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Quantitative resistance (QR) to Leptosphaeria maculans in Brassica napus has proven to be very useful for breeding resistant varieties with increased potential for durability. To date, however, QR has been assessed primarily in the field after a long plant developmental cycle and its expression is influenced by environmental conditions. An evaluation method in controlled conditions would offer the possibility to evaluate the level of QR in a shorter time and to study the kinetics of fungal growth in the stem in relation to susceptibility/resistance levels conferred by quantitative trait loci (QTL). We used a cut-petiole inoculation method coupled with both visual and PCR-based phenotyping at different time-points after inoculation, which revealed different kinetics of symptom development and pathogen growth among varieties. The results obtained with this method were partly consistent with field resistance phenotyping data, and allowed detection of the effect of at least two resistance QTL alleles introduced in near-isogenic lines. This method would be useful to compare a wider range of host varieties, to study the spectrum effect of resistance QTL or to decipher the mechanisms underlying this quantitative resistance. [ABSTRACT FROM AUTHOR]

Camalexin has been reported to play defensive functions against several pathogens in Arabidopsis. In this study, we investigated the possible role of camalexin accumulation in two Arabidopsis genotypes with different levels of basal resistance to the compatible eH strain of the clubroot agent Plasmodiophora brassicae. Camalexin biosynthesis was induced in infected roots of both Col-0 (susceptible) and Bur-0 (partially resistant) accessions during the secondary phase of infection. However, the level of accumulation was four-to-seven times higher in Bur-0 than Col-0. This was associated with the enhanced transcription of a set of camalexin biosynthetic P450 genes in Bur-0: CYP71A13, CYP71A12, and CYP79B2. This induction correlated with slower P. brassicae growth in Bur-0 compared to Col-0, thus suggesting a relationship between the levels of camalexin biosynthesis and the different levels of resistance. Clubroot-triggered biosynthesis of camalexin may also participate in basal defense in Col-0, as gall symptoms and pathogen development were enhanced in the pad3 mutant (Col-0 genetic background), which is defective in camalexin biosynthesis. Clubroot and camalexin responses were then studied in Heterogeneous Inbred Families (HIF) lines derived from a cross between Bur-0 and Col-0. The Bur/Col allelic substitution in the region of the previously identified clubroot resistance QTL PbAt5.2 (Chromosome 5) was associated with both the enhanced clubroot-triggered induction of camalexin biosynthesis and the reduced P. brassicae development. Altogether, our results suggest that high levels of clubroot-triggered camalexin biosynthesis play a role in the quantitative control of partial resistance of Arabidopsis to clubroot. [ABSTRACT FROM AUTHOR]