Characterisation of Temperature-Sensitivity of Brassica Resistance Against Leptosphaeria maculans

Abstract

Variety resistance is an important tool in controlling against plant pathogens. As temperatures increase due to global warming, temperature-resilient resistance may play an important role in crop protection. However, the mechanisms behind temperature-sensitivity of the resistance response is poorly understood in crop species, and most of our knowledge comes from studies on the model plant Arabidopsis thaliana. This project aims to further understand the temperature-sensitivity of Brassica napus resistance against Leptosphaeria maculans. Field and controlled environment experiments were done to investigate the effects of cultivar R gene-mediated and quantitative resistance and weather on the severity of phoma stem canker. Higher maximum June temperatures were found to be related to phoma stem canker severity; this effect was reduced in cultivars with good quantitative resistance. The hypothesis that R genes operate in the stems of adult winter oilseed rape plants, providing some protection against L. maculans was supported by a controlled environment stem inoculation assay. A constantly elevated temperature of 25°C appeared to reduce this effect. Suppressor of NPR1, constitutive (BrSNC1) (an ortholog of an R-gene that mediates high temperature inhibition of resistance in A. thaliana), and FocBr1, which confers resistance to Fusarium oxysporum in Brassica rapa, were investigated for their roles in the temperature-sensitivity of resistance in B. rapa. TILLING mutations in the genes showed phenotypic differences in L. maculans inoculation assays at 20oC and 25oC. FocBr1-1 mutants displayed higher susceptibility at 20oC compared to the wild-type, whereas Brsnc1 mutants showed what is hypothesised to be an autoimmune response at 20oC and 25oC. It is postulated that, analogous to the mechanism in A. thaliana, BrSNC1 is a temperature-sensitive component in the B. napus resistance response. B. napus R-gene introgression lines Topas-Rlm7 and Topas-Rlm4 were found to be temperature-sensitive and -resilient, respectively, from cotyledon inoculation assays at 20°C and 25°C. Differences in defence related gene expression, including pathogenesis-related protein 1 (PR1) (a salicylic acid inducible marker gene) were explored using qPCR and RNA-seq. Lower levels of PR1 expression were found in Topas-Rlm7 lines compared to the temperature-resilient Topas-Rlm4 line at 25°C. Further identification of other genes expressed differentially in Topas-Rlm4 compared to Topas-Rlm7 may provide useful molecular markers that could be used by breeders to guide their selection of B. napus cultivars with temperature-resilient resistance to L. maculans. These three studies provide greater understanding of different aspects of Brassica defence at elevated temperatures; specifically in how quantitative resistance is affected by high temperatures, the roles of two specific defence related genes BrSNC1 and FocBr1 and gene expression differences in temperature -sensitive and -resilient R genes. These findings provide valuable information that increases the scope for developing oilseed crops better suited to a warming climate due to greater temperature-resilient resistance to phoma stem.