Glycerol-3-phosphate levels are associated with basal resistance to the hemibiotrophic fungus Colletotrichum higginsianum in Arabidopsis

SOS - too many signals for systemic acquired resistance?

Following pathogen infection, activation of systemic acquired resistance (SAR) in uninfected tissues requires transmission of a signal(s) from the infected tissue via the vasculature. Several candidates for this long-distance signal have been identified, including methyl salicylate (MeSA), an SFD1/GLY1-derived glycerol-3-phosphate (G3P)-dependent signal, the lipid-transfer protein DIR1, the dicarboxylic acid azelaic acid (AzA), the abietane diterpenoid dehydroabietinal (DA), jasmonic acid (JA), and the amino acid-derivative pipecolic acid (Pip). Some of these signals work cooperatively to activate SAR and/or regulate MeSA metabolism. However, Pip appears to activate SAR via an independent pathway that may impinge on these other signaling pathway(s) during de novo salicylic acid (SA) biosynthesis in the systemic tissue. Thus, a complex web of cross-interacting signals appears to activate SAR.

Study in vitro of the impact of endophytic bacteria isolated from Centella asiatica on the disease incidence caused by the hemibiotrophic fungus Colletotrichum higginsianum

Thirty-one endophytic bacteria isolated from healthy leaves of Centella asiatica were screened in vitro for their ability to reduce the growth rate and disease incidence of Colletotrichum higginsianum, a causal agent of anthracnose. Isolates of Cohnella sp., Paenibacillus sp. and Pantoea sp. significantly stimulated the growth rate of C. higginsianum MUCL 44942, while isolates of Achromobacter sp., Acinetobacter sp., Microbacterium sp., Klebsiella sp. and Pseudomonas putida had no influence on this plant pathogen. By contrast, Bacillus subtilis BCA31 and Pseudomonas fluorescens BCA08 caused a marked inhibition of C. higginsianum MUCL 44942 growth by 46 and 82 %, respectively. Cell-free culture filtrates of B. subtilis BCA31 and P. fluorescens BCA08 were found to contain antifungal compounds against C. higginsianum MUCL 44942. Inoculation assays on in vitro-cultured plants of C. asiatica showed that foliar application of B. subtilis BCA31, three days before inoculation with C. higginsianum MUCL 44942, significantly reduced incidence and severity of the disease. The role of endophytic bacteria in maintaining the apparent inactivity of C. higginsianum MUCL 44942 in C. asiatica grown in the wild is discussed.

Biochemical and Molecular-Genetic Characterization of SFD1's Involvement in Lipid Metabolism and Defense Signaling

The Arabidopsis thaliana SFD1 (suppressor of fatty acid desaturase deficiency1) gene (also known as GLY1) is required for accumulation of 34:6 (i.e., 18:3-16:3) monogalactosyldiacylglycerol (MGDG) and for the activation of systemic acquired resistance (SAR), an inducible defense mechanism that confers resistance against a broad spectrum of pathogens. SFD1, which has been suggested to be involved in lipid-based signaling in SAR, contains a putative chloroplast transit peptide and has glycerol-3-phosphate synthesizing dihydroxyacetone phosphate (DHAP) reductase (also referred as glycerol-3-phosphate dehydrogenase) activity. The goals of this study were to determine if the DHAP reductase activity and chloroplast localization are required for SFD1's involvement in galactolipid metabolism and SAR signaling. The crystal structure of a Leishmania mexicana glycerol-3-phosphate dehydrogenase was used to model SFD1 structure and identify Lys194, Lys279, and Asp332 as potential catalytic site residues in SFD1. Mutational analysis of SFD1 confirmed that Lys194, Lys279, and Asp332 are critical for SFD1's DHAP reductase activity, and its involvement in SAR. SFD1 proteins with these residues individually substituted by Ala lacked DHAP reductase activity and were unable to complement the SAR defect of the sfd1 mutant. The SFD1-Ala279 protein was also unable to restore 34:6-MGDG content when expressed in the sfd1 mutant. In vivo imaging of a green fluorescent protein-tagged SFD1 protein demonstrated that SFD1 is targeted to the chloroplast. The N-terminal 43 amino acids, which are required for proper targeting of SFD1 to the chloroplast, are also required for SFD1's function in lipid metabolism and SAR. Taken together, these results demonstrate that SFD1's DHAP reductase activity is required in the chloroplast for lipid metabolism and defense signaling.

