Plasmodiophora brassicae (P. brassicae) is a soil-born pathogen worldwide and can infect most cruciferous plants, which causes great yield decline and economic losses. It is not well known how microbial diversity and community composition change during P. brassicae infecting plant roots. Here, we employed a resistant and a susceptible pakchoi cultivar with and without inoculation with P. brassicae to analyze bacterial and fungal diversity using 16S rRNA V3-V4 and ITS_V1 regions, respectively. 16S rRNA V3-V4 and ITS_V1 regions were amplified and sequenced separately. Results revealed that both fungal and bacterial diversity increased, and composition was changed in the rhizosphere soil of the susceptible pakchoi compared with the resistant cultivar. In the four groups of R_mock, S_mock, R_10d, and S_10d, the most relatively abundant bacterium and fungus was Proteobacteria, accounting for 61.92%, 58.17%, 48.64%, and 50.00%, respectively, and Ascomycota, accounting for 75.11%, 63.69%, 72.10%, and 90.31%, respectively. A total of 9488 and 11,914 bacteria were observed uniquely in the rhizosphere soil of resistant and susceptible pakchoi, respectively, while only 80 and 103 fungi were observed uniquely in the correlated soil. LefSe analysis showed that 107 and 49 differentially abundant taxa were observed in bacteria and fungi. Overall, we concluded that different pakchoi cultivars affect microbial diversity and community composition, and microorganisms prefer to gather around the rhizosphere of susceptible pakchoi. These findings provide a new insight into plant–microorganism interactions.

Downy mildew caused by the obligate parasite Hyaloperonospora brassicae is a devastating disease for Brassica species. Infection of Hyaloperonospora brassicae often leads to yellow spots on leaves, which significantly impacts quality and yield of pakchoi. In the present study, we conducted a comparative transcriptome between the resistant and susceptible pakchoi cultivars in response to Hyaloperonospora brassicae infection. A total of 1073 disease-resistance-related differentially expressed genes were identified using a Venn diagram. The Gene Ontology and Kyoto Encyclopedia of Genes and Genomes pathway analyses revealed that these genes were mainly involved in plant−pathogen interaction, plant hormone signal transduction, and other photosynthesis-related metabolic processes. Analysis of the phytohormone content revealed that salicylic acid increased significantly in the resistant material after inoculation with Hyaloperonospora brassicae, whereas the contents of jasmonic acid, abscisic acid, and 1-aminocyclopropane-1-carboxylic acid decreased. Exogenous salicylic acid treatment also significantly upregulated Hyaloperonospora brassicae-induced genes, which further confirmed a crucial role of salicylic acid during pakchoi defense against Hyaloperonospora brassicae. Based on these findings, we suggest that the salicylic-acid-mediated signal transduction contributes to the resistance of pakchoi to downy mildew, and PAL1, ICS1, NPR1, PR1, PR5, WRKY70, WRKY33, CML43, CNGC9, and CDPK15 were involved in this responsive process. Our findings evidently contribute to revealing the molecular mechanism of pakchoi defense against Hyaloperonospora brassicae.

Hydroponics is currently one of the primary methods for soilless cultivation. Although the phenotype and quality of vegetables differ between hydroponic and soil-based systems, limited research has been conducted on the selection and breeding of pakchoi cultivars specifically suited for hydroponics. In this study, principal component analysis (PCA) and cluster analysis were performed on the commercial traits, agronomic characteristics, and nutritional quality of 20 pakchoi parental lines grown under hydroponic conditions to classify and screen suitable germplasm for breeding. PCA reduced the 11 agronomic traits into two independent principal components, accounting for a cumulative contribution of 79.22%. Cluster analysis grouped the 20 parental lines into four categories based on the composite scores of agronomic traits and nutritional quality. Group 3 was selected for breeding programs aiming to develop high-yielding cultivars with a desirable morphology. For breeding targets emphasizing darker leaves and petiole coloration, Group 4 presented the most suitable germplasm. Group 1 was ideal for enhancing nutritional quality by offering parent lines rich in calcium, magnesium, vitamin C, and amino acids. Alternatively, Group 2 contained lines with high levels of soluble proteins, amino acids, and soluble sugars.

