Comparative analyses of morphology and temporal floral organ transcriptome provide insights into the development of staminodes in Globba racemosa (Zingiberaceae)

Abstract

Staminode, the most conspicuous floral organ in Zingiberaceae, which greatly contributes to the ornamental value of flowers in this family. Meanwhile, staminode is a key innovation in Zingiberaceae, which is hypothesized to have originated from the fertile stamen. Previous morphological and gene expression analyses have provided evidence for this hypothesis. However, in Zingiberaceae, little is known about the gene expression dynamics of the staminode compared to other floral organs at transcriptomic level, and the molecular mechanisms underlying identity specification of the staminodes remain unresolved. In this study, by using G. racemosa, an ornamental plant in Zingiberaceae, we first traced the flower development of G. racemosa, with special attention to the development of the two types of staminodes (the labellum and the outer androecial member), to explore the morphological differences between staminodes and the fertile stamen. Then, by combining a full-length transcriptome and comparative transcriptome data from seven types of floral organs at four developmental stages, we identified candidate genes that are specifically, preferentially, or differentially expressed in the labellum and the outer androecial member compared to other floral organs. Using weighted gene co-expression network analysis (WGCNA), we further identified several modules that are significantly correlated with the labellum and the outer androecial member. Lastly, by examining the expression patterns of four well-known gene regulatory networks, which, according to previous studies, are presumed to be involved in the identity specification and morphogenesis of staminodes in Zingiberaceae, we found other potential regulators for the development of staminodes of G. racemosa. Notably, we found that on the one hand, the labellum and the outer androecial member shared some genes with the fertile stamen, providing evidence for the stamen origin of staminodes in Zingiberaceae; on the other hand, the labellum, outer androecial member, and petal also share many genes, explaining the morphological similarity among labellum, outer androecial member, and petal. Thus, in terms of regulatory mechanisms, the staminodes in G. racemosa may represent a complex of stamen and petal characteristics. In summary, our results offer valuable resources for further research on gene functions and lay the foundation for future analyses of the molecular mechanisms underlying staminode development in Zingiberaceae.

Keywords: Flower organ development; Globba racemosa; Staminode; Transcriptome.