Enhancing health-promoting isothiocyanates in Chinese kale sprouts via manipulating BoESP.

Glucosinolates (GSLs) are a group of sulfur-containing secondary metabolites, which are abundant in Brassica vegetables. GSL breakdown products (GBPs), especially isothiocyanates (ITCs) benefit human health. Chinese kale is a native Brassica vegetable in China, and its sprouts are rich in GSLs and nutritional substances. ITCs are the predominant GBPs while alternative products are formed in the presence of specifier proteins. However, fewer ITCs are formed in the sprouts. Epithiospecifier (ESP) promotes the formation of epithionitriles at the expense of ITCs in Arabidopsis, but a systematic study of different isoforms of ESPs in most vegetables is still missing. In this study, changes in the content of GBPs and the precursor GSLs, as well as thiols per plant were monitored during sprout development. The proportions of epithionitriles and ITCs in total GBPs were found to be increased and decreased, respectively. RNA-seq showed enhanced expression of numerous genes involved in GSLs biosynthesis and degradation, as well as sulfur assimilation in sprouts compared to seeds. Four copies of BoESPs were isolated and BoESP2 was the most abundant isoform. Generally, transcription of BoESPs showed a strong response to abscisic acid and gibberellin, and consequently epithionitriles increased under these treatments. Knockdown of BoESP2 expression through virus-induced gene silencing system could effectively increase total ITCs and decrease total epithionitriles. Overall, dynamic GSL metabolic flux exists in the sprouting period, and the expression of BoESPs determines the pattern of GBPs, suggesting that improving the health-promoting ITCs in Chinese kale sprouts through manipulating BoESPs by metabolic engineering is feasible.

WRKY33-mediated indolic glucosinolate metabolic pathway confers resistance against Alternaria brassicicola in Arabidopsis and Brassica crops.

The tryptophan (Trp)-derived plant secondary metabolites, including camalexin, 4-hydroxy-indole-3-carbonylnitrile, and indolic glucosinolate (IGS), show broad-spectrum antifungal activity. However, the distinct regulations of these metabolic pathways among different plant species in response to fungus infection are rarely studied. In this study, our results revealed that WRKY33 directly regulates IGS biosynthesis, notably the production of 4-methoxyindole-3-ylmethyl glucosinolate (4MI3G), conferring resistance to Alternaria brassicicola, an important pathogen which causes black spot in Brassica crops. WRKY33 directly activates the expression of CYP81F2, IGMT1, and IGMT2 to drive side-chain modification of indole-3-ylmethyl glucosinolate (I3G) to 4MI3G, in both Arabidopsis and Chinese kale (Brassica oleracea var. alboglabra Bailey). However, Chinese kale showed a more severe symptom than Arabidopsis when infected by Alternaria brassicicola. Comparative analyses of the origin and evolution of Trp metabolism indicate that the loss of camalexin biosynthesis in Brassica crops during evolution might attenuate the resistance of crops to Alternaria brassicicola. As a result, the IGS metabolic pathway mediated by WRKY33 becomes essential for Chinese kale to deter Alternaria brassicicola. Our results highlight the differential regulation of Trp-derived camalexin and IGS biosynthetic pathways in plant immunity between Arabidopsis and Brassica crops.

Involvement of glucosinolates in the resistance to zinc oxide nanoparticle-induced toxicity and growth inhibition in Arabidopsis.

Zinc oxide nanoparticles (ZnO NPs) are widely used to manufacture textile fibers, synthetic rubber, and paint. However, crop yields and quality are threatened by the increased use of metallic NPs in industry, which has resulted in their accumulation in agricultural land. Many studies have shown that plants defend against biotic and abiotic stresses through the activities of metabolites and hormones. However, whether glucosinolates (GSs) are involved in plant responses to ZnO NP-related stress remains unknown. In this study, wild-type (WT) and GS mutant (myb28/29 and cyp79B2/B3) Arabidopsis plants were subjected to ZnO NP stress to address this question. Our results showed that exposure to ZnO NPs promoted GS accumulation and induced the relative messenger RNA (mRNA) expression levels of GS biosynthesis-related genes. Moreover, ZnO NP treatment adversely affected root length, the number of lateral roots, chlorophyll contents, and plant biomass. Importantly, our results showed that root growth, chlorophyll contents, and plant biomass were all decreased in the GS mutants compared with those in WT plants. Overall, our results showed that WT plants tolerated ZnO NP-induced stress more efficiently than the GS mutants, suggesting that GSs are involved in plant resistance to ZnO NP-induced toxicity.

The flavor of Chinese kale sprouts is affected by genotypic variation of glucosinolates and their breakdown products.

Metabolic profiling of glucosinolates and their breakdown products in sprouts of 22 Chinese kale (Brassica oleracea var. alboglabra, BOA) varieties were investigated by using high-performance liquid chromatography (HPLC) and gas chromatography-mass spectrometry (GC-MS). Relationships between glucosinolate metabolites and flavor of Chinese kale sprouts were also analyzed. Results showed that compositions and contents of both glucosinolates and their breakdown products varied greatly among different varieties of Chinese kale sprouts. Gluconapin and 4,5-Epithio-pentanenitrile were the dominant glucosinolate and glucosinolate breakdown product in Chinese kale sprouts, respectively. Gluconapin and glucobrassicin were significantly related to bitterness (r = 0.577, 0.648, respectively; p < 0.05). BOA 1 and BOA 13, BOA 3 and BOA 10 are good candidates for future breeding programs since the former two varieties have light bitterness and pungency, and the latter two varieties contain high levels of glucosinolate breakdown products such as isothiocyanates and epithionitriles in sprouts.