Salicylic Acid, an Ambimobile Molecule Exhibiting a High Ability to Accumulate in the Phloem

International audienceThe ability of exogenous salicylic acid (SA) to accumulate in castor bean (Ricinus communis) phloem was evaluated by HPLC and liquid scintillation spectrometry analyses of phloem sap collected from the severed apical part of seedlings. Time-course experiments indicated that SA was transported to the root system via the phloem and redistributed upward in small amounts via the xylem. This helps to explain the peculiarities of SA distribution within the plant in response to biotic stress and exogenous SA application. Phloem loading of SA at 1, 10, or 100 mM was dependent on the pH of the cotyledon incubating solution, and accumulation in the phloem sap was the highest (about 10-fold) at the most acidic pH values tested (pH 4.6 and 5.0). As in animal cells, SA uptake still occurred at pH values close to neutrality (i.e. when SA is only in its dissociated form according to the calculations made by ACD LogD suite software). The analog 3,5-dichlorosalicylic acid, which is predicted to be nonmobile according to the models of Bromilow and Kleier, also moved in the sieve tubes. These discrepancies and other data may give rise to the hypothesis of a possible involvement of a pH-dependent carrier system translocating aromatic monocarboxylic acids in addition to the ion-trap mechanism

L'arsenal phytosanitaire face aux ennemis des plantes. Considérations générales

Les plantes sont attaquées, non seulement par divers types de micro-organismes pathogènes, mais aussi par d'autres ennemis, parmi lesquels des mollusques, des nématodes, des acariens et des insectes. Elles peuvent développer des stratégies complexes et efficaces pour faire face aux pathogènes (réaction hypersensible et résistance systémique acquise) et aux herbivores (émissions de produits volatils attirant des parasites de l'agresseur, synthèse d'inhibiteurs de protéinases). Cependant, la confrontation de la plante avec les agresseurs peut aussi tourner à l'avantage de ces derniers. Aussi, dans le passé, l'attaque des cultures a-telle régulièrement généré des pertes économiques considérables. À partir de la seconde guerre mondiale, l'essor de la chimie organique, associé à la prise en considération des problèmes du monde agricole, a débouché sur la mise au point d'un nombre sans cesse croissant de produits phytosanitaires. Ils se répartissent actuellement en une dizaine de classes, les herbicides, les fongicides et les insecticides–acaricides, représentant plus de 90% du marché mondial. La plupart des produits phytosanitaires mis sur le marché au cours de ces trois dernières décennies sont actifs à faible dose et bénéficient d'un profil toxicologique et cotoxicologique nettement plus favorable que celui des premières familles de pesticides, ces dernières étant maintenant totalement ou partiellement retirées du marché. Plusieurs familles plus ou moins récentes sont des analogues de métabolites produits par divers types d'organismes. Les progrès réalisés dans la protection des cultures sont donc remarquables, mais les déséquilibres environnementaux créés ont généré une dépendance vis-à-vis des phytosanitaires. Fort judicieusement, des stratégies alternatives (éliciteurs, ingénierie génétique par exemple) ont été initiées ou développées lors de la dernière décennie

Vectorisation of agrochemicals via amino acid carriers: influence of the spacer arm structure on the phloem mobility of phenylpyrrole conjugates in the Ricinus system: Vectorization of agrochemicals via amino acid carriers in the Ricinus system

International audienceBACKGROUND: Excessive agrochemical use causes significant threats to environmental safety and human health. Reducing pesticide use without reducing yield is necessary for sustainable agriculture. Therefore we developed a vectorization strategy to enhance agrochemical delivery through plant amino acid carriers.RESULTS: In addition to a fenpiclonil conjugate recently described, three new amino acid conjugates were synthesized by coupling fenpiclonil to an L-α-amino acid. Phloem mobility of these conjugates which exhibit different structures of the spacer arm introduced between fenpiclonil and the α-amino acid function, was studied using the Ricinus model. Conjugate L-14 which contains a triazole ring with the shortest amino acid chain showed the best phloem systemicity among the four conjugates. By contrast, removing the triazole ring in the spacer arm did not improve systemicity. L-14 exhibited phloem systemicity at all reported pH values (pHs from 5.0 to 6.5) of the foliar apoplast, while acidic derivatives of fenpiclonil were translocated only at pH values near 5.0.CONCLUSION: The conjugates were recognized by a pH-dependent transporter system and translocated at distance in the phloem. They exhibited a broader ability to phloem systemicity than fenpiclonil acidic derivatives within the pH value range of the foliar apoplast

Phloem sap intricacy and interplay with aphid feeding

Aphididae feed upon the plant sieve elements (SE), where they ingest sugars, nitrogen compounds and other nutrients. For ingestion, aphid stylets penetrate SE, and because of the high hydrostatic pressure in SE, phloem sap exudes out into the stylets. Severing stylets to sample phloem exudates (i.e. stylectomy) has been used extensively for the study of phloem contents. Alternative sampling techniques are spontaneous exudation upon wounding that only works in a few plant species, and the popular EDTA-facilitated exudation technique. These approaches have allowed fundamental advances on the understanding of phloem sap composition and sieve tube physiology, which are surveyed in this review. A more complete picture of metabolites, ions, proteins and RNAs present in phloem sap is now available, which has provided large evidence for the phloem role as a signalling network in addition to its primary role in partitioning of photo-assimilates. Thus, phloem sap sampling methods can have remarkable applications to analyse plant nutrition, physiology and defence responses. Since aphid behaviour is suspected to be affected by phloem sap quality, attempts to manipulate phloem sap content were recently undertaken based on deregulation in mutant plants of genes controlling amino acid or sugar content of phloem sap. This opens up new strategies to control aphid settlement on a plant host

Aphids as biological models and agricultural pests

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