DnFCA Isoforms Cooperatively Regulate Temperature-Related Flowering in Dendrobium nobile

SIMPLE SUMMARY: A low temperature is required for flowering in Dendrobium nobile. However, this process can be suppressed or disrupted by a high ambient temperature. Little is currently known about the regulation networks and the mechanisms behind this process. Here, we report two isoforms from the DnFCA (FLOWERING CONTROL LOCUS C in D. nobile) gene locus, DnFCAγ and DnFCAβ, which cooperatively regulate temperature-related flowering in D. nobile. The overexpression in Arabidopsis indicated that both isoforms can partially rescue the late flowering of fca-1 but tend to delay the flowering time and downregulate the APETALA1 (AP1) expression in wild-type plants. When introduced into the detached axillary buds and seedlings of D. nobile, only DnFCAγ was able to suppress the transcription of DnAPL1 (AP1-LIKE 1 in D. nobile) in axillary buds, but both of them activated DnAGL19 (AGAMOUS LIKE 19 in D. nobile) in seedlings. Vernalization induced the accumulation of DnFCAβ in leaves, which allowed the activation of DnAGL19 and DnFT for the initiation of the inflorescence meristem (IM) in axillary buds. The synchronous enrichment of DnFCAγ in axillary buds may result in the suppression of DnAPL1, avoiding the premature development of floral organs until the floral primordium is produced after a long period of low temperatures. A high ambient temperature induced the long-standing accumulation of DnFCAγ in axillary buds, which can lead to the loss of flowering competence due to the lack of DnAPL1 activation. ABSTRACT: Timely flowering is a determinative trait for many economically valuable species in the Dendrobium genus of the Orchidaceae family, some of which are used for ornamental and medicinal purposes. D. nobile, a representative species of nobile-type Dendrobium, normally flowers in spring after exposure to sufficient low temperatures in winter. However, flowering can be stopped or disrupted by the untimely application of high temperatures. Little is known about the regulation and the mechanisms behind this switch. In this study, we report two isoforms from the KFK09_017173 locus of the D. nobile genome, named DnFCAγ and DnFCAβ, respectively, that cooperatively regulate flowering in D. nobile. These two isoforms are generated by alternative 3′ polyadenylation of DnFCA (FLOWERING CONTROL LOCUS C in D. nobile) pre-mRNA and contain a distinct 3′-terminus. Both can partially rescue late flowering in the Arabidopsis fca-1 mutant, while in wild-type Arabidopsis, they tend to delay the flowering time. When introduced into the detached axillary buds or young seedlings of D. nobile, both were able to induce the transcription of DnAGL19 (AGAMOUS LIKE 19 in D. nobile) in seedlings, whereas only DnFCAγ was able to suppress the transcription of DnAPL1 (AP1-LIKE 1 in D. nobile) in axillary buds. Furthermore, the time-course change of DnFCAγ accumulation was opposite to that of DnAPL1 in axillary buds, which was remarkable under low temperatures and within a short time after the application of high temperatures, supporting the suggestion that the expression of DnAPL1 can be inhibited by a high accumulation of DnFCAγ in floral buds. In leaves, the accumulation of DnFCAβ was in accordance with that of DnAGL19 and DnFT (FLOWERING LOCUS T in D. nobile) to a large extent, suggesting the activation of the DnAGL19–DnFT pathway by DnFCAβ. Taken together, these results suggest that the DnFCAγ–DnAPL1 pathway in axillary buds and the DnFCAβ–DnAGL19 pathway in the leaves cooperatively promote flowering under low temperatures. The long-term and constant, or untimely, application of high temperatures leads to the constitutive suppression of DnAPL1 by a high level of DnFCAγ in axillary buds, which consequently delays floral development.