High temperatures have a serious impact on the quality and yield of cold-loving Chinese cabbage, which has evolved to have a unique set of stress mechanisms. To explore the relationship between these mechanisms and the heat-tolerance of Chinese cabbage, the physiological indicators of the heat-tolerant ‘268’ line and heat-sensitive ‘334’ line were measured. Under heat stress, the proline (Pro), soluble sugar (SS), and superoxide dismutase (SOD) indexes of the ‘268’ line increased significantly. When additionally using transcriptome analysis, we found that the identified 3,360 DEGs were abundantly enriched in many metabolic pathways including ‘plant hormone signal transduction’, ‘carbon metabolism’, and ‘glycolysis/gluconeogenesis’. Dynamic gene expression patterns showed that HKL1 in Cluster 15 may be a key factor in the regulation of sugar homeostasis. The interaction network screened four ABA-related genes in Cluster 15, suggesting that high temperatures lead to changes in hormonal signaling, especially an increase in ABA signaling. Compared with the ‘334’ line, the expressions of Prx50, Prx52, Prx54, SOD1, and SOD2 in the ‘268’ line were significantly upregulated, and these genes were actively involved in the reactive oxygen species (ROS) scavenging process. In summary, our results revealed the relationship between plant heat tolerance, physiology, and biochemistry and may also provide ideas for the future development of high-quality and heat-tolerant Chinese cabbage germplasm resources.

In order to make the identification and utilization of nutritional quality components in Chinese cabbage more predictive, to obtain ideal raw materials, and to help screen functional Chinese cabbage varieties that have high nutritional value, we conducted a comprehensive evaluation of the nutritional quality of different Chinese cabbage varieties. In this study, 17 nutritional quality indexes of 35 Chinese cabbage varieties, including crude fiber (CF), crude protein (CP), vitamin C (VC), glucose (Glc), fructose (Fru), malic acid (MA), citric acid (CA), oxalic acid (OA), total amino acid (TAA) and CA, K, Mg, P, Cu, Fe, Mn and Zn, were analyzed using diversity analysis, correlation analysis, principal component analysis, membership function analysis and cluster analysis. The results showed that there were different degrees of variation in the 17 nutritional quality indexes, and the coefficients of variation ranged from 11.45% to 91.47%. The correlation analysis found that there were significant or extremely significant correlations between different nutrient elements of Chinese cabbage, which indicated that principal component analysis could be carried out, and the comprehensive score (D value) of different materials could be obtained using principal component analysis and the membership function method. The nutritional quality of Chinese cabbage was classified into five grades by cluster analysis. Finally, a mathematical model for evaluating the nutritional quality of Chinese cabbage was established using the D value and multiple stepwise regression methods, and 10 key indexes were selected from the 17 indexes, which could be used for the rapid identification of the nutritional quality of Chinese cabbage. This study provided a theoretical basis for the nutritional quality evaluation and variety breeding of Chinese cabbage.

Introgression breeding is a widely used method for the genetic improvement of crop plants; however, the mechanism underlying candidate gene flow patterns during hybridization is poorly understood. In this study, we used a powerful pipeline to investigate a Chinese cabbage (Brassica rapa L. ssp. pekinensis) introgression line with the anthocyanin overaccumulation phenotype. Our purpose was to analyze the gene flow patterns during hybridization and elucidate the genetic factors responsible for the accumulation of this important pigment compound. We performed RNA-seq analysis by using two pipelines, one with and one without a reference sequence, to obtain transcriptome data. We identified 930 significantly differentially expressed genes (DEGs) between the purple-leaf introgression line and B. rapa green cultivar, namely, 389 up-regulated and 541 down-regulated DEGs that mapped to the B. rapa reference genome. Since only one anthocyanin pathway regulatory gene was identified, i.e., Bra037887 (bHLH), we mined unmapped reads, revealing 2,031 de novo assembled unigenes, including c3563g1i2. Phylogenetic analysis suggested that c3563g1i2, which was transferred from the Brassica B genome of the donor parental line Brassica juncea, may represent an R2R3-MYB transcription factor that participates in the ternary transcriptional activation complex responsible for the anthocyanin overaccumulation phenotype of the B. rapa introgression line. We also identified genes involved in cold and light reaction pathways that were highly upregulated in the introgression line, as confirmed using quantitative real-time PCR analysis. The results of this study shed light on the mechanisms underlying the purple leaf trait in Brassica plants and may facilitate the use of introgressive hybridization for many traits of interest.

Wucai is a biannual leafy vegetable that is more sensitive to cold than most Brassicaceae vegetables. Here, phenotypic differences in wucai were investigated to identify those genotypes with high cold tolerance at low temperature, which could allow their cultivation in cold climates. In all, on the basis of PIabs, 20 wucai genotypes (10 lower PIabs and 10 higher PIabs) were selected from a collection of 124 genotypes. Both W16-13 and SW-3 (higher PIabs) and the LS-6 and W15-16 (lower PIabs) were screened from the 20 wucai genotypes according to several key plant physiological traits: net photosynthetic rate, electrolyte leakage, chilling injury index, specific leaf area, malondialdehyde contents, and PIabs. To further verify the reliability of PIabs, four genotypes were grown under low temperature gradient conditions and their morphological indexes, chlorophyll content, total antioxidant capacity, fluorescence transients (OJIP transients), leaf cell ultrastructure, and gene expression were measured. These indicators strongly demonstrated that the W16-13 and SW-3, which had higher PIabs, possessed higher resistance to cold stress, while both LS-6 and W15-16 were sensitive to cold. As this study shows, the easily measured performance index, PIabs, could be applied to wucai genotypes to screen for one or more varieties characterized by higher cold tolerance.

Carotenoids are isoprenoid compounds synthesized by all photosynthetic organisms. Despite much research on carotenoid biosynthesis in the model plant Arabidopsis thaliana, there is a lack of information on the carotenoid pathway in Brassica rapa. To better understand its carotenoid biosynthetic pathway, we performed a systematic analysis of carotenoid biosynthetic genes at the genome level in B. rapa. We identified 67 carotenoid biosynthetic genes in B. rapa, which were orthologs of the 47 carotenoid genes in A. thaliana. A high level of synteny was observed for carotenoid biosynthetic genes between A. thaliana and B. rapa. Out of 47 carotenoid biosynthetic genes in A. thaliana, 46 were successfully mapped to the 10 B. rapa chromosomes, and most of the genes retained more than one copy in B. rapa. The gene expansion was caused by the whole-genome triplication (WGT) event experienced by Brassica species. An expression analysis of the carotenoid biosynthetic genes suggested that their expression levels differed in root, stem, leaf, flower, callus, and silique tissues. Additionally, the paralogs of each carotenoid biosynthetic gene, which were generated from the WGT in B. rapa, showed significantly different expression levels among tissues, suggesting differentiated functions for these multi-copy genes in the carotenoid pathway. This first systematic study of carotenoid biosynthetic genes in B. rapa provides insights into the carotenoid metabolic mechanisms of Brassica crops. In addition, a better understanding of carotenoid biosynthetic genes in B. rapa will contribute to the development of conventional and transgenic B. rapa cultivars with enriched carotenoid levels in the future.