The new view of the primary cell wall

Duplication and neofunctionalization of a horizontally transferred xyloglucanase as a facet of the Red Queen coevolutionary dynamic

Oomycete protists share phenotypic similarities with fungi, including the ability to cause plant diseases, but branch in a distant region of the tree of life. It has been suggested that multiple horizontal gene transfers (HGTs) from fungi-to-oomycetes contributed to the evolution of plant-pathogenic traits. These HGTs are predicted to include secreted proteins that degrade plant cell walls, a barrier to pathogen invasion and a rich source of carbohydrates. Using a combination of phylogenomics and functional assays, we investigate the diversification of a horizontally transferred xyloglucanase gene family in the model oomycete species Phytophthora sojae. Our analyses detect 11 xyloglucanase paralogs retained in P. sojae. Using heterologous expression in yeast, we show consistent evidence that eight of these paralogs have xyloglucanase function, including variants with distinct protein characteristics, such as a long-disordered C-terminal extension that can increase xyloglucanase activity. The functional variants analyzed subtend a phylogenetic node close to the fungi-to-oomycete transfer, suggesting the horizontally transferred gene was a bona fide xyloglucanase. Expression of three xyloglucanase paralogs in Nicotiana benthamiana triggers high-reactive oxygen species (ROS) generation, while others inhibit ROS responses to bacterial immunogens, demonstrating that the paralogs differentially stimulate pattern-triggered immunity. Mass spectrometry of detectable enzymatic products demonstrates that some paralogs catalyze the production of variant breakdown profiles, suggesting that secretion of variant xyloglucanases increases efficiency of xyloglucan breakdown as well as diversifying the damage-associated molecular patterns released. We suggest that this pattern of neofunctionalization and the variant host responses represent an aspect of the Red Queen host-pathogen coevolutionary dynamic.

Duplication and neofunctionalization of a horizontally-transferred xyloglucanase as a facet of the red queen co-evolutionary dynamic

Oomycetes are heterotrophic protists that share phenotypic similarities with fungi, including the ability to cause plant diseases, but branch in a separate and distant region of the eukaryotic tree of life. It has been suggested that multiple horizontal gene transfers (HGTs) from fungi-to-oomycetes contributed to the evolution of plant-pathogenic traits. These HGTs are predicted to include secreted proteins that degrade plant cell walls. This is a key trait in the pathology of many oomycetes, as the plant cell wall represents a primary barrier to pathogen invasion and a rich source of carbohydrates. Many of the HGT gene families identified have undergone multiple rounds of duplication. Using a combination of phylogenomic analysis and functional assays, we investigate the diversification of a horizontally-transferred xyloglucanase gene family in the model oomycete species Phytophthora sojae. Our analyses detect 11 genes retained in P. sojae among a complex pattern of gene duplications and losses. Using a phenotype assay, based on heterologous expression in yeast, we show that eight of these paralogs have xyloglucanase function, including variants with distinct protein characteristics, such as a long-disordered C-terminal extension that can increase xyloglucanase activity. The functional xyloglucanase variants analysed subtend an ancestral node close to the fungi-oomycetes gene transfer, suggesting the horizontally-transferred gene was a bona fide xyloglucanase. Expression of xyloglucanase paralogs in Nicotiana benthamiana triggers distinct patterns of reactive oxygen species (ROS) generation, demonstrating that enzyme variants differentially stimulate pattern-triggered immunity in plants. Mass spectrometry of detectable enzymatic products demonstrates that some paralogs catalyze production of variant breakdown profiles, suggesting that secretion of multiple xyloglucanase variants increases efficiency of xyloglucan breakdown, as well as potentially diversifying the range of Damage-Associated Molecular Patterns (DAMPs) released during pathogen attack. We suggest that such patterns of protein neofunctionalization, and variant host responses, represent an aspect of the Red Queen host-pathogen co-evolutionary dynamic.

Significance Statement

The oomycetes are a diverse group of eukaryotic microbes that include some of the most devastating pathogens of plants. Oomycetes perceive, invade, and colonize plants in similar ways to fungi, in part because they acquired the genes to attack and feed on plants from fungi. These genes are predicted to be useful to oomycete plant pathogens because they have undergone multiple rounds of gene duplication. One key enzyme for attacking plant cell wall structures is called xyloglucanase. Xyloglucanase in the oomycetes has undergone multiple rounds of gene duplication, leading to variants including an enzyme with a C-terminal extension that increases activity. Some xyloglucanase variants trigger unique patterns of reactive oxygen species (ROS) in planta, and generate different profiles of cell wall breakdown products - such outcomes could act to mystify and increase the workload of the plant immune system, allowing successful pathogens to proliferate.

