Anders K. Nilsson

Cyclophilin 20-3 relays a 12-oxo-phytodienoic acid signal during stress responsive regulation of cellular redox homeostasis

The jasmonate family of phytohormones plays central roles in plant development and stress acclimation. However, the architecture of their signaling circuits remains largely unknown. Here we describe a jasmonate family binding protein, cyclophilin 20-3 (CYP20-3), which regulates stress-responsive cellular redox homeostasis. (+)-12-oxo-phytodienoic acid (OPDA) binding promotes CYP20-3 to form a complex with serine acetyltransferase 1, which triggers the formation of a hetero-oligomeric cysteine synthase complex with O-acetylserine(thiol)lyase B in chloroplasts. The cysteine synthase complex formation then activates sulfur assimilation that leads to increased levels of thiol metabolites and the buildup of cellular reduction potential. The enhanced redox capacity in turn coordinates the expression of a subset of OPDA-responsive genes. Thus, we conclude that CYP20-3 is a key effector protein that links OPDA signaling to amino acid biosynthesis and cellular redox homeostasis in stress responses.

Action of growth hormone: current views.

Growth hormone (GH) plays an important regulatory role in determining the rate of postnatal somatic growth in many species, and ultimately determines final statural size. Other hormones, such as insulin and thyroid hormones, also influence body growth, but G H is the only known hormone that stimulates proportional body growth in a dose-dependent manner over a wide range of doses (1). Furthermore, G H is the only hormone that can cause excessive stimulation of body growth and gigantism (1, 2). Growth of tissues is the result of an increase in cell number, cell size or both. It has been convincingly demonstrated that G H stimulates growth of various tissues by increasing the number of cells rather than by increasing cell size (3-5). Thus, the decreased DNA/protein ratio that is characteristic of G H deficient individuals is normalized after G H replacement therapy (1). A controversial question concerning the mechanism of action of the growth-promoting effect of GH is whether the hormone acts directly on cells in different tissues, or indirectly by regulating the circulating level of somatomedins (insulin-like growth factors IGFs). The somatomedin hypothesis implies that the actions of G H on somatic growth are not due to a direct interaction between G H and skeletal tissues, but are mediated by IGF-I, which is synthesized in non-skeletal tissues, such as the liver (68) . With the recent recognition that GH regulatks the,production of IGF-I in many tissues, the somatomedin hypothesis has been modified, and an autocrine and paracrine function of IGF-I has been suggested as an important component of G H action (9). The aim of this presentation is to discuss the cellular mechanisms by which G H stimulates longitudinal bone growth. Special emphasis will be placed on recent reports suggesting that GH promotes postnatal tissue growth by stimulating precursor cells in various tissues, including epiphgseal cartilage.

Role of the penetration-resistance genes PEN1, PEN2 and PEN3 in the hypersensitive response and race-specific resistance in Arabidopsis thaliana

Plants are highly capable of recognizing and defending themselves against invading microbes. Adapted plant pathogens secrete effector molecules to suppress the hosts immune system. These molecules may be recognized by host-encoded resistance proteins, which then trigger defense in the form of the hypersensitive response (HR) leading to programmed cell death of the host tissue at the infection site. The three proteins PEN1, PEN2 and PEN3 have been found to act as central components in cell wall-based defense against the non-adapted powdery mildew Blumeria graminis fsp. hordei (Bgh). We found that loss of function mutations in any of the three PEN genes cause decreased hypersensitive cell death triggered by recognition of effectors from oomycete and bacterial pathogens in Arabidopsis. There were considerable additive effects of the mutations. The HR induced by recognition of AvrRpm1 was almost completely abolished in the pen2 pen3 and pen1 pen3 double mutants and the loss of cell death could be linked to indole glucosinolate breakdown products. However, the loss of the HR in pen double mutants did not affect the plants ability to restrict bacterial growth, whereas resistance to avirulent isolates of the oomycete Hyaloperonospora arabidopsidis was strongly compromised. In contrast, the double and triple mutants demonstrated varying degrees of run-away cell death in response to Bgh. Taken together, our results indicate that the three genes PEN1, PEN2 and PEN3 extend in functionality beyond their previously recognized functions in cell wall-based defense against non-host pathogens.

Expression of plasminogen activator inhibitor-1 mRNA in healthy, atherosclerotic and thrombotic human arteries and veins.

