Slow germination speed caused by low temperature stress intensifies the risk posed by adverse environmental factors, contributing to low germination rate and reduced production of rapeseed. The purpose of this study was to understand the transcriptional regulation mechanism for rapid germination of rapeseed. The results showed that seed components and size do not determine the seed germination speed. Different temporal transcriptomic profiles were generated under normal and low temperature conditions in genotypes with fast and slow germination speeds. Using weight gene co-expression network analysis, 37 823 genes were clustered into 15 modules with different expression patterns. There were 10 233 and 9111 differentially expressed genes found to follow persistent tendency of up- and down-regulation, respectively, which provided the conditions necessary for germination. Hub genes in the continuous up-regulation module were associated with phytohormone regulation, signal transduction, the pentose phosphate pathway, and lipolytic metabolism. Hub genes in the continuous down-regulation module were involved in ubiquitin-mediated proteolysis. Through pairwise comparisons, 1551 specific upregulated DEGs were identified for the fast germination speed genotype under low temperature stress. These DEGs were mainly enriched in RNA synthesis and degradation metabolisms, signal transduction, and defense systems. Transcription factors, including WRKY, bZIP, EFR, MYB, B3, DREB, NAC, and ERF, are associated with low temperature stress in the fast germination genotype. The aquaporin NIP5 and late embryogenesis abundant (LEA) protein genes contributed to the water uptake and transport under low temperature stress during seed germination. The ethylene/H2O2-mediated signal pathway plays an important role in cell wall loosening and embryo extension during germination. The ROS-scavenging system, including catalase, aldehyde dehydrogenase, and glutathione S-transferase, was also upregulated to alleviate ROS toxicity in the fast germinating genotype under low temperature stress. These findings should be useful for molecular assisted screening and breeding of fast germination speed genotypes for rapeseed.

To cite this paper: Xian, M., T. Luo, M.N. Khan, L. Hu and Z. Xu, 2017. Identifying differentially expressed genes associated with tolerance against low temperature stress in Brassica napus through transcriptome analysis. AbstractUnder direct-seeding method of sowing, seedling survival is adversely affected due to low temperature. The purpose of this study was to understand the morphological, physiological and molecular response of rapeseed resistance to low temperature stress at seedling establishment. Two varieties Qinyou No.7 (resistant biotype) and Fengyou 730 (sensitive biotype) were used for morphological and physiological experiments under low temperature stress and the resistant variety was used for transcriptome analysis based on the Illumina HiSeq 2000 platform. Results showed resistant variety initiated osmotic regulation system firstly and then triggered the antioxidant enzyme system when exposed to low temperature stress for long duration. The MDA content firstly tended to increase and then decreased in resistant variety, but it continued to increase in sensitive variety. Through transcriptome analysis of Qinyou No. 7 under low temperature for 0 h, 24 h, 48 h and 96 h, 2254 differentially expressed unigenes clustering in 7 expression patterns were participated in low temperature stress. Proteinserine/threonine kinases, myo-inositol-1-phosphate synthases and calmodulins, as the members of ABA and IP3/Ca2+ signal transduction pathway, played an important role in the low temperature stress. Different expressed genes dataset provides useful candidate genes for functional analysis of rapeseed resistance to low temperature.

Low temperature is a main adverse factor affecting successful seedling establishment in the Yangtze River basin, and thus low‐temperature stress tolerance at the germination and seedling emergence stages is an important attribute for rapeseed (Brassica napus L.) production. The present study was conducted to evaluate the low‐temperature stress tolerance indices (STIs) of a large conventional rapeseed genotype population at the germination and seedling emergence stages. Twelve seed germination‐ and seedling emergence‐related indices were investigated under normal and low‐temperature conditions, and the broad‐sense heritability of these indices ranged from 25.4 to 57.6%. Therefore, the selection of genotypes with strong low temperature tolerance is a feasible way to guarantee seedling establishment. Using principal component analysis, the STIs during the seed germination and seedling emergence stages have been divided into three main categories: dry matter weight, germination speed and final germination percentage, and root length with a seedling vigor index. The top three genotypes ranked by comprehensive STI score were Huayou No. 2, SWU44, and 2012‐K8053; the bottom three genotypes ranked by comprehensive STI score were Chuanyou18, wx1025, and 97096. The results of the present study could provide valuable information for breeders to utilize the rapeseed germplasm resources in the Yangtze River basin to achieve the goal of low‐temperature resistance breeding.

The naturally drought-prone climate of the Tibetan Plateau has produced highly drought-resistant Brassica juncea. The objective of the present study was to examine improvement in drought resistance in B. napus by distant hybridisation between B. juncea and B. napus. Distant hybridisation was performed to generate F1 hybrids, which were open-pollinated by a set of breeding lines of B. napus. Continuous self-crossing was then performed to produce the F2–F6 generations, and 74 lines of new-type Brassica napus with stable fertility and morphological phenotypes were selected. The drought resistance of the 74 lines was evaluated during the germination stage by simulating drought stress at 15% PEG-6000, and a wide range of genetic variation in drought resistance was scored. Cytological identification of four lines chosen from strongly, intermediate and weakly drought-resistant clusters demonstrated that their chromosomes had gradually stabilised to B. napus (2n = 38) after advanced self-crossing. A drought-resistant line (line 290) and a drought-susceptible line (line 299) were selected to determine the physiological response to drought stress at the seedling stage. The results showed that proline, soluble protein and malondialdehyde contents of the drought-resistant line were always lower than those of the drought-susceptible line and other common rapeseed variety under drought stress and rewatering conditions. This indicates that the drought-resistant line may have a better reactive oxygen species scavenging system with a less extreme reaction to drought stress. Additionally, the results revealed that the genetic diversity of B. napus under drought resistance was broadened by distant hybridisation, which could encourage breeders to utilise the germplasm resources of B. juncea in the Tibetan Plateau to achieve the goal of drought resistance.