Effect of volatiles derived from Brassica plants on the growth of Sclerotinia sclerotiorum

Abstract

Secondary metabolites play an important role in plant resistance mechanisms against fungal pathogens. Glucosinolate procures of isothiocyanates exist in many species of plant families and they are specific to Brassicaceae. Release of toxic volatile compounds (isothiocyanates) by the hydrolysis of glucosinolates catalysed by myrosinase enzyme was studied on Sclerotinia sclerotiorum, the causal agent of oilseed rape stem rot. The fungal colonies were exposed to hydrolysed freeze dried powder of Brassica species including: Brassica juncea var. Cutlass, Brassica rapa varieties Parkland and Echo and Brassica napus varieties Hyola401 and RGS003. The different shoot parts of the plants, including seeds, leaves, stems, petioles and roots, as sources of glucosinolates, demonstrated significant differences in the production of toxic compounds affecting the mycelial growth of S. sclerotiorum in vitro. These inhibitions suggest that the glucosinolate–myrosinase system in Brassica species is activated and produce toxic volatiles (isothiocyanates) with different potentials. Maximum inhibition was related to B. juncea var. Cutlass. To determine the volatile compounds present in Brassica plants, the powder of their shoot parts was used for GC-MS experiments and available isothiocyanates were monitored. The results of the chromatographic studies on the seeds of B juncea var. Cutlass showed the highest peak, corresponding to the 1-propene, 3-isothiocyanates, which is Allyl ITC. The fresh part of leaves of this variety demonstrated the same peak as 1-propene, 3-ITC reflecting Allyl ITC as well. To study the effect of hydrolysis substrate variations, including oxalic acid (a pathogenicity factor of pathogen) concentrations and pH levels, the pathogen mycelial colonies were exposed to volatiles derived from seed meal of variety Cutlass. The released toxic volatile compounds inhibited significantly the radial growth of the fungus, whilst only acidic pH levels affected the system and vice versa. This shows that oxalic acid concentrations and pH levels, even at physiological rates of ambient concentration and pH, did not affect the GSL-M defence system during fungal activity in the host tissues.