Leaf Mustard (Brassica juncea) Germplasm Resources Showed Diverse Characteristics in Agro-Morphological Traits and Glucosinolate Levels

Leaf mustard, characterized by its purple/red/green leaves with a green/white midrib, is known for its thick, tender, and spicy leaves with a unique taste and flavor. There were only a few studies reported on leaf mustard for its morphological and biochemical traits from Korea. A total of 355 leaf mustard accessions stored at the GenBank of the National Agrobiodiversity Center were evaluated for 25 agro-morphological traits and seven intact glucosinolates (GSLs). The accessions showed a wide variation in terms of most of the traits. The quantitative agro-morphological traits varied from 16.0 (leaf length) to 48.7% (petiole width) of the coefficient of variation (CV). The highest variation was observed in glucoiberin (299.5%, CV), while the total GSL showed a CV of 66.1%. Sinigrin, followed by gluconapin and gluconasturtiin, was the most abundant GSL, accounting for as high as 75% of the total GSLs, while glucobrassicanapin and glucoiberin were the least abundant, contributing 0.7% and 0.1% on average, respectively. Sinigrin had a positive significant correlation with all GSLs but gluconasturtiin, while glucobarbarin and gluconasturtiin were highly positively correlated to each other, but least correlated with other GSLs. The leaf length was negatively correlated with sinigrin and glucoiberin. The width of the petiole showed a positive correlation with gluconapin, glucobrassicanapin, and glucobrassicin, while the length of the petiole had a negative correlation with sinigrin, glucobrassicanapin, glucoiberin, glucobrassicin, and the total GSLs. A higher width of the midrib was associated with higher contents of gluconapin, glucobrassicanapin, and glucobrassicin. A PCA analysis based on the agro-morphological traits showed that the first and second principal components accounted for 65.2% of the overall variability. Accessions that form a head tend to exhibit a longer leaf length, a larger plant weight, a thicker midrib, and higher widths of the midrib, petiole, and leaf. The GSLs showed inconsistent inter-and intra-leaf variation. Accessions that identified for various traits in their performance, such as, for example, Yeosu66 and IT259487 (highest total glucosinolates) and IT228984 (highest plant weight), would be promising lines for developing new varieties.

First report of Pectobacterium aroidearum causing soft rot on leaf mustard in China

Leaf mustard (Brassica juncea [L.] Czern. et Coss.) is widely planted in China as an important leaf vegetable. In March 2022, water-soaked and mushy rot symptoms were observed on leaf mustard plants in the field of Zhaotong (27.85°N; 105.05°E), Yunnan, China. The incidence of symptomatic leaf mustard was approximately 10%. The isolation of the causal agent followed the method of Peng et al. (2023). Briefly, infected tissues from four diseased plants were mixed and teased apart, and homogenized by vortex shaking. The bacterial suspension was diluted and spread on nutrient agar (NA). About 10 single colonies exhibiting different colony morphologies were picked and purified separately by successive streaking. A pinprick method was used for pathogenicity tests with an inoculum concentration of 108 CFU/ml (Singh et al. 2013). Among 10 isolates, only strain YKX exhibited soft rot symptoms on detached mustard leaves. In addition, ten two-month-old leaf mustard plants grown in the greenhouse were used for in vivo pathogenicity tests. Briefly, sterilized pins were dipped in the bacterial suspension, and then leaf mustard petioles were pricked with these pins. After inoculation, each plant was kept in a plastic bag for 12 hours to maintain high humidity. As expected, strain YKX caused obvious rot symptoms on eight plants at 1-2 days post-inoculation while the control group including two plants treated with sterile water showed no symptoms. The colonies of strain YKX on NA were white, roughly circular, and convex. For a preliminary identification, total DNA was extracted and used as the template in PCR amplification of 16S rDNA with the universal PCR primer pair 27F/1492R (Weisburg et al. 1991). The quality-filtered DNA sequence (871 bp) showed 100% query coverage and 99.47% identity to the 16S rDNA sequences of type strain Pectobacterium aroidearum SCRI 109T (GenBank: NR_159926) found in the NCBI rRNA/ITS database. Whole-genome sequencing of strain YKX was then performed using the Illumina and Nanopore sequencing platforms by Tsingke Biotechnology Co., Ltd. (Beijing, China). A single contig (GenBank: CP129239) with a length of approximately 4.9 Mb was obtained by de novo hybrid assembly using Unicycler v0.5.0 (Wick et al. 2017). The quality of the genomic data was evaluated by BUSCO v5.4.7 (Manni et al. 2021) against the gammaproteobacteria_odb10 dataset. A BUSCO complete score of 99.5% indicated high assembly quality. The genome sequence of strain YKX was uploaded to the Type Strain Genome Server for a genome-based taxonomic analysis (Meier-Kolthoff et al. 2022). The distance-based phylogeny showed that strain YKX and P. aroidearum L6 (GenBank: CP065044) and P. aroidearum PC1 (GenBank: NC_012917) form a clade. When comparing strain YKX with L6 or PC1, the digital DNA-DNA hybridization value (83.5-83.8%) was above the species delineation threshold (70% for DDH), clearly indicating that strain YKX should be classified as P. aroidearum. Additionally, P. aroidearum was reisolated from inoculated leaves and identified based on morphological similarities and 16S rDNA sequencing, thus fulfilling Koch's postulates. It is worth noting that a previous study reported occurrences of soft rot disease on leaf mustard attributed to Rhizopus microsporus var. chinensis (Wang et al. 2020). To our knowledge, this is the first report of P. aroidearum causing soft rot on leaf mustard in China, which expands the known host range of this pathogen and benefits the control of this disease.

multiceps) under natural and inoculated fermentation

Due to the uncontrolled fermentation process and unstable quality of naturally fermented leaf mustard, inoculated fermentation is receiving more attention. Here, the physicochemical properties, volatile compounds, and microbial community in leaf mustard under natural fermentation (NF) and inoculated fermentation (IF) were analyzed and compared. The contents of total acid, crude fiber, and nitrite of leaf mustard were measured. Headspace-solid phase microextraction-gas chromatography-mass spectrometry and orthogonal projection on latent structure-discriminant analysis were used to analyze the differences of volatile compounds in NF and IF leaf mustard. Moreover, Illumina MiSeq high-throughput sequencing technology was employed to reveal the composition of microbiota. The results showed that the nitrite content in leaf mustard after IF (3.69 mg/kg) was significantly lower than that after NF (4.43 mg/kg). A total of 31 and 25 kinds of volatile components were identified in IF and NF, respectively. Among the detected compounds, 11 compounds caused the differences between IF and NF leaf mustard. The results of inter-group difference analysis showed that there were significant differences in fungal flora between IF and NF samples. Saccharomycetes, Kazachstania, and Ascomycota were the landmark microorganisms in IF leaf mustard and the landmark microorganisms in NF were Mortierellomycota, Sordariomycetes, and Eurotiomycetes. The abundance of probiotics (such as Lactobacillus) in IF leaf mustard (51.22%) was higher than that in NF (35.20%) and the abundance of harmful molds (such as Mortierella and Aspergillus) was opposite. Therefore, IF leaf mustard showed the potential to reduce the content of nitrite and harmful molds and increase the beneficial volatile compounds and probiotics. PRACTICAL APPLICATION: Leaf mustard of inoculated fermentation (IF) showed better fermented characteristics than natural fermentation in terms of lower nitrite content, greater beneficial volatile substances, and better potential for increasing probiotics and reducing harmful molds. These results provided a theoretical basis for IF leaf mustard and contributed to the industrial production of fermented leaf mustard.