Traits linked to natural variation of sulfur content in Arabidopsis thaliana.

Sulfur (S) is an essential mineral nutrient for plant growth and development; it is important for primary and specialized plant metabolites that are crucial for biotic and abiotic interactions. Foliar S content varies up to 6-fold under a controlled environment, suggesting an adaptive value under certain natural environmental conditions. However, a major quantitative regulator of S content in Arabidopsis thaliana has not been identified yet, pointing to the existence of either additional genetic factors controlling sulfate/S content or of many minor quantitative regulators. Here, we use overlapping information of two separate ionomics studies to select groups of accessions with low, mid, and high foliar S content. We quantify series of metabolites, including anions (sulfate, phosphate, and nitrate), thiols (cysteine and glutathione), and seven glucosinolates, gene expression of 20 genes, sulfate uptake, and three biotic traits. Our results suggest that S content is tightly connected with sulfate uptake, the concentration of sulfate and phosphate anions, and glucosinolate and glutathione synthesis. Additionally, our results indicate that the growth of pathogenic bacteria is enhanced in the A. thaliana accessions containing higher S in their leaves, suggesting a complex regulation between S homeostasis, primary and secondary metabolism, and biotic pressures.

N-hydroxypipecolic acid primes plants for enhanced microbial pattern-induced responses.

The bacterial elicitor flagellin induces a battery of immune responses in plants. However, the rates and intensities by which metabolically-related defenses develop upon flagellin-sensing are comparatively moderate. We report here that the systemic acquired resistance (SAR) inducer N-hydroxypipecolic acid (NHP) primes Arabidopsis thaliana plants for strongly enhanced metabolic and transcriptional responses to treatment by flg22, an elicitor-active peptide fragment of flagellin. While NHP powerfully activated priming of the flg22-induced accumulation of the phytoalexin camalexin, biosynthesis of the stress hormone salicylic acid (SA), generation of the NHP biosynthetic precursor pipecolic acid (Pip), and accumulation of the stress-inducible lipids γ-tocopherol and stigmasterol, it more modestly primed for the flg22-triggered generation of aromatic and branched-chain amino acids, and expression of FLG22-INDUCED RECEPTOR-KINASE1. The characterization of the biochemical and immune phenotypes of a set of different Arabidopsis single and double mutants impaired in NHP and/or SA biosynthesis indicates that, during earlier phases of the basal immune response of naïve plants to Pseudomonas syringae infection, NHP and SA mutually promote their biosynthesis and additively enhance camalexin formation, while SA prevents extraordinarily high NHP levels in later interaction periods. Moreover, SA and NHP additively contribute to Arabidopsis basal immunity to bacterial and oomycete infection, as well as to the flagellin-induced acquired resistance response that is locally observed in plant tissue exposed to exogenous flg22. Our data reveal mechanistic similarities and differences between the activation modes of flagellin-triggered acquired resistance in local tissue and the SAR state that is systemically induced in plants upon pathogen attack. They also corroborate that the NHP precursor Pip has no independent immune-related activity.

N-hydroxypipecolic acid induces systemic acquired resistance and transcriptional reprogramming via TGA transcription factors.

N-hydroxypipecolic acid (NHP) accumulates in pathogen-inoculated and distant leaves of the Arabidopsis shoot and induces systemic acquired resistance (SAR) in dependence of the salicylic acid (SA) receptor NPR1. We report here that SAR triggered by exogenous NHP treatment requires the function of the transcription factors TGA2/5/6 in addition to NPR1, and is further positively affected by TGA1/4. Consistently, a tga2/5/6 triple knockout mutant is fully impaired in NHP-induced SAR gene expression, while a tga1/4 double mutant shows an attenuated, partial transcriptional response to NHP. Moreover, tga2/5/6 and tga1/4 exhibited fully and strongly impaired pathogen-triggered SAR, respectively, while SA-induced resistance was more moderately compromised in both lines. At the same time, tga2/5/6 was not and tga1/4 only partially impaired in the accumulation of NHP and SA at sites of bacterial attack. Strikingly, SAR gene expression in the systemic tissue induced by local bacterial inoculation or locally applied NHP fully required functional TGA2/5/6 and largely depended on TGA1/4 factors. The systemic accumulation of NHP and SA was attenuated but not abolished in the SAR-compromised and transcriptionally blocked tga mutants, suggesting their transport from inoculated to systemic tissue. Our results indicate the existence of a critical TGA- and NPR1-dependent transcriptional module that mediates the induction of SAR and systemic defence gene expression by NHP.

