Wood (i.e. secondary xylem produced by cambial activity) of poplars represents an important raw material of great economic value (Balatinecz, Kretschmann & Leclercq 2001). Poplar biomass also provides a promising bioenergy feedstock that could help to reduce our dependency on fossil fuels (Sannigrahi, Ragauskas & Tuskan 2010). From a biological perspective, wood serves three main functions that are fundamental for plant growth. These functions are (1) long-distance transport of water and nutrients from roots to transpiring leaves, (2) providing mechanical support to the plant body and (3) storage of carbohydrates, water and various other specialized compounds. In a typical hardwood such as poplar, these three functions are divided between three different cell types – vessel elements, fibres and living parenchyma.
Vessel elements and fibres represent 85–90% (vol/vol) of mature wood in poplar (Mellerowicz et al. 2001). Hydraulic and mechanical properties of wood are closely associated with the physical structure of these cells. For instance, xylem hydraulic conductivity is proportional to the vessel diameter to the fourth power as predicted by the Hagen–Poiseuille equation (Tyree & Zimmermann 2002), and mechanical parameters such as modulus of rupture have been linked with wood density and fibre lumen diameters (Woodrum, Ewers & Telewski 2003; Pratt et al. 2007; Onoda, Richards & Westoby 2010). Moreover, hydraulic and mechanical functions appear to be closely integrated as strong mechanical support may be required to prevent implosion of xylem conduits under high xylem tension (Hacke et al. 2001; Jacobsen et al. 2005; Pittermann et al. 2006; Coleman et al. 2008). Furthermore, wood density and fibre length together with the chemical composition of wood are critical factors that determine its material properties and hence its suitability for a specific end use in the wood-processing industry.
Wood structure is established during xylogenesis as a result of cambial activity. Wood is formed through a series of precisely regulated developmental steps that include cell division, cell expansion, secondary cell wall deposition and programmed cell death (Samuels, Kaneda & Rensing 2006). Many genes influencing xylem differentiation in poplar have been recently identified, including several key regulatory and structural genes (e.g. Aspeborg et al. 2005; Groover et al. 2010; Zhong et al. 2011). The process of xylogenesis and the resulting xylem phenotype are strongly affected by environmental conditions such as water (Arend & Fromm 2007), nutrient (Lautner et al. 2007; Hacke et al. 2010) and light availability (Plavcová, Hacke & Sperry 2011). The developmental programme giving rise to a specific physiological and anatomical xylem phenotype is underpinned by changes in gene expression as demonstrated by recent studies describing transcriptional changes in developing xylem of poplars subjected to drought (Berta et al. 2010) and high salinity (Janz et al. 2012). However, more research is needed to better understand the molecular mechanisms underlying xylem phenotypic plasticity as it is likely that expression of different genes is altered by different environmental triggers.
In this study, we used nitrogen (N) fertilization to perturb the xylem phenotype of hybrid poplar (Populus trichocarpa × deltoides, clone H11-11) saplings and to investigate corresponding changes in gene expression. Nitrogen fertilization has a profound effect on poplar growth and development including xylogenesis (Harvey & van den Driessche 1999; Pitre, Cooke & Mackay 2007a; Pitre et al. 2007b; Hacke et al. 2010). The influence of nitrogen supply on the expression of selected genes has been evaluated in poplar leaves, roots, phloem and bulk xylem (Cooke et al. 2003; Cooke, Martin & Davis 2005; Ehlting et al. 2007; Hacke et al. 2010). To our knowledge, there is only one genome-wide study focused on the expression of nitrogen availability-related genes in the cambial region of poplar (Pitre et al. 2010), and this study was specifically designed to compare the effects of nitrogen fertilization and stem leaning on wood formation. In contrast, our study was designed to explore changes in gene expression that may underlie traits related to xylem water transport. Our goal was to identify candidate genes that may be linked with increased radial growth, wide vessel diameters and decreased wood density that we expected to be differentially expressed in poplars growing under high N availability.