Herbivore stress could impair the induced-defense system in plants: A case study with Brassica. sp

Background/Question/Methods

Plants produce a multitude of secondary metabolites that enhance their fitness in different environmental conditions. The production of these metabolites varies in response to environmental cues, resource availabilities and defense requirements. Although the production of chemical defenses involves high metabolic cost, under nutrient limitations a higher concentration of defense compounds in plant tissues are evolutionarily favored as those tissues are better protected from pests and pathogens. While this is particularly true with regard to carbon based defenses, few studies have evaluated the effects of nutrient deficiency in the production of defense compounds that contain nitrogen. Glucosinolates (GLS) are a major class of nitrogen containing secondary compounds produced by Brassica sp. The conversion of GLS to the more toxic isotiocyanates (ITC) is the major process contributing to the defense in brassica species. We conducted a greenhouse experiments to evaluate the production of GLS and conversion efficiency of GLS to ITC in B. juncea and B.nigra under nutrient deficiency (N, K) and methyl jasmonate application. 

Results/Conclusions

In both B. juncea and B.nigra, nitrogen deficiency and application of JA increased the production of sinigrin compared to the control. However the production of ITC with different treatments varied between B. juncea and B.nigra. In B. juncea, application of methyl jasmonate decreased ITC production under both nitrogen and potassium deficiency. In B.nigra ITC production increased in plants exposed to methyl jasmonate under potassium deficiency. The activity of myrosinase decreased under both nitrogen deficiency and methyl jasmonate application. Consequently, the conversion of GLS to ITC decreased with methyl jasmonate application in both B. juncea and B.nigra. These responses will be further discussed with relation to the catalytic efficiency of myrosinase enzyme under nutrient limitation and methyl jasmonate application. Our results reveals that stressed plants might not be well defended due to the partial conversion efficiency of GLS to ITC.