Bactéries

نعمة
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Joined: Wed Feb 21, 2018 7:50 pm

Bactéries

Post by نعمة » Tue Jul 17, 2018 7:28 pm

Procaryotes de 0,8 à 2 µm.
Génome porté par un chromosome bactérien appelé nucléoïde (~ 106 pb, circulaire) et par des plasmides (ADN circulaire, n x 106 pb) pouvant être transmis horizontalement entre individus d’espèces identiques ou non (transmission de gènes de résistance).

Les plasmides portent parfois les gènes impliqués dans l’interaction du pathogène et de la plante (Agrobacterium, Rhizobium, Pseudomonas).

Les mécanismes de pathogénèse sont très variés : synthèse de toxines (certaines Corynébactéries), d’antibiotiques, hormones et régulateurs de croissance (Agrobacterium, Corynebacterium), enzymes lytiques (pectate lyase et méthylestérases d’Erwinia carotovora), transfert d’ADN dans la cellule végétale (Agrobacterium).


Les infections bactériennes provoquent des symptômes variés (dépérissements, pourritures, tumeurs, nécroses, chancres, flétrissements) et leur identification
repose sur l’isolement et l’observation en microscopie (morphologie, Gram, présence de flagelles, ...), les tests biochimiques, sérologiques et moléculaires (hybridation, PCR,...).

Six genres bactériens sont principalement impliqués dans les maladies de plantes :
- Agrobacterium : galle du collet (crown gall) (A. tumefaciens) chevelu racinaire (hairy root) (A. rhizogenes).
- Erwinia : feu bactérien des pomoïdes (E. amylovora), flétrissement des cucurbitacées (E. tracheiphila), jambe noire de la pomme de terre (E. carotovora subsp atroseptica)
- Pseudomonas : nécroses sur arbres fruitiers (Burkholderia, Comamonas, Pseudomonas,
Ralstonia, P. syringae).
- Xanthomonas : nécrose bactérienne de la vigne (X. ampelina), pourriture noire du chou ou chancre des agrumes (X. campestris).
- Streptomyces : galle commune de la pomme de terre (S. scabies)
- Corynebacterium : (Clavibacter, Corynebacterium, Curtobacterium, Rhodococcus)
croissance anormale (C. fascians).



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