Wood is formed by the successive addition of secondary xylem, which consists of cells with a conspicuously thickened secondary wall composed mainly of lignin and cellulose. Several genes involved in lignin and cellulose biosynthesis have been characterized, but the factors that regulate the formation of secondary walls in woody tissues remain to be identified. In this study, we show that plant-specific transcription factors, designated NAC SECONDARY WALL THICKENING PROMOTING FACTOR1 (NST1) and NST3, are key regulators of the formation of secondary walls in woody tissues of Arabidopsis thaliana. In nst1-1 nst3-1 double knockout plants, the secondary wall thickenings in interfascicular fibers and secondary xylem, except for vascular vessels, were completely suppressed without affecting formation of cells destined to be woody tissues. Conversely, as shown previously for NST1, overexpression of NST3 induced ectopic secondary wall thickenings in various aboveground tissues. Furthermore, the expression of chimeric repressors derived from NST1 and NST3 suppressed secondary wall thickenings in the presumptive interfascicular fibers. Because putative orthologs of NST1 and NST3 are present in the genome of poplar, our results suggest that they are also key regulators of the formation of secondary walls in woody plants and could be used as a tool for the genetic engineering of wood and its derivatives.

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The crystal transformation of Valonia cellulose induced in a dilute alkaline solution at high temperatures has been examined in detail by high-resolution solid-state 13C NMR spectroscopy. The Cl and C4 resonance lines, which are characteristic triplets for the celluloses from primitive organisms, markedly •undergo changes in relative intensities of the constituent lines with increasing annealing temperature, and those lines are finally converted to doublets with almost equivalent intensities. Such spectral changes have been successfully analyzed in terms of the composite crystal model in which native cellulose crystals are assumed to be composites of two allomorphs, celluloses IQ and Ig, resulting in confirmation of the validity of the composite crystal model. Moreover, the fraction of cellulose I" is found to be reduced from 0.64 for intact Valonia cellulose to about 0.12 by annealing as a result of the crystal transformation from cellulose L to Ig.

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Summary— In order to clarify the role of microfibrils in the generation of growth stresses in trees, an experimental analysis was carried out on 7 Appalachian hardwood species which were with or without gelatinous fiber in the upper region of the leaning stem. In the species that had gelatinous fibers, large longitudinal tensile stresses appeared in the region where the cross-sectional area of gelatinous layers were large. In the species that had no gelatinous fibers the following relationships were observed: (a) the smaller the microfibril angle, the larger the longitudinal tensile stress; (b) the larger the tensile stress, the larger the α-cellulose content; (c)

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The mechanism for tree orientation in angiosperms is based on the production of high tensile stress on the upper side of the inclined axis. In many species, the stress level is strongly related to the presence of a peculiar layer, called the G-layer, in the fibre cell wall. The structure of the G-layer has recently been described as a hydrogel thanks to N(2) adsorption-desorption isotherms of supercritically dried samples showing a high mesoporosity (pores size from 2-50 nm). This led us to revisit the concept of the G-layer that had been, until now, only described from anatomical observation. Adsorption isotherms of both normal wood and tension wood have been measured on six tropical species. Measurements show that mesoporosity is high in tension wood with a typical thick G-layer while it is much less with a thinner G-layer, sometimes no more than normal wood. The mesoporosity of tension wood species without a G-layer is as low as in normal wood. Not depending on the amount of pores, the pore size distribution is always centred around 6-12 nm. These results suggest that, among species producing fibres with a G-layer, large structural differences of the G-layer exist between species.

ing solar energy conversion.3-5 Carefully controlled experimental conditions are required, however, for the preparation and stabilization of ultrasmall colloidal semiconductor particles. Inspired by the ability of commercially available Nafion membranes to incorporate colloidal semiconductors and catalysts,6-9 we have developed functionalized, ultrathin, polymer-blend membranes (PBMs) as matrices for size-quantized semiconductor particles. Our work shows that miscible polymer blends provide an excel-