First steps in studying the origins of secondary woodiness in Begonia (Begoniaceae): Combining anatomy, phylogenetics, and stem transcriptomics

Since Darwin’s observation that secondary woodiness is common on islands, the evolution of woody plants fromherbaceous ancestors has been documented in numerous angiosperm groups. However, the evolutionary processesthat give rise to this phenomenon are poorly understood. To begin addressing this we have used a range ofapproaches to study the anatomical and genetic changes associated with the evolution and development ofsecondary woodiness in a tractable group. Begonia is a large, mainly herbaceous, pantropical genus that showsmultiple shifts towards secondarily woody species inhabiting mainly tropical montane areas throughout the world.Molecular phylogenies, including only a sample of the woody species in Begonia, indicated at least eight instancesof a herbaceous–woody transition within the genus. Wood anatomical observations of the five woody speciesstudied revealed protracted juvenilism that further support the secondary derived origin of wood within Begonia.To identify potential genes involved in shifts towards secondary woodiness, stem transcriptomes of wooddevelopment in B. burbidgei were analysed and compared with available transcriptome datasets for the non-woodyB. venustra, B. conchifolia, and Arabidopsis, and with transcriptome datasets for wood development in Populus.Results identified a number of potential regulatory genes as well as variation in expression of key biosyntheticenzymes. </div

Wood formation and transcript analysis with focus on tension wood and ethylene biology

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

Cytokinin Regulation of Auxin Synthesis in Arabidopsis Involves a Homeostatic Feedback Loop Regulated via Auxin and Cytokinin Signal Transduction[W][OA]

This study demonstrates that auxin and cytokinin regulate each other's biosynthesis, providing an intrinsic mechanism for optimizing the relative intracellular concentrations of both hormones

Downregulation of cinnamoyl-coenzyme a reductase in poplar: multiple-level phenotyping reveals effects on cell wall polymer metabolism and structure

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