Identification of MAM1s in Regulation of 3C Glucosinolates Accumulation in Allopolyploid Brassica juncea

Allopolyploid Brassica juncea is particularly enriched in sinigrin,a kind of 3C aliphatic glucosinolates(GSLs),giving rise to characteristic taste after picking.However,the molecular mechanism underlying 3C aliphatic GSLs biosynthesis in this species remains unknown.In this study,we genome-widely identified GSLs metabolic genes,indicating different evolutionary rate of GSLs metabolic genes between subgenomes of B.juncea.Eight methythioalkylmalate synthase(MAMs)homologs were identified from B.juncea,in which six MAM1s were located in chloroplast and the other two were not detected with any expression.Furthermore,BjMAM1-4,BjMAM1-5,and BjMAM1-6 displayed higher expression levels in leaves than other tissues.Silenced expression analysis revealed that BjMAM1-4 and BjMAM1-6 function in 3C and 4C aliphatic GSLs accumulation.The specificity of the substrate selection for the second cycle reaction is much lower than that of the first cycle,suggesting these genes may preferentially catalyze 3C aliphatic GSLs biosynthesis.Our study provides insights into the molecular mechanism underlying the accumulation of 3C aliphatic GSLs,thereby facilitating the manipulation of aliphatic GSLs content in B.juncea.

Effect of CO_2 enrichment on the glucosinolate contents under different nitrogen levels in bolting stem of Chinese kale (Brassica alboglabra L.)

The effects of CO2 enrichment on the growth and glueosinolate （GS） concentrations in the bolting stem of Chinese kale （Brassica alboglabra L.） treated with three nitrogen （N） concentrations （5, 10, and 20 mmol/L） were investigated. Height, stem thickness, and dry weights of the total aerial parts, bolting stems, and roots, as well as the root to shoot ratio, significantly increased as CO2 concentration was elevated from 350 to 800 μl/L at each N concentration. In the edible part of the bolting stem, 11 individual GSs were identified, including 7 aliphatic and 4 indolyl GSs. GS concentration was affected by the elevated CO2 concentration, N concentration, and CO2×N interaction. At 5 and 10 mmol N/L, the concentrations of aliphatic GSs and total GSs significantly increased, whereas those ofindolyl GSs were not affected, by elevated atmospheric CO2. However, at 20 mmol N/L, elevated CO2 had no significant effects on the concentrations of total GSs and total indolyl GSs, but the concentrations of total aliphatic GSs significantly increased. Moreover, the bolting stem carbon （C） content increased, whereas the N and sulfur （S） contents decreased under elevated CO2 concentration in the three N treatments, resulting in changes in the C/N and N/S ratios. Also the C/N ratio is not a reliable predictor of change of GS concentration, while the changes in N and S contents and the N/S ratio at the elevated CO2 concentration may influence the GS concentration in Chinese kale bolting stems. The results demonstrate that high nitrogen supply is beneficial for the growth of Chinese kale, but not for the GS concentration in bolting stems, under elevated CO2 condition.

Promotion of Arabidopsis immune responses by a rhizosphere fungus via supply of pipecolic acid to plants and selective augment of phytoalexins

The ascomycete insect pathogenic fungi such as Metarhizium species have been demonstrated with the abilities to form the rhizosphere or endophytic relationships with different plants for nutrient exchanges.In this study,after the evident infeasibility of bacterial disease development in the boxed sterile soils,we established a hydroponic system for the gnotobiotic growth of Arabidopsis thaliana with the wild-type and transgenic strain of Metarhizium robertsii.The transgenic fungus could produce a high amount of pipecolic acid(PIP),a pivotal plant-immune-stimulating metabolite.Fungal inoculation experiments showed that M.robertsii could form a non-selective rhizosphere relationship with Arabidopsis.Similar to the PIP uptake by plants after exogenous application,PIP level increased in Col-0 and could be detected in the PIP-non-producing Arabidopsis mutant(ald1)after fungal inoculations,indicating that plants can absorb the PIP produced by fungi.The transgenic fungal strain had a better efficacy than the wild type to defend plants against the bacterial pathogen and aphid attacks.Contrary to ald1,fmo1 plants could not be boosted to resist bacterial infection after treatments.After fungal inoculations,the phytoalexins camalexin and aliphatic glucosinolate were selectively increased in Arabidopsis via both PIP-dependent and-independent ways.This study unveils the potential mechanism of the fungus-mediated beneficial promotion of plant immunity against biological stresses.The data also highlight the added values of M.robertsii to plants beyond the direct suppression of insect pest populations.