Transcriptional Regulations on the Low-Temperature-Induced Floral Transition in an Orchidaceae Species, Dendrobium nobile: An Expressed Sequence Tags Analysis
Vernalization-induced flowering is a cold-relevant adaptation in many species, but little is known about the genetic basis behind in Orchidaceae species. Here, we reported a collection of 15017 expressed sequence tags (ESTs) from the vernalized axillary buds of an Orchidaceae species, Dendrobium nobile, which were assembled for 9616 unique gene clusters. Functional enrichment analysis showed that genes in relation to the responses to stresses, especially in the form of low temperatures, and those involving in protein biosynthesis and chromatin assembly were significantly overrepresented during 40 days of vernalization. Additionally, a total of 59 putative flowering-relevant genes were recognized, including those homologous to known key players in vernalization pathways in temperate cereals or Arabidopsis, such as cereal VRN1, FT/VRN3, and Arabidopsis AGL19. Results from this study suggest that the networks regulating vernalization-induced floral transition are conserved, but just in a part, in D. nobile, temperate cereals, and Arabidopsis. 1. Introduction Transition from the vegetative phase to the flowering phase is crucial to both development and reproduction in plants. Besides endogenous signals, this process is also affected by external cues such as day length and temperature. Vernalization, an exposure to low temperature extending over a period differing from species to species, is an adaptive nature to ensure some plants survival in harsh winters and flowers under a favourable condition in spring. In the dicot model Arabidopsis, vernalization-regulated flowering is mediated by both FLC-dependant and -independent pathways [1, 2], in which the gene VIN3 (which interacts with VRN2), a key upstream component, is probably activated by exposure to low temperature and subsequently leads to changes in histone methylation of downstream gene regions [3]. For example, the expression of FLC is suppressed by vernalization through enrichment of H3K27m3 on the chromatin [3], which consequently releases FT and SOC1 from inhibition by FLC to promote the transition to flowering. This FLC-dependant pathway is regulated by both temperature and day length [4]. AGL19, a close relative of SOC1, is believed to mediate an FLC-independent pathway that activates flowering under vernalization in Arabidopsis [1]. This process is associated with a cold-induced decrease of H3K27m3 on AGL19 locus and probably also with the loss of function of CLF and MSI1 or the involved complex [5]. Ectopically expression of Arabidopsis AGL19 leads to only mild abnormalities, suggesting that
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