ABSTRACT
Xylogenesis involves successive developmental processes--cambial division, cell expansion and differentiation, cell death--each occurring along a gradient from the cambium to the pith of the stem. Taking advantage of the high level of organisation of wood tissues, we isolated cambial zone (CZ), differentiating xylem (DX) and mature xylem (MX) from both tension wood (TW) and opposite wood (OW) of bent poplars. Four different cDNA libraries were then constructed and used to generate 10,062 EST, reflecting the genes expressed in the different wood tissues. For the most abundant clusters, the EST distributions were compared between libraries in order to identify genes specific or over-represented at some specific developmental stages. They clearly showed a developmental shift between CZ and DX, whereas there is a continuity of development between DX and MX. CZ was mainly characterized by clusters of genes involved in cell cycle, protein synthesis and fate. Interestingly, two clusters with no assigned function were found specific to the cambial zone. In DX and MX, clusters were mostly involved in methylation of lignin precursors and microtubule cytoskeleton. In addition, in DX, EST from TW and OW were compared: five clusters of arabinogalactan proteins, one for sucrose synthase and one for fructokinase were specific or over-represented in TW. Moreover, a putative transcription factor and a cluster of unknown function were also identified in DX-TW. The informative comparison of multiple libraries prepared from wood tissues led to the identification of genes--some with still unknown functions--putatively involved in xylogenesis and tension wood formation.
Subject(s)
Populus/genetics , Cell Cycle/genetics , Cluster Analysis , DNA, Complementary/genetics , DNA, Complementary/isolation & purification , DNA, Plant/genetics , DNA, Plant/isolation & purification , Expressed Sequence Tags , Gene Library , Genes, Plant , Genome, Plant , Lignin/metabolism , Methylation , Mucoproteins/genetics , Plant Proteins/biosynthesis , Plant Proteins/genetics , Populus/growth & development , Populus/metabolism , RNA, Plant/genetics , RNA, Plant/isolation & purification , WoodABSTRACT
We evaluated lignin profiles and pulping performances of 2-year-old transgenic poplar (Populus tremula x Populus alba) lines severely altered in the expression of caffeic acid/5-hydroxyferulic acid O-methyltransferase (COMT) or cinnamyl alcohol dehydrogenase (CAD). Transgenic poplars with CAD or COMT antisense constructs showed growth similar to control trees. CAD down-regulated poplars displayed a red coloration mainly in the outer xylem. A 90% lower COMT activity did not change lignin content but dramatically increased the frequency of guaiacyl units and resistant biphenyl linkages in lignin. This alteration severely lowered the efficiency of kraft pulping. The Klason lignin level of CAD-transformed poplars was slightly lower than that of the control. Whereas CAD down-regulation did not change the frequency of labile ether bonds or guaiacyl units in lignin, it increased the proportion of syringaldehyde and diarylpropane structures and, more importantly with regard to kraft pulping, of free phenolic groups in lignin. In the most depressed line, ASCAD21, a substantially higher content in free phenolic units facilitated lignin solubilization and fragmentation during kraft pulping. These results point the way to genetic modification of lignin structure to improve wood quality for the pulp industry.
