Mutation in the Arabidopsis regulatory-associated protein TOR 1B (RAPTOR1B) leads to decreased jasmonates levels in leaf tissue

The Target of Rapamycin (TOR) complex (TORC) regulates plant growth and development by modulation of metabolism in response to environmental cues. TORC contains in its core the TOR kinase and two interacting partners, namely; regulatory-associated partner of TOR (RAPTOR) and lethal with sec thirteen protein 8 (LST8). RAPTOR is described to act as a scaffold protein which recruits substrates for phosphorylation to the TOR kinase. In the current manuscript we show that mutation of Arabidopsis RAPTOR1B leads to significantly decreased levels of free jasmonic acid (JA), jasmonoyl-(L)-isoleucine (JA-Ile) as well as its biosynthetic precursor 12-oxo-phytodienoic acid (OPDA). Although raptor1b leaves showed decreased basic JA level compared to WT, the mutant responded substantially to wounding stress by producing the same amount of JA as WT. Furthermore, we could show that the chemical inhibition of TOR by AZD-8055 led to an opposite response. AZD-treated WT and raptor1b leaves accumulated high JA levels. These results strongly imply that the TOR signaling pathway is responding differentially to the inhibition of the TOR kinase as compared to the inhibition of the scaffold protein RAPTOR.

The activity of HYDROPEROXIDE LYASE 1 regulates accumulation of galactolipids containing 12-oxo-phytodienoic acid in Arabidopsis

Arabidopsis produces galactolipids containing esters of 12-oxo-phytodienoic acid (OPDA) and dinor-12-oxo-phytodienoic acid (dnOPDA). These lipids are referred to as arabidopsides and accumulate in response to abiotic and biotic stress. We explored the natural genetic variation found in 14 different Arabidopsis accessions to identify genes involved in the formation of arabidopsides. The accession C24 was identified as a poor accumulator of arabidopsides whereas the commonly used accession Col-0 was found to accumulate comparably large amounts of arabidopsides in response to tissue damage. A quantitative trait loci analysis of an F2 population created from a cross between C24 and Col-0 located a region on chromosome four strongly linked to the capacity to form arabidopsides. Expression analysis of HYDROPEROXIDE LYASE 1 (HPL1) showed large differences in transcript abundance between accessions. Transformation of Col-0 plants with the C24 HPL1 allele under transcriptional regulation of the 35S promoter revealed a strong negative correlation between HPL1 expression and arabidopside accumulation after tissue damage, thereby strengthening the view that HPL1 competes with ALLENE OXIDE SYNTHASE (AOS) for lipid-bound hydroperoxide fatty acids. We further show that the last step in the synthesis of galactolipid-bound OPDA and dnOPDA from unstable allene oxides is exclusively enzyme-catalyzed and not the result of spontaneous cyclization. Thus, the results presented here together with previous studies suggest that all steps in arabidopside biosynthesis are enzyme-dependent and apparently all reactions can take place with substrates being esterified to galactolipids.

The Alphabet of Galactolipids in Arabidopsis thaliana

Galactolipids constitute the major lipid class in plants. In recent years oxygenated derivatives of galactolipids have been detected. They are discussed as signal molecules during leaf damage, since they accumulate in wounded leaves in high levels. Using different analytical methods such as nuclear magnetic resonance, infra-red spectroscopy, and high performance liquid chromatography/mass spectrometry (HPLC/MS) earlier reports focused on the analysis of either oxidized or non-oxidized species and needed high levels of analytes. Here, we report on the analysis of the galactolipid subfraction of the Arabidopsis leaf lipidome by an improved HPLC/MS(2)-based method that is fast, robust, and comparatively simple in its performance. Due to a combination of phase partitioning, solid phase fractionation, liquid chromatography, and MS(2) experiments this method has high detection sensitivity and requires only low amounts of plant material. With this method 167 galactolipid species were detected in leaves of Arabidopsis thaliana. Out of these 79 being newly described species. From all species the head group and acyl side chains were identified via MS(2) experiments. Moreover, the structural identification was supported by HPLC/time-of-flight (TOF)-MS and gas chromatography (GC)/MS analysis. The quantification of different galactolipid species that accumulated 30 min after a mechanical wounding in A. thaliana leaves showed that the oxidized acyl side chains in galactolipids are divided into 65% cyclopentenones, 27% methyl-branched ketols, 3.8% hydroperoxides/straight-chain ketols, 2.0% hydroxides, and 2.6% phytoprostanes. In comparison to the free cyclopentenone derivatives, the esterified forms occur in a 149-fold excess supporting the hypothesis that galactolipids might function as storage compounds for cyclopentenones. Additional analysis of the ratio of non-oxidized to oxidized galactolipid species in leaves of wounded plants was performed resulting in a ratio of 2.0 in case of monogalactosyl diacylglycerol (MGD), 8.1 in digalactosyl diacylglycerol (DGD), and 0.6 in the acylated MGD. This indicates that galactolipid oxidation is a major and rapid metabolic process that occurs class specific.