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

Nitric oxide (NO) and reactive oxygen intermediates (ROIs) play key roles in the activation of disease resistance mechanisms both in animals and plants. In animals NO cooperates with ROIs to kill tumor cells and for macrophage killing of bacteria. Such cytotoxic events occur because unregulated NO levels drive a diffusionlimited reaction with O 2 ؊ to generate peroxynitrite (ONOO ؊ ), a mediator of cellular injury in many biological systems. Here we show that in soybean cells unregulated NO production at the onset of a pathogen-induced hypersensitive response (HR) is not sufficient to activate hypersensitive cell death. The HR is triggered only by balanced production of NO and ROIs. Moreover, hypersensitive cell death is activated after interaction of NO not with O 2 ؊ but with H 2O2 generated from O2 ؊ by superoxide dismutase. Increasing the level of O 2 ؊ reduces NO-mediated toxicity, and ONOO ؊ is not a mediator of hypersensitive cell death. During the HR, superoxide dismutase accelerates O 2 ؊ dismutation to H2O2 to minimize the loss of NO by reaction with O 2 ؊ and to trigger hypersensitive cell death through NO͞H 2O2 cooperation. However, O2 ؊ rather than H 2O2 is the primary ROI signal for pathogen induction of glutathione S-transferase, and the rates of production and dismutation of O 2 ؊ generated during the oxidative burst play a crucial role in the modulation and integration of NO͞H 2O2 signaling in the HR. Thus although plants and animals use a similar repertoire of signals in disease resistance, ROIs and NO are deployed in strikingly different ways to trigger host cell death.A ttempted infection of plants by an avirulent pathogen elicits a battery of defenses often accompanied by the collapse of challenged host cells. This hypersensitive cell death results in a restricted lesion delimited from surrounding healthy tissue and is thought to contribute to pathogen restriction. An early event in this hypersensitive response (HR) is the generation of superoxide (O 2 Ϫ ) and accumulation of hydrogen peroxide (H 2 O 2 ) in an oxidative burst reminiscent of that producing such reactive oxygen intermediates (ROIs) in activated macrophages (1).Activation of the oxidative burst in the plant HR is part of a highly amplified and integrated signal system that also involves salicylic acid and perturbations of cytosolic Ca 2ϩ to trigger defense mechanisms (2) and to mediate the establishment of systemic immunity (3). The oxidative burst is necessary but not sufficient to trigger host cell death, and recent data indicate that nitric oxide (NO) cooperates with ROIs in the activation of hypersensitive cell death (4).NO and ROIs also interact in the mammalian native immune system where macrophage killing of pathogens and tumor cells involves the diffusion-limited reaction of NO and O 2 to generate ONOO Ϫ , a long lived and highly reactive oxidant species that freely crosses membranes (5), which may modulate NO signal functions (6). ONOO Ϫ induces apoptosis in some human tumor cells (7), and it is also directly cytotoxic...

Following a previous microbial inoculation, plants can induce broad-spectrum immunity to pathogen infection, a phenomenon known as systemic acquired resistance (SAR). SAR establishment in Arabidopsis thaliana is regulated by the Lys catabolite pipecolic acid (Pip) and flavin-dependent-monooxygenase1 (FMO1). Here, we show that elevated Pip is sufficient to induce an FMO1-dependent transcriptional reprogramming of leaves that is reminiscent of SAR. In planta and in vitro analyses demonstrate that FMO1 functions as a pipecolate N-hydroxylase, catalyzing the biochemical conversion of Pip to N-hydroxypipecolic acid (NHP). NHP systemically accumulates in plants after microbial attack. When exogenously applied, it overrides the defect of NHP-deficient fmo1 in acquired resistance and acts as a potent inducer of plant immunity to bacterial and oomycete infection. Our work has identified a pathogen-inducible L-Lys catabolic pathway in plants that generates the N-hydroxylated amino acid NHP as a critical regulator of systemic acquired resistance to pathogen infection.

We investigated the relationships of the two immune-regulatory plant metabolites, salicylic acid (SA) and pipecolic acid (Pip), in the establishment of plant systemic acquired resistance (SAR), SAR-associated defense priming, and basal immunity. Using SA-deficient sid2, Pip-deficient ald1, and sid2 ald1 plants deficient in both SA and Pip, we show that SA and Pip act both independently from each other and synergistically in Arabidopsis thaliana basal immunity to Pseudomonas syringae. Transcriptome analyses reveal that SAR establishment in Arabidopsis is characterized by a strong transcriptional response systemically induced in the foliage that prepares plants for future pathogen attack by preactivating multiple stages of defense signaling and that SA accumulation upon SAR activation leads to the downregulation of photosynthesis and attenuated jasmonate responses systemically within the plant. Whereas systemic Pip elevations are indispensable for SAR and necessary for virtually the whole transcriptional SAR response, a moderate but significant SA-independent component of SAR activation and SAR gene expression is revealed. During SAR, Pip orchestrates SA-dependent and SA-independent priming of pathogen responses in a FLAVIN-DEPENDENT-MONOOXYGENASE1 (FMO1)-dependent manner. We conclude that a Pip/FMO1 signaling module acts as an indispensable switch for the activation of SAR and associated defense priming events and that SA amplifies Pip-triggered responses to different degrees in the distal tissue of SAR-activated plants.