Plants are able to trigger multiple signaling pathways to cope with pathogenic invasion. Hypersensitive response (HR), one of the effective mechanisms, is triggered by the interaction between pathogenic Avr effectors from the pathogen and plant R proteins (also known as gene-for-gene interaction). Plant tissues induce distinct activities when they have, or have no HR, and those differences may help scientists to find out the crucial factors in efficient defense against plant pathogens. The general objective of this Ph.D. thesis is based on that background and applied on the studies of Brassica napus – Leptosphaeria maculans pathosystem. Three cultivars of B. napus (Westar, Surpass400 and 01-23-2-1) were inoculated by two L. maculans isolates (HCRT75 8-1 and HCRT77 7-2) to cause three distinct levels of severity: susceptible, intermediate and resistant. Expression studies (by RT-qPCR), histochemical assays (such as trypan blue staining) were applied on the cotyledons of those cultivars to search the differences in defense response from those cultivars (with distinct severities). Histochemical assays transcriptional analysis suggested that the intermediate and resistant genotypes (i.e. Surpass400 and 01-23-2-1) displayed earlier H2O2 accumulation and cell death on the cotyledons. and activation of the genes related to salicylic acid (SA) and ROS (as early as 3 and 5 dpi). The results indicated that the early activation of SA/ROS signaling is one of the crucial components for B. napus to defend against L. maculans. Environmental factors are essential components for plant growth/development; therefore, it is reasonable that the environmental changes are able to alter the actions in plant defense. The third part of this study was to explore the association between temperature and HR resistance. Lesion measurements suggested that high incubation temperature resulted in larger lesion size. RT – qPCR results reflected the distinct expression levels of putative temperature – sensitive genes among three incubating temperature conditions (28 oC/ 22 oC). The results indicated that 22 oC/16 oC condition is the peak point for PR1/2 expression. This study suggested that the defense in B. napus was affected by the temperature and might have an optimal temperature to elicit robust defense signals.