SEC14-GOLD protein PATELLIN2 binds IRON-REGULATED TRANSPORTER1 linking root iron uptake to vitamin E.

Organisms require micronutrients, and Arabidopsis (Arabidopsis thaliana) IRON-REGULATED TRANSPORTER1 (IRT1) is essential for iron (Fe2+) acquisition into root cells. Uptake of reactive Fe2+ exposes cells to the risk of membrane lipid peroxidation. Surprisingly little is known about how this is avoided. IRT1 activity is controlled by an intracellular variable region (IRT1vr) that acts as a regulatory protein interaction platform. Here, we describe that IRT1vr interacted with peripheral plasma membrane SEC14-Golgi dynamics (SEC14-GOLD) protein PATELLIN2 (PATL2). SEC14 proteins bind lipophilic substrates and transport or present them at the membrane. To date, no direct roles have been attributed to SEC14 proteins in Fe import. PATL2 affected root Fe acquisition responses, interacted with ROS response proteins in roots, and alleviated root lipid peroxidation. PATL2 had high affinity in vitro for the major lipophilic antioxidant vitamin E compound α-tocopherol. Molecular dynamics simulations provided insight into energetic constraints and the orientation and stability of the PATL2-ligand interaction in atomic detail. Hence, this work highlights a compelling mechanism connecting vitamin E with root metal ion transport at the plasma membrane with the participation of an IRT1-interacting and α-tocopherol-binding SEC14 protein.

OXIDATIVE SIGNAL-INDUCIBLE1 induces immunity by coordinating N-hydroxypipecolic acid, salicylic acid, and camalexin synthesis.

Expression of OXIDATIVE SIGNAL-INDUCIBLE1 (OXI1) is induced by a number of stress conditions and regulates the interaction of plants with pathogenic and beneficial microbes. In this work, we generated Arabidopsis OXI1 knockout and genomic OXI1 overexpression lines and show by transcriptome, proteome, and metabolome analysis that OXI1 triggers ALD1, SARD4, and FMO1 expressions to promote the biosynthesis of pipecolic acid (Pip) and N-hydroxypipecolic acid (NHP). OXI1 contributes to enhanced immunity by induced SA biosynthesis via CBP60g-induced expression of SID2 and camalexin accumulation via WRKY33-targeted transcription of PAD3. OXI1 regulates genes involved in reactive oxygen species (ROS) generation such as RbohD and RbohF. OXI1 knock out plants show enhanced expression of nuclear and chloroplast genes of photosynthesis and enhanced growth under ambient conditions, while OXI1 overexpressing plants accumulate NHP, SA, camalexin, and ROS and show a gain-of-function (GOF) cell death phenotype and enhanced pathogen resistance. The OXI1 GOF phenotypes are completely suppressed when compromising N-hydroxypipecolic acid (NHP) synthesis in the fmo1 or ald1 background, showing that OXI1 regulation of immunity is mediated via the NHP pathway. Overall, these results show that OXI1 plays a key role in basal and effector-triggered plant immunity by regulating defense and programmed cell death via biosynthesis of salicylic acid, N-hydroxypipecolic acid, and camalexin.

Insect eggs trigger systemic acquired resistance against a fungal and an oomycete pathogen.

