•We study how poplar genotype mediates water deficit effects on a gall-inducing aphid.•Three irrigation levels were applied to a susceptible and a resistant host genotype.•For both genotypes, water ...deficit affected plant growth and gall anatomy.•The effect of water deficit on aphid performance differed according to host genotype.•The medium irrigation level favored aphid development on the resistant host.
Successful plant manipulation by herbivores requires a reconfiguration of the primary and secondary metabolisms of the host-plant. Water deficit is generally predicted to negatively affect the development of gall-inducing insects, by impairing their ability to remodel the primary metabolism of their host. We assessed whether host genotype could modulate water deficit effects on a gall-inducing aphid, Phloeomyzus passerinii, developing on poplars, which are known to be among the most susceptible trees to water deficit. Stem-cuttings of two poplar genotypes, with a different resistance level to P. passerinii, were grown under three irrigation treatments, and subsequently infested with the same aphid clone. Plant growth parameters and aphid colonies development were recorded, and the development and organization of the pseudogall induced by the aphid was observed using histological investigations. The three irrigation treatments resulted in three levels of water deficit. Mean predawn leaf water potential reached −0.10, −0.49 and −0.75MPa in the optimal, intermediate and low irrigation treatments respectively. In both genotypes, growth, architecture and photosynthesis were similarly affected. The severe water deficit reduced the development of aphids on both genotypes, which is in agreement with the general prediction for gall-inducing insects. Nonetheless, while the intermediate water deficit did not affect the development of aphids on the susceptible genotype, it enhanced their development on the resistant one. This latter observation contradicts the prediction for gall-inducing insects, but is in agreement with the plant stress hypothesis for sucking insects. The size of the pseudogall induced by the aphids in the bark of their host progressively diminished when water deficit increased. Lignification, believed to be an important component of poplar resistance to P. passerinii, also progressively decreased with water deficit. The involvement of physiological and architecture modifications due to water deficit on the development of aphids is discussed. The results demonstrate that host genotype can modulate water deficit effects on the development of a galling aphid, thereby compromising predictions on the outcome of droughts on stand resistance to plant-manipulating herbivores.
The lignification process in poplar tension wood lignified cell wall layers, specifically the S(1) and S(2) layers and the compound middle lamella (CML), was analysed using ultraviolet (UV) and ...transmission electron microscopy (TEM). Variations in the thickness of the gelatinous layer (G-layer) were also measured to clarify whether the lignified cell wall layers had completed their lignification before the deposition of G-layers, or, on the contrary, if lignification of these layers was still active during G-layer formation. Observations using UV microscopy and TEM indicated that both UV absorbance and the degree of potassium permanganate staining increased in the CML and S(1) and S(2) layers during G-layer formation, suggesting that the lignification of these lignified layers is still in progress during G-layer formation. In the context of the cell-autonomous monolignol synthesis hypothesis, our observations suggest that monolignols must go through the developing G-layer during the lignification of CML and the S(1) and S(2) layers. The alternative hypothesis of external synthesis (in the rays) does not require that monolignols go through the G-layer before being deposited in the CML, or the S(1) and S(2) layers. Interestingly, the previous observation of lignin in the poplar G-layer was not confirmed with the microscopy techniques used in the present study.
KEY MESSAGE : Phloeomyzus passerinii can induce a pseudogall within the bark of susceptible poplar genotypes, while in resistant genotypes the induction seems to be impeded by lignification ...processes. Phloeomyzus passerinii is a major pest of poplar stands in Europe, North Africa and the Near East. This aphid feeds in the cortical parenchyma of different poplar species and hybrids, and can affect their growth and survival through unknown mechanisms. In some genotypes, however, resistance prevents either the settlement or the development of aphid colonies. For a better understanding of tree reactions to aphid probing, we compared the anatomical and biochemical modifications undergone within the bark of stem cuttings, after different delays of either aphid colonization or mechanical wounding. To assess how resistance may modulate tree reactions, the comparison was performed using three poplar genotypes exhibiting different resistance levels. In these three genotypes, mechanical wounding induced a similar, localized, wound periderm. In contrast, aphid colonization triggered more extended reactions, which differed among genotypes. In the susceptible genotype, aphids induced a reaction tissue, characterized after a month by thin-walled hypertrophied cells and a depletion of soluble phenolic compounds and starch. Anatomical features of this reaction tissue suggest that the aphid initiates a pseudogall in the cortical tissues of its susceptible host. In the resistant genotypes, however, the differentiation of the reaction tissue was totally or partially inhibited, probably because of extended lignification processes. The implications of a pseudogall induction on susceptible hosts’ physiology, and the impact of lignification on aphid development and behavior, are discussed.
