Molecular framework underlying floral bilateral symmetry and nectar spur development in Tropaeolum, an atypical member of the Brassicales

Abstract

Premise: Floral spurs are key innovations associated with elaborate pollination mechanisms that have evolved independently several times across angiosperms. Spur formation can shift the floral symmetry from radial to bilateral, as it is the case in Tropaeolum, the only member of the Brassicales with floral nectar spurs. The genetic mechanisms underlying both spur and bilateral symmetry in the family have not yet been investigated. Methods: We studied flower development and morphoanatomy of Tropaeolum longifolium. We also generated a reference transcriptome and isolated all candidate genes involved in adaxial-abaxial differential growth during spur formation. Finally, we evaluated the evolution of the targeted genes across Brassicales and examined their expression in dissected floral parts. Results: Five sepals initiate spirally, followed by five petals alternate to the sepals, five antesepalous stamens, three antepetalous stamens, and three carpels. Intercalary growth at the common base of sepals and petals forms a floral tube. The spur is an outgrowth from the adaxial region of the tube, lined up with the medial sepal. We identified Tropaeolum specific duplications in the TCP3/4L and STM gene lineages, which are critical for spur formation in other taxa. In addition, we found that TM6 (MADS-box), RL2 (RAD-like7), and KN2/6L2 and OSH6L (KNOX1 genes), have been lost in core Brassicales but retained in Tropaeolum. Conclusions: Three genes are pivotal during the extreme adaxial-abaxial asymmetry of the floral tube, namely, TlTCP4L2 restricted to the adaxial side where the spur is formed, and TlTCP12 and TlSTM1 to the abaxial side, lacking a spur. K E Y W O R D S bilateral floral symmetry, Brassicales, CINCINNATA, CYCLOIDEA, DIVARICATA, floral tube, KNOX, RADIALIS, spur development, TropaeolaceaeFloral spurs are hollow outgrowths that produce and accumulate nectar inside. They have evolved independently in multiple angiosperm lineages and develop through distinct ontogenetic pathways from any floral organ, although they are more frequently sepal-or petal-derived (Hodges and Arnold, 1995;Hodges, 1997). The monogeneric family Tropaeolaceae comprising ca. 90 species (Andersson and Andersson, 2000) native to the Americas is the only family of the highly diversified Brassicales with a floral medial nectar spur (Ronse De Craene and Smets, 2001;Ronse De Craene, 2018). Spurs in Tropaeolaceae have been considered either as directly formed from the floral receptacle (Ronse De Craene and Smets, 2001;Ronse De Craene, 2018), from sepals alone (e.g., Astié, 1962;Rachmilevitz and Fahn, 1975) or from sepals and petals (Rama Devi and Narayana, 1994). Additionally, in Tropaeolum, spur formation results in strong bilateral symmetry (Figure 1).Genes that control spur development have been identified in Ranunculaceae (

The cellular and molecular basis of the spur development in Impatiens uliginosa

“…In contrast, no KNOX gene expression was detected in the transcriptome of early spurs and in in situ hybridization in a broader development stage [ 27 ]. Studies on Tropaeolum showed that both TCP and KNOX genes may be involved in spur development and play a crucial role in extreme adaxial-abaxial asymmetry [ 28 ]. Furthermore, coregulated gene groups that mediate hormone synthesis or response also participate in spur development.…”