CRYPTOGEIN INDUCED VASCULAR DEFENSE RESPONSES INRAPESEED SIMILAR TO THAT PRODUCED DURING THE BR FOLLOWING INFECTION BY AN AVIRULENT ISOLATE OF LEPTOSPHAERIA MACULANS
CRYPTOGEIN INDUCED VASCULAR DEFENSE RESPONSES IN RAPESEED SIMILAR TO THAT PRODUCED DURING THE BR FOLLOWING INFECTION BY AN AVIRULENT ISOLATE OF LEPTOSPHAERIA MACULANS S ROUSSELl, M NICOLE2, F LOPEZ2, JP GEIGER2, P RICCI 3, RENARD Ml and H BRUNl l, Plant Pathology, BP 29, 35653, Le Rheu, France., 2ORSTOM, Plant Pathology, BP 5045, 34032, Montpellier, France, 3, INRA, Plant Pathology, BP 2078, 06606, Antibes, France. Background and objectives Blackleg caused by the vascular fungus Leptosphaeria maculans (Lm) is one of the most threatening disease of oilseed rape (Brassica napus). Rapidly, after initial establishment in the mesophyll, the fungus displayed a systemic phase throughout vessels. Differential interactions revealed an hypersensitive reaction (HR) in incompatible systems. It was demonstrated that treatment of B. napus cotyledons with cryptogein, a fungal elicitor, induced HR-like symptoms. Although, several physiological, biochemical and molecular mechanisms were evidenced in elicited cell suspensions, very little information is available on the expression of plant defense genes in vascular tissues following plant infiltration with elicitin. In the present study, we have investigated microscopically vascular responses of rapeseed cotyledons following treatment with cryptogein, or infection by an avirulent Lm isolate. Results and conclusions In elicited or Lm-inoculated cotyledons, parenchyma vascular cells displayed ultrastructural changes such as vesiculation of the cytoplasm, morphological modifications of nuclei and disorganization of organelles. Observations revealed that the lumen of vessels was occluded by a fibrillar-like material that was pink-stained with toluidine blue. This accumulated material positively immunoreacted with anti-pectin antibodies. Vessel-associated parenchyma cells were characterized by a strong fluorescence indicating the presence of phenolics, histochen-fically identified as flavonoids. None of these observations were reported in susceptible infected plants and controls. These data (1) show that cryptogein triggers responses at the vascular level in rapeseed cotyledons, (2) reveal that these responses are similar to that observed in cotyledons showing Lm -induced HR, (3) confirm that parenchyma vascular cells of plants play a key role in the resistance to vascular pathogens.