Etude physiologique, biochimique, moléculaire et agronomique du stress salin chez le melon (Cucumis melo L.)

Chaque année sur son site de Malaga (Espagne), certaines parcelles de la société Rouge-Gorge, qui produit des melons, subissent une perte de rendement de 5 tonnes/ha. Des analyses indiquent que ces parcelles ont une teneur élevée en ion Na+ dans le sol et dans l eau d irrigation. Les problèmes de stress salin sont très étudiés sur de nombreuses espèces d intérêt agronomique, mais aucune étude n a encore été menée sur le melon. Un long travail préliminaire a permis de mettre en place les différents outils indispensables pour la suite (notamment la mise au point du système de culture reproduisant ce stress). Les résultats obtenus montrent que lors d un stress salin, chez cette variété de melon, il y a activation des systèmes de production des AOS (NADPH oxydases), mais pas des mécanismes de détoxication (peroxydases). Il y a aussi activation de la galactinol synthase et accumulation de sucres (stachyose en particulier) dans les feuilles. Un traitement à l acide ascorbique permet de restaurer, en grande partie, le rendement. Ce traitement influe négativement sur l état d activation des NADPH oxydases, positivement sur les peroxydases et négativement sur le niveau d expression de la galactinol synthase. L accumulation des sucres dans les feuilles est aussi diminuée. La présence d éléments de réponse (NFkb et AP-1 like) au potentiel redox sur le promoteur de la galactinol synthase permet de lier le stress salin (qui favorise la production d AOS) à la perte de rendement (due à l accumulation de sucre dans les feuilles). La vitamine C en limitant la production d AOS permet aussi d atténuer l effet du stress salin sur la plante.The company which funded this work cultivates melon in 3 different places: in the Center West and South East in France, as well as in South of Spain. In this third location, the proximity of the sea, and the irrigation technique make the water salty (approximatively 30 mM NaCl in water and soil extract). This NaCl concentration decreases the growth of melon plants and the yield from 13 tons/ha to 8 tons/ha. The present work begun with the preparation of the tools needed for further experiments, especially the culture systems. A publication of Shalata and Neumann (Shalata and Neumann, 2001) shows that an exogenous application of ascorbic acid could increase salt tolerance in tomato. Exogenous application of ascorbic acid during hydroponic culture of melon improved salt tolerance in melon, and allowed a partial recovery of yield (near 20% more than in salt stress plant without ascorbic acid). Biochemical evidences shows that NADPH oxidases , but not peroxidases, are activated during salt stress. Addition of ascorbic acid (0,5 mM) in the medium prevented the activation of NADPH oxidases and increased the activity of peroxidases. RNA blot experiments revealed activation of galactinol synthase gene, which encodes a key enzyme of stachyose synthesis. Stachyose is the major transported sugar in melon. The activation of this gene probably creates a metabolic deviation and contributes to the loss of yield. The promoter of galactinol synthase contains a NFkB response box, which senses redox status of the cell. The relationship between ascorbic acid and galactinol synthase is not fully understood, but it is still under investigation in the lab. Field experiments confirmed that AA increases salt tolerance.POITIERS-BU Sciences (861942102) / SudocSudocFranceF

Sugar-regulated expression of a putative hexose transport gene in grape (Vitis vinifera)

Different lengths of the promoter of grape (Vitis vinifera) VvHT1 (Hexose Transporter 1) gene, which encodes a putative hexose transporter expressed during the ripening of grape, have been transcriptionally fused to the -glucuronidase reporter gene. In transgenic tobacco (Nicotiana tabacum) transformed with these constructs, VvHT1 promoters were clearly responsible for the sink organ preferential expression. The potential sugar effectors of VvHT1 promoter were studied in tobacco cv Bright-Yellow 2 cells transformed with chimeric constructs. Glucose (56 mm), sucrose (Suc; 58 mm), and the non-transported Suc isomer palatinose doubled the -glucuronidase activity conferred by the VvHT1 promoter, whereas fructose did not affect it. These effects were the strongest with the 2.4-kb promoter, which contains all putative sugar-responsive elements (activating and repressing), but they were also significant with the 0.3-kb promoter, which contains only activating sugar boxes. The induction of VvHT1 expression by both Suc and palatinose was confirmed in the homologous grape berry cell culture. The data provide the first example of a putative sugar transporter, which is induced by both glucose and Suc in higher plants. Although induction of VvHT1 expression by Suc does not require transport, the presence of glucosyl moiety is necessary for Suc sensing. These results provide new insights into sugar sensing and signaling in plants

Sugar-Regulated Expression of a Putative Hexose Transport Gene in Grape

Different lengths of the promoter of grape (Vitis vinifera) VvHT1 (Hexose Transporter 1) gene, which encodes a putative hexose transporter expressed during the ripening of grape, have been transcriptionally fused to the β-glucuronidase reporter gene. In transgenic tobacco (Nicotiana tabacum) transformed with these constructs, VvHT1 promoters were clearly responsible for the sink organ preferential expression. The potential sugar effectors of VvHT1 promoter were studied in tobacco cv Bright-Yellow 2 cells transformed with chimeric constructs. Glucose (56 mm), sucrose (Suc; 58 mm), and the non-transported Suc isomer palatinose doubled the β-glucuronidase activity conferred by the VvHT1 promoter, whereas fructose did not affect it. These effects were the strongest with the 2.4-kb promoter, which contains all putative sugar-responsive elements (activating and repressing), but they were also significant with the 0.3-kb promoter, which contains only activating sugar boxes. The induction of VvHT1 expression by both Suc and palatinose was confirmed in the homologous grape berry cell culture. The data provide the first example of a putative sugar transporter, which is induced by both glucose and Suc in higher plants. Although induction of VvHT1 expression by Suc does not require transport, the presence of glucosyl moiety is necessary for Suc sensing. These results provide new insights into sugar sensing and signaling in plants

Non-specific lipid-transfer protein genes expression in grape (Vitis sp.) cells in response to fungal elicitor treatments

basic cystein-rich proteins believed to be involved in plant defense mechanisms. Three cDNAs coding nsLTPs from grape (Vitis vinifera sp.) were cloned by reverse-transcriptase-polymerase chain reaction (RT-PCR) and PCR. The expression of nsLTP genes was investigated in 41Brootstock grape cell suspension, in response to various defense-related signal molecules. Ergosterol (a fungispecific sterol) and a proteinaceous elicitor purified from Botrytis cinerea strongly and rapidly induced the accumulation of nsLTP mRNAs. Jasmonic acid, cholesterol, and sitosterol also promoted nsLTPs mRNA accumulation, although to a lesser extent, whereas salicylic acid had no effect. High performance liquid chromatography analysis indicated that the amounts of three LTP isoforms (previously named P1, P2, and P4) were increased by ergosterol. None of the four isoforms displayed any significant antifungal properties, with the exception of the P4 isoform, which reduced Botrytis mycelium growth in vitro, but only in calcium- free medium. The results are discussed in the context of plant–pathogen interactions