Chinese cabbage, which is a cold season crop, can still be damaged at an overly low temperature. It is crucial to study the mechanism of the resistance to low temperature of Chinese cabbage. In this study, the Chinese cabbage ‘XBJ’ was used as the material, and nine different low temperatures and control samples were treated. Using RNA-seq and lignin content determination, we analyzed 27 samples, and the stained sections of them were observed. A total of 8845 genes were screened for the WGCNA analysis, yielding 17 modules. The GO and KEGG analyses of the modules was highly associated with a low-temperature treatment. The pathways such as ‘starch and sucrose metabolism’ and ‘plant hormone signal transduction’ were enriched in modules related to low temperature. Interestingly, L-15DAT-associated MEcoral2 was found to have 14 genes related to the ‘lignin biosynthetic process’ in the GO annotation. The combination of the determination of the lignin content and the treatment of the stained sections showed that the lignin content of the low-temperatures samples were indeed higher than that of the control. We further explored the expression changes of the lignin synthesis pathway and various genes and found that low temperature affects the expression changes of most genes in the lignin synthesis pathway, leading to the speculation that the lignin changes at low temperature are a defense mechanism against low temperatures. The 29 BrCOMT gene sequence derived from the RNA-seq was non-conserved, and eight BrCOMT genes were differentially expressed. This study provides a new insight into how lignin is affected by low temperature.

The purple-leaf phenotype in pak choi is due to the accumulation of anthocyanin. The main regulated genes are unclear. The gene controlling the purple-leaf phenotype was mapped on A03 using BSA-seq, but the candidate interval was not further narrowed with InDel markers. Based on our previous study, we hypothesized that the candidate gene that regulates purple leaves in pak choi may also be the Dark_Pur gene from B. juncea. Using the Dark_Pur-1 marker to identify P₁, P₂, F₁, and F₂, it was confirmed that the purple trait in purple-leaf pak choi was controlled by the Dark_Pur gene from B. juncea through distant hybridization. A DNA segment of approximately 514 Kb containing the Dark_Pur gene was reintroduced into pak choi from B. juncea. Meanwhile, a new purple pak choi germplasm line was created with green pak choi × purple B. juncea via distant hybridization, which proved that distant hybridization is an effective method for creating new germplasms. Furthermore, the purple-leaf phenotypes of 20 pak choi varieties were identified, and the purple-leaf traits of all lines were derived from B. juncea via distant hybridization. At present, few studies have focused on the background of the purple trait in pak choi; however, in this study, our results suggest that there is a high probability that the purple trait in pak choi may be completely derived from purple B. juncea via distant hybridization. This study also lays a good foundation for research on the creation of new germplasms through distant hybridization among the Brassica species.

Ogura cytoplasmic male sterility (CMS) is one of the important methods for hybrid seed production in cruciferous crops. The lack of a restorer of fertility gene (Rfo) in Brassica rapa L. restricts the development and utilization of its germplasm resources. In this research, Brassica napus with the Rfo gene was used to restore the fertility of Ogura CMS B. rapa with the golden heart trait. Through the distant cross of two B. rapa and four B. napus, six interspecific hybrid combinations received F1 seeds. The six combinations were different in seed receiving. By morphological observation and molecular marker-assisted selection (MAS), in F1, individuals containing the Rfo gene all appeared fertile, while those without it remained male-sterile. The pollen viability of the fertile individuals was measured, and the fertile lines of the six interspecific hybrid combinations were different (40.68–80.49%). Three individuals (containing both Rfo and GOLDEN genes) with the highest pollen vitality (≥60%) were backcrossed with fertile cytoplasmic B. rapa, resulting in a total of 800 plants. Based on the MAS, a total of 144 plants with GOLDEN but no Rfo were screened (18%). Moreover, through morphological investigation, one individual with normal cytoplasm, stable fertility but without the restoring gene Rfo, the GOLDEN gene, and morphological characteristics similar to those of B. rapa was obtained. These results increased the diversity of B. rapa germplasm and provided a new method for the utilization of CMS germplasm in Brassica crops.

Leaf-color variation in plants should be associated with chlorophyll metabolism and chloroplast development. Here, we characterized a low-temperature-sensitive pakchoi DH line, 1197, which exhibited green leaves at 25 °C, but showed yellowing at 4 °C. Low temperature significantly reduced chlorophyll accumulation and disrupted chloroplast ultrastructure. After transfer from 4 °C to 25 °C for 7 days, yellow leaves partially regreened, and chlorophyll a content increased by 366.67%. RNA-seq analysis identified 3058 core DEGs associated with the yellowing–regreening transition, which were significantly enriched in photosynthesis–antenna proteins, photosynthesis, and porphyrin metabolism pathways. Leaf yellowing was characterized by repression of chlorophyll biosynthesis genes (e.g., CHLD, CHLM, PORC) and induction of degradation genes (SGR1, SGR2, NYC1, PAO), together with widespread downregulation of chloroplast function-related genes. In addition, GLK2, HBI1, NAC047, and NAC029 were identified as candidate regulators of temperature-dependent leaf-color conversion. This study provides candidate molecular insights into low-temperature-induced yellowing and regreening in pakchoi and offers candidate genes for future functional validation and Brassica breeding.