Glycerol-3-phosphate and systemic immunity

Glycerol-3-phosphate (G3P), a conserved three-carbon sugar, is an obligatory component of energy-producing reactions including glycolysis and glycerolipid biosynthesis. G3P can be derived via the glycerol kinase-mediated phosphorylation of glycerol or G3P dehydrogenase (G3Pdh)-mediated reduction of dihydroxyacetone phosphate. Previously, we showed G3P levels contribute to basal resistance against the hemibiotrophic pathogen, Colletotrichum higginsianum. Inoculation of Arabidopsis with C. higginsianum correlated with an increase in G3P levels and a concomitant decrease in glycerol levels in the host. Plants impaired in GLY1 encoded G3Pdh accumulated reduced levels of G3P after pathogen inoculation and showed enhanced susceptibility to C. higginsianum. Recently, we showed that G3P is also a potent inducer of systemic acquired resistance (SAR) in plants. SAR is initiated after a localized infection and confers whole-plant immunity to secondary infections. SAR involves generation of a signal at the site of primary infection, which travels throughout the plants and alerts the un-infected distal portions of the plant against secondary infections. Plants unable to synthesize G3P are defective in SAR and exogenous G3P complements this defect. Exogenous G3P also induces SAR in the absence of a primary pathogen. Radioactive tracer experiments show that a G3P derivative is translocated to distal tissues and this requires the lipid transfer protein, DIR1. Conversely, G3P is required for the translocation of DIR1 to distal tissues. Together, these observations suggest that the cooperative interaction of DIR1 and G3P mediates the induction of SAR in plants.

Common Motifs in the Response of Cereal Primary Metabolism to Fungal Pathogens are not Based on Similar Transcriptional Reprogramming

During compatible interactions with their host plants, biotrophic plant-pathogens subvert host metabolism to ensure the sustained provision of nutrient assimilates by the colonized host cells. To investigate, whether common motifs can be revealed in the response of primary carbon and nitrogen metabolism toward colonization with biotrophic fungi in cereal leaves, we have conducted a combined metabolome and transcriptome study of three quite divergent pathosystems, the barley powdery mildew fungus (Blumeria graminis f.sp. hordei), the corn smut fungus Ustilago maydis, and the maize anthracnose fungus Colletotrichum graminicola, the latter being a hemibiotroph that only exhibits an initial biotrophic phase during its establishment. Based on the analysis of 42 water-soluble metabolites, we were able to separate early biotrophic from late biotrophic interactions by hierarchical cluster analysis and principal component analysis, irrespective of the plant host. Interestingly, the corresponding transcriptome dataset could not discriminate between these stages of biotrophy, irrespective, of whether transcript data for genes of central metabolism or the entire transcriptome dataset was used. Strong differences in the transcriptional regulation of photosynthesis, glycolysis, the TCA cycle, lipid biosynthesis, and cell wall metabolism were observed between the pathosystems. However, increased contents of Gln, Asn, and glucose as well as diminished contents of PEP and 3-PGA were common to early post-penetration stages of all interactions. On the transcriptional level, genes of the TCA cycle, nucleotide energy metabolism and amino acid biosynthesis exhibited consistent trends among the compared biotrophic interactions, identifying the requirement for metabolic energy and the rearrangement of amino acid pools as common transcriptional motifs during early biotrophy. Both metabolome and transcript data were employed to generate models of leaf primary metabolism during early biotrophy for the three investigated interactions.

First off the mark: early seed germination

Most plant seeds are dispersed in a dry, mature state. If these seeds are non-dormant and the environmental conditions are favourable, they will pass through the complex process of germination. In this review, recent progress made with state-of-the-art techniques including genome-wide gene expression analyses that provided deeper insight into the early phase of seed germination, which includes imbibition and the subsequent plateau phase of water uptake in which metabolism is reactivated, is summarized. The physiological state of a seed is determined, at least in part, by the stored mRNAs that are translated upon imbibition. Very early upon imbibition massive transcriptome changes occur, which are regulated by ambient temperature, light conditions, and plant hormones. The hormones abscisic acid and gibberellins play a major role in regulating early seed germination. The early germination phase of Arabidopsis thaliana culminates in testa rupture, which is followed by the late germination phase and endosperm rupture. An integrated view on the early phase of seed germination is provided and it is shown that it is characterized by dynamic biomechanical changes together with very early alterations in transcript, protein, and hormone levels that set the stage for the later events. Early seed germination thereby contributes to seed and seedling performance important for plant establishment in the natural and agricultural ecosystem.

