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Wood formation is a complex developmental process driven by the annual activity of the vascular cambium. Conifers usually produce juvenile wood at young ages followed by mature wood for the rest of their lifetime. Juvenile wood exhibits poorer wood quality (i.e., lower density) compared with mature wood and can account for up to 50% of short-rotation harvested logs, thus representing a major challenge for commercial forestry globally. Wood density is an important quality trait for many timber-related products. Understanding the molecular mechanisms involved in the regulation of juvenile wood density is critical for the improvement of juvenile wood quality via marker-aided selection. A previous study has identified several candidate genes affecting mature wood density in Picea sitchensis (Bong.) Carr.; however, genes associated with juvenile wood density in conifers remain poorly characterized. Here, cDNA microarrays containing 3320 xylem unigenes were used to investigate genes differentially transcribed in juvenile wood with high (HD) and low density (LD) in Pinus radiata D.Don. In total, 814 xylem unigenes with differential transcription were identified in at least one of two microarray experiments and 73 genes (45 for HD, 28 for LD) were identified in both experiments, thus representing putative candidate genes for juvenile wood density. Interestingly, cellulose synthases (PrCesA3, PrCesA11) and sucrose synthase (SuSy), which are involved in secondary cell wall formation, had stronger transcription in juvenile wood with HD, while genes functioning in primary wall formation (pectin synthesis, cell expansion and other modifications) were more transcribed in LD wood. Cell wall genes encoding monolignol biosynthesis enzymes, arabinogalactan proteins, actins and tubulins were differentially transcribed in either HD or LD juvenile wood; however, the latter had exclusively greater transcription of genes involved in monolignol polymerization (laccase and peroxidase). The identified candidate genes also included many non-cell-wall genes (transcription factors, environmental-responsive genes, hormone signalling, etc.) and genes with unknown functions, suggesting complex gene pathways in the regulation of juvenile wood density. Interestingly, 19 out of 73 candidate genes for wood density were among the 108 candidate genes previously identified for microfibril angle, and 16 genes appeared to influence both traits in a synergistic manner for wood stiffness.

The ancient cell adhesion fasciclin (FAS) domain is found in bacteria, fungi, algae, insects and animals, and occurs in a large family of fasciclin-like arabinogalactan proteins (FLAs) in higher plants. Functional roles for FAS-containing proteins have been determined for insects, algae and vertebrates; however, the biological functions of the various higher-plant FLAs are not clear. Expression of some FLAs has been correlated with the onset of secondary-wall cellulose synthesis in Arabidopsis stems, and also with wood formation in the stems and branches of trees, suggesting a biological role in plant stems. We examined whether FLAs contribute to plant stem biomechanics. Using phylogenetic, transcript abundance and promoter-GUS fusion analyses, we identified a conserved subset of single FAS domain FLAs (group A FLAs) in Eucalyptus and Arabidopsis that have specific and high transcript abundance in stems, particularly in stem cells undergoing secondary-wall deposition, and that the phylogenetic conservation appears to extend to other dicots and monocots. Gene-function analyses revealed that Arabidopsis T-DNA knockout double mutant stems had altered stem biomechanics with reduced tensile strength and a reduced tensile modulus of elasticity, as well as altered cell-wall architecture and composition, with increased cellulose microfibril angle and reduced arabinose, galactose and cellulose content. Using materials engineering concepts, we relate the effects of these FLAs on cell-wall composition with stem biomechanics. Our results suggest that a subset of single FAS domain FLAs contributes to plant stem strength by affecting cellulose deposition, and to the stem modulus of elasticity by affecting the integrity of the cell-wall matrix.

In response to gravitational stresses, angiosperm trees form tension wood in the upper sides of branches and leaning stems in which cellulose content is higher, microfibrils are typically aligned closely with the fibre axis and the fibres often have a thick inner gelatinous cell wall layer (G-layer). Gene expression was studied in Eucalyptus nitens branches oriented at 45 degrees using microarrays containing 4900 xylem cDNAs, and wood fibre characteristics revealed by X-ray diffraction, chemical and histochemical methods. Xylem fibres in tension wood (upper branch) had a low microfibril angle, contained few fibres with G-layers and had higher cellulose and decreased Klason lignin compared with lower branch wood. Expression of two closely related fasciclin-like arabinogalactan proteins and a beta-tubulin was inversely correlated with microfibril angle in upper and lower xylem from branches. Structural and chemical modifications throughout the secondary cell walls of fibres sufficient to resist tension forces in branches can occur in the absence of G-layer enriched fibres and some important genes involved in responses to gravitational stress in eucalypt xylem are identified.

• In response to gravitational stresses, angiosperm trees form tension wood in the upper sides of branches and leaning stems in which cellulose content is higher, microfibrils are typically aligned closely with the fibre axis and the fibres often have a thick inner gelatinous cell wall layer (G-layer). • Gene expression was studied in Eucalyptus nitens branches oriented at 45° using microarrays containing 4900 xylem cDNAs, and wood fibre characteristics revealed by X-ray diffraction, chemical and histochemical methods. • Xylem fibres in tension wood (upper branch) had a low microfibril angle, contained few fibres with G-layers and had higher cellulose and decreased Klason lignin compared with lower branch wood. Expression of two closely related fasciclin-like arabinogalactan proteins and a β-tubulin was inversely correlated with microfibril angle in upper and lower xylem from branches. • Structural and chemical modifications throughout the secondary cell walls of fibres sufficient to resist tension forces in branches can occur in the absence of G-layer enriched fibres and some important genes involved in responses to gravitational stress in eucalypt xylem are identified. [ABSTRACT FROM AUTHOR]