Characterization of systemic acquired resistance in Brassica napus

Abstract

Plants activate an array of defense mechanisms upon pathogen attack. Systemic acquired resistance (SAR) is an induced disease resistance phenomenon deployed after infection by a necrogenic pathogen and is dependent on endogenous accumulation of salicylic acid. The objectives of my research were to characterize SAR in the crop plant, Brassica napus (canola), and study the effects of overexpressing genes involved in SAR on disease resistance. Biological induction of SAR using necrogenic Pseudomonas syringae and chemical induction using benzo (1,2,3) thiadiazole-7-carbothionic acid reduced growth of the bacterial pathogen P. syringae and the fungal pathogen Leptosphaeria maculans. This growth reduction was associated with an increase in transcript levels of pathogenesis-related (PR) genes, one of the characteristic features of SAR. Transgenic plants expressing a bacterial salicylate hydroxylase gene (NahG), were more susceptible to the above pathogens and were delayed in accumulating PR gene transcripts, indicating a need for SA accumulation for SAR in B. napus. Expression of two SAR genes from Arabidopsis, DEFECTIVE IN INDUCED RESISTANCE 1 (DIR1) and NON EXPRESSOR OF PATHOGENESIS-RELATED 1 (NPR1), in B. napus enhanced resistance against virulent P. syringae without SAR pre-treatments. Putative orthologs of DIR1 and NPR1 (BnDIR1 and BnNPR1) were isolated from B. napus based on EST sequences. BnDIR1 and BnNPR1 display 71% and 66% amino acid sequence similarities, respectively, to the corresponding Arabidopsis proteins. Expression of BnNPR1 in Arabidopsis npr1 mutant backgrounds indicated that it was able to functionally complement these mutations. Expression of BnDIR1 enhanced disease resistance in both Arabidopsis wild-type and dir1-1 mutant backgrounds. Expression of DIR1, NPR1, BnDIR1 and BnNPR1, separately, in B. napus plants enhanced resistance against P. syringae. SAR pre-treatments further enhanced resistance of transgenic B. napus plants expressing DIR1 and BnDIR1 to P. syringae, indicating an additive effect. Expression of DIR1 in B. napus did not provide resistance against L. maculans. These results provide the first in-depth molecular characterization of SAR in B. napus, and in particular, provide new insight into DIR1 function not previously reported in Arabidopsis.