Feng Gao

Rapeseed (Brassica napus L.) is an important crop for healthy edible oil and stockfeed worldwide. However, its growth and yield are severely hampered by black rot, a destructive disease caused by Xanthomonas campestris pv. campestris (Xcc). Despite the identification of several quantitative trait loci (QTLs) associated with resistance to black rot in Brassica crops, the underlying molecular mechanisms remain largely unexplored. In this study, we investigated Xcc-induced transcriptomic and metabolic changes in the leaves of two rapeseed varieties: Westar (susceptible) and ZS5 (resistant). Our findings indicated that Xcc infection elicited more pronounced overall transcriptomic and metabolic changes in Westar compared to ZS5. Transcriptomic analyses revealed that the phenylpropanoid biosynthesis, cutin, suberine and wax biosynthesis, tryptophan metabolism, and phenylalanine metabolism were enriched in both varieties. Notably, photosynthesis was down-regulated in Westar after infection, whereas this down-regulation occurred at a later stage in ZS5. Integrated analyses of transcriptome and metabolome revealed that the tryptophan metabolism pathway was enriched in both varieties. Indolic glucosinolates and indole-3-acetic acid (IAA) are two metabolites derived from tryptophan. The expression of genes involved in the indolic glucosinolate pathway and the levels of indolic glucosinolates were significantly elevated in both varieties post-infection. Additionally, exogenous application of IAA promoted the development of black rot, whereas the use of an IAA synthesis inhibitor attenuated black rot development in both resistant and susceptible rapeseed varieties. These findings provide valuable molecular insights into the interactions between rapeseed and Xcc, facilitating the advancement of black rot resistance breeding in Brassica crops.

E3 ligases promote protein ubiquitination and degradation, which regulate every aspect of eukaryotic life. The Ariadne (ARI) proteins of RBR (ring between ring fingers) protein subfamily has been discovered as a group of potential E3 ubiquitin ligases. Only a few available research studies show their role in plant adaptations processes against the external environment. Presently, the functions of ARI proteins are largely unknown in plants. Therefore, in this study, we performed genome-wide analysis to identify the ARI gene family and explore their potential importance in B. napus. A total of 39 ARI genes were identified in the B. napus genome and were classified into three subfamilies (A, B and C) based on phylogenetic analysis. The protein–protein interaction networks and enrichment analysis indicated that BnARI genes could be involved in endoreduplication, DNA repair, proteasome assembly, ubiquitination, protein kinase activity and stress adaptation. The transcriptome data analysis in various tissues provided us an indication of some BnARI genes’ functional importance in tissue development. We also identified potential BnARI genes that were significantly responsive towards the abiotic stresses. Furthermore, eight BnARI genes were identified as candidate genes for multiple agronomic traits through association mapping analysis in B. napus; among them, BnaA02g12100D, which is the ortholog of AtARI8, was significantly associated with ten agronomic traits. This study provided useful information on BnARI genes, which could aid targeted functional research and genetic improvement for breeding in B. napus.

In plants, phloem is not only a vital structure that is used for nutrient transportation, but it is also the location of a response that defends against various stresses, named phloem-based defense (PBD). Phloem proteins (PP2s) are among the predominant proteins in phloem, indicating their potential functional role in PBD. Sclerotinia disease (SD), which is caused by the necrotrophic fungal pathogen S. sclerotiorum (Sclerotinia sclerotiorum), is a devastating disease that affects oil crops, especially Brassica napus (B. napus), mainly by blocking nutrition and water transportation through xylem and phloem. Presently, the role of PP2s in SD resistance is still largely estimated. Therefore, in this study, we identified 62 members of the PP2 gene family in the B. napus genome with an uneven distribution across the 19 chromosomes. A phylogenetic analysis classified the BnPP2s into four clusters (I–IV), with cluster I containing the most members (28 genes) as a consequence of its frequent genome segmental duplication. A comparison of the gene structures and conserved motifs suggested that BnPP2 genes were well conserved in clusters II to IV, but were variable in cluster I. Interestingly, the motifs in different clusters displayed unique features, such as motif 6 specifically existing in cluster III and motif 1 being excluded from cluster IV. These results indicated the possible functional specification of BnPP2s. A transcriptome data analysis showed that the genes in clusters II to IV exhibited dynamic expression alternation in tissues and the stimulation of S. sclerotiorum, suggesting that they could participate in SD resistance. A GWAS analysis of a rapeseed population comprising 324 accessions identified four BnPP2 genes that were potentially responsible for SD resistance and a transgenic study that was conducted by transiently expressing BnPP2-6 in tobacco (Nicotiana tabacum) leaves validated their positive role in regulating SD resistance in terms of reduced lesion size after inoculation with S. sclerotiorum hyphal plugs. This study provides useful information on PP2 gene functions in B. napus and could aid elaborated functional studies on the PP2 gene family.

SET domain group encoding proteins function as histone lysine methyltransferases. These proteins are involved in various biological processes, including plant development and adaption to the environment by modifying the chromatin structures. So far, the SET domain genes (SDGs) have not been systematically investigated in Brassica napus (B. napus). In the current study, through genome-wide analysis, a total of 122 SDGs were identified in the B. napus genome. These BnSDGs were subdivided into seven (I–VII) classes based on phylogeny analysis, domain configurations, and motif distribution. Segmental duplication was involved in the evolution of this family, and the duplicated genes were under strong purifying selection. The promoter sequence of BnSDGs consisted of various growth, hormones, and stress-related cis-acting elements along with transcription factor binding sites (TFBSs) for 20 TF families in 59 of the 122 BnSDGs. The gene ontology (GO) analysis revealed that BnSDGs were closely associated with histone and non-histone methylation and metal binding capacity localized mostly in the nucleus. The in silico expression analysis at four developmental stages in leaf, stem root, floral organ, silique, and seed tissues showed a broad range of tissue and stage-specific expression pattern. The expression analysis under four abiotic stresses (dehydration, cold, ABA, and salinity) also provided evidence for the importance of BnSDGs in stress environments. Based on expression analysis, we performed reverse transcription-quantitative PCR for 15 target BnSDGs in eight tissues (young leaf, mature leaf, root, stem, carpel, stamen, sepal, and petals). Our results were in accordance with the in silico expression data, suggesting the importance of these genes in plant development. In conclusion, this study lays a foundation for future functional studies on SDGs in B. napus.