Integrated transcriptomics and metabolomics analyses provide insights into the mechanisms of capitulum architecture in Argyranthemum frutescens (Asteraceae)

The capitulum is a crucial Asteraceae innovation; however, the molecular mechanisms of its development remain elusive. Here, we performed transcriptome and metabolome analyses to explore the molecular mechanisms underlying floret development of Argyranthemum frutescens, whose capitula produce ray (R), disk (D) and trans (T) florets. Up-regulated differentially expressed genes (DEGs) related to cutin, suberine and wax biosynthesis, fatty acid elongation and lipid accumulation were significantly enriched in T. DEGs in the tryptophan metabolism pathway were up-regulated in R, and DEGs in the abscisic acid synthesis pathway were up-regulated in D. Auxin and abscisic acid occurred as a reverse distribution gradient in the three floret types. TCP, WOX transcription factors and ABCE genes were identified and analyzed. Our results suggest that cutin, suberine and wax biosynthesis regulates the formation of petal cuticle by upregulating the content of lipids in T, thereby improving the reproductive efficiency of A. frutescens. Auxin and abscisic acid co-regulate floret development in a concentration dependent manner in A. frutescens, and expression of flower development genes was strongly correlated with the IAA content. These results enable a comprehensive understanding of the molecular mechanisms of capitulum formation in A. frutescens and increase our understanding of capitulum diversity in Asteraceae.