Omp16) have also been described in other plant-bacteria associations, indicating a common protein expression pattern as a result of symbiosis. A high protein content of 60kDa chaperonin isoforms was detected as non-differentially expressed proteins of the bacteria proteome. These results allow the assessment of the physiological significance of specific proteins to G. diazotrophicus metabolism and to the pathways involved in bacteria-host endophytic interaction.

Mechanism of auxin-regulated gene expression in plants

Plant hormones control most aspects of the plant life cycle by regulating genome expression. Expression of auxin-responsive genes involves interactions among auxin-responsive DNA sequence elements, transcription factors and trans-acting transcriptional repressors. Transcriptional output from these auxin signaling complexes is regulated by proteasome-mediated degradation that is triggered by interaction with auxin receptor-E3 ubiquitin ligases such SCF(TIR1). Auxin signaling components are conserved throughout land plant evolution and have proliferated and specialized to control specific developmental processes.

Velocity estimates for signal propagation leading to systemic jasmonic acid accumulation in wounded Arabidopsis

The wound response prohormone jasmonic acid (JA) accumulates rapidly in tissues both proximal and distal to injury sites in plants. Using quantitative liquid chromatography-mass spectrometry after flash freezing of tissues, we found that JA accumulated within 30 s of injury in wounded Arabidopsis leaves (p = 3.5 e(-7)). JA augmentation distal to wounds was strongest in unwounded leaves with direct vascular connections to wounded leaves wherein JA levels increased significantly within 120 s of wounding (p = 0.00027). This gave conservative and statistically robust temporal boundaries for the average velocity of the long distance signal leading to distal JA accumulation in unwounded leaves of 3.4-4.5 cm min(-1). Like JA, transcripts of the JA synthesis gene LIPOXYGENASE2 (LOX2) and the jasmonate response gene JAZ10.3 also accumulated to higher levels in directly interconnected leaves than in indirectly connected leaves. JA accumulation in a lox2-1 mutant plant was initiated rapidly after wounding then slowed progressively compared with the wild type (WT). Despite this, JAZ10.3 expression in the two genotypes was similar. Free cyclopentenone jasmonate levels were similar in both resting WT and lox2-1. In contrast, bound cyclopentenone jasmonates (arabidopsides) were far lower in lox2-1 than in the WT. The major roles of LOX2 are to generate arabidopsides and the large levels of JA that accumulate proximal to the wound. LOX2 is not essential for some of the most rapid events elicited by wounding.

A gene expression analysis of syncytia laser microdissected from the roots of the Glycine max (soybean) genotype PI 548402 (Peking) undergoing a resistant reaction after infection by Heterodera glycines (soybean cyst nematode)

The syncytium is a nurse cell formed within the roots of Glycine max by the plant parasitic nematode Heterodera glycines. Its development and maintenance are essential for nematode survival. The syncytium appears to undergo two developmental phases during its maturation into a functional nurse cell. The first phase is a parasitism phase where the nematode establishes the molecular circuitry that during the second phase ensures a compatible interaction with the plant cell. The cytological features of syncytia undergoing susceptible or resistant reactions appear the same during the parasitism phase. Depending on the outcome of any defense response, the second phase is a period of syncytium maintenance (susceptible reaction) or failure (resistant reaction). In the analyses presented here, the localized gene expression occurring at the syncytium during the resistant reaction was studied. This was accomplished by isolating syncytial cells from Glycine max genotype Peking (PI 548402) by laser capture microdissection. Microarray analyses using the Affymetrix soybean GeneChip directly compared Peking syncytia undergoing a resistant reaction to those undergoing a susceptible reaction during the parasitism phase of the resistant reaction. Those analyses revealed lipoxygenase-9 and lipoxygenase-4 as the most highly induced genes in the resistant reaction. The analysis also identified induced levels of components of the phenylpropanoid pathway. These genes included phenylalanine ammonia lyase, chalcone isomerase, isoflavone reductase, cinnamoyl-CoA reductase and caffeic acid O-methyltransferase. The presence of induced levels of these genes implies the importance of jasmonic acid and phenylpropanoid signaling pathways locally at the site of the syncytium during the resistance phase of the resistant reaction. The analysis also identified highly induced levels of four S-adenosylmethionine synthetase genes, the EARLY-RESPONSIVE TO DEHYDRATION 2 gene and the 14-3-3 gene known as GENERAL REGULATORY FACTOR 2. Subsequent analyses studied microdissected syncytial cells at 3, 6 and 9 days post infection (dpi) during the course of the resistant reaction, resulting in the identification of signature gene expression profiles at each time point in a single G. max genotype, Peking.

