New conditions for matrix-assisted laser desorption/ionization mass spectrometry of native bacterial R-type lipopolysaccharides

A matrix-assisted laser desorption/ionization mass spectrometry approach to the lipid A from Mesorhizobium loti

The isolation, purification and analysis of the lipid A obtained from Mesorhizobium loti Ayac 1 BII strain is presented. Analysis of the carbohydrate moiety after acid hydrolysis by high-pH anion-exchange chromatography with pulse amperometric detection (HPAEC-PAD) showed the presence of glucosamine and galacturonic acid as the only sugar components. Gas chromatographic (GC) and GC/mass spectrometric (MS) analysis of the fatty acids revealed the presence of 3-OH-C12:0; 3-OH-C13:0; 3-OH-C20:0 and 27-OH-C28:0 among the major hydroxylated species. In addition, C16:0, C17:0, C18:0 and C 20:0 were shown as main saturated fatty acids. Different polyacylated species were evidenced by thin layer chromatography of lipid A, allowing the purification of two fractions. Ultraviolet matrix-assisted laser desorption/ionization time-of-flight (UV-MALDI-TOF) MS analysis with different matrices, in the positive- and negative-ion mode, was performed. The fast moving component revealed the presence of hexa-acylated species, varying in the fatty acid composition. Species containing three 3-OH fatty acids and a 27-OH-C28:0 fatty acid were observed. Individual ions within this family differ by +/-14 mass units. The slow moving component was enriched mainly in penta-acylated species. Among them, three subgroups were detected: the major one compatible with the sugar core bearing two 3-OH 20:0 fatty acids, a 3-OH 13:0 or a 3-OH 12:0 fatty acid, a 27-OH 28:0 fatty acid and one saturated fatty acid. Each signal differs in a C18:0 acyl unit from the corresponding hexa-acylated family. On the other hand, a subgroup bearing one 3-OH 20:0 fatty acid, one 27-OH 28:0 fatty acid and two non-polar fatty acids was shown. A minor subgroup compatible with structures containing two hydroxylated and three non-polar fatty acids was also detected. The results obtained showed that nor-harmane was an excellent matrix for charged lipid A structural studies in both, positive and negative ion modes.

The elicitation of plant innate immunity by lipooligosaccharide of Xanthomonas campestris

Lipopolysaccharides (LPSs) and lipooligosaccharides (LOSs) are major components of the cell surface of Gram-negative bacteria with diverse roles in bacterial pathogenesis of animals and plants that include elicitation of host defenses. Little is known about the mechanisms of perception of these molecules by plants and about the associated signal transduction pathways that trigger plant immunity. Here we address the issue of the molecular basis of elicitation of plant defenses through the structural determination of the LOS of the plant pathogen Xanthomonas campestris pv. campestris strain 8004 and examination of the effects of LOS and fragments obtained by chemical treatments on the immune response in Arabidopsis thaliana. The structure shows a strong accumulation of negatively charged groups in the lipid A-inner core region and has a number of novel features, including a galacturonyl phosphate attached at a 3-deoxy-D-manno-oct-2-ulosonic acid residue and a unique phosphoramide group in the inner core region. Intact LOS and the lipid A and core oligosaccharides derived from it were all able to induce the defense-related genes PR1 and PR2 in Arabidopsis and to prevent the hypersensitive response caused by avirulent bacteria. Although LOS induced defense-related gene transcription in two temporal phases, the core oligosaccharide induced only the earlier phase, and lipid A induced only the later phase. These findings suggest that plant cells can recognize lipid A and core oligosaccharide structures within LOS to trigger defensive cellular responses and that this may occur via two distinct recognition events.