Epsilon Open Archive

Abstract

New molecular tools were used to study wood formation in hybrid aspen (Populus tremula L. x tremuloides Michx.), with a focus on tension wood (TW) formation. TW is a gravitational response, and forms on the upper side of the stem to maintain an optimal position. TW formation is associated with an increased growth rate, modified fiber cell walls with low lignin, and high cellulose content. The plant hormone ethylene is induced during TW formation, but mechanisms regulating its level in wood forming tissues, and its exact function in wood development, are not well known. Several ethylene related transcripts were found in expression sequence tags (EST) libraries from wood forming tissues. A 1-aminocyclopropane-1-carboxylate oxidase (ACO) was cloned, observed to be expressed in developing xylem, and highly up-regulated exclusively in (upper side) TW forming tissues. Simultaneously, the precursor of ethylene, ACC, accumulated in opposite (lower) wood. This finding demonstrated that ACO expression, rather than ACC availability, regulated ethylene production in this system. Further, an ACC deaminase (ACD) that metabolizes ACC was identified from EST libraries. Previously thought to be specific to microorganisms, this is the first report of an ACD endogenous to plants. The poplar ACD was mainly expressed in the cambial zone and phloem, complementing the expression of ACO in the xylem, which suggests that ethylene synthesis within the stem, is tightly controlled. A poplar microarray was used to study gene regulation in TW. The analysis was focused on genes related to the flux of carbohydrates to cell wall components, plant hormones and a group of highly up-regulated fasciclin-like arabinogalactan proteins. The study demonstrates several genes important for the flux of carbohydrates from lignin and hemicelluloses towards cellulose production. Microarray analysis was also used to study the effect of gene expression in transgenic trees down-regulated in lignin biosynthesis genes, either the enzyme caffeic acid O-methyl transferase or cinnamyl alcohol dehydrogenase (CAD). Interestingly, several of the genes uniquely affected by CAD down-regulation where related the plant clock function. This finding appears to be caused by a secondary event, due to the red coloration of the xylem observed in the transgenic lines.