Liriodendron tulipifera,also known as tuliptree,is a popular ornamental horticultural plant with extraordinary tulipshaped flowers characterized by an orange band near their base.The mechanisms underlying petal band-specific pigmentation during L.tulipifera flower development are unclear.Here,we combined nontargeted and targeted metabolomics and transcriptomics to identify a pathway cascade leading to carotenoid biosynthesis that is specifically activated in the petal band.The comparative analysis of carotenoid metabolites between L.tulipifera and Liriodendron hybrids indicates thatγ-carotene,a rare carotene in plants,is the most likely orange pigment responsible for the coloration of the petal band.Phenotypic and transcriptomic analyses of developing petals reveal that the band area is first predefined by the loss of green color.Later,the band is maintained by locally activating and repressing carotenoid and chlorophyll biosynthesis genes,respectively.Two rate-limiting genes of carotene biosynthesis,carotenoid isomerase(CRTISO)and epsilon lycopene cyclase(ε-LCY),encode the core enzymes responsible for petal band-specific orange pigmentation in L.tulipifera.In particular,a putative additionalε-LCY copy specific to L.tulipifera may contribute to the distinct petal coloration pattern,compared with L.chinense.Taken together,our work provides a first glimpse of the metabolome and transcriptome dynamics in tuliptree flower coloration and provides a valuable resource for flower breeding or metabolic engineering as well as for understanding flower evolution in an early woody angiosperm.
Flowering is crucial to plant reproduction and controlled by multiple factors.However,the mechanisms underlying the regulation of flowering in perennial plants are still largely unknown.Here,we first report a super long blooming 1(slbl)mutant of the relict tree Liriodendron chinense possessing a prolonged blooming period of more than 5 months,in contrast to the 1 month blooming period in the wild type(WT).Phenotypic characterization showed that earlier maturation of lateral shoots was caused by accelerated axillary bud fate,leading to the phenotype of continuous flowering in slbl mutants.The transcriptional activity of genes related to hormone signaling(auxin,cytokinin,and strigolactone),nutrient availability,and oxidative stress relief further indicated active outgrowth of lateral buds in slbl mutants.Interestingly,we discovered a unique FT splicing variant with intron retention specific to slbl mutants,representing a potential causal mutation in the slbl mutants.Surprisingly,most slbl inbred offspring flowered precociously with shorter juvenility(~4 months)than that(usually 8-10 years)required in WT plants,indicating heritable variation underlying continuous flowering in slbl mutants.This study reports an example of a perennial tree mutant that flowers continuously,providing a rare resource for both breeding and genetic research.