Arabidopsis OPT6 is an oligopeptide transporter with exceptionally broad substrate specificity

Oligopeptide transporters (OPTs) are found in fungi, bacteria and plants. The nine Arabidopsis thaliana OPT genes are expressed mainly in the vasculature and are thought to transport tetra- and pentapeptides, and peptide-like substrates such as glutathione. Expression of AtOPT6 in Xenopus laevis oocytes demonstrated that AtOPT6 transports many tetra- and pentapeptides. In addition, AtOPT6 transported reduced glutathione (GSH), a tripeptide, but not oxidized glutathione (GSSG). Recent data showed that Candida albicans OPTs can transport peptides up to eight amino acids in length. AtOPT6 transported mammalian signaling peptides up to 10 amino acids in length and, in addition, known plant development- and nematode pathogenesis-associated peptides up to 13 amino acids long. AtOPT6 displayed high affinity for penta- and dodecapeptides, but low affinity for GSH. In comparison the Saccharomyces cerevisiae ScOPT1 was incapable of transporting any of the longer peptides tested. These data demonstrate the necessity of experimentally determining substrate specificity of individual OPTs, and lay a foundation for structure/function studies. Characterization of the AtOPT6 substrate range provides a basis for investigating the possible physiological function of AtOPT6 in peptide signaling and thiol transport in response to stress.

The jasmonate pathway: the ligand, the receptor and the core signalling module

Jasmonates regulate specific developmental processes and plant adaptation to environment by controlling responses to external biotic or abiotic stimuli. The core events of jasmonate signalling are now defined. After hormone perception by SCF(COI1), JAZ (JAsmonate ZIM domain) repressors are targeted for proteasome degradation, releasing MYC2 and de-repressing transcriptional activation. JAZs are homomeric and heteromeric proteins and have been instrumental in recent advances in the field, such as the identification of COI1 as a critical component of the jasmonate receptor and the discovery of the bioactive jasmonate in Arabidopsis, (+)-7-iso-JA-Ile. Small changes in jasmonate structure result in hormone inactivation and might be the key to switching-off signalling for specific responses to stimulus and for long-distance signalling events.

Kinetics of the accumulation of jasmonic acid and its derivatives in systemic leaves of tobacco (Nicotiana tabacum cv. Xanthi nc) and translocation of deuterium-labeled jasmonic acid from the wounding site to the systemic site

In plants, the mobile signal needed for wound-induced systemic acquired resistance (WSR) has been elusive. The signal compound involved in WSR is supposed to be JA or its derivatives. On the basis of kinetic study of the accumulation of JA or its derivatives, it was discovered that JA, JA-Ile, tuberonic acid (TA, 12-OH epi-JA), and tuberonic acid glucoside (TAG) accumulated in systemic tissues in response to mechanical wounding stress in the tobacco plant (Nicotiana tabacum). Attempts to recover deuterium-labeled JA in systemic leaves after feeding the wounded leaves with deuterium-labeled JA were successfully done. It was also found that the translocated deuterium-labeled JA was metabolized to TA in systemic leaves under feeding of deuterium-labeled JA to the wounding leaves.

Top hits in contemporary JAZ: an update on jasmonate signaling

The phytohormone jasmonate (JA) regulates a wide range of growth, developmental, and defense-related processes during the plant life cycle. Identification of the JAZ family of proteins that repress JA responses has facilitated rapid progress in understanding how this lipid-derived hormone controls gene expression. Recent analysis of JAZ proteins has provided insight into the nature of the JA receptor, the chemical specificity of signal perception, and cross-talk between JA and other hormone response pathways. Functional diversification of JAZ proteins by alternative splicing, together with the ability of JAZ proteins to homo- and heterodimerize, provide mechanisms to enhance combinatorial diversity and versatility in gene regulation by JA.

