Effect of diamondback month (Plutella xylostella) feeding on glucosinolate profile and content change in two mustard (Brassica juncea) varieties

Mustard (Brassica juncea Coss.) is an original Chinese vegetable with agricultural importance in brassica family of Cruciferae. There are about 16 varieties, including B. juncea var. rugosa, B. juncea var. latipa and B. juncea var. tumida, and so on. They distribute widely in Sichuan and Zhejiang in China. Mustard is often attacked by herbivorous insects (e.g. diamondback month, Plutella xylostella) during different development stages. Glucosinolates (GSs) are believed to have multiple functions in plant-pest interactions, providing not only feeding and ovipositioning stimulants but also compounds acting as toxins and feeding deterrents to generalist insect herbivores. However, little was known about the effect of diamondback month feeding on glucosinolate profile and content in mustard.In the present study, in order to investigate the effect of GSs on plant resistance to herbivores, the larval preference of diamondback moth was studied in two mustard varieties, B. juncea var. rugosa and B. juncea var. latipa, and the changes in glucosinolate profile and content were analyzed by high performance liquid chromatography (HPLC) in diamondback moth fed leaves and mechanical wounded leaves.The uniform and healthy seedlings with 45 true leaves from the two varieties were selected and divided into two groups. One group, including the seedlings from each of the different varieties were placed in a randomly assigned position in the same diagonal grid within the cage. Six fourth instar larvae which were newly emerged and had been starved for four hours were placed in the centre of cage. The area of the damaged leaves and the number of larvae on each plant were recorded 24 hours after treatments. The seedlings of other group were cut by 1/3 of the whole leaf with scissors as mechanical wounding, and the samples were harvested 24 hours after treatment. Prior to the larvae feeding and mechanical wounding, 15-20 g of the untreated samples were taken from the seedlings of each variety as control, and all of the samples were harvested, freeze-dried and analyzed for GSs.The results showed that there was significantly heavier injury in B. juncea var. rugosa than in B. juncea var. latipa. Six and eight GSs were detected in healthy leaves of B. juncea var. latipa and B. juncea var. rugosa, respectively, and 2-propenyl GS was the major profile in two varieties, accounting for more than 92% of the total GSs content. Significantly lower total GS content was observed in healthy leaves of B. juncea var. latipa (7.21 μmol/g) than that in B. juncea var. rugosa (13.31 μmol/g). The content of total GSs in two varieties increased significantly after attracted by diamondback moth for 24 h, and the increasing extent was significantly higher in B. juncea var. latipa than in B. juncea var. rugosa, reaching 20.49 μmol/g and 21.35 μmol/g, respectively. Furthermore, the highest increase was observed in the content of 2-propenyl GS in both two varieties, and the increasing extent was also significantly higher in B. juncea var. latipa than in B. juncea var. rugosa. In addition, several glucosinolate profiles were induced after larvae feeding, an indole GS (4-hydroxyindol-3-ylmethyl GS) was induced in B. juncea var. rugosa at low level, while an aliphatic GS (10-methylsulfinyldecyl GS) and an indole GS (1-methoxy-3-indolymethyl GS) with low level were newly detected in B. juncea var. latipa. Likewise, the content of total GSs, as well as 2-propenyl GS increased significantly at similar rate between two varieties tested after mechanical wounding treatment. Furthermore, 10-methylsulfinyldecyl GS was also detected in B. juncea var. latipa at low level after mechanical wounding treatment, but 1-methoxy-3-indolymethyl GS was not detected.4- hydroxyindol-3-ylmethyl GS was not identified in B. juncea var. rugosa.In conclusion, our results indicate that the mustard variety with higher GS content is more susceptible to diamondback moth. Both diamondback moth feeding and mechanical wounding could induce the biosynthesis of glucosinolates with difference in glucosinolate profile and increasing rate. It is also suggested that 2-propenyl GS might play an important role in plant-environment interactions, and 2-propenyl GS in the resistant mustard variety seems to be more susceptible to the feeding of herbivorous insects.