Glycerol-3-phosphate is a critical mobile inducer of systemic immunity in plants

Glycerol-3-phosphate (G3P) is an important metabolite that contributes to the growth and disease-related physiologies of prokaryotes, plants, animals and humans alike. Here we show that G3P serves as the inducer of an important form of broad-spectrum immunity in plants, termed systemic acquired resistance (SAR). SAR is induced upon primary infection and protects distal tissues from secondary infections. Genetic mutants defective in G3P biosynthesis cannot induce SAR but can be rescued when G3P is supplied exogenously. Radioactive tracer experiments show that a G3P derivative is translocated to distal tissues, and this requires the lipid transfer protein, DIR1. Conversely, G3P is required for the translocation of DIR1 to distal tissues, which occurs through the symplast. These observations, along with the fact that dir1 plants accumulate reduced levels of G3P in their petiole exudates, suggest that the cooperative interaction of DIR1 and G3P orchestrates the induction of SAR in plants.

Metabolic and transcriptional responses of glycerolipid pathways to a perturbation of glycerol 3-phosphate metabolism in Arabidopsis

Glycerolipid synthesis in plants involves two major metabolic pathways compartmentalized in the chloroplasts and cytosol, respectively. Although these two parallel pathways are regulated with considerable flexibility, the factors mediating this process remain unclear. To investigate the influence of glycerol 3-phosphate (Gly-3-P) on the interactions of the glycerolipid pathways, we generated transgenic Arabidopsis lines with a feedback-resistant Gly-3-P dehydrogenase gene (gpsA(FR)) from Escherichia coli. gpsA(FR) was detected in the cytosol, but augmented Gly-3-P levels were observed in the cytosol as well as in chloroplasts. Glycerolipid composition and fatty acid positional distribution analyses revealed an altered fatty acid flux that affected not only the molar ratios of glycerolipid species but also their fatty acid composition. To decipher this complex pathway, a transgenic line was subjected to lipidomic analysis and a global gene-expression survey. The results revealed that changes in Gly-3-P metabolism caused altered expression of a broad array of genes. When viewed from the perspective of glycerolipid metabolism, coherent networks emerged, revealing that many enzymatic components of the glycerolipid pathways operate in a modular manner under the influence of Gly-3-P. Transcript levels of the enzymes involved in the prokaryotic pathway were mostly induced, whereas genes of the eukaryotic pathway enzymes were largely suppressed. Hence, the gene-expression changes were consistent with the detected biochemical phenotype. Our results suggest that Gly-3-P modulates the balance of the two glycerolipid pathways in Arabidopsis by influencing both metabolic flux and gene transcription.

The common metabolite glycerol-3-phosphate is a novel regulator of plant defense signaling

Conversion of glycerol to glycerol-3-phosphate (G3P) is one of the highly conserved steps of glycerol metabolism in evolutionary diverse organisms. In plants, G3P is produced either via the glycerol kinase (GK)-mediated phosphorylation of glycerol, or via G3P dehydrogenase (G3Pdh)-mediated reduction of dihydroxyacetone phosphate (DHAP). We have recently shown that G3P levels contribute to basal resistance against the hemibiotrophic pathogen, Colletotrichum higginsianum. Since a mutation in the GLY1-encoded G3Pdh conferred more susceptibility compared to a mutation in the GLI1-encoded GK, we proposed that GLY1 is the major contributor of the total G3P pool that participates in defense against C. higginsianum.

Fatty Acid-derived signals in plant defense

Fatty acids (FAs) consist of long hydrophobic, often unbranched chains of hydrocarbons, with hydrophilic carboxylic acid groups at one end. They are an important source of reserve energy and essential components of membrane lipids in all living organisms. In plants, FA metabolic pathways play significant roles in pathogen defense. Historically, FAs were only assigned passive roles in plant defense such as biosynthetic precursors for cuticular components or the phytohormone jasmonic acid. However, recent discoveries demonstrate more direct roles for FAs and their breakdown products in inducing various modes of plant defenses. Both 16- and 18-carbon FAs participate in defense to modulate basal, effector-triggered, and systemic immunity in plants. Studies of FA metabolic mutants also reveal an active signaling role for the cuticle in plant defense. This review summarizes the current knowledge of the involvement of FAs, FA-derived oxylipins, and enzymes catalyzing FA metabolism in plant defense.