Responses of physiological indexes and leaf epicuticular waxes of Brassica napus to Sclerotinia sclerotiorum infection

Two Brassica napus genotypes with different resistances to Sclerotinia sclerotiorum were used to clarify whether or not the epicuticular wax is involved in the defence mechanism of B. napus to S. sclerotiorum. The total wax and wax constituents were significantly higher in the susceptible cultivar than in the resistant cultivar, except for esters. Infection by S. sclerotiorum increased the content of aldehydes in both cultivars, while increasing the content of alkanes and unknowns in the resistant cultivar. More crystalloid rods were observed on adaxial surfaces of leaves of the susceptible cultivar than on those of the resistant cultivar. The resistance to S. sclerotiorum was correlated to the responses of wax content but not the amount of total wax. The up-regulation of transcription of a wax-related gene and a pathogenesis-related gene (PR1) in the pathogen-challenged resistant cultivar also supported this observation. After inoculation, the increase in phenylalanine ammonia lyase (PAL) activity in the resistant cultivar and the decrease in the susceptible cultivar were correlated with their resistance to disease. The content of alkanes, alcohols, ketones and total wax were negatively correlated with the activities of PAL and polyphenoloxidase. After wax removal, the resistant cultivar developed more necrotic spots compared to seedlings with intact wax, whilst no significant change was observed for susceptible seedlings. These results show that epicuticular wax contributed more to the defence of resistant cultivars than susceptible cultivars. The leaf epicuticular wax, defence enzymes and salicylic acid-dependent signalling pathway all contribute to defence against S. sclerotiorum in B. napus