Comparative Transcriptomic Analyses The Formation Mechanism Of Cluster Buds Mutant In Brassica Napus

As an important oil crop in China,plant type breeding of Brassica napus is an important task for scientific research workers.After several years of screening,our research team got a B.napus mutant,of which buds were clustered,and named as “cluster buds mutant”.Cluster buds mutant is suitable for high density planting,which is an important direction for plant type breeding.In this study,we explored the molecular mechanisms of cluster buds mutant from two sides,phenotypic observation and transcriptomic analyses.We found that the leaf initiation rate of cluster buds mutant was faster in the seedling stage,and cluster buds phenotype have already formed in the bolting period.So we could conclude that cluster buds phenotype was formed before bolting period.In addition,the pods density of cluster buds mutant increased significantly and the yield of cluster buds mutant had a certain increase.Through microscopic observation,it was found that multiple main inflorescences meristem grew together during floral differentiation period.The results showed that the formation of cluster buds was due to the abnormal differentiation of shoot apical meristem(SAM),which caused the formation of extra stem cell niche(SCN)in B.napus.The molecular mechanism of cluster buds mutant was analyzed by RNA-Seq.Three SAM developmental stages(20,60 and 90 days after sowing)in the mutant and the wild type were selected for transcriptome sequencing,which generated 5331,9006 and 7862 differentially expressed genes(DEGs),respectively.First,functional analysis foun revealed that the genes related to WUSCHEL(WUS)and CLAVATA(CLV)feedback loop were not included in these DEGs,indicating that the formation of our cluster buds mutant was independent of WUS and CLV feedback loop.Second,a large number of DEGs,especially the SCF complexes of E3 ligase,related to ubiquitin proteasome system were down-regulated in buds cluster mutant.Third,transcription factors classification results showed that many NAC family transcription factors were enriched.The CUP-SHAPED COTYLEDON1(CUC1),CUC2 and CUC3 genes belong to NAC family transcription factors,played a key role in maintaining the boundaries between plant lateral organs and the SAM.The transcription factors interaction analysis revealed that KAN1 had a higher betweenness centrality in the interaction network,whose function is promoting the formation of abaxial identity in both leaves and carpels.From the phenotypic observation and transcriptomic analyses,we assumed a possible mechanism for the ectopic SCN in the SAM of buds cluster mutant.The growth inhibition of the boundaries between plant lateral organs and the SAM is missing,which leads the loss of SAM dominance and the change of leaf primordia initiating sites and induces multiple shoot tips.