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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 –

The meristem genes SHOOT MERISTEMLESS (STM), CLAVATA1 (CLV1) and ZWILLE (ZLL) are essential for the formation and maintenance of the shoot apical meristem (SAM). As an extension of that work, this thesis examines the function of these genes during seed oil accumulation and microspore-derived embryogenesis. Using a transgenic approach it is demonstrated that only the over-expression of BnSTM increases the percentage of total seed oil and a reduction of glucosinolate (GLS) levels achieved by transcriptional regualtion. The over-expression of BnSTM also affected in vitro embryogenesis by increasing the number and quality of microspore-derived embryos (MDEs) in contrast to the MDEs down-regulating BnSTM. The MDEs with ectopic expression of BnSTM were found to regulate embryonic SAM by altering cytokinin synthesis, catabolism, perception and signaling. Taken together, these findings provide evidence for a novel function of BnSTM in promoting desirable changes in seed oil and GLS levels and enhancing in vitro embryogenesis.