Plasminogen activator inhibitor-1 (PAI-1), specific inhibitor of plasminogen activators (PA), plays an important role in the regulation of fibrinolysis. Increased levels of PAI-1 have been associated with vascular disease such as thrombosis and atherosclerosis. In the present study the expression of PAI-1 mRNA in human healthy, atherosclerotic and thrombotic blood vessel walls was quantified by RNA-RNA hybridization in solution and localized by in situ hybridization. The mean expression of PAI-1 mRNA was significantly higher in healthy arteries (0.86 pg/microgram total RNA) than in healthy veins (0.29 pg/microgram total RNA), p < 0.01. The mean PAI-1 mRNA expression in thrombotic arteries (1.72 pg/micrograms total RNA) was significantly higher than that in healthy arteries, p < 0.05, and the mean PAI-1 mRNA expression in thrombotic veins (1.29 pg/micrograms total RNA) was significantly higher than that in healthy veins, p < 0.01. By in situ hybridization PAI-1 mRNA was detected in the intima, media and adventitia of healthy arteries and healthy veins. In atherosclerotic arteries PAI-1 mRNA was detected in the atherosclerotic plaque and in the medial and adventitial layers below the plaque. An increased expression of PAI-1 mRNA was found in the intimal layer of a thrombotic vein. The increased expression of PAI-1 mRNA in thrombotic arteries and veins indicates a role for PAI-1 in thrombogenesis.

Oxo-phytodienoic acid (OPDA) is formed on fatty acids esterified to galactolipids after tissue disruption in Arabidopsis thaliana

Biotic and abiotic stress induces the formation of galactolipids esterified with the phytohormones 12‐oxo‐phytodienoic acid (OPDA) and dinor‐oxo‐phytodienoic acid (dnOPDA) in Arabidopsis thaliana. The biosynthetic pathways of free (dn)OPDA is well described, but it is unclear how they are incorporated into galactolipids. We herein show that (dn)OPDA containing lipids are formed rapidly after disruption of cellular integrity in leaf tissue. Five minutes after freeze‐thawing, 60–70% of the trienoic acids esterified to chloroplast galactolipids are converted to (dn)OPDA. Stable isotope labeling with 18O‐water provides strong evidence for that the fatty acids remain attached to galactolipids during the enzymatic conversion to (dn)OPDA.

A voltage-dependent chloride channel fine-tunes photosynthesis in plants

In natural habitats, plants frequently experience rapid changes in the intensity of sunlight. To cope with these changes and maximize growth, plants adjust photosynthetic light utilization in electron transport and photoprotective mechanisms. This involves a proton motive force (PMF) across the thylakoid membrane, postulated to be affected by unknown anion (Cl−) channels. Here we report that a bestrophin-like protein from Arabidopsis thaliana functions as a voltage-dependent Cl− channel in electrophysiological experiments. AtVCCN1 localizes to the thylakoid membrane, and fine-tunes PMF by anion influx into the lumen during illumination, adjusting electron transport and the photoprotective mechanisms. The activity of AtVCCN1 accelerates the activation of photoprotective mechanisms on sudden shifts to high light. Our results reveal that AtVCCN1, a member of a conserved anion channel family, acts as an early component in the rapid adjustment of photosynthesis in variable light environments.

Cartilage as a Target Tissue for Growth Hormone and Insulin-Like Growth Factor I

Skeletal growth of most mammals during the post-pubertal stage is dependent upon differentiation/proliferation of chondrocytes in the specialized region of long bone known as the epiphyseal growth plate or physis. Previous studies have demonstrated that growth hormone (GH) and insulin-like growth factor I (IGF-I) have important regulatory roles in the rate of differentiation and proliferation of these cells. This review focuses on recent developments in our understanding of the interrelationship between GH and IGF-I in promoting longitudinal bone growth with emphasis on the relative contribution of circulating (endocrine-acting) IGF-I versus locally produced (paracrine/autocrine-acting) IGF-I. In addition, a differentiation-clonal expansion theory of GH action is proposed, which is a slight modification of the dual effector theory originally proposed by Green et al. (1).

