Finally, a radial system of rays comprised primarily of parenchyma cells traverses both the secondary phloem and xylem and serves to transport and store substances, including photosynthate and water ( Mellerowicz et al., 2001 Chaffey et al., 2002). In addition, secondary phloem contains phloem fiber cells with thick secondary cell walls organized in bundles, which form discontinuous concentric layers around the stem diameter ( Evert, 2006). Secondary phloem consists of sieve elements, companion cells, and phloem parenchyma cells with thin primary cell walls, which facilitate their transport functions. Secondary xylem is made up of fibers and vessel elements that both form thick secondary (lignified) cell walls. Typically, secondary xylem is produced inwards and secondary phloem outwards of the vascular cambium, although deviations from this pattern do occur (reviewed in Spicer and Groover, 2010). Secondary growth originates in the vascular cambium, a lateral meristem that consists of meristem cells (cambial initials) organized in a single-celled radial file that forms a continuous cylinder around the stem ( Mellerowicz et al., 2001). In poplar, for example, primary growth occurs in the top four internodes, and at the ninth internode secondary growth is already completely established ( Dharmawardhana et al., 2010). The stem's primary and secondary growth zones are spatiotemporally separated, easily discernable, and develop rapidly in fast-growing species like poplar (within 1 to 2 months). In the stem of poplar ( Populus spp) and many other tree species, the primary growth then gives way to a secondary (woody) growth that produces secondary xylem and secondary phloem. The primary growth occurs at the top of the stem a few internodes below the SAM. The vasculature develops from the procambium of the primary stem. In the stem, primary growth originates in the shoot apical meristem (SAM) and is responsible for production of leaves and stems. Many plants, especially trees, show two distinct types of growth patterns, known as primary and secondary growth (reviewed in Baucher et al., 2007). In the plant, wood serves many functions in water/nutrient transport, mechanical support, and storage of organic compounds, water, and gases ( Brunner et al., 2004). From ecological and evolutionary perspectives, wood plays an important role in the global carbon biogeochemical cycle as it dominates terrestrial ecosystem biomass ( Kirilenko and Sedjo, 2007 Bonan, 2008). Wood provides a renewable resource for the production of pulp, paper, and construction timber ( Skog and Nicholson, 1998) and is growing in importance as a lignocellulosic feedstock for biofuel production ( Ragauskas et al., 2006). A set of four LBD genes, including the LBD1 gene, was predominantly expressed in wood-forming tissues, suggesting a broader regulatory role of these transcription factors during secondary woody growth in poplar. Two key Class I KNOTTED1-like homeobox genes that promote meristem identity in the cambium were downregulated, while an Altered Phloem Development gene that is known to promote phloem differentiation was upregulated in the mutant. In wild-type plants, LBD1 was most highly expressed in the phloem and cambial zone. Dominant-negative suppression of Pta LBD1 via translational fusion with the repressor SRDX domain caused decreased diameter growth and suppressed and highly irregular phloem development. Homology analysis showed highest similarity to an uncharacterized LBD1 gene from Arabidopsis thaliana, and we consequently named it Populus tremula × Populus alba (Pta) LBD1. Molecular characterization of the mutation through positioning of the tag and retransformation experiments shows that the phenotype is conditioned by activation of an uncharacterized gene that encodes a novel member of the LATERAL ORGAN BOUNDARIES DOMAIN (LBD) family of transcription factors. We identified and subsequently characterized an activation-tagged poplar ( Populus tremula × Populus alba) mutant with enhanced woody growth and changes in bark texture caused primarily by increased secondary phloem production. Regulation of secondary (woody) growth is of substantial economic and environmental interest but is poorly understood.
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