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.
• 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.
Wood is a complex and highly variable tissue, the formation of which is developmentally and environmentally regulated. In reaction to gravitropic stimuli, angiosperm trees differentiate tension wood, ...a wood with specific anatomical, chemical and mechanical features. In poplar the most significant of these features is an additional layer that forms in the secondary wall of tension wood fibres. This layer is mainly constituted of cellulose microfibrils oriented nearly parallel to the fibre axis. Tension wood formation can be induced easily and strongly by bending the stem of a tree. Located at the upper side of the bent stem, tension wood can be compared with the wood located on its lower side. Therefore tension wood represents an excellent model for studying the formation of xylem cell walls. This review summarizes results recently obtained in the field of genomics on tension wood. In addition, we present an example of how the application of functional genomics to tension wood can help decipher the molecular mechanisms responsible for cell wall characteristics such as the orientation of cellulose microfibrils.
Hardwood trees are able to reorient their axes owing to tension wood differentiation. Tension wood is characterised by important ultrastructural modifications, such as the occurrence in a number of ...species, of an extra secondary wall layer, named gelatinous layer or G-layer, mainly constituted of cellulose microfibrils oriented nearly parallel to the fibre axis. This G-layer appears directly involved in the definition of tension wood mechanical properties. This review gathers the data available in the literature about lignification during tension wood formation. Potential roles for lignin in tension wood formation are inferred from biochemical, anatomical and mechanical studies, from the hypotheses proposed to describe tension wood function and from data coming from new research areas such as functional genomics.
In this work, a cDNA encoding a defensin from
Picea glauca, named
PgD1, was isolated. The corresponding
PgD1 cDNA encodes a putative protein of 83 amino acids with a calculated molecular mass of 8.9
...kDa. The mature protein of 50 amino acids is characterized by the presence of specific highly conserved residues common to all plant defensins.
PgD1 is developmentally expressed during seed germination and is also up-regulated by wounding and by jasmonic acid treatment, suggesting a role in both the constitutive and induced defense mechanisms. The recombinant protein was found to cause extensive growth inhibition of three fungal pathogens (
Cylindrocladium floridanum,
Fusarium oxysporum and
Nectria galligena) at 2.5
μM.
Six Populus deltoides Bartr. ex Marsh. x P. nigra L. genotypes were selected to investigate whether stem xylem anatomy correlated with gas exchange rates, water-use efficiency (WUE) and growth ...performance. Clonal copies of the genotypes were grown in a two-plot common garden test under contrasting water regimes, with one plot maintained irrigated and the other one subjected to moderate summer water deficit. The six genotypes displayed a large range of xylem anatomy, mean vessel and fibre diameter varying from about 40 to 60 microm and from 7.5 to 10.5 microm, respectively. Decreased water availability resulted in a reduced cell size and an important rise in vessel density, but the extent of xylem plasticity was both genotype and trait dependent. Vessel diameter and theoretical xylem-specific hydraulic conductivity correlated positively with stomatal conductance, carbon isotope discrimination and growth performance-related traits and negatively with intrinsic WUE, especially under water deficit conditions. Vessel diameter and vessel density measured under water deficit conditions correlated with the relative losses in biomass production in response to water deprivation; this resulted from the fact that a more plastic xylem structure was generally accompanied by a larger loss in biomass production.
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