Summary Fifteen poplar cDNA encoding fasciclin‐like arabinogalactan proteins (PopFLAs) were finely characterized, whereas the presence of arabinogalactan proteins (AGPs) was globally assessed during wood formation. PopFLAs transcript accumulation was analysed through EST distribution in cDNA libraries, semi‐quantitative RT‐PCR, microarray experiment and Northern blot analysis. Similarly, AGPs contents were globally quantified by rocket electrophoresis. AGPs accumulation was further examined by Western blotting and immunocytolocalization. Ten PopFLAs were specifically expressed in tension wood (TW) and not expressed in the cambial zone. Rocket electrophoresis revealed important AGPs accumulation in TW xylem. An anti‐AGPs specific antibody recognized two proteins preferentially present in the cell wall‐bound fraction from TW. Immunocytochemistry revealed a strong labelling close to the inner part of the G‐layer of TW fibres. PopFLAs are expressed in xylem and many are up‐regulated in TW. It is suggested that some PopFLAs accumulating at the inner side of the G‐layer may have a specific function in the building of this layer. PopFLAs expression may therefore be linked to the specific mechanical properties of TW.

We describe in this study punchless, a nonpathogenic mutant from the rice blast fungus M. grisea, obtained by plasmid-mediated insertional mutagenesis. As do most fungal plant pathogens, M. grisea differentiates an infection structure specialized for host penetration called the appressorium. We show that punchless differentiates appressoria that fail to breach either the leaf epidermis or artificial membranes such as cellophane. Cytological analysis of punchless appressoria shows that they have a cellular structure, turgor, and glycogen content similar to those of wild type before penetration, but that they are unable to differentiate penetration pegs. The inactivated gene, PLS1, encodes a putative integral membrane protein of 225 aa (Pls1p). A functional Pls1p-green fluorescent protein fusion protein was detected only in appressoria and was localized in plasma membranes and vacuoles. Pls1p is structurally related to the tetraspanin family. In animals, these proteins are components of membrane signaling complexes controlling cell differentiation, motility, and adhesion. We conclude that PLS1 controls an appressorial function essential for the penetration of the fungus into host leaves. P lant pathogenic fungi are a major threat for crop production worldwide. Several of these fungal pathogens depend on the formation of a specialized cell, called an appressorium, for the successful penetration of host plant surfaces (1, 2). This penetration process is typical of fungal leaf spots such as rice blast caused by the filamentous ascomycete Magnaporthe grisea (3). This disease is disseminated by water-splashed conidia (ref. 4; Fig. 1A). After their adhesion to the host leaf by means of a strong glue (ref. 5; Fig. 1B), conidia germinate and differentiate appressoria at the tips of their germ tubes (Fig. 1C). This developmental process is induced by chemical and physical stimuli from the host surface such as hardness, hydrophobicity, or cutin monomers (2, 3). This dome-shaped, thick-walled, and melanized cell (Fig. 1D) promotes the differentiation of a specialized hypha called the penetration peg that is involved in penetration of plant surfaces (ref. 6; pp in Fig. 1E). The differentiation of appressoria and penetration pegs is the result of complex morphogenetic processes that involve major developmental and metabolic changes (2,3,6). Inside the epidermal plant cell, the penetration peg differentiates into a bulbous infection hypha that invades the whole cell and colonizes the leaf.To isolate genes involved in this infection process, we generated a collection of M. grisea pathogenicity mutants by using plasmid-mediated insertional mutagenesis. This strategy was successful for the cloning of pathogenicity genes in Colletotrichum lindemuthianum (7), Ustilago maydis,** and M. grisea (8,9). In this study, we isolated punchless, a nonpathogenic mutant defective in penetration of plant surfaces. We show that the PLS1 gene is inactivated in punchless and encodes a putative membrane protein related to the tetraspanin family (10...

We examined the relationships between xylem resistance to cavitation and 16 structural and functional traits across eight unrelated Populus deltoides ¥ Populus nigra genotypes grown under two contrasting water regimes. The xylem water potential inducing 50% loss of hydraulic conductance (Y50) varied from -1.60 to -2.40 MPa. Droughtacclimated trees displayed a safer xylem, although the extent of the response was largely genotype dependant, with Y50 being decreased by as far as 0.60 MPa. At the tissue level, there was no clear relationship between xylem safety and either xylem water transport efficiency or xylem biomechanics; the only structural trait to be strongly associated with Y50 was the double vessel wall thickness, genotypes exhibiting a thicker double wall being more resistant. At the leaf level, increased cavitation resistance was associated with decreased stomatal conductance, while no relationship could be identified with traits associated with carbon uptake or bulk leaf carbon isotope discrimination, a surrogate of intrinsic water-use efficiency. At the whole-plant level, increased safety was associated with higher shoot growth potential under well-irrigated regime only. We conclude that common trade-offs between xylem resistance to cavitation and other physiological traits that are observed across species may not necessarily hold true at narrower scales.

In the search for renewable energy sources, genetic engineering is a promising strategy to improve plant cell wall composition for biofuel and bioproducts generation. Lignin is a major factor determining saccharification efficiency and, therefore, is a prime target to engineer. Here, lignin content and composition were modified in poplar (Populus tremula 3 Populus alba) by specifically down-regulating CINNAMYL ALCOHOL DEHYDROGENASE1 (CAD1) by a hairpin-RNA-mediated silencing approach, which resulted in only 5% residual CAD1 transcript abundance. These transgenic lines showed no biomass penalty despite a 10% reduction in Klason lignin content and severe shifts in lignin composition. Nuclear magnetic resonance spectroscopy and thioacidolysis revealed a strong increase (up to 20-fold) in sinapaldehyde incorporation into lignin, whereas coniferaldehyde was not increased markedly. Accordingly, ultra-high-performance liquid chromatography-mass spectrometry-based phenolic profiling revealed a more than 24,000-fold accumulation of a newly identified compound made from 8-8 coupling of two sinapaldehyde radicals. However, no additional cinnamaldehyde coupling products could be detected in the CAD1-deficient poplars. Instead, the transgenic lines accumulated a range of hydroxycinnamate-derived metabolites, of which the most prominent accumulation (over 8,500-fold) was observed for a compound that was identified by purification and nuclear magnetic resonance as syringyl lactic acid hexoside. Our data suggest that, upon down-regulation of CAD1, coniferaldehyde is converted into ferulic acid and derivatives, whereas sinapaldehyde is either oxidatively coupled into S9(8-8)S9 and lignin or converted to sinapic acid and derivatives. The most prominent sink of the increased flux to hydroxycinnamates is syringyl lactic acid hexoside. Furthermore, low-extent saccharification assays, under different pretreatment conditions, showed strongly increased glucose (up to +81%) and xylose (up to +153%) release, suggesting that down-regulating CAD1 is a promising strategy for improving lignocellulosic biomass for the sugar platform industry.