Abstract
Flower bud formation and flowering in chrysanthemums occur under short day conditions (SD), but the molecular basis for the switch to reproductive growth is less well understood than in model plants. Here, a spontaneous mutant able to flower under long days is described. In an attempt to reveal the pathway(s) involved in the formation of flower buds under contrasting daylengths, transcriptome sequencing was carried out in plants grown both under SD and long day conditions (LD). A number of differentially transcribed genes involved in the various known flowering pathways were identified. Both circadian clock genes and Chrysanthemum FLOWERING LOCUS T Like3 (CmFTL3) were up-regulated under SD, thereby inducing floral bud formation and flowering. The gibberellin (GA) signaling pathway-related genes Gibberellin 20-oxidase (GA20ox) and Gibberellin receptor (GID1) were up-regulated in the mutant under LD, while the catabolic genes Gibberellin 2-oxidase (GA2ox) and GA-INSENSITIVE (GAI) were down-regulated, thereby inducing the transcription of SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1 (SOC1) and LEAFY (LFY). The GA content of the leaf was higher in the mutant than in the wild type (WT) under LD and SD, and the mutant has more branching than WT plants under LD or SD. When treated with GA, the mutant flowered earlier under both SD and LD relative to WT, but there was no detectable phenotype difference between the two lines. The indication was that the photoperiod pathway majorly regulates flower bud formation and flowering time in chrysanthemums under SD. The GA signaling pathway only plays a subsidiary role for flowering. However, the GA signaling pathway predominated for flowering under LD.
Highlights
Flowering is a key event in both seed and ornamental crops [1]
The endogenous concentration of GA was determined in both young leaves of wild type (WT) and Mutant (M) plants grown under either long day conditions (LD) or short day conditions (SD) and those of WT and M plants still in the vegetative stage, sampled prior to the imposition of a controlled photoperiod (Figure 2)
Under LD, the GA concentration was substantially higher in M than in WT plants, but there was no significant difference between M and WT in plants grown under SD (Figure 2)
Summary
The switch to reproductive growth occurs in response to a variety of endogenous and environmental cues, and in the model plant Arabidopsis thaliana at least, six major pathways (photoperiod, vernalization, autonomous, gibberellin (GA), ambient temperature and aging) are known to regulate the floral transition process [2,3]. GA regulates SOC1 and LFY gene activity; overexpression of SOC1 rescued GA-deficient mutant ga to induce flowering [14]. FLOWERING LOCUS C (FLC) transcription is regulated by the MADS box transcription factor SHORT VEGETATIVE PHASE (SVP), the product of which binds to the promoters of FT and SOC1 [3,23]
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