Hypocotyl length is a critical determinant for the efficiency of mechanical harvesting in pakchoi production, but the knowledge on the molecular regulation of hypocotyl growth is very limited. Here, we report a spontaneous mutant of pakchoi, lhy7.1, and identified its characteristics. We found that it has an elongated hypocotyl phenotype compared to the wild type caused by the longitudinal growth of hypocotyl cells. Different light quality treatments, transcriptome, and proteomic analyses were performed to reveal the molecular mechanisms of hypocotyl elongation. The data showed that the hypocotyl length of lhy7.1 was significantly longer than that of WT under red, blue, and white lights but there was no significant difference under dark conditions. Furthermore, we used transcriptome and label-free proteome analyses to investigate differences in gene and protein expression levels between lhy7.1 and WT. At the transcript level, 4568 differentially expressed genes (DEGs) were identified, which were mainly enriched in “plant hormone signal transduction”, “photosynthesis”, “photosynthesis–antenna proteins”, and “carbon fixation in photosynthetic organisms” pathways. At the protein level, 1007 differentially expressed proteins (DEPs) were identified and were mainly enriched in photosynthesis-related pathways. The comprehensive transcriptome and proteome analyses revealed a regulatory network of hypocotyl elongation involving plant hormone signal transduction and photosynthesis-related pathways. The findings of this study help elucidate the regulatory mechanisms of hypocotyl elongation in lhy7.1.

Clubroot disease is a kind of soil-borne disease that seriously infects Brassica species. In this study, we collected 121 varieties of non-heading Chinese cabbages. In order to better understand the genetic variation and to screen suitable clubroot disease-resistant parental material, we re-sequenced them to examine the population genetic structure, population genetic diversity, population differentiation index, and selective sweep based on SNPs. The mapping rate with the reference genome was high, and data quality analysis revealed that the sequencing quality was good. The annotated data indicated that intronic and intergenic areas held the majority of SNPs and indels. Four subgroups of 121 non-heading Chinese cabbages were identified using principal component analysis, phylogenetic tree, and genetic structure analysis. An examination of genetic diversity revealed that while selfing may happen in subgroups C and D, heterozygosity may exist in subgroups A and B. In subgroup B, self-fertilization is not possible. There was a moderate degree of genetic differentiation between subgroups B and C (Fst = 0.0744347). For genes in certain sweep regions, we also ran GO enrichment and KEGG enrichment analysis. Two disease resistance-related genes, BraA01g042910.3.5C and BraA06g019360.3.5C, were examined. These findings will serve as a theoretical foundation for developing novel, clubroot disease-resistant types of non-heading Chinese cabbages.

Pakchoi is a kind of nonheading Chinese cabbage being widely cultivated not only in China but also all over Asia. High temperature is a major limiting factor influencing the yield and quality of pakchoi, while the mechanism of pakchoi dealing with high-temperature challenges remains largely elusive. In the present study, we conducted a comparative transcriptomic analysis, which was also validated by qPCR, of the heat-tolerant Xinxiaqing (XXQ) variant and Suzhouqing (SZQ) variant, which are heat-sensitive under high-temperature treatment. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses suggest that high-temperature-induced phytohormones signal transduction, especially auxin signal transduction, regulates the heat responses of pakchoi. Our further investigations imply that high-temperature-activated auxin signal plays a positive role in helping pakchoi deal with high-temperature challenge; IAA-pretreated pakchoi plants exhibited greater resistance to the high-temperature treatment, probably due to the induction of antioxidant activity. In addition, our study also identified six heat shock proteins/factors (HSPs/HSFs) whose up-regulation correlates with the elevated heat tolerance of pakchoi. Notably, among these high-temperature-induced heat-responsive factors, HSP20 and HSP26.5 are under the regulation of auxin signal, and this signal cascade contributes to enhancing the thermostability of pakchoi. In the present study, we identified crucial high-temperature-responsive factors and signaling pathways in pakchoi, which help in understanding the mechanism of pakchoi coping with high-temperature challenge.