Immunolocalization of hemicelluloses in Arabidopsis thaliana stem. Part I: temporal and spatial distribution of xylans

Biochemical Characterization of Rice Xylan Biosynthetic Enzymes in Determining Xylan Chain Elongation and Substitutions

Grass xylan consists of a linear chain of β-1,4-linked xylosyl residues that often form domains substituted only with either arabinofuranose (Araf) or glucuronic acid (GlcA)/methylglucuronic acid (MeGlcA) residues, and it lacks the unique reducing end tetrasaccharide sequence found in dicot xylan. The mechanism of how grass xylan backbone elongation is initiated and how its distinctive substitution pattern is determined remains elusive. Here, we performed biochemical characterization of rice xylan biosynthetic enzymes, including xylan synthases, glucuronyltransferases and methyltransferases. Activity assays of rice xylan synthases demonstrated that they required short xylooligomers as acceptors for their activities. While rice xylan glucuronyltransferases effectively glucuronidated unsubstituted xylohexaose acceptors, they transferred little GlcA residues onto (Araf)-substituted xylohexaoses and rice xylan 3-O-arabinosyltransferase could not arabinosylate GlcA-substituted xylohexaoses, indicating that their intrinsic biochemical properties may contribute to the distinctive substitution patterns of rice xylan. In addition, we found that rice xylan methyltransferase exhibited a low substrate binding affinity, which may explain the partial GlcA methylation in rice xylan. Furthermore, immunolocalization of xylan in xylem cells of both rice and Arabidopsis showed that it was deposited together with cellulose in secondary walls without forming xylan-rich nanodomains. Together, our findings provide new insights into the biochemical mechanisms underlying xylan backbone elongation and substitutions in grass species.

Cynipid wasps systematically reprogram host metabolism and restructure cell walls in developing galls

Many insects have evolved the ability to manipulate plant growth to generate extraordinary structures called galls, in which insect larva can develop while being sheltered and feeding on the plant. In particular, cynipid (Hymenoptera: Cynipidae) wasps have evolved to form morphologically complex galls and generate an astonishing array of gall shapes, colors, and sizes. However, the biochemical basis underlying these remarkable cellular and developmental transformations remains poorly understood. A key determinant in plant cellular development is cell wall deposition that dictates the physical form and physiological function of newly developing cells, tissues, and organs. However, it is unclear to what degree cell walls are restructured to initiate and support the formation of new gall tissue. Here, we characterize the molecular alterations underlying gall development using a combination of metabolomic, histological, and biochemical techniques to elucidate how valley oak (Quercus lobata) leaf cells are reprogrammed to form galls. Strikingly, gall development involves an exceptionally coordinated spatial deposition of lignin and xylan to form de novo gall vasculature. Our results highlight how cynipid wasps can radically change the metabolite profile and restructure the cell wall to enable the formation of galls, providing insights into the mechanism of gall induction and the extent to which plants can be entirely reprogrammed to form unique structures and organs.

Maize WI5 encodes an endo-1,4-β-xylanase required for secondary cell wall synthesis and water transport in xylem

Water transport from roots to leaves through xylem is important for plant growth and development. Defects in water transport can cause drought stress, even when there is adequate water in the soil. Here, we identified the maize (Zea mays) wilty5 (wi5) mutant, which exhibits marked dwarfing and leaf wilting throughout most of its life cycle under normal growth conditions. wilty5 seedlings exhibited lower xylem conductivity and wilted more rapidly under drought, NaCl, and high temperature treatments than wild-type plants. Map-based cloning revealed that WI5 encodes an active endo-1,4-β-xylanase from glycosyl dehydration family 10, which mainly functions in degrading and reorganizing cell wall xylan. Reverse-transcription polymerase chain reaction and β-glucuronidase assays revealed that WI5 is highly expressed in stems, especially in internodes undergoing secondary wall assembly. RNA sequencing suggested that WI5 plays a unique role in internode growth. Immunohistochemistry and electron microscopy confirmed that wi5 is defective in xylan deposition and secondary cell wall thickening. Lignin deposition and xylan content were markedly reduced in wi5 compared to the wild-type plants. Our results suggest that WI5 functions in xylem cell wall thickening through its xylanase activity and thereby regulates xylem water transport, the drought stress response, and plant growth in maize.

