Systemic acquired resistance (SAR) is a mechanism of induced defense that confers long-lasting protection against a broad spectrum of microorganisms. SAR requires the signal molecule salicylic acid (SA) and is associated with accumulation of pathogenesis-related proteins, which are thought to contribute to resistance. Much progress has been made recently in elucidating the mechanism of SAR. Using the model plant Arabidopsis, it was discovered that the isochorismate pathway is the major source of SA during SAR. In response to SA, the positive regulator protein NPR1 moves to the nucleus where it interacts with TGA transcription factors to induce defense gene expression, thus activating SAR. Exciting new data suggest that the mobile signal for SAR might be a lipid molecule. We discuss the molecular and genetic data that have contributed to our understanding of SAR and present a model describing the sequence of events leading from initial infection to the induction of defense genes.

Systemic acquired resistance

Phosphatidic acid (PtdOH) is emerging as an important signalling lipid in abiotic stress responses in plants. The effect of cold stress was monitored using 32P-labelled seedlings and leaf discs of Arabidopsis thaliana. Low, non-freezing temperatures were found to trigger a very rapid 32P-PtdOH increase, peaking within 2 and 5 min, respectively. In principle, PtdOH can be generated through three different pathways, i.e. i) via de novo phospholipid biosynthesis (through acylation of lyso-PtdOH), ii) via phospholipase D hydrolysis of structural phospholipids or iii) via phosphorylation of diacylglycerol (DAG) by DAG kinase (DGK). Using a differential 32P-labelling protocol and a PLD-transphosphatidylation assay, evidence is provided that the rapid 32P-PtdOH response was primarily generated through DGK. A simultaneous decrease in the levels of 32P-PtdInsP, correlating in time, temperature dependency and magnitude with the increase in 32P-PtdOH, suggested that a PtdInsP-hydrolyzing PLC generated the DAG in this reaction. Testing T-DNA insertion lines available for the seven DGK genes, revealed no clear changes in 32P-PtdOH responses, suggesting functional redundancy. Similarly, known cold-stress mutants were analyzed to investigate whether the PtdOH response acted downstream of the respective gene products. The hos1, los1 and fry1 mutants were found to exhibit normal PtdOH responses. Slight changes were found for ice1, snow1, and the overexpression line Super-ICE1, however, this was not cold-specific and likely due to pleiotropic effects. A tentative model illustrating direct cold effects on phospholipid metabolism is presented.

/pdf/rapid-phosphatidic-acid-accumulation-in-response-to-low-3xktru1vuv.pdf

Rapid phosphatidic acid accumulation in response to low temperature stress in Arabidopsis is generated through diacylglycerol kinase

https://www.cell.com/trends/plant-science/pdf/S1360-1385(07)00215-4.pdf

Cold stress regulation of gene expression in plants.

Thermogenic mechanisms in brown fat.

Lignin is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine-tuning of multiple "upstream" (i.e., lignin bioengineering, lignin isolation and "early-stage catalytic conversion of lignin") and "downstream" (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a "beginning-to-end" analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignin's biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance.

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis.

Lipid-transfer proteins (LTP) are basic, 9-kDa proteins present in high amounts (as much as 4% of the total soluble proteinss) in higher plants. LTPs can enhance the in vitro transfer of phospholipids between membranes and can bind acyl chains. On the basis of these properties, LTPs were thought to participate in membrane biogenesis and regulation of the intracellular fatty acid pools. However, the isolation of several cDNAs and genes revealed the presence of a signal peptide indicating that LTPs could enter the secretory pathway. They were found to be secreted and located in the cell wall. Thus, novel roles were suggested for plant LTPs: participation in cutin formation, embryogenesis, defense reactions against phytopathogens, symbiosis, and the adaptation of plants to various environmental conditions. The validity of these suggestions needs to be determined, in the hope that they will elucidate the role of this puzzling family of plant proteins.

Lipid-transfer proteins in plants

Mitochondrial protein alteration in active brown fat: A sodium dodecyl sulfate-polyacrylamide gel electrophoretic study

http://directory.umm.ac.id/Data%20Elmu/jurnal/P/PlantScience/PlantScience_Elsevier/Vol157.Issue1.2000/5358.pdf

Lipid transfer proteins are encoded by a small multigene family in Arabidopsis thaliana.

A phospholipid transfer protein has been purified 195-fold from an extract of spinach leaves. This protein is capable of transferring phosphatidylcholine, phosphatidylinositol, phosphatidylglycerol and phosphatidyl-ethanolamine from liposomes to mitochondria. In addition to this protein, a minor part of the total activity was associated with a less purified fraction.

The pure protein has an isoelectric point of 9.0 ± 0.2 determined by a chromatofocusing technique. Electrophoresis on sodium dodecyl sulfate/polyacrylamide gel showed that the protein is homogeneous and has an apparent molecular weight of 9000 ± 1000, in agreement with the value (8832) calculated from the amino acid composition. This composition is characterized by a high amount of alanine and glycine and by the absence of phenylalanine, whereas arginine, glutamine, histidine and methionine are minor components.

The spinach protein is also able to transfer phosphatidylcholine and phosphatidylglycerol from liposomes to intact chloroplasts. This observation reinforces the hypothesis that plastid phospholipids are partly imported from outside the organelle by a transfer process.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1111/j.1432-1033.1984.tb08020.x

Purification and characterization of a spinach‐leaf protein capable of transferring phospholipids from liposomes to mitochondria or chloroplasts

In plants, a temperature downshift represents a major stress that will lead to the induction or repression of many genes. Therefore, the cold signal has to be perceived and transmitted to the nucleus. In response to a cold exposure, we have shown that the phospholipase D (PLD) and the phospholipase C (PLC)/diacylglycerol kinase pathways are simultaneously activated. The role of these pathways in the cold response has been investigated by analyzing the transcriptome of cold-treated Arabidopsis (Arabidopsis thaliana) suspension cells in the presence of U73122 or ethanol, inhibitors of the PLC/diacylglycerol kinase pathway and of the phosphatidic acid produced by PLD, respectively. This approach showed that the expression of many genes was modified by the cold response in the presence of such agents. The cold responses of most of the genes were repressed, thus correlating with the inhibitory effect of U73122 or ethanol. We were thus able to identify 58 genes that were regulated by temperature downshift via PLC activity and 87 genes regulated by temperature downshift via PLD-produced phosphatidic acid. Interestingly, each inhibitor appeared to affect different cold response genes. These results support the idea that both the PLC and PLD pathways are upstream of two different signaling pathways that lead to the activation of the cold response. The connection of these pathways with the CBF pathway, currently the most understood genetic system playing a role in cold acclimation, is discussed.

/pdf/the-cold-induced-early-activation-of-phospholipase-c-and-d-35ylowffso.pdf

The cold-induced early activation of phospholipase C and D pathways determines the response of two distinct clusters of genes in Arabidopsis cell suspensions.

Jean-Claude Kader

Papers

Lipid-transfer proteins in plants

Mitochondrial protein alteration in active brown fat: A sodium dodecyl sulfate-polyacrylamide gel electrophoretic study

Lipid transfer proteins are encoded by a small multigene family in Arabidopsis thaliana.

Purification and characterization of a spinach‐leaf protein capable of transferring phospholipids from liposomes to mitochondria or chloroplasts

The cold-induced early activation of phospholipase C and D pathways determines the response of two distinct clusters of genes in Arabidopsis cell suspensions.