Abstract
Double-spikes Phalaenopsis orchids have greater market value than those with single-spike. In this study, a gene designated as Spike Activator 1 (SPK1), which encodes a basic helix-loop-helix (bHLH) transcription factor, was isolated and characterized from Phalaenopsis aphrodite (moth orchid). SPK1 was highly expressed in the meristematic tissues. In the axillary bud, SPK1 was highly upregulated by a moderately low temperature of 20 °C but downregulated by a spike inhibition temperature of 30 °C. SPK1 protein is localized in the nucleus. Another bHLH, bHLH35, which is also highly expressed in young tissues in the same way as SPK1 was also identified. In contrast to SPK1, bHLH35 transcripts are downregulated at 20 °C but upregulated at 30 °C. Bimolecular florescence complementation assay and yeast two-hybrid assays indicated that SPK1 interacts with bHLH35 and forms a heterodimer. Virus-induced gene silencing (VIGS) showed that 7 out of 15 vector control plants produced double spikes but that only 1 out of 15 VIGS-spk1 plants produced double spikes. RT-qPCR results indicated that VIGS-spk1 downregulated gene expression levels of SPK1, FT, CYCB, and EXPA8. Overall, we propose that SPK1 plays an essential role in early axillary bud development and spike initiation of P. aphrodite.
Highlights
Phalaenopsis aphrodite is one of the most important export ornamental plants in the world
Our team is interested in identifying the gene function of basic helix-loop-helix (bHLH) transcription factors (TFs), and we found that a bHLH gene was significantly upregulated at 20 ◦C but inhibited at 30 ◦C, which piqued our interest to do further study
In the axillary bud of AB4, some Virus-induced gene silencing (VIGS)-spk1 plants have less gene silencing effects and expressed high Spike Activator 1 (SPK1), CYCB, EXPA8, and FLOWERING LOCUS T (FT) transcripts (Figure S4). These results suggest that AB4 is already mature before VIGS treatment and that the mature axillary buds immediately respond to 20 ◦C spike induction, turn on spiking signal, and upregulate spiking genes
Summary
Phalaenopsis aphrodite (moth orchid) is one of the most important export ornamental plants in the world. Floral spikes of Phalaenopsis orchids are initiated from axillary buds at the leaf base. After a period of moderately low-temperature induction for one and a half months, axillary buds enlarge, break dormancy, and protrude from the leaf base to develop into young floral spikes. Temperature is a critical factor in spike induction of Phalaenopsis orchids. Some indicated that Phalaenopsis orchid can initiate spikes at constant 14 to 23 ◦C or fluctuating day/night temperatures of 20 ◦C/14 ◦C or 23 ◦C/17 ◦C. Researchers concluded that day/night temperature fluctuation is not necessary for spike initiation [2]. Once orchid spiking was initiated, the plants took 10 to 15 weeks to flower at 20 to 23 ◦C [2,6]
Sign-in/Register to view highlights & summary Sign-in/Register to access full text options 
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Schedule a call
