Radial or Bilateral? The Molecular Basis of Floral Symmetry

Abstract

In the plant kingdom, the flower is one of the most relevant evolutionary novelties. Floral symmetry has evolved multiple times from the ancestral condition of radial to bilateral symmetry. During evolution, several transcription factors have been recruited by the different developmental pathways in relation to the increase of plant complexity. The MYB proteins are among the most ancient plant transcription factor families and are implicated in different metabolic and developmental processes. In the model plant Antirrhinum majus, three MYB transcription factors (DIVARICATA, DRIF, and RADIALIS) have a pivotal function in the establishment of floral dorsoventral asymmetry. Here, we present an updated report of the role of the DIV, DRIF, and RAD transcription factors in both eudicots and monocots, pointing out their functional changes during plant evolution. In addition, we discuss the molecular models of the establishment of flower symmetry in different flowering plants.

Plant DNA Methylation: An Epigenetic Mark in Development, Environmental Interactions, and Evolution

“…One well-known example of gene silencing affecting plant morphology is the methylation of the CYCLOIDEA gene ( CYC ) in the common toadflax ( Linaria vulgaris ). In many angiosperms, the CYC gene has a conserved role in establishing bilateral symmetry, promoting the development of the dorsal identity of the flower organs [ 145 ]. During the angiosperm evolution, many flower symmetry transitions have occurred [ 146 ].…”

“…These reversions depend on the modifications of expression profile or mutations in the genes involved in determining flower symmetry. The loss or the reduced expression of the CYC gene in many angiosperms, such as Conandron ramondioides [ 147 ] or Plantago lanceolata [ 148 ], is associated with the ventralization of the flower [ 145 ]. In L. vulgaris, the CYC methylation, and the consequent reduction of its expression, causes the loss of bilateral symmetry, and the flower assumes a peloric mutant phenotype with radial symmetry [ 149 ].…”

Floral symmetry: the geometry of plant reproduction

“…For example, five petals are arranged along the adaxial-abaxial axis in three distinct positionstwo dorsal, two lateral and one ventralthanks to the activity of two teosinte BRANCHED1-CYCLOIDEA-PCF (TCP) domain-containing TFs, namely CYCLOIDEA (CYC) and its paralog DICHOTOMA (DICH), that control dorsal-ventral axis asymmetry (Figure 3c). Genetic studies of the cyc dich 'peloric' mutant in snapdragon (Antirrhinum majus), which produce radially symmetric flowers with ectopic ventral identity, demonstrated their role in controlling flower bilateral symmetry by patterning the dorsal region and restricting the ventral domain [25][26][27]. Accordingly, the gene CYC is expressed at very early stages of flower development, in the dorsal domain of the flower meristem, where it affects growth rate and primordium initiation, thus establishing dorsoventral asymmetry [26].…”

“…The expansion of TF families during plant evolution is positively correlated with flower complexity [27,28]. The MYB TF family is one of the most ancient found in animals and plants, and their role in flower development has been extensively studied in the past three decades.…”

“…The MYB domain is the key feature of this protein family and is necessary for protein-protein interactions and DNA-binding. Three MYB TFs, DIVARICATA (DIV), RADIALIS (RAD), and DIV-and-RAD-interacting-factors (DRIF), compose the core DDR module for (a)symmetry establishment along the dorsal-ventral (abaxial-adaxial) axis [27] (Figure 3c). DIV and DRIF form heterodimers in the ventral region and activate unknown downstream target genes to promote ventralization.…”