Abstract
Enhanced resistance is a key strategy of controlling ‘Verticillium stem striping’ in Brassica napus caused by the soil-borne vascular pathogen Verticillium longisporum. The present study analyses the role of a broad range of components in the phenylpropanoid and salicylic acid (SA) pathways in basal and cultivar-related resistance of B. napus towards V. longisporum. A remarkable increase of susceptibility to V. longisporum in SA-deficient transgenic NahG plants indicated an essential role of SA in basal resistance of B. napus to V. longisporum. Accordingly, elevated SA levels were also found in a resistant and not in a susceptible cultivar during early asymptomatic stages of infection (7 dpi), which was associated with increased expression of PR1 and PR2. In later symptomatic stages (14 or 21 dpi), SA responses did not differ anymore between cultivars varying in resistance. In parallel, starting at 7 dpi, an overall increase in phenylpropanoid syntheses developed in the resistant cultivar, including the activity of some key enzymes, phenylalanine ammonium lyase (PAL), cinnamyl alcohol dehydrogenase (CAD) and peroxidase (POX) and the expression of key genes, PAL4, CCoAMT, CCR, POX. As a consequence, a remarkable increase in the levels of phenolic acids (t-cinnamic acid, p-coumaric acid, caffeic acid, ferulic acid, sinapic acid) occurred associated with cultivar resistance. A principal component analysis including all 27 traits studied indicated that component 1 related to SA synthesis (PR1, PR2, POX, level of free SA) and component 2 related to lignin synthesis (level of free ferulic acid, free p-coumaric acid, conjugated t-cinnamic acid) were the strongest factors to determine cultivar-related resistance. This study provides evidence that both SA and phenolic acid synthesis are important in cultivar-related resistance, however, with differential roles during asymptomatic and symptomatic stages of infection.
Highlights
Verticillium longisporum (VL) is a soil-borne vascular fungal pathogen with host specificity to Brassicaceae [1]
Except for the first week after inoculation, significantly higher levels of disease severity were recorded in V. longisporum infected
Overexpression of glucosyl and methyl transferases in A. thaliana suppressed the accumulation of salicylic acid (SA) and SA-glucoside, and the AtSGT1 and OsBSMT1 mutants became more susceptible to disease [21,22]
Summary
Verticillium longisporum (VL) is a soil-borne vascular fungal pathogen with host specificity to Brassicaceae [1]. The pathogen widely occurs in oilseed rape production regions in Europe and North. America [2,3,4,5]. Oilseed rape (Brassica napus) is the most important crop for oil production in Europe and Canada and the prevalent host of V. longisporum [6]. Due to the relatively short crop rotation and increased area of oilseed rape cultivation, incidence of ‘Verticillium stem striping’ is on the rise and threatens oilseed rape production. In the absence of their host, the melanized microsclerotia of V. longisporum remain dormant and viable in soil for several years [6,7,8].
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