Herbivory-induced signalling in plants: perception and action

Plants and herbivores have been interacting for millions of years. Over time, plants have evolved mechanisms to defend against herbivore attacks. Herbivore-challenged plants reconfigure their metabolism to produce compounds that are toxic, repellant or anti-digestive for the herbivores. Some compounds are volatile signals that attract the predators of herbivores. All these responses are tightly regulated by a signalling network triggered by the plant's perception machinery. Several compounds that specifically elicit herbivory-induced responses in plants have been isolated from herbivore oral secretions and oviposition fluids. Elicitor perception is rapidly followed by cell membrane depolarization, calcium influx and mitogen-activated protein kinase (MAPK) activation; plants also elevate the concentrations of reactive oxygen and nitrogen species, and modulate phytohormone levels accordingly. In addition to these reactions in the herbivore-attacked regions of a leaf, defence responses are also mounted in unattacked parts of the attacked leaf and as well in unattacked leaves. In this review, we summarize recent progress in understanding how plants recognize herbivory, the involvement of several important signalling pathways that mediate the responses to herbivore attack and the signals that transduce local into systemic responses.

Methods for the analysis of oxylipins in plants

Plant oxylipins comprise a highly diverse and complex class of molecules that are derived from lipid oxidation. The initial oxidation of unsaturated fatty acids may either occur by enzymatic or chemical reactions. A large variety of oxylipin classes are generated by an array of alternative reactions further converting hydroperoxy fatty acids. The structural diversity of oxylipins is further increased by their occurrence either as free fatty acid derivatives or as esters in complex lipids. Lipid peroxidation is common to all biological systems, appearing in developmentally regulated processes and as a response to environmental changes. The oxylipins formed may perform various biological roles; some of them have signaling functions. In order to elucidate the roles of oxylipins in a given biological context, comprehensive analytical assays are available for determining the oxylipin profiles of plant tissues. This review summarizes indirect methods to estimate the general peroxidation state of a sample and more sophisticated techniques for the identification, structure determination and quantification of oxylipins.

Agrobacterium tumefaciens promotes tumor induction by modulating pathogen defense in Arabidopsis thaliana

Agrobacterium tumefaciens causes crown gall disease by transferring and integrating bacterial DNA (T-DNA) into the plant genome. To examine the physiological changes and adaptations during Agrobacterium-induced tumor development, we compared the profiles of salicylic acid (SA), ethylene (ET), jasmonic acid (JA), and auxin (indole-3-acetic acid [IAA]) with changes in the Arabidopsis thaliana transcriptome. Our data indicate that host responses were much stronger toward the oncogenic strain C58 than to the disarmed strain GV3101 and that auxin acts as a key modulator of the Arabidopsis-Agrobacterium interaction. At initiation of infection, elevated levels of IAA and ET were associated with the induction of host genes involved in IAA, but not ET signaling. After T-DNA integration, SA as well as IAA and ET accumulated, but JA did not. This did not correlate with SA-controlled pathogenesis-related gene expression in the host, although high SA levels in mutant plants prevented tumor development, while low levels promoted it. Our data are consistent with a scenario in which ET and later on SA control virulence of agrobacteria, whereas ET and auxin stimulate neovascularization during tumor formation. We suggest that crosstalk among IAA, ET, and SA balances pathogen defense launched by the host and tumor growth initiated by agrobacteria.

The power of mutants for investigating jasmonate biosynthesis and signaling

Mutant analysis includes approaches that range from traditional screening of mutant populations (forward genetics), to identifying mutations in known genes (reverse genetics), to examining the effects of site-specific mutations that encode modified proteins. All these methodologies have been applied to study jasmonate synthesis and signaling, and their use has led to important discoveries. The fad3 fad7 fad8 mutant of Arabidopsis, and other mutants defective in jasmonate synthesis, revealed the roles of jasmonate in flower development and plant defense against necrotrophic fungal pathogens. The coi1 mutant identified the F-box protein that is now known to be the receptor for jasmonoyl-isoleucine, the active form of jasmonate hormone. Investigations of how JASMONATE-ZIM DOMAIN (JAZ) proteins bind to COI1 and facilitate jasmonate perception have relied on the jai3 mutant, on JAZDeltaJas constructs, and on site-specific mutations in the Jas and ZIM domains of these proteins.