Pectin may hinder the unfolding of xyloglucan chains during cell deformation: implications of the mechanical performance of Arabidopsis hypocotyls with pectin alterations

Plant cell walls, like a multitude of other biological materials, are natural fiber-reinforced composite materials. Their mechanical properties are highly dependent on the interplay of the stiff fibrous phase and the soft matrix phase and on the matrix deformation itself. Using specific Arabidopsis thaliana mutants, we studied the mechanical role of the matrix assembly in primary cell walls of hypocotyls with altered xyloglucan and pectin composition. Standard microtensile tests and cyclic loading protocols were performed on mur1 hypocotyls with affected RGII borate diester cross-links and a hindered xyloglucan fucosylation as well as qua2 exhibiting 50% less homogalacturonan in comparison to wild-type. As a control, wild-type plants (Col-0) and mur2 exhibiting a specific xyloglucan fucosylation and no differences in the pectin network were utilized. In the standard tensile tests, the ultimate stress levels (approximately tensile strength) of the hypocotyls of the mutants with pectin alterations (mur1, qua2) were rather unaffected, whereas their tensile stiffness was noticeably reduced in comparison to Col-0. The cyclic loading tests indicated a stiffening of all hypocotyls after the first cycle and a plastic deformation during the first straining, the degree of which, however, was much higher for mur1 and qua2 hypocotyls. Based on the mechanical data and current cell wall models, it is assumed that folded xyloglucan chains between cellulose fibrils may tend to unfold during straining of the hypocotyls. This response is probably hindered by geometrical constraints due to pectin rigidity.

Changes in cell wall composition associated with maturation in the gymnosperm Araucaria angustifolia

A general structural characterization and an investigation on the dynamics of formation of cell wall polysaccharides was performed, using plantlets stem samples from a typical gymnosperm from southern Brazil, Araucaria angustifolia, as experimental model. Microscopic examination and monosaccharide composition of plantlet segments at different heights were carried out to show the representative portions of stem cell wall development. The plantlets were divided in portions (tip, middle and base) which were submitted to sequential extractions. The extraction with water gave rise to large amounts of pectic material in the three portions and more highly substituted pectins occurred in the tip portion of the stems. Increase in alkali concentration extracted, respectively, higher amounts of xyloglucan structurally similar to those from dicotyledons. However, oligosaccharides containing galactose and fucose where found in higher amounts in base than tip portion. The changes in cell wall composition suggest that the development in gymnosperm cell walls follow the same key events as found in dicotyledon walls (type I).

Regular Papers / Articles OrdinairesIntine wall modifications during germination ofZygophyllum fabago(Zygophyllaceae) pollen grains

The composition of the inner layer (intine) of mature, activated, and germinated Zygophyllum fabago L. (Zygophyllaceae) pollen grains was studied. Cytochemical techniques showed neutral and acidic polysaccharides to be the major component of the thin and unlayered intine. The intine lacks lipids, with only scattered lipid globules being observed near the plasma membrane. Immunocytochemical localization of esterified and unesterified pectins in the intine was performed to determine the behaviour (permeability and elasticity) of germinal apertures. The high density of unesterified pectins in the intine of Z. fabago may be related to harmomegathic changes, which increase the elasticity of the intine during hydration and germination processes. A new layer was deposited in germinated pollen grains, recognized by 1,3-β-glucan (callose) antibodies; this layer plays a role in keeping the grains swollen during the germination process and probably forms a selective barrier to control the movement of substances through the pollen walls. Indeed, the composition of the Z. fabago intine was related to both the hydration process preceding germination and the passage of allergenic proteins through it.Key words: callose, germination, intine, pectins, pollen grains, Zygophyllum fabago.

Xyloglucan from the leaves of Hymenaea courbaril

A fucosylated xyloglucan was isolated from the leaves of Hymenaea courbaril by alkaline extraction, followed by ethanol precipitation and ion-exchange chromatography. The isolated polysaccharide showed Glc:Xyl:Gal:Fuc in molar ratio of 8:5:2.5:1 and (D)(25) +40.5 degrees. Composition and linkage analyses, supported by NMR spectroscopic measurements, showed that the polysaccharide has a glucan backbone which is highly substituted at O-6 with D-xylopyranose residues, about a half of which are substituted at O-2 by D-galactopyranosyl units. Some of the galactose residues are further substituted by L-fucopyranose at O-2. The M(r), as determined by HPSEC, was 49,500.