Variation for myrosinase activity in some crop Brassica species

摘要

Glucosinolates are the class of sulphur containing secondary metabolites present in all cruciferous plants. These release a variety of compounds (isothiocyanates, thiocyanates and nitriles etc.) following hydrolysis by enzyme myrosinase. Some of these metabolites are defensive against insects and other attractants. The nature of breakdown products is regulated by an associated epithiospecifier protein (ESP). Studies in Arabidopsis have shown ESP/myrosinase system can be modified to increase its defense responses. In this communication, we report variation for myrosinase enzyme activity in various Brassica species. Inheritance of myrosinase activity in B. juncea was also investigated. 128 diverse Brassica accessions were assayed for myrosinase activity and glucosinolate content at various development stages. Significant genotypic variation was observed, myrosinase activity was highest in leaves, followed by developing seeds and least in the mature seeds. B. napus leaves had the maximum mean value 1.29±0.59 n mole glucose released mg-1 min-1, followed by B. juncea (0.63 ± 0.05), B. nigra (0.52 ± 0.09) and B. rapa (0.30 ±0.03). B. napus cv. ACN 50 (2.07) had the highest myrosinase activity and ACN 40 (0.68) the least. In B. juncea, it ranged from 0.08 (KH 2099) to 1.50 (ALM 129). Genotypes with appreciably low myrosinase activity were KH 2099 (0.09) of B. juncea,VKS 11/29 (0.08) of B. rapa and FRG 2 (0.17) of B. nigra. B. rapa showed strong positive correlation among myrosinase activity recorded at all the development stages. For B. nigra, it was reverse. In B. juncea and B. napus, variable results were indicated.Myrosinase activity in green tissues was negatively correlated with total glucosinolates content. It had a negative association with but 3-ethyl glucosinolate (gluconapin) and a positive correlation with 2-0H-but-3-ethyl glucosinolates (progoitrin). Two to three bands of myrosinase isoenzymes were detected after ultrascanning, when crude extract of myrosinase was subjected of PAGE.Genetic studies in B. juncea, indicated the involvement of 3-4 genes, acting in additive fashion, in myrosinase heredity.