Evolution and genetic control of the floral ground plan

Abstract

Contents Summary70I.Introduction70II.What is the floral ground plan?71III.Diversity and evolution of the floral ground plan72IV.Genetic mechanisms77V.What's next?82Acknowledgements83References83 Summary The floral ground plan is a map of where and when floral organ primordia arise. New results combining the defined phylogeny of flowering plants with extensive character mapping have predicted that the angiosperm ancestor had whorls rather than spirals of floral organs in large numbers, and was bisexual. More confidently, the monocot ancestor likely had three organs in each whorl, whereas the rosid and asterid ancestor (Pentapetalae) had five, with the perianth now divided into sepals and petals. Genetic mechanisms underlying the establishment of the floral ground plan are being deduced using model species, the rosid Arabidopsis, the asterid Antirrhinum, and in grasses such as rice. In this review, evolutionary and genetic conclusions are drawn together, especially considering how known genes may control individual processes in the development and evolution of ground plans. These components include organ phyllotaxis, boundary formation, organ identity, merism (the number or organs per whorl), variation in the form of primordia, organ fusion, intercalary growth, floral symmetry, determinacy and, finally, cases where the distinction between flowers and inflorescences is blurred. It seems likely that new pathways of ground plan evolution, and new signalling mechanisms, will soon be uncovered by integrating morphological and genetic approaches.

A design principle for floral organ number and arrangement in flowers with bilateral symmetry

“…Our model can be applied to the phylogenetic relationship within clades. The co-existence of merosity, especially that of pentamery and tetramery, is widely found in the eudicot clades (Smyth, 2018), not only in Plantaginaceae (Lamiales, Fig. 4A) but also in Dipsacales (e.g.…”

“…Merosity describes the common organ number of perianth whorls and is usually clade specific ( Fig. 1C; Ronse De Craene, 2010; Smyth, 2018). In eudicots, the largest clade of flowering plants, the common number is usually four or five, whereas in monocots, the sister clade to eudicots, it is three (Ronse De Craene and Brockington, 2013;Endress, 2010;Remizowa et al, 2010).…”

Reconstructing development of the earliest seed integuments raises a new hypothesis for the evolution of ancestral seed‐bearing structures

“…Integument lobe fusion and lobe number were both variable in Genomosperma . These characteristics are highly reminiscent of development in extant floral meristems (reviewed by Smyth, 2018) in which the number of floral organs (merosity) is usually stable and stereotypical, but variability (including the occurrence of fused organs) has been observed between individual flowers in Arabidopsis thaliana (Plackett et al ., 2018b). Both merosity and phyllotaxy are far less stable in extant early‐diverging angiosperms such as Amborella trichopoda and the Austrobaileyales (Specht & Bartlett, 2009).…”

Homologs of LEAFY and UNUSUAL FLORAL ORGANS Promote the Transition From Inflorescence to Floral Meristem Identity in the Cymose Aquilegia coerulea

“…In the flower itself, one possible explanation for the additional sepal whorls is that they are connected to the initiation of B gene expression. Both LFY and UFO typically play roles in activating the B class genes, particularly AP3 , which then participates with the other B gene PISTILLATA ( PI ) in an auto-regulatory feedback loop that stabilizes their mutual expression (reviewed Smyth, 2018). Consistent with this, it is common to see classic “B class” homeosis in both LFY and UFO mutants (reviewed Moyroud et al, 2010), resulting in the transformation of petals into sepals and stamens into carpels.…”