Analyse des polysaccharides de la paroi cellulaire de phytophthora megakarya et Pythium myriotylum deux Oomycètes phytopathogènes de Theobroma cacao l. et de Xanthosoma sagittifolium L. Schott.

Abstract

Oomycetes are formidable pathogenic agents for cultivated plants and the annual losses cause by this organism is considerable. Among them we have Pythium myriotylum (causative agent of cocoyam root rot disease) and phytophthora megakarya (agent responsible of black pod disease of cocoa) cause losses in order of 80%. Just like the superior fungi, oomycetes possess a complex and dynamic cell wall which plays an essential role in protection and pathogenicity. To enhance the current knowledge on the role that occupies this wall, as well as its change during the mycelial growth, the polysaccharides of wall (AIS) and the various parietal fractions (ALS and ALI) of mycelium of P.myriotylium and P. megakarya was analyzed in the 3rd day and 9th day of his growth. Three days and nine days after inoculation of a sporale solution calibrated of P. myriotylum and P. megakarya, the mycelial masses were collected for extraction of the cell wall (AIS), as well as its various fraction(ALS and ALI); then the monosaccharide profile of different parietal fractions was analyzed. Our results show that the mycelial masses increase in time to these two oomycetes. The yield on extraction shows that there is no quantitative difference at the level of the cell wall to these two oomycetes. In an essential way, the fraction ALS and ALI is more plentiful to P. megakarya than at P. myriotylum. The analysis by gas chromatography of parietal fraction (AIS) shows at these both oomycetes, a predominance in glucose which increases in time (63.20% to 66.22% at P. megakarya and 58.95% to 59.66% at P. myriotylum). We also note the presence of the galactose and the mannose as well as the small proportion of N-acétylglucosamine (4.82% to 4.14% at P. megakarya and 5.31% to5.37% at P. myriotylum). The fraction ALS and ALI are constituted also mainly by glucose which is more important in the ALI fraction than in the ALS fraction and decreases according to time. We also note in these fractions the presence of minor sugars met at the level of the wall following the example of N-acétylglucosamine which the abundance is marked in the ALI fraction (8.49% to 7.05% at P. megakarya and 9.85% to 10.90% at P. myriotylum) that in the fraction ALS (1.16% to 5.23% at P. megakarya and 4.25 % to 1.28% at P. myriotylum). All these results showed a change and continuous remodeling of the oomycetes cell wall during mycelial growth. The discovery of small proportions of N-acetylglucosamine in the cell wall and especially its strong presence in the ALI fraction than the ALS fraction could be determining in the development of new strategies against Oomycetes, in particularly against P. megakarya and P. myriotylum.