Danny Rioux Les plantes répondent à différentes agressions en formant des structures de défense et en produisant des substances toxiques visant à repousser ou à limiter l’action d’insectes et d’agents pathogènes. L’action combinée du scolyte de l’écorce (Ips typographus) et du champignon de bleuissement Ceratocystis polonica, dont il est un vecteur, cause des dommages chez l’épinette de Norvège (Picea abies) allant même jusqu’à provoquer la mort de l’arbre. Deux clones d’épinette de Norvège furent inoculés avec C. polonica et les examens microscopiques révèlent que les arbres du clone résistant accumulent plus de phénols dans des cellules de parenchyme du phloème comparativement aux arbres du clone susceptible. L’amidon était présent en plus grande quantité et était transformé plus rapidement en composés phénoliques lorsque la résistance se manifestait. L’article montre que l’enzyme clé menant à la production des flavonoïdes, la chalcone synthase, est exprimée plus fortement chez le clone résistant. Les flavonoïdes sont des phénols pouvant se transformer en tanins et autres polymères insolubles, renforçant ainsi les structures de défense. Les auteurs ont également noté parallèlement à la résistance l’expression plus intense d’une peroxydase, autre enzyme essentielle à la synthèse phénolique. Comme souligné dans d’autres systèmes, c’est la rapidité avec laquelle les arbres répondent à l’agression qui est primordiale pour l’issue de cette interaction. Danny Rioux Plants respond to various stress by forming defensive structures or by producing toxic compounds to repel or limit the action of insects and pathogens. Attacks by the bark beetle (Ips typographus) and its fungal associate Ceratocystis polonica, the latter producing a blue stain, cause serious damage to Norway spruce (Picea abies), and can even kill healthy trees. Two Norway spruce clones were inoculated with C. polonica and microscopic observations revealed that trees of the resistant clone accumulate more phenols in phloem parenchyma cells than those from the susceptible clone. Starch is found in higher concentrations before infection and is rapidly transformed in inoculated resistant trees. Transcript levels of chalcone synthase, the key enzyme in flavonoid synthesis, are higher in the resistant clone. Flavonoids represent a class of phenolics that can be converted in tannins and other insoluble phenols to strengthen various defensive structures. Concomitantly with resistance, the authors have also noted that the activity of a peroxidase isoform is greatly enhanced. Peroxi-dases are also important enzymes in phenol synthesis. As reported for many plants after challenge with pests, it is the rapidity to which the activation of defense responses takes place that is crucial for the outcome of this interaction. Kent Harrison Blackshaw et al. (2004) recently investigated the relative responses of wheat, canola, and 22 agricultural weeds to P fertilizer. In a controlled study using pot-grown plants in the greenhouse, each species was grown in field soil amended with P at rates ranging from 5 to 60 mg kg-1 soil. Shoot and root biomass of all weed species increased in response to increasing P fertilization, although the magnitude of the response was variable and depended upon the species. Compared to wheat or canola, more than half of the weed species tested exhibited similar or greater responses in shoot and root biomass to increasing levels of soil P. This effect was most apparent at the highest levels of soil P, where 17 of the weed species extracted more of the available soil P than wheat or canola. As soil P levels increased, P uptake increased as much as 50% or more in species like downy brome and wild oat, but remained virtually unaltered in other species such as green foxtail and barnyardgrass. One of the …