Abstract
Plant cell suspension culture systems are valuable for the study of complex biological systems such as inducible defence responses and aspects of plant innate immunity. Perturbations to the cellular metabolome can be investigated using metabolomic approaches in order to reveal the underlying metabolic mechanism of cellular responses. Lipopolysaccharides from the sorghum pathogen, Burkholderia andropogonis (LPSB.a.), were purified, chemically characterised and structurally elucidated. The lipid A moiety consists of tetra- and penta-acylated 1,4’-bis-phosphorylated disaccharide backbone decorated by aminoarabinose residues, while the O-polysaccharide chain consists of linear trisaccharide repeating units of [→2)-α-Rha3CMe-(1 → 3)-α-Rha-(1 → 3)-α-Rha-(1 → ]. The effect of LPSB.a. in triggering metabolic reprogramming in Sorghum bicolor cells were investigated using untargeted metabolomics with liquid chromatography coupled to mass spectrometry detection. Cells were treated with LPSB.a. and the metabolic changes monitored over a 30 h time period. Alterations in the levels of phytohormones (jasmonates, zeatins, traumatic-, azelaic- and abscisic acid), which marked the onset of defence responses and accumulation of defence-related metabolites, were observed. Phenylpropanoids and indole alkaloids as well as oxylipins that included di- and trihydroxyoctadecedienoic acids were identified as signatory biomarkers, with marked secretion into the extracellular milieu. The study demonstrated that sorghum cells recognise LPSB.a. as a ‘microbe-associated molecular pattern’, perturbing normal cellular homeostasis. The molecular features of the altered metabolome were associated with phytohormone-responsive metabolomic reconfiguration of primary and secondary metabolites originating from various metabolic pathways, in support of defence and immunity.
Highlights
Plant cell suspension culture systems are valuable for the study of complex biological systems such as inducible defence responses and aspects of plant innate immunity
LPSB.a. consists of a hydrophilic hetero-polysaccharide OPS covalently linked through a core oligosaccharide (COS) to the lipid A (LA) of Mr approximately 1.5 kDa
The composition, size and structure of the OPS might be dependable indicators of virulence potential. It appears to partake in the molecular communication between host plants and bacteria
Summary
Plant cell suspension culture systems are valuable for the study of complex biological systems such as inducible defence responses and aspects of plant innate immunity. Plants employ several layers of defence to counter potential pathogens[3,4] These passive (preformed) and active (induced) responses are triggered upon detection of conserved immunogenic motifs, epitopes or ‘patterns’ such www.nature.com/scientificreports as bacterial lipopolysaccharides (LPS), flagellin and fungal ergosterol, β-glucans and chitin[5]. These ‘microbe/ pathogen-associated molecular patterns, M/PAMPs’ are perceived via surface-located ‘pattern-recognition receptors, PRRs’ to initiate signal transduction events linked to MAMP-triggered immunity or MTI6 that can be acquired in a systemic manner, contributing to broad-spectrum and durable resistance[1]. Environmental and growth conditions may alter its structure, affecting biological activity of the whole LPS molecule[9,15]
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