G C A T T A C G G C A T genes Review Radial or Bilateral? The Molecular Basis of Floral Symmetry Francesca Lucibelli y, Maria Carmen Valoroso y and Serena Aceto * Department of Biology, University of Naples Federico II, 80126 Napoli, Italy; [email protected] (F.L.); [email protected] (M.C.V.) * Correspondence: [email protected]; Tel.: +39-081-2535190 These authors equally contributed to the work. y Received: 19 March 2020; Accepted: 3 April 2020; Published: 6 April 2020 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. Keywords: DIVARICATA; DRIF; RADIALIS; MYB transcription factors; flower symmetry 1. Introduction The success of flowering plants is strictly related to the evolutionary innovations enclosed in the flower, whose ancestral form can be dated back 140–250 million years ago (Mya) [1–4]. The ≈ extraordinary morphological diversity displayed by flowers, affects the shape, size, and color of the perianth, as well as the disposition of the floral organs, resulting in different symmetry types, among which radial symmetry (actinomorphy) represents the ancestral state [4,5]. During the first angiosperm radiation (late Cretaceous, 93–89 Mya), the first changes from radially symmetric to asymmetric flower ≈ appeared [6,7]. The asymmetric flower is considered plesiomorphic to the bilaterally symmetric flower (zygomorphic), which evolved later during flower evolution (Paleogene, 65–34 Mya) (Figure1)[ 6]. ≈ However, the transition from radial to bilateral symmetry has occurred independently many times during flower evolution [8], possibly as a result of the adoption of different pollination strategies. In fact, radial symmetry allows pollination by many types of insects, while bilateral symmetry tends to promote interactions only with specific pollinators that have often coevolved with the bilaterally symmetric flowers; for example, the bilateral flowers of the orchid genus Ophrys are visited by male wasps attracted by the high resemblance of the lip to female insects [9–11]. The molecular basis of floral symmetry, such as that of other relevant processes regarding the development, life cycle, and metabolism of plants, is regulated by the action of specific transcription factors (TFs). During evolution, the number of plant TF families has expanded, ranging from 36 in Chlorophyta to 58 in Eudicots (Figure2)[ 12]. The progressive increment of the number of TF families is probably linked to the increase of plant and flower complexity, to the number of genome duplications [12] and, more generally, to the evolution of plant genome complexity [13]. Genes 2020, 11, 395; doi:10.3390/genes11040395 www.mdpi.com/journal/genes Genes 2020, 11, x FOR PEER REVIEW 2 of 14 Genes 2020, 11, x FOR PEER REVIEW 2 of 14 Genes 2020, 11, 395 2 of 14 Figure 1. Different types of floral symmetry; (A) radially symmetric flower (actinomorphic), (B) asymmetrical flower, (C) bilaterally symmetric flower (zygomorphic). The molecular basis of floral symmetry, such as that of other relevant processes regarding the development, life cycle, and metabolism of plants, is regulated by the action of specific transcription factors (TFs). During evolution, the number of plant TF families has expanded, ranging from 36 in Chlorophyta to 58 in Eudicots (Figure 2) [12]. The progressive increment of the number of TF families is probably linked to the increase of plant and flower complexity, to the number of genome duplicationsFigure 1. [12Di]ff and,erent more types general of florally, symmetry;to the evolution (A) radially of plant symmetric genome complexity flower (actinomorphic), [13]. (B) asymmetrical flower, (C) bilaterally symmetric flower (zygomorphic). Figure 1. Different types of floral symmetry; (A) radially symmetric flower (actinomorphic), (B) asymmetrical flower, (C) bilaterally symmetric flower (zygomorphic). The molecular basis of floral symmetry, such as that of other relevant processes regarding the development, life cycle, and metabolism of plants, is regulated by the action of specific transcription factors (TFs). During evolution, the number of plant TF families has expanded, ranging from 36 in Chlorophyta to 58 in Eudicots (Figure 2) [12]. The progressive increment of the number of TF families is probably linked to the increase of plant and flower complexity, to the number of genome duplications [12] and, more generally, to the evolution of plant genome complexity [13]. Figure 2. The evolution of the number of transcription factors (TF) families from Chlorophytae to Eudicots. Starting