Forward and reverse genetics to identify genes involved in the age-related resistance response in Arabidopsis thaliana

SUMMARY Age-related resistance (ARR) occurs in numerous plant species, often resulting in increased disease resistance as plants mature. ARR in Arabidopsis to Pseudomonas syringae pv. tomato is associated with intercellular salicylic acid (SA) accumulation and the transition to flowering. Forward and reverse genetic screens were performed to identify genes required for ARR and to investigate the mechanism of the ARR response. Infiltration of SA into the intercellular space of the ARR-defective mutant iap1-1 (important for the ARR pathway) partially restored ARR function. Inter- and intracellular SA accumulation was reduced in the mutant iap1-1 compared with the wild-type, and the SA regulatory gene EDS1 was also required for ARR. Combining microarray analysis with reverse genetics using T-DNA insertion lines, four additional ARR genes were identified as contributing to ARR: two plant-specific transcription factors of the NAC family [ANAC055 (At3g15500) and ANAC092 (At5g39610)], a UDP-glucose glucosyltransferase [UGT85A1 (At1g22400)] and a cytidine deaminase [CDA1 (At2g19570)]. These four genes and IAP1 are also required for ARR to Hyaloperonospora parasitica. IAP1 encodes a key component of ARR that acts upstream of SA accumulation and possibly downstream of UGT85A1, CDA1 and the two NAC transcription factors (ANAC055, ANAC092).

The wound hormone jasmonate

Plant tissues are highly vulnerable to injury by herbivores, pathogens, mechanical stress, and other environmental insults. Optimal plant fitness in the face of these threats relies on complex signal transduction networks that link damage-associated signals to appropriate changes in metabolism, growth, and development. Many of these wound-induced adaptive responses are triggered by de novo synthesis of the plant hormone jasmonate (JA). Recent studies provide evidence that JA mediates systemic wound responses through distinct cell autonomous and non-autonomous pathways. In both pathways, bioactive JAs are recognized by an F-box protein-based receptor system that couples hormone binding to ubiquitin-dependent degradation of transcriptional repressor proteins. These results provide a framework for understanding how plants recognize and respond to tissue injury.

Regulation of gene expression by jasmonate hormones

Plants possess inducible defense systems to oppose attack by pathogens and herbivores. Jasmonates are important signaling molecules produced by plants which regulate in positive or negative crosstalk with ethylene subsets of genes involved in defense against necrotrophic microorganisms or herbivorous insects, respectively. This review presents an overview of promoter sequences and transcription factors involved in jasmonate-responsive gene expression with the most important components summarized here. Frequently occurring jasmonate-responsive promoter sequences are the GCC motif, which is commonly found in promoters activated synergistically by jasmonate and ethylene, and the G-box, which is commonly found in promoters activated by jasmonates and repressed by ethylene. Important transcription factors conferring jasmonate-responsive gene expression in Arabidopsis are ORA59 and AtMYC2. ORA59 interacts with the GCC motif and controls the expression of genes that are synergistically induced by jasmonates and ethylene, whereas AtMYC2 interacts with the G-box and related sequences, and controls genes activated by jasmonate alone. AtMYC2 can interact with JAZ proteins, which are hypothesized to act as repressors. The bioactive jasmonate (+)-7-iso-JA-l-Ile promotes the interaction between the ubiquitin ligase complex SCF(COI1) and JAZ proteins, resulting in their degradation by the 26S proteasome, thereby liberating AtMYC2 from repression according to the prevailing model. Literature up to 1 June 2009 was used for this review.

A rapid wound signal activates the systemic synthesis of bioactive jasmonates in Arabidopsis

Jasmonic acid (JA) and its biologically active derivatives (bioactive JAs) perform a critical role in regulating plant responses to wound stress. The perception of bioactive JAs by the F-box protein COI1 triggers the SCF(COI1)/ubiquitin-dependent degradation of JASMONATE ZIM-DOMAIN (JAZ) proteins that repress the expression of JA-response genes. JA is required for many wound-inducible systemic defense responses, but little is known about the role of the hormone in long-distance signal relay between damaged and undamaged leaves. Here, we show that the wounding of Arabidopsis thaliana leaves results in the rapid (<5 min) accumulation of jasmonoyl-l-isoleucine (JA-Ile), the bioactive form of JA, in leaves distal to the wound site. The rapid systemic increase in JA-Ile preceded the onset of early transcriptional responses, and was associated with JAZ degradation. Wound-induced systemic production of JA-Ile required the JA biosynthetic enzyme 12-oxo-phytodienoic acid (OPDA) reductase 3 (OPR3) in undamaged responding leaves, but not in wounded leaves, and was largely dependent on the JA-conjugating enzyme JAR1. Interestingly, the wound-induced synthesis of JA/JA-Ile in systemic leaves was correlated with a rapid decline in OPDA levels. These results are consistent with a model in which a rapidly transmitted wound signal triggers the systemic synthesis of JA, which, upon conversion to JA-Ile, activates the expression of early response genes by the SCF(COI1)/JAZ pathway.