Plants are able to detect insect eggs deposited on leaves. In Arabidopsis, eggs of the butterfly species Pieris brassicae (common name large white) induce plant defenses and activate the salicylic acid (SA) pathway. We previously discovered that oviposition triggers a systemic acquired resistance (SAR) against the bacterial hemibiotroph pathogen Pseudomonas syringae. Here, we show that insect eggs or treatment with egg extract (EE) induce SAR against the fungal necrotroph Botrytis cinerea BMM and the oomycete pathogen Hyaloperonospora arabidopsidis Noco2. This response is abolished in ics1, ald1 and fmo1, indicating that the SA pathway and the N-hydroxypipecolic acid (NHP) pathway are involved. Establishment of EE-induced SAR in distal leaves potentially involves tryptophan-derived metabolites, including camalexin. Indeed, SAR is abolished in the biosynthesis mutants cyp79B2 cyp79B3, cyp71a12 cyp71a13 and pad3-1, and camalexin is toxic to B. cinerea in vitro. This study reveals an interesting mechanism by which lepidopteran eggs interfere with plant-pathogen interactions.

Metabolic regulation of systemic acquired resistance.

Plants achieve an optimal balance between growth and defense by a fine-tuned biosynthesis and metabolic inactivation of immune-stimulating small molecules. Recent research illustrates that three common hubs are involved in the cooperative regulation of systemic acquired resistance (SAR) by the defense hormones N-hydroxypipecolic acid (NHP) and salicylic acid (SA). First, a common set of regulatory proteins is involved in their biosynthesis. Second, NHP and SA are glucosylated by the same glycosyltransferase, UGT76B1, and thereby inactivated in concert. And third, NHP confers immunity via the SA receptor NPR1 to reprogram plants at the level of transcription and primes plants for an enhanced defense capacity. An overview of SA and NHP metabolism is provided, and their contribution to long-distance signaling in SAR is discussed.

UGT76B1, a promiscuous hub of small molecule-based immune signaling, glucosylates N-hydroxypipecolic acid, and balances plant immunity.

Glucosylation modulates the biological activity of small molecules and frequently leads to their inactivation. The Arabidopsis thaliana glucosyltransferase UGT76B1 is involved in conjugating the stress hormone salicylic acid (SA) as well as isoleucic acid (ILA). Here, we show that UGT76B1 also glucosylates N-hydroxypipecolic acid (NHP), which is synthesized by FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1) and activates systemic acquired resistance (SAR). Upon pathogen attack, Arabidopsis leaves generate two distinct NHP hexose conjugates, NHP-O-ß-glucoside and NHP glucose ester, whereupon only NHP-O-ß-glucoside formation requires a functional SA pathway. The ugt76b1 mutants specifically fail to generate the NHP-O-ß-glucoside, and recombinant UGT76B1 synthesizes NHP-O-ß-glucoside in vitro in competition with SA and ILA. The loss of UGT76B1 elevates the endogenous levels of NHP, SA, and ILA and establishes a constitutive SAR-like immune status. Introgression of the fmo1 mutant lacking NHP biosynthesis into the ugt76b1 background abolishes this SAR-like resistance. Moreover, overexpression of UGT76B1 in Arabidopsis shifts the NHP and SA pools toward O-ß-glucoside formation and abrogates pathogen-induced SAR. Our results further indicate that NHP-triggered immunity is SA-dependent and relies on UGT76B1 as a common metabolic hub. Thereby, UGT76B1-mediated glucosylation controls the levels of active NHP, SA, and ILA in concert to balance the plant immune status.

The mobile SAR signal N-hydroxypipecolic acid induces NPR1-dependent transcriptional reprogramming and immune priming.

N-hydroxypipecolic acid (NHP) accumulates in the plant foliage in response to a localized microbial attack and induces systemic acquired resistance (SAR) in distant leaf tissue. Previous studies indicated that pathogen inoculation of Arabidopsis (Arabidopsis thaliana) systemically activates SAR-related transcriptional reprogramming and a primed immune status in strict dependence of FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1), which mediates the endogenous biosynthesis of NHP. Here, we show that elevations of NHP by exogenous treatment are sufficient to induce a SAR-reminiscent transcriptional response that mobilizes key components of immune surveillance and signal transduction. Exogenous NHP primes Arabidopsis wild-type and NHP-deficient fmo1 plants for a boosted induction of pathogen-triggered defenses, such as the biosynthesis of the stress hormone salicylic acid (SA), accumulation of the phytoalexin camalexin and branched-chain amino acids, as well as expression of defense-related genes. NHP also sensitizes the foliage systemically for enhanced SA-inducible gene expression. NHP-triggered SAR, transcriptional reprogramming, and defense priming are fortified by SA accumulation, and require the function of the transcriptional coregulator NON-EXPRESSOR OF PR GENES1 (NPR1). Our results suggest that NPR1 transduces NHP-activated immune signaling modes with predominantly SA-dependent and minor SA-independent features. They further support the notion that NHP functions as a mobile immune regulator capable of moving independently of active SA signaling between leaves to systemically activate immune responses.