The Living Fossil Psilotum nudum Has Cortical Fibers With Mannan-Based Cell Wall Matrix

Cell wall thickening and development of secondary cell walls was a major step in plant terrestrialization that provided the mechanical support, effective functioning of water-conducting elements and fortification of the surface tissues. Despite its importance, the diversity, emergence and evolution of secondary cell walls in early land plants have been characterized quite poorly. Secondary cell walls can be present in different cell types with fibers being among the major ones. The necessity for mechanical support upon increasing plant height is widely recognized; however, identification of fibers in land plants of early taxa is quite limited. In an effort to partially fill this gap, we studied the fibers and the composition of cell walls in stems of the sporophyte of the living fossil Psilotum nudum. Various types of light microscopy, combined with partial tissue maceration demonstrated that this perennial, rootless, fern-like vascular plant, has abundant fibers located in the middle cortex. Extensive immunodetection of cell wall polymers together with various staining and monosaccharide analysis of cell wall constituents revealed that in P. nudum, the secondary cell wall of its cortical fibers is distinct from that of its tracheids. Primary cell walls of all tissues in P. nudum shoots are based on mannan, which is also common in other extant early land plants. Besides, the primary cell wall contains epitope for LM15 specific for xyloglucan and JIM7 that binds methylesterified homogalacturonans, two polymers common in the primary cell walls of higher plants. Xylan and lignin were detected as the major polymers in the secondary cell walls of P. nudum tracheids. However, the secondary cell wall in its cortical fibers is quite similar to their primary cell walls, i.e., enriched in mannan. The innermost secondary cell wall layer of its fibers but not its tracheids has epitope to bind the LM15, LM6, and LM5 antibodies recognizing, respectively, xyloglucan, arabinan and galactan. Together, our data provide the first description of a mannan-based cell wall in sclerenchyma fibers, and demonstrate in detail that the composition and structure of secondary cell wall in early land plants are not uniform in different tissues.

Synchrotron FTIR and Raman spectroscopy provide unique spectral fingerprints for Arabidopsis floral stem vascular tissues

Cell walls are highly complex structures that are modified during plant growth and development. For example, the development of phloem and xylem vascular cells, which participate in the transport of sugars and water as well as providing support, can be influenced by cell-specific wall composition. Here, we used synchrotron radiation-based Fourier-transform infrared (SR-FTIR) and Raman spectroscopy to analyse the cell wall composition of floral stem vascular tissues of wild-type Arabidopsis and the double-mutant sweet11-1 sweet12-1, which has impaired sugar transport. The SR-FTIR spectra showed that in addition to modified xylem cell wall composition, phloem cell walls in the double-mutant line were characterized by modified hemicellulose composition. Combining Raman spectroscopy with a classification and regression tree (CART) method identified combinations of Raman shifts that could distinguish xylem vessels and fibers. In addition, the disruption of the SWEET11 and SWEET12 genes impacted on xylem wall composition in a cell-specific manner, with changes in hemicelluloses and cellulose observed at the xylem vessel interface. These results suggest that the facilitated transport of sugars by transporters that exist between vascular parenchyma cells and conducting cells is important in ensuring correct phloem and xylem cell wall composition.

Xylan epitope profiling: an enhanced approach to study organ development-dependent changes in xylan structure, biosynthesis, and deposition in plant cell walls

BACKGROUND

Xylan is a major hemicellulosic component in the cell walls of higher plants especially in the secondary walls of vascular cells which are playing important roles in physiological processes and overall mechanical strength. Being the second most abundant cell wall polymer after cellulose, xylan is an abundant non-cellulosic carbohydrate constituent of plant biomass. Xylan structures have been demonstrated to contribute to plant biomass recalcitrance during bioenergy applications. A critical understanding of xylan composition, structure, and biosynthesis in developing plant stems will allow an increased understanding of how cell walls are put together in this organ in a basic research, and, in applied research, will improve strategies in xylan engineering to reduce biomass recalcitrance for economically feasible biofuel production.