Plant oxylipins: COI1/JAZs/MYC2 as the core jasmonic acid-signalling module

Jasmonic acid (JA) and its derivates, collectively known as jasmonates (JAs), are essential signalling molecules that coordinate the plant response to biotic and abiotic challenges, in addition to several developmental processes. The COI1 F-box and additional SCF modulators have long been known to have a crucial role in the JA-signalling pathway. Downstream JA-dependent transcriptional re-programming is regulated by a cascade of transcription factors and MYC2 plays a major role. Recently, JAZ family proteins have been identified as COI1 targets and repressors of MYC2, defining the 'missing link' in JA signalling. JA-Ile has been proposed to be the active form of the hormone, and COI1 is an essential component of the receptor complex. These recent discoveries have defined the core JA-signalling pathway as the module COI1/JAZs/MYC2.

Plant hormone conjugation: a signal decision

Tight regulation of the auxin hormone indole-3-acetic acid (IAA) is crucial for plant development. Newly discovered IAA antagonists are the amide-linked tryptophan conjugates of IAA and jasmonic acid (JA). JA-Trp and IAA-Trp interfered with root gravitropism in Arabidopsis, and inhibited several responses to exogenously supplied IAA. Relatively low concentrations of the inhibitors occurred in Arabidopsis, but Pisum sativum flowers contained over 300 pmole g(-1) FW of JA-Trp. DihydroJA was an even more effective inhibitor than JA-Trp, suggesting that Trp conjugates with other JA derivatives may also be functional. JA-Trp and IAA-Trp add to the list of documented bioactive amide hormone conjugates. The only other example is JA-Ile, the recently discovered jasmonate signal. These examples establish that conjugation not only inactivates hormones, but in some cases creates novel compounds that function in hormone signaling.

Plant cytochrome CYP74 family: biochemical features, endocellular localisation, activation mechanism in plant defence and improvements for industrial applications

Not just another P450: Shown here is a model of the overall structure of CYP74C3 with the putative membrane-binding region that is required for enzyme activation. Members of the CYP74 family of cytochrome P450 enzymes are specialised in the metabolism of hydroperoxides and play an important role in oxylipin metabolism, which is one of the main defence mechanisms employed by plants. In order to respond to their rapidly changing environments, plants have evolved complex signalling pathways, which enable tight control over stress responses. Recent work has shed new light on one of these pathways that involves the different classes of plant oxylipins that are produced through the CYP74 pathway. These phytochemicals play an important role in plant defence, and can act as direct antimicrobials or as signalling molecules that inducing the expression of defence genes. The fine-tuning regulation of defence responses, which depends on the precise cross-talk among different signalling pathways, has important consequences for plant fitness and is a new, challenging area of research. In this review we focus on new data relating to the physiological significance of different phyto-oxylipins and related enzymes. Moreover, recent advances in the biotechnological production of oxylipins are also discussed.

The tryptophan conjugates of jasmonic and indole-3-acetic acids are endogenous auxin inhibitors

Most conjugates of plant hormones are inactive, and some function to reduce the active hormone pool. This study characterized the activity of the tryptophan (Trp) conjugate of jasmonic acid (JA-Trp) in Arabidopsis (Arabidopsis thaliana). Unexpectedly, JA-Trp caused agravitropic root growth in seedlings, unlike JA or nine other JA-amino acid conjugates. The response was dose dependent from 1 to 100 microm, was independent of the COI1 jasmonate signaling locus, and unlike the jasmonate signal JA-isoleucine, JA-Trp minimally inhibited root growth. The Trp conjugate with indole-3-acetic acid (IAA-Trp) produced a similar response, while Trp alone and conjugates with benzoic and cinnamic acids did not. JA-Trp and IAA-Trp at 25 microm nearly eliminated seedling root inhibition caused by 2 microm IAA. The TIR1 auxin receptor is required for activity because roots of tir1-1 grew only approximately 60% of wild-type length on IAA plus JA-Trp, even though tir1-1 is auxin resistant. However, neither JA-Trp nor IAA-Trp interfered with IAA-dependent interaction between TIR1 and Aux/IAA7 in cell-free assays. Trp conjugates inhibited IAA-stimulated lateral root production and DR5-beta-glucuronidase gene expression. JA-deficient mutants were hypersensitive to IAA and a Trp-overaccumulating mutant was less sensitive, suggesting endogenous conjugates affect auxin sensitivity. Conjugates were present at 5.8 pmol g(-1) fresh weight or less in roots, seedlings, leaves, and flowers, and the values increased approximately 10-fold in roots incubated in 25 microm Trp and IAA or JA at 2 microm. These results show that JA-Trp and IAA-Trp constitute a previously unrecognized mechanism to regulate auxin action.