from Chlorophytae, the number of TF families increased in number and type. The number of ancestral TF families present in Chlorophytae was 36. New TF families have originated in Charophyta (11) and in Marchantiophyta (7). The last appearance of new TF families occurred before the origin of Bryophyta, with the recruitment of 4 new families (adapted from [12]). The classification of the TFs in different families is based on the presence of specific domains able to bind to a regulatory target sequence. Some TF families are very ancient, are present in all the plant lineages, and have assumed different roles during evolution. Among them, the MADS-box family is one of the most ancient, present since the evolution of the Chlorophyta [12]. The MADS-box TFs are involved in a wide range of developmental pathways, from spore germination [14], gametophyte and sporophyte generation [14,15], the formation of motile flagella in sperms of non-seed plants [16], to root development [17–19], abiotic stress responses [20], tuber dormancy [21], and fruit expansion [22]. In addition, they have been recruited in the pathway that drives the determination of flower organs, as explained by the canonical ABCDE model (mainly in eudicots) and its modifications (e.g., the fading borders in basal angiosperms, magnoliids, and basal eudicots, and the orchid code model in orchids) [23–28]. 2. The MYB Transcription Factors The MYB TF family, together with the MADS-box TFs, was present during plant evolution, starting from the Chlorophyta lineage (Figure2)[ 12]. The MYB TFs have been initially identified in species Genes 2020, 11, 395 3 of 14 distantly related to the plant kingdom (e.g., human, chicken, mouse, and fruit fly) as involved in the oncogenic process [29–34]. The first MYB TF isolated in plants was COLORED1 from Zea mays, which was involved in anthocyanin synthesis [35], and its homolog has been found in Arabidopsis thaliana [36]. Subsequently, MYB TFs have been identified in all eukaryotic organisms, revealing a protein structure conserved during evolution. All the MYB proteins are characterized by the presence of a variable number (from one to four and more) of MYB repeats (R). The R sequence is composed of ~ 52 amino acids and includes three regularly spaced residues of tryptophan or other aliphatic amino acids [37]; this structure forms a hydrophobic core composed of α-helices, where two helices adopt a helix–turn–helix (HTH) conformation [37,38]. This structure is necessary for DNA binding and protein–protein interactions [38,39]. Based on the number of R repeats, the MYB TFs are classified into 4R, 3R, 2R, and 1R-MYB types. The 3R-MYBs have three R repeats (R1R2R3) and originated before the divergence between animals and plants [40–42]. They are mainly involved in the regulation of the cell cycle both in animals (as in humans, zebrafish, and fruit fly) [43–45] and plants (as in A. thaliana)[46]. The 2R-MYBs are plant-specific, have two R repeats (R2R3), and are the largest group of plant MYB TFs [47–50]. They are involved in various plant-specific processes such as the response to hormones, the identity of specific cell types, and regulation of secondary metabolism [51]. Two hypotheses have been proposed on the evolution of the 2R- and 3R-MYB types, known as ‘the gain model’ and ‘the loss model’ (Figure3). The gain model hypothesizes the existence of an ancient 2R-MYB before the animal-plant divergence. A subsequent intragenic domain duplication resulted in the origin of the 3R-MYB type, with the gain of another R repeat (R1), followed by the lineage-specificGenes 2020 extinction, 11, x FOR PEER of REVIEW the 2R-MYB type in animals [47,48]. In contrast, the loss model4 of 14 considers the 3R-MYB type the ancestor of the whole MYB superfamily. After the divergence between animals circadian clock and epidermal cell differentiation in A. thaliana, and binding telomeric DNA regions and plants,in thePetroselinum lineage-specific crispum and loss Z. mays of the[54]. R1 repeat in duplicated 3R-MYBs gave rise to the 2R-MYB type in plants [42,47,49,50]. Figure 3. The evolution of the different types of MYB TFs. The alternative ‘gain’ or ‘loss’ model explains Figure 3. The evolution of the different types of MYB TFs. The alternative ‘gain’ or ‘loss’ model the origin of the 3R- and 2R-MYB types through the acquisition or deletion of the R1 repeat, respectively. explains the origin of the 3R- and 2R-MYB types through the acquisition or deletion of the R1 repeat, The 4R-MYBsrespectively.