Natural variation in temperature-modulated immunity uncovers transcription factor bHLH059 as a thermoresponsive regulator in Arabidopsis thaliana.

Temperature impacts plant immunity and growth but how temperature intersects with endogenous pathways to shape natural variation remains unclear. Here we uncover variation between Arabidopsis thaliana natural accessions in response to two non-stress temperatures (22°C and 16°C) affecting accumulation of the thermoresponsive stress hormone salicylic acid (SA) and plant growth. Analysis of differentially responding A. thaliana accessions shows that pre-existing SA provides a benefit in limiting infection by Pseudomonas syringae pathovar tomato DC3000 bacteria at both temperatures. Several A. thaliana genotypes display a capacity to mitigate negative effects of high SA on growth, indicating within-species plasticity in SA-growth tradeoffs. An association study of temperature x SA variation, followed by physiological and immunity phenotyping of mutant and over-expression lines, identifies the transcription factor bHLH059 as a temperature-responsive SA immunity regulator. Here we reveal previously untapped diversity in plant responses to temperature and a way forward in understanding the genetic architecture of plant adaptation to changing environments.

Putrescine elicits ROS-dependent activation of the salicylic acid pathway in Arabidopsis thaliana.

Polyamines are small amines that accumulate during stress and contribute to disease resistance through as yet unknown signaling pathways. Using a comprehensive RNA-sequencing analysis, we show that early transcriptional responses triggered by each of the most abundant polyamines (putrescine, spermidine, spermine, thermospermine and cadaverine) exhibit specific quantitative differences, suggesting that polyamines (rather than downstream metabolites) elicit defense responses. Signaling by putrescine, which accumulates in response to bacteria that trigger effector triggered immunity (ETI) and systemic acquired resistance (SAR), is largely dependent on the accumulation of hydrogen peroxide, and is partly dependent on salicylic acid (SA), the expression of ENHANCED DISEASE SUSCEPTIBILITY (EDS1) and NONEXPRESSOR of PR GENES1 (NPR1). Putrescine elicits local SA accumulation as well as local and systemic transcriptional reprogramming that overlaps with SAR. Loss-of-function mutations in arginine decarboxylase 2 (ADC2), which is required for putrescine synthesis and copper amine oxidase (CuAO), which is involved in putrescine oxidation, compromise basal defenses, as well as putrescine and pathogen-triggered systemic resistance. These findings confirm that putrescine elicits ROS-dependent SA pathways in the activation of plant defenses.

Inducible biosynthesis and immune function of the systemic acquired resistance inducer N-hydroxypipecolic acid in monocotyledonous and dicotyledonous plants.

Recent work has provided evidence for the occurrence of N-hydroxypipecolic acid (NHP) in Arabidopsis thaliana, characterized its pathogen-inducible biosynthesis by a three-step metabolic sequence from l-lysine, and established a central role for NHP in the regulation of systemic acquired resistance. Here, we show that NHP is biosynthesized in several other plant species in response to microbial attack, generally together with its direct metabolic precursor pipecolic acid and the phenolic immune signal salicylic acid. For example, NHP accumulates locally in inoculated leaves and systemically in distant leaves of cucumber in response to Pseudomonas syringae attack, in Pseudomonas-challenged tobacco and soybean leaves, in tomato inoculated with the oomycete Phytophthora infestans, in leaves of the monocot Brachypodium distachyon infected with bacterial (Xanthomonas translucens) and fungal (Magnaporthe oryzae) pathogens, and in M. oryzae-inoculated barley. Notably, resistance assays indicate that NHP acts as a potent inducer of acquired resistance to bacterial and fungal infection in distinct monocotyledonous and dicotyledonous species. Pronounced systemic accumulation of NHP in leaf phloem sap of locally inoculated cucumber supports a function for NHP as a phloem-mobile immune signal. Our study thus generalizes the existence and function of an NHP resistance pathway in plant systemic acquired resistance.

