SummaryStems and branches of angiosperm trees form tension wood (TW) when exposed to a gravitational stimulus. One of the main characteristics of TW, which distinguishes it from normal wood, is the formation of fibers with a thick inner gelatinous cell wall layer mainly composed of crystalline cellulose. Hence TW is enriched in cellulose, and deficient in lignin and hemicelluloses. An expressed sequence tag library made from TW-forming tissues in Populus tremula (L.) · tremuloides (Michx.) and data from transcript profiling using microarray and metabolite analysis were obtained during TW formation in Populus tremula (L.) in two growing seasons. The data were examined with the aim of identifying the genes responsible for the change in carbon (C) flow into various cell wall components, and the mechanisms important for the formation of the gelatinous cell wall layer (G-layer). A specific effort was made to identify carbohydrate-active enzymes with a putative function in cell wall biosynthesis. An increased C flux to cellulose was suggested by a higher abundance of sucrose synthase transcripts. However, genes related to the cellulose biosynthetic machinery were not generally affected, although the expression of secondary wall-specific CesA genes was modified in both directions. Other pathways for which the data suggested increased activity included lipid and glucosamine biosynthesis and the pectin degradation machinery. In addition, transcripts encoding fasciclin-like arabinogalactan proteins were particularly increased and found to lack true Arabidopsis orthologs. Major pathways for which the transcriptome and metabolome analysis suggested decreased activity were the pathway for C flux through guanosine 5¢-diphosphate (GDP) sugars to mannans, the pentose phosphate pathway, lignin biosynthesis, and biosynthesis of cell wall matrix carbohydrates. Several differentially expressed auxin-and ethylene-related genes and transcription factors were also identified.

The plant hormone ethylene is an important signal in plant growth responses to environmental cues. In vegetative growth, ethylene is generally considered as a regulator of cell expansion, but a role in the control of meristem growth has also been suggested based on pharmacological experiments and ethylene-overproducing mutants. In this study, we used transgenic ethylene-insensitive and ethylene-overproducing hybrid aspen (Populus tremula ؋ tremuloides) in combination with experiments using an ethylene perception inhibitor [1-methylcyclopropene (1-MCP)] to demonstrate that endogenous ethylene produced in response to leaning stimulates cell division in the cambial meristem. This ethylenecontrolled growth gives rise to the eccentricity of Populus stems that is formed in association with tension wood.plant hormones ͉ secondary xylem ͉ tension wood ͉ vascular cambium ͉ wood development T he vascular cambium is the meristem that produces secondary xylem and phloem by periclinal cell divisions, and it is responsible for wood production and stem diameter growth in trees. Cambial growth rate along the trunk is correlated with leaf biomass and crown structure (1). Superimposed on this intrinsic control, cambial growth is also strongly influenced by environment, where mechanical and gravitational loads imposed by wind and leaning are important (2). Wind sway induces increased diameter growth that protects against stem breakage (3), whereas a static lean results in a localized growth response known as tension wood (TW) in dicotyledonous angiosperms. TW is formed on the upper side of the leaning stem, resulting in characteristic asymmetric growth, and it serves to correct the stem position (4). In addition to the striking increase in cambial cell divisions, TW has an altered anatomy, and the fibers form an additional inner, cellulose-rich, gelatinous secondary cell wall layer (G layer) (5).Experiments with ethylene applications have revealed that this volatile plant hormone has the potential to both inhibit and stimulate growth (6). Its synthesis from S-adenosylmethionine through the action of 1-aminocyclopropane-1-carboxylate (ACC) synthase (ACS) and ACC oxidase (ACO) is triggered in vegetative tissues in response to many environmental cues. To date, concepts of ethylene function in vegetative growth have mainly implicated its role in primary tissue and cell expansion (6). However, ethylene biosynthesis increases during TW formation because of an asymmetric induction of ACO (7,8), and application of ethylene has been shown to stimulate cambial growth both in trees and herbaceous species (9, 10). Additional observations also support a potential role for ethylene in cell division. Applied ethylene stimulated endoreduplication in cucumber hypocotyls and growth of the intercalary meristem in deepwater rice (11-13). Moreover, the ethylene-overproducing Arabidopsis mutant eto1 exhibits aberrant cell divisions in the quiescent center of the root (14). However, experiments where ethylene homeostasis is artificially manipulated by a...

SummaryEthylene Response Factors (ERFs) are a large family of transcription factors that mediate responses to ethylene. Ethylene affects many aspects of wood development and is involved in tension wood formation. Thus ERFs could be key players connecting ethylene action to wood development.We identified 170 gene models encoding ERFs in the Populus trichocarpa genome. The transcriptional responses of ERF genes to ethylene treatments were determined in stem tissues of hybrid aspen (Populus tremula 9 tremuloides) by qPCR. Selected ethylene-responsive ERFs were overexpressed in wood-forming tissues and characterized for growth and wood chemotypes by FT-IR.Fifty ERFs in Populus showed more than five-fold increased transcript accumulation in response to ethylene treatments. Twenty-six ERFs were selected for further analyses. A majority of these were induced during tension wood formation. Overexpression of ERFs 18, 21, 30, 85 and 139 in wood-forming tissues of hybrid aspen modified the wood chemotype. Moreover, overexpression of ERF139 caused a dwarf-phenotype with altered wood development, and overexpression of ERF18, 34 and 35 slightly increased stem diameter.We identified ethylene-induced ERFs that respond to tension wood formation, and modify wood formation when overexpressed. This provides support for their role in ethylenemediated regulation of wood development.