The evolution of nuclear auxin signalling

BACKGROUND

The plant hormone auxin directs many aspects of plant growth and development. To understand the evolution of auxin signalling, we compared the genes encoding two families of crucial transcriptional regulators, AUXIN RESPONSE FACTOR (ARF) and AUXIN/INDOLE-3-ACETIC ACID (Aux/IAA), among flowering plants and two non-seed plants, Physcomitrella patens and Selaginella moellendorffii.

RESULTS

Comparative analysis of the P. patens, S. moellendorffii and Arabidopsis thaliana genomes suggests that the well-established rapid transcriptional response to auxin of flowering plants, evolved in vascular plants after their divergence from the last common ancestor shared with mosses. An N-terminally truncated ARF transcriptional activator is encoded by the genomes of P. patens and S. moellendorffii, and suggests a supplementary mechanism of nuclear auxin signalling, absent in flowering plants. Site-specific analyses of positive Darwinian selection revealed relatively high rates of synonymous substitution in the A. thaliana ARFs of classes IIa (and their closest orthologous genes in poplar) and Ib, suggesting that neofunctionalization in important functional regions has driven the evolution of auxin signalling in flowering plants. Primary auxin responsive gene families (GH3, SAUR, LBD) show different phylogenetic profiles in P. patens, S. moellendorffii and flowering plants, highlighting genes for further study.

CONCLUSION

The genome of P. patens encodes all of the basic components necessary for a rapid auxin response. The spatial separation of the Q-rich activator domain and DNA-binding domain suggests an alternative mechanism of transcriptional control in P. patens distinct from the mechanism seen in flowering plants. Significantly, the genome of S. moellendorffii is predicted to encode proteins suitable for both methods of regulation.

Quantitative structure-property relationship studies on amino acid conjugates of jasmonic acid as defense signaling molecules

Jasmonates and related compounds, including amino acid conjugates of jasmonic acid, have regulatory functions in the signaling pathway for plant developmental processes and responses to the complex equilibrium of biotic and abiotic stress. But the molecular details of the signaling mechanism are still poorly understood. Statistically significant quantitative structure-property relationship models (r(2) > 0.990) constructed by genetic function approximation and molecular field analysis were generated for the purpose of deriving structural requirements for lipophilicity of amino acid conjugates of jasmonic acid. The best models derived in the present study provide some valuable academic information in terms of the 2/3D-descriptors influencing the lipophilicity, which may contribute to further understanding the mechanism of exogenous application of jasmonates in their signaling pathway and designing novel analogs of jasmonic acid as ecological pesticides.

Illuminated behaviour: phytochrome as a key regulator of light foraging and plant anti-herbivore defence

In many ecological scenarios, the success of an individual plant is defined by the behavioural decisions that it makes when confronted with the risks of competition with other plants, and biomass losses to insect herbivores. These decisions involve expression of shade avoidance responses and induced chemical defences. Because these responses are costly, they frequently engender resource allocation dilemmas. In this review, I discuss the mechanisms that trigger adaptive responses to competitors and herbivores, highlighting the role of phytochromes as central organizers of the overall resource allocation strategy of plants. Phytochromes sense the reduction in the red to far-red (R : FR) ratio of sunlight caused by the proximity of other plants. Shade-intolerant plants respond to low R : FR ratios with shade avoidance behaviours and reduced investment in defence. Pfr depletion leads to increased stability of growth-promoting phytochrome-interacting factors (PIFs), and results in the production of auxins and gibberellins, degradation of DELLA proteins, which are repressors of PIFs, and reduced sensitivity to jasmonates. Thus, phytochrome appears to fulfil its organizational role by regulating the relative strength of the signalling circuits controlled by growth-related and defence-related hormones. I point out cases of signalling redundancy and discuss the significance of recent work on hormone signalling for our understanding of the mechanisms that control adaptive plant behaviour.

Hormone (dis)harmony moulds plant health and disease

Diseased plants often display phenotypes consistent with hormone perturbations. We review recent data that have revealed roles in plant-microbe interactions for cellular components and signaling molecules that previously were associated only with hormone signaling. A better understanding of cross-talk between hormonal and defense signaling pathways should reveal new potential targets for microbial effectors that attenuate host resistance mechanisms.