Genetic insights into the regulatory pathways for continuous flowering in a unique orchid Arundina graminifolia

Abstract

Background: Manipulation of flowering time and frequency of blooming is key to enhancing the ornamental value of orchids. Arundina graminifolia is a unique orchid that flowers year round, although the molecular basis of this flowering pattern remains poorly understood.

Results: We compared the A. graminifolia transcriptome across tissue types and floral developmental stages to elucidate important genetic regulators of flowering and hormones. Clustering analyses identified modules specific to floral transition and floral morphogenesis, providing a set of candidate regulators for the floral initiation and timing. Among candidate floral homeotic genes, the expression of two FT genes was positively correlated with flower development. Assessment of the endogenous hormone levels and qRT-PCR analysis of 32 pathway-responsive genes supported a role for the regulatory networks in floral bud control in A. graminifolia. Moreover, WGCNA showed that flowering control can be delineated by modules of coexpressed genes; especially, MEgreen presented group of genes specific to flowering.

Conclusions: Candidate gene selection coupled with hormonal regulators brings a robust source to understand the intricate molecular regulation of flowering in precious orchids.

Keywords: Arundina graminifolia; Flowering; Hormone signaling; Transcriptome; WGCNA.

Fig. 1

Stages of flower development in Arundina graminifolia. A-D: Scanning electron micrograph (SEM) of early floral developmental stages. A: Stage 0 (IM: inflorescence meristem); B-C: Stage 1 (IM: inflorescence meristem, PP: petal primordia, SP: sepal primordia); D: Stage 2 (PP: petal primordia, SP: sepal primordia, LP: lip primordia, CP: carpel primordia); E-H: Stage 3; I-L: Stage 4; M-P: Stage 5; P: mature flower

Fig. 2

DEGs relating to flowering control. A: heatmap of flowering-related DEGs; B: Phylogenetic tree of AgFT genes with FT, MFT, and TFL genes from other species; C: qRT-PCR expression of AgFT1 and AgFT2 in flower development stages; D: qRT-PCR analysis of AgFT1 and AgFT2 in tissues other than flower

Fig. 3

A: DEGs involved in auxin biosynthesis and signaling, cytokinins biosynthesis and signaling, strigolactone biosynthesis and signaling, gibberellin biosynthesis and signaling, and ABA signaling and biosynthesis; B: Comparison of hormone levels in A. graminifolia, C. ensifolium and C. sinense

Fig. 4

Weighted Gene Coexpression Network Analysis (WGCAN). A: Gene modules related to flowering and tissue types; B: Gene significance to flowering; C: Identification of four hub genes from green module

Fig. 5

Hub genes from coexpressed modules related to flowering and hormonal regulation (A) (blue circles show the key hormonal regulators and the red circles show floral regulators), qRT-PCR analyses of 30 selected genes related to multiple pathways (B) and the hypothetical model of flowering regulation by different pathways (C)