A Role for Tocopherol Biosynthesis in Arabidopsis Basal Immunity to Bacterial Infection.

Tocopherols are lipid-soluble antioxidants synthesized in plastids of plants and other photosynthetic organisms. The four known tocopherols, α-, ß-, γ-, and δ-tocopherol, differ in number and position of methyl groups on their chromanol head group. In unstressed Arabidopsis (Arabidopsis thaliana) leaves, α-tocopherol constitutes the main tocopherol form, whereas seeds predominantly contain γ-tocopherol. Here, we show that inoculation of Arabidopsis leaves with the bacterial pathogen Pseudomonas syringae induces the expression of genes involved in early steps of tocopherol biosynthesis and triggers strong accumulation of γ-tocopherol, moderate production of δ-tocopherol, and generation of the benzoquinol precursors of tocopherols. The pathogen-inducible biosynthesis of tocopherols is promoted by the immune regulators ENHANCED DISEASE SUSCEPTIBILITY1 and PHYTOALEXIN-DEFICIENT4. In addition, tocopherols accumulate in response to bacterial flagellin and reactive oxygen species. By quantifying tocopherol forms in inoculated wild-type plants and biosynthetic pathway mutants, we provide biochemical insights into the pathogen-inducible tocopherol pathway. Notably, vitamin E deficient2 (vte2) mutant plants, which are compromised in both tocopherol and benzoquinol precursor accumulation, exhibit increased susceptibility toward compatible P. syringae and possess heightened levels of markers of lipid peroxidation after bacterial infection. The deficiency of triunsaturated fatty acids in vte2-1 fatty acid desaturase3-2 (fad3-2) fad7-2 fad8 quadruple mutants prevents increased lipid peroxidation in the vte2 background and restores pathogen resistance to wild-type levels. Therefore, the tocopherol biosynthetic pathway positively influences salicylic acid accumulation and guarantees effective basal resistance of Arabidopsis against compatible P. syringae, possibly by protecting leaves from the pathogen-induced oxidation of trienoic fatty acid-containing lipids.

ABCG1 contributes to suberin formation in Arabidopsis thaliana roots.

Diffusion barriers enable plant survival under fluctuating environmental conditions. They control internal water potential and protect against biotic or abiotic stress factors. How these protective molecules are deposited to the extracellular environment is poorly understood. We here examined the role of the Arabidopsis ABC half-size transporter AtABCG1 in the formation of the extracellular root suberin layer. Quantitative analysis of extracellular long-chain fatty acids and aliphatic alcohols in the atabcg1 mutants demonstrated altered root suberin composition, specifically a reduction in longer chain dicarboxylic acids, fatty alcohols and acids. Accordingly, the ATP-hydrolyzing activity of heterologous expressed and purified AtABCG1 was strongly stimulated by fatty alcohols (C26-C30) and fatty acids (C24-C30) in a chain length dependent manner. These results are a first indication for the function of AtABCG1 in the transport of longer chain aliphatic monomers from the cytoplasm to the apoplastic space during root suberin formation.

Root-specific camalexin biosynthesis controls the plant growth-promoting effects of multiple bacterial strains.

