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Cytological Investigation of Resistance to Leptosphaeria maculans Conferred to Brassica napus by Introgressions Originating from B. juncea or B. nigra B Genome

First and sixth authors: Station de Pathologie Végétale; and fourth and fifth authors: Station d'Amélioration des Plantes, INRA, BP29, 35653 Le Rheu Cedex, France; and second and third authors: Genetrop Laboratoire de Phytopathologie, IRD, BP 5045, 34032 Montpellier, France

Introgressions into Brassica napus from the B genome, either the B. nigra chromosome B4 or the B. juncea fragment carrying the Jlm1 gene, have given rise to the B. napus-B. nigra addition line (LA4+) and the B. napus-B. juncea recombinant line (MXS), respectively. The resistance of these two lines to Leptosphaeria maculans is characterized by a hypersensitive reaction (HR) on both the cotyledons and leaves, while the collar displays a high degree of resistance. Responses induced in cotyledons of the two lines by L. maculans inoculation were investigated with emphasis on cytological events underlying the HR and on host defense reactions. Features of host cell changes including condensation and lobing of nuclei, fragmentation of chromatin, disruption of the nuclear membranes, and plasma membrane withdrawal were reminiscent of HR cell death in MXS and LA4+ plants. Restriction of pathogen growth to the infection areas in LA4+ was correlated to reinforcement of cell wall barriers, including wall apposition, papillae, and vessel plugging. In MXS, the lower expression of resistance was associated with a delay in plant responses. These results indicate that mechanisms underlying the HR in the B. napus recombinant and addition lines are differently controlled according to the introgressed genes.

Additional keywords: HR-type symptoms, resistance gene.