A novel rice fragile culm 24 mutant encodes a UDP-glucose epimerase that affects cell wall properties and photosynthesis

UDP-glucose epimerases (UGEs) are essential enzymes for catalysing the conversion of UDP-glucose (UDP-Glc) into UDP-galactose (UDP-Gal). Although UDP-Gal has been well studied as the substrate for the biosynthesis of carbohydrates, glycolipids, and glycoproteins, much remains unknown about the biological function of UGEs in plants. In this study, we selected a novel rice fragile culm 24 (Osfc24) mutant and identified it as a nonsense mutation of the FC24/OsUGE2 gene. The Osfc24 mutant shows a brittleness phenotype with significantly altered cell wall composition and disrupted orientation of the cellulose microfibrils. We found significantly reduced accumulation of arabinogalactan proteins in the cell walls of the mutant, which may consequently affect plant growth and cell wall deposition, and be responsible for the altered cellulose microfibril orientation. The mutant exhibits dwarfism and paler leaves with significantly decreased contents of galactolipids and chlorophyll, resulting in defects in plant photosynthesis. Based on our results, we propose a model for how OsUGE2 participates in two distinct metabolic pathways to co-modulate cellulose biosynthesis and cell wall assembly by dynamically providing UDP-Gal and UDP-Glc substrates.

Characterizing compression wood formed in radiata pine branches

The formation of reaction wood is an adaptive feature of trees in response to various mechanical forces. In gymnosperms, reaction wood consists of compression wood (CW) and opposite wood (OW) that are formed on the underside and upperside of bent trunks and branches. Although reaction wood formed in bent trunks has been extensively investigated, relatively little has been reported from conifer branches. In this study SilviScan® technology was used to characterize radiata pine branches at high resolution. Compared to OW formed in the branches, CW showed greater growth, darker colour, thicker tracheid walls, higher coarseness, larger microfibril angle (MFA), higher wood density, lower extensional stiffness and smaller internal specific surface area. However, tracheids of CW were similar to those of OW in their radial and tangential diameters. These results indicated that gravity influenced tracheid cell division and secondary wall formation but had limited impact on primary wall expansion. Furthermore, seasonal patterns of CW formation were not observed in the branches from cambial age 4 while earlywood and latewood were clearly separated in all rings of OW. The marked change of MFA during reaction wood formation suggested that branches could be ideal materials for further study of cellulose microfibril orientation.

Ultrastructural development of the softwood cell wall during pyrolysis

The pyrolytic conversion of pine wood at mild temperatures between 200°C and 300°C was investigated by transmission electron microscopy (TEM). Based on TEM imaging and image analysis, a novel method was developed for determining the local orientation of the cellulose microfibrils in the secondary wall S2 which gives a measure for the progression of pyrolytic conversion of the cell wall. Elemental composition of pyrolysed specimens was determined up to 600°C. TEM imaging together with the evaluation of the elemental composition shows that first the polyoses are degraded, while the cellulose microfibril orientation is still visible up to 225°C. The cellulose microfibrils could not be observed at temperatures higher than 250°C, while lignin containing compound middle lamella (CML) was still visible. After a gradual decrease of the CML up to 275°C, the cell wall became entirely isotropic beginning at 300°C. Based on the presented results, we propose an early degradation of the supramolecular structure of the cell wall.

A comprehensive analysis of the relation of cellulose microfibril orientation and lignin content in the S2 layer of different tissue types of spruce wood (Picea abies (L.) Karst.)

A possible relation between cellulose microfibril angle and lignin content in the S2 layer was investigated by X-ray diffraction and cellular UV microspectrophotometry on spruce tissues [Picea abies] with different structural features and chemical composition. A strong correlation was not found, neither for the individual tissue types nor for the compiled data of all tissues. As the data did not confirm the findings in former studies, further examinations are necessary concerning a possible general interrelation between microfibril angle and lignin content.