Plants in their natural ecosystems interact with numerous microorganisms, but how they influence their microbiota is still elusive. We observed that sulfatase activity in soil, which can be used as a measure of rhizosphere microbial activity, is differently affected by Arabidopsis accessions. Following a genome-wide association analysis of the variation in sulfatase activity we identified a candidate gene encoding an uncharacterized cytochrome P450, CYP71A27 Loss of this gene resulted in 2 different and independent microbiota-specific phenotypes: A lower sulfatase activity in the rhizosphere and a loss of plant growth-promoting effect by Pseudomonas sp. CH267. On the other hand, tolerance to leaf pathogens was not affected, which agreed with prevalent expression of CYP71A27 in the root vasculature. The phenotypes of cyp71A27 mutant were similar to those of cyp71A12 and cyp71A13, known mutants in synthesis of camalexin, a sulfur-containing indolic defense compound. Indeed, the cyp71A27 mutant accumulated less camalexin in the roots upon elicitation with silver nitrate or flagellin. Importantly, addition of camalexin complemented both the sulfatase activity and the loss of plant growth promotion by Pseudomonas sp. CH267. Two alleles of CYP71A27 were identified among Arabidopsis accessions, differing by a substitution of Glu373 by Gln, which correlated with the ability to induce camalexin synthesis and to gain fresh weight in response to Pseudomonas sp. CH267. Thus, CYP71A27 is an additional component in the camalexin synthesis pathway, contributing specifically to the control of plant microbe interactions in the root.

Nitrite and nitric oxide are important in the adjustment of primary metabolism during the hypersensitive response in tobacco.

Nitrate and ammonia deferentially modulate primary metabolism during the hypersensitive response in tobacco. In this study, tobacco RNAi lines with low nitrite reductase (NiRr) levels were used to investigate the roles of nitrite and nitric oxide (NO) in this process. The lines accumulate NO2-, with increased NO generation, but allow sufficient reduction to NH4+ to maintain plant viability. For wild-type (WT) and NiRr plants grown with NO3-, inoculation with the non-host biotrophic pathogen Pseudomonas syringae pv. phaseolicola induced an accumulation of nitrite and NO, together with a hypersensitive response (HR) that resulted in decreased bacterial growth, increased electrolyte leakage, and enhanced pathogen resistance gene expression. These responses were greater with increases in NO or NO2- levels in NiRr plants than in the WT under NO3- nutrition. In contrast, WT and NiRr plants grown with NH4+ exhibited compromised resistance. A metabolomic analysis detected 141 metabolites whose abundance was differentially changed as a result of exposure to the pathogen and in response to accumulation of NO or NO2-. Of these, 13 were involved in primary metabolism and most were linked to amino acid and energy metabolism. HR-associated changes in metabolism that are often linked with primary nitrate assimilation may therefore be influenced by nitrite and NO production.

N-hydroxypipecolic acid and salicylic acid: a metabolic duo for systemic acquired resistance.

Recent research has established that the pipecolate pathway, a three-step biochemical sequence from l-lysine to N-hydroxypipecolic acid (NHP), is central for plant systemic acquired resistance (SAR). NHP orchestrates SAR establishment in concert with the immune signal salicylic acid (SA). Here, we outline the biochemistry of NHP formation from l-Lys and address novel progress on SA biosynthesis in Arabidopsis and other plant species. In Arabidopsis, the pathogen-inducible pipecolate and salicylate pathways are activated by common and distinct regulatory elements and mutual interactions between both metabolic branches exist. The mode of action of NHP in SAR involves direct induction of SAR gene expression, signal amplification, priming for enhanced defense activation and positive interplay with SA signaling to ensure elevated plant immunity.

Fluctuating Light Interacts with Time of Day and Leaf Development Stage to Reprogram Gene Expression.

