Revealing the Response Mechanism of Winter Rapeseed (Brassica rapa L. subsp. oleifera ) to Freezing Stress Based on Transcriptome Analysis

Winter rapeseed ( Brassica rapa L. subsp.oleifera ) is an important oilseed crop in northern China, where low-temperature stress imposes severe constraints on its yield. Its leaves dry up and wither during the overwintering period, but its roots stay deeply embedded in the soil. Following March of the next year, new leaves grow from the growing point and it enters reproductive growth. Therefore, we carried out this research for exploring the regulatory mechanisms and physiological-biochemical responses of roots in different rapeseed varieties as the temperature decrease before wintering. This study used a strong cold-tolerant variety "Longyou 7 (L7)" and a moderate cold-tolerant variety "Longyou 99 (L99)" as experimental materials. The materials were planted in field to analyze the effects of natural low-temperature stress on rapeseed growth, physiological characteristics, hormone levels, and transcriptional levels before wintering. Additionally, transcriptome sequencing (RNA-seq) was combined to decipher the molecular regulatory mechanisms. Results showed that as temperature decrease, contents of proline (Pro), soluble sugar (SS), soluble protein (SP), and salicylic acid (SA) in roots of the strong cold-tolerant variety L7 were all significantly higher than those in L99, while contents of malondialdehyde (MDA) and gibberellin (GA₃) in L7 were significantly lower than those in L99. RNA-seq results revealed that differentially expressed genes (DEGs) were significantly involved in phenylpropane biosynthesis, carbon metabolism, starch and sucrose metabolism, MAPK signaling pathway, ribosome, proteasome, and protein processing. Among these, the differences in the expression of genes related to cell signal transduction (MAPK signaling pathway) and metabolism were particularly prominent. Through weighted gene co-expression network analysis (WGCNA), 9 candidate genes related to protein kinases, plant hormones, transcription factors, and signal transduction were identified in the MAPK and 2 other modules.