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.     

Limitation of nocturnal ATP import into chloroplasts seems to affect hormonal crosstalk, prime defense, and enhance disease resistance in Arabidopsis thaliana

When grown under short-day conditions at low light, leaves of an Arabidopsis thaliana (accession Col-0) mutant with defects in the two genes encoding plastid ATP/ADP antiporters (so-called ntt1-2 null mutants) display a variety of physiological changes. These include the formation of necrotic lesions and the accumulation of hydrogen peroxide in the leaves. Here, we show that, under short-day conditions, leaves of the ntt1-2 mutant display enhanced resistance to Hyaloperonospora arabidopsidis, Botrytis cinerea, and Pseudomonas syringae pv. tomato DC3000. Resistance to these pathogens was associated with constitutively elevated levels of the plant hormone salicylic acid and, eventually, jasmonic acid, and constitutive or primed activation after pathogen attack of various defense genes that are dependent on these hormones. In addition, the antagonistic crosstalk between the salicylic acid and jasmonic acid signaling pathways seems to be affected in ntt1-2. Because the enhanced resistance of ntt1-2 to H. arabidopsidis was not seen when the mutant was grown under long-day conditions, our findings argue that nocturnal ATP import into chloroplasts is crucial to keep A. thaliana from runaway activation of pathogen resistance.     

Jasmonate-induced gene expression of barley (Hordeum vulgare) leaves -- the link between jasmonate and abscisic acid

In barley leaves a group of genes is expressed in response to treatment with jasmonates and abscisic acid (ABA) [21]. One of these genes coding for a jasmonate-induced protein of 23 kDa (JIP-23) was analyzed to find out the link between ABA and jasmonates by recording its expression upon modulating independently, the endogenous level of both of them. By use of inhibitors of JA synthesis and ABA degradation, and the ABA-deficient mutant Az34, as well as of cultivar-specific differences, it was shown that endogenous jasmonate increases are necessary and sufficient for expression of this gene. The endogenous rise of ABA did not induce synthesis of JIP-23, whereas exogenous ABA did not act via jasmonates. Different signalling pathways are suggested and discussed.     

The glycosyltransferase UGT76B1 modulates N-hydroxy-pipecolic acid homeostasis and plant immunity

The tradeoff between growth and defense is a critical aspect of plant immunity. Therefore, the plant immune response needs to be tightly regulated. Salicylic acid (SA) is an important plant hormone regulating defense against biotrophic pathogens. Recently, N-hydroxy-pipecolic acid (NHP) was identified as another regulator for plant innate immunity and systemic acquired resistance (SAR). Although the biosynthetic pathway leading to NHP formation is already been identified, how NHP is further metabolized is unclear. Here, we present UGT76B1 as a uridine diphosphate-dependent glycosyltransferase (UGT) that modifies NHP by catalyzing the formation of 1-O-glucosyl-pipecolic acid in Arabidopsis thaliana. Analysis of T-DNA and clustered regularly interspaced short palindromic repeats (CRISPR) knock-out mutant lines of UGT76B1 by targeted and nontargeted ultra-high performance liquid chromatography coupled to high-resolution mass spectrometry (UHPLC-HRMS) underlined NHP and SA as endogenous substrates of this enzyme in response to Pseudomonas infection and UV treatment. ugt76b1 mutant plants have a dwarf phenotype and constitutive defense response which can be suppressed by loss of function of the NHP biosynthetic enzyme FLAVIN-DEPENDENT MONOOXYGENASE 1 (FMO1). This suggests that elevated accumulation of NHP contributes to the enhanced disease resistance in ugt76b1. Externally applied NHP can move to distal tissue in ugt76b1 mutant plants. Although glycosylation is not required for the long-distance movement of NHP during SAR, it is crucial to balance growth and defense.     

A lipoxygenase is the main lipid body protein in cucumber and soybean cotyledons during the stage of triglyceride mobilization.

The 90-kDa lipid body protein characterized earlier by its high expression during the stage of fat degradation was identified as a form of lipoxygenase. This organelle form was compared with lipoxygenase species purified from the cytosol. It is further shown that the antibodies raised against the lipid body membrane lipoxygenase from cucumber cotyledons cross-react with both cytosolic and lipid body lipoxygenase from soybean.