Natural light environments are highly variable. Flexible adjustment between light energy utilization and photoprotection is therefore of vital importance for plant performance and fitness in the field. Short-term reactions to changing light intensity are triggered inside chloroplasts and leaves within seconds to minutes, whereas long-term adjustments proceed over hours and days, integrating multiple signals. While the mechanisms of long-term acclimation to light intensity have been studied by changing constant growth light intensity during the day, responses to fluctuating growth light intensity have rarely been inspected in detail. We performed transcriptome profiling in Arabidopsis (Arabidopsis thaliana) leaves to investigate long-term gene expression responses to fluctuating light (FL). In particular, we examined whether responses differ between young and mature leaves or between morning and the end of the day. Our results highlight global reprogramming of gene expression under FL, including that of genes related to photoprotection, photosynthesis, and photorespiration and to pigment, prenylquinone, and vitamin metabolism. The FL-induced changes in gene expression varied between young and mature leaves at the same time point and between the same leaves in the morning and at the end of the day, indicating interactions of FL acclimation with leaf development stage and time of day. Only 46 genes were up- or down-regulated in both young and mature leaves at both time points. Combined analyses of gene coexpression and cis-elements pointed to a role of the circadian clock and light in coordinating the acclimatory responses of functionally related genes. Our results also suggest a possible cross talk between FL acclimation and systemic acquired resistance-like gene expression in young leaves.

A MPK3/6-WRKY33-ALD1-Pipecolic Acid Regulatory Loop Contributes to Systemic Acquired Resistance.

Plants induce systemic acquired resistance (SAR) upon localized exposure to pathogens. Pipecolic acid (Pip) production via AGD2-LIKE DEFENSE RESPONSE PROTEIN1 (ALD1) is key for SAR establishment. Here, we report a positive feedback loop important for SAR induction in Arabidopsis thaliana We showed that local activation of the MAP kinases MPK3 and MPK6 is sufficient to trigger Pip production and mount SAR. Consistent with this, mutations in MPK3 or MPK6 led to compromised Pip accumulation upon inoculation with the bacterial pathogen Pseudomonas syringae pv tomato DC3000 (Pto) AvrRpt2, which triggers strong sustained MAPK activation. By contrast, P. syringae pv maculicola and Pto, which induce transient MAPK activation, trigger Pip biosynthesis and SAR independently of MPK3/6. ALD1 expression, Pip accumulation, and SAR were compromised in mutants defective in the MPK3/6-regulated transcription factor WRKY33. Chromatin immunoprecipitation showed that WRKY33 binds to the ALD1 promoter. We found that Pip triggers activation of MPK3 and MPK6 and that MAPK activation after Pto AvrRpt2 inoculation is compromised in wrky33 and ald1 mutants. Collectively, our results reveal a positive regulatory loop consisting of MPK3/MPK6, WRKY33, ALD1, and Pip in SAR induction and suggest the existence of distinct SAR activation pathways that converge at the level of Pip biosynthesis.

l-lysine metabolism to N-hydroxypipecolic acid: an integral immune-activating pathway in plants.

l-lysine catabolic routes in plants include the saccharopine pathway to α-aminoadipate and decarboxylation of lysine to cadaverine. The current review will cover a third l-lysine metabolic pathway having a major role in plant systemic acquired resistance (SAR) to pathogen infection that was recently discovered in Arabidopsis thaliana. In this pathway, the aminotransferase AGD2-like defense response protein (ALD1) α-transaminates l-lysine and generates cyclic dehydropipecolic (DP) intermediates that are subsequently reduced to pipecolic acid (Pip) by the reductase SAR-deficient 4 (SARD4). l-pipecolic acid, which occurs ubiquitously in the plant kingdom, is further N-hydroxylated to the systemic acquired resistance (SAR)-activating metabolite N-hydroxypipecolic acid (NHP) by flavin-dependent monooxygenase1 (FMO1). N-hydroxypipecolic acid induces the expression of a set of major plant immune genes to enhance defense readiness, amplifies resistance responses, acts synergistically with the defense hormone salicylic acid, promotes the hypersensitive cell death response and primes plants for effective immune mobilization in cases of future pathogen challenge. This pathogen-inducible NHP biosynthetic pathway is activated at the transcriptional level and involves feedback amplification. Apart from FMO1, some cytochrome P450 monooxygenases involved in secondary metabolism catalyze N-hydroxylation reactions in plants. In specific taxa, pipecolic acid might also serve as a precursor in the biosynthesis of specialized natural products, leading to C-hydroxylated and otherwise modified piperidine derivatives, including indolizidine alkaloids. Finally, we show that NHP is glycosylated in Arabidopsis to form a hexose-conjugate, and then discuss open questions in Pip/NHP-related research.