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Abstract
The MYB transcription factors DIVARICATA (DIV), DIV-and-RAD-Interacting-Factor (DRIF), and the small interfering peptide RADIALIS (RAD) can interact, forming a regulatory module that controls different plant developmental processes. In the snapdragon Antirrhinum majus, this module, together with the TCP transcription factor CYCLOIDEA (CYC), is responsible for the establishment of floral dorsoventral asymmetry. The spatial gene expression pattern of the OitDIV, OitDRIF, and OitRAD homologs of Orchis italica, an orchid with zygomorphic flowers, has suggested a possible conserved role of these genes in bilateral symmetry of the orchid flower. Here, we have identified four DRIF genes of orchids and have reconstructed their genomic organization and evolution. In addition, we found snapdragon transcriptional cis-regulatory elements of DIV and RAD loci generally conserved within the corresponding orchid orthologues. We have tested the biochemical interactions among OitDIV, OitDRIF1, and OitRAD of O. italica, showing that OitDRIF1 can interact both with OitDIV and OitRAD, whereas OitDIV and OitRAD do not directly interact, as in A. majus. The analysis of the quantitative expression profile of these MYB genes revealed that in zygomorphic orchid flowers, the DIV, DRIF1, and RAD transcripts are present at higher levels in the lip than in lateral inner tepals, whereas in peloric orchid flowers they show similar expression levels. These results indicate that MYB transcription factors could have a role in shaping zygomorphy of the orchid flower, potentially enriching the underlying orchid developmental code.
Highlights
The MYB proteins DIVARICATA (DIV), RADIALIS (RAD), and DIV-and-RAD-Interacting-Factor (DRIF) are part of a regulatory module involved in distinct developmental processes of plants (Machemer et al, 2011; Raimundo et al, 2013)
The DRIF genes of orchids have been identified only in O. italica, where the expression pattern of OitDRIF1 and OitDRIF2 was analysed in floral tissues (Valoroso et al, 2017)
The DRIF homolog number in the examined species is generally five (Raimundo et al, 2018). These findings led us to search for other DRIF genes expressed in O. italica, to identify their homologs in other orchids and to verify the number of DRIF genes within the available genome of orchid species, currently restricted to the only subfamilies Epidendroideae and Apostasioideae
Summary
The MYB proteins DIVARICATA (DIV), RADIALIS (RAD), and DIV-and-RAD-Interacting-Factor (DRIF) are part of a regulatory module involved in distinct developmental processes of plants (Machemer et al, 2011; Raimundo et al, 2013). The MYB domain has undergone successive rearrangements resulting in the acquisition of specific interaction abilities: the MYB domain of DRIF can interact with the MYBI domain of DIV or with the MYB domain of RAD (Machemer et al, 2011; Raimundo et al, 2013; Raimundo et al, 2018). The small RAD proteins (less than 100 amino acids in size) have an antagonistic effect on the formation of the DIV/DRIF complex and have been classified as small-interfering peptides (siPEP) or microproteins (Seo et al, 2011; Staudt and Wenkel, 2011; Eguen et al, 2015)
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