METHODS

We describe an approach to enable the monitoring of xylan epitope structures in cell walls during the stem maturation process in Arabidopsis. The technique integrates glycome profiling, an in vitro immunoanalytical platform, and in situ immunolocalisation to provide comprehensive details on the presence, relative abundances, and dynamics with which diverse xylan epitope structures are integrated to the cell walls throughout the stem maturation process.

RESULTS

Our experimental results and the supporting in silico analysis demonstrated that xylan deposition in stems occurs early on in stem development; however, xylan epitope types (representing substituted and unsubstituted regions on xylan backbone made of β-(1,4)-linked xylose residues) and the strength of their integration into the final wall structure vary during stem maturation.

CONCLUSIONS

Our novel approach thus provides a method to comprehensively survey the differences in xylan epitope patterning and deposition occurring in stem development and thereby providing a robust tool for characterising altered xylan integration patterns in cell walls during the stem maturation process in diverse plant cell wall biosynthetic mutants. Our findings also suggest that this approach could rapidly and reliably delineate xylan deposition patterns in the cell walls of plants belonging to diverse phylogenetic classes providing novel insights into the functional roles of xylans in overall growth and development.

Tissue specific response of Miscanthus×giganteus to dilute acid pretreatment for enhancing cellulose digestibility

The recalcitrance in grasses varies according to cell type and tissue. In this study, dilute acid pretreatment was performed on Miscanthus×giganteus internodes that include rind and pith regions which showing heterogeneous structural and chemical changes. Pretreatment on pith effectively hydrolyzed 73.33% hemicelluloses and separated cohesive cell walls from the compound middle lamella due to lignin migration. Lignin droplets with an average diameter of 49.5±29.3nm were concurrently coalesced on wall surface, that in turn exposed more microfibrils deep in walls to be enzymatically hydrolyzed reaching 82.55%. By contrast, the rind with a relatively intergrated cell structure was covered by larger lignin droplets (101.2±44.1nm) and filled with inaccessible microfibrils limiting enzymatic sacchrification (31.50%). Taken together, the cellulose digestibility of biomass was not majorly influenced by cellulose crystallinity, while it was strongly correlated with the positive effects of hemicelluloses degradation, lignin redistribution, cellulose exposure and loosening cell wall structure.

GUX1 and GUX2 glucuronyltransferases decorate distinct domains of glucuronoxylan with different substitution patterns

Xylan comprises up to one-third of plant cell walls, and it influences the properties and processing of biomass. Glucuronoxylan in Arabidopsis is characterized by a linear β-(1,4)-linked backbone of xylosyl residues substituted by glucuronic acid and 4-O-methylglucuronic acid (collectively termed [Me]GlcA). The role of these substitutions remains unclear. GUX1 (glucuronic acid substitution of xylan 1) and GUX2, recently identified as glucuronyltransferases, are both required for substitution of the xylan backbone with [Me]GlcA. Here, we demonstrate clear differences in the pattern of [Me]GlcA substitution generated by each of these glucuronyltransferases. GUX1 decorates xylan with a preference for addition of [Me]GlcA at evenly spaced xylosyl residues. Intervals of eight or 10 residues dominate, but larger intervals are observed. GUX2, in contrast, produces more tightly clustered decorations with most frequent spacing of five, six or seven xylosyl residues, with no preference for odd or even spacing. Moreover, each of these GUX transferases substitutes a distinct domain of secondary cell wall xylan, which we call the major and minor domains. These major and minor xylan domains were not separable from each other by size or charge, a finding that suggests that they are tightly associated. The presence of both differently [Me]GlcA decorated domains may produce a xylan molecule that is heterogeneous in its properties. We speculate that the major and minor domains of xylan may be specialised, such as for interaction with cellulose or lignin. These findings have substantial implications for our understanding of xylan synthesis and structure, and for models of the molecular architecture of the lignocellulosic matrix of plant cell walls.