Involvement of the Electrophilic Isothiocyanate Sulforaphane in Arabidopsis Local Defense Responses

The secondary metabolite sulforaphane is produced during the hypersensitive response and is involved in protection against infections. Plants defend themselves against microbial pathogens through a range of highly sophisticated and integrated molecular systems. Recognition of pathogen-secreted effector proteins often triggers the hypersensitive response (HR), a complex multicellular defense reaction where programmed cell death of cells surrounding the primary site of infection is a prominent feature. Even though the HR was described almost a century ago, cell-to-cell factors acting at the local level generating the full defense reaction have remained obscure. In this study, we sought to identify diffusible molecules produced during the HR that could induce cell death in naive tissue. We found that 4-methylsulfinylbutyl isothiocyanate (sulforaphane) is released by Arabidopsis (Arabidopsis thaliana) leaf tissue undergoing the HR and that this compound induces cell death as well as primes defense in naive tissue. Two different mutants impaired in the pathogen-induced accumulation of sulforaphane displayed attenuated programmed cell death upon bacterial and oomycete effector recognition as well as decreased resistance to several isolates of the plant pathogen Hyaloperonospora arabidopsidis. Treatment with sulforaphane provided protection against a virulent H. arabidopsidis isolate. Glucosinolate breakdown products are recognized as antifeeding compounds toward insects and recently also as intracellular signaling and bacteriostatic molecules in Arabidopsis. The data presented here indicate that these compounds also trigger local defense responses in Arabidopsis tissue.

Redundancy among phospholipase D isoforms in resistance triggered by recognition of the Pseudomonas syringae effector AvrRpm1 in Arabidopsis thaliana

Plants possess a highly sophisticated system for defense against microorganisms. So called MAMP (microbe-associated molecular patterns) triggered immunity (MTI) prevents the majority of non-adapted pathogens from causing disease. Adapted plant pathogens use secreted effector proteins to interfere with such signaling. Recognition of microbial effectors or their activity by plant resistance (R)-proteins triggers a second line of defense resulting in effector triggered immunity (ETI). The latter usually comprises the hypersensitive response (HR) which includes programmed cell death at the site of infection. Phospholipase D (PLD) mediated production of phosphatidic acid (PA) has been linked to both MTI and ETI in plants. Inhibition of PLD activity has been shown to attenuate MTI as well as ETI. In this study, we systematically tested single and double knockouts in all 12 genes encoding PLDs in Arabidopsis thaliana for effects on ETI and MTI. No single PLD could be linked to ETI triggered by recognition of effectors secreted by the bacterium Pseudomonas syringae. However, repression of PLD dependent PA production by n-butanol strongly inhibited the HR following Pseudomonas syringae effector recognition. In addition some pld mutants were more sensitive to n-butanol than wild type. Thus, the effect of mutations of PLDs could become detectable, and the corresponding genes can be proposed to be involved in the HR. Only knockout of PLDδ caused a loss of MTI-induced cell wall based defense against the non-host powdery mildew Erysiphe pisi. This is thus in stark contrast to the involvement of a multitude of PLD isoforms in the HR triggered by AvrRpm1 recognition.

Formation of oxidized phosphatidylinositol and 12-oxo-phytodienoic acid containing acylated phosphatidylglycerol during the hypersensitive response in Arabidopsis

Plant membranes are composed of a wide array of polar lipids. The functionality of these extends far beyond a pure structural role. Membrane lipids function as enzyme co-factors, establish organelle identity and as substrates for enzymes such as lipases and lipoxygenases. Enzymatic degradation or oxidation (enzymatic or non-enzymatic) of membrane lipids leads to the formation of a diverse group of bioactive compounds. Plant defense reactions provoked by pathogenic microorganisms are often associated with substantial modifications of the lipidome. In this study, we profiled changes in phospholipids during the hypersensitive response triggered by recognition of the bacterial effector protein AvrRpm1 in Arabidopsis thaliana. A simple and robust LC-MS based method for profiling plant lipids was designed to separate all the major species of glycerolipids extracted from Arabidopsis leaf tissue. The method efficiently separated several isobaric and near isobaric lipid species, which otherwise are difficult to quantify in direct infusion based profiling. In addition to the previously reported OPDA-containing galactolipids found to be induced during hypersensitive response in Arabidopsis, three OPDA-containing sulfoquinovosyl diacylglycerol species, one phosphatidylinositol species as well as two acylated OPDA-containing phosphatidylglycerol species were found to accumulate during the hypersensitive response in Arabidopsis. Our study confirms and extends on the notion that the hypersensitive response in Arabidopsis triggers a unique profile of Allene Oxide Synthase dependent oxidation of membrane lipids. Primary targets of this oxidation seem to be uncharged and anionic lipid species.