In Arabidopsis thaliana, flowers are determinate, showing a fixed number of whorls. Here, we report on three independent genes, a novel gene REBELOTE (RBL; protein of unknown function), SQUINT (SQN; a cyclophilin), and ULTRAPETALA1 (ULT1; a putative transcription factor) that redundantly influence floral meristem (FM) termination. Their mutations, combined with each other or with crabs claw, the genetic background in which they were isolated, trigger a strong FM indeterminacy with reiterations of extra floral whorls in the center of the flower. The range of phenotypes suggests that, in Arabidopsis, FM termination is initiated from stages 3 to 4 onwards and needs to be maintained through stage 6 and beyond, and that RBL, SQN, and ULT1 are required for this continuous regulation. We show that mutant phenotypes result from a decrease of AGAMOUS (AG) expression in an inner 4th whorl subdomain. However, the defect of AG activity alone does not explain all reported phenotypes, and our genetic data suggest that RBL, SQN, and, to a lesser extent, ULT1 also influence SUPERMAN activity. Finally, from all the molecular and genetic data presented, we argue that these genes contribute to the more stable and uniform development of flowers, termed floral developmental homeostasis.

The molecular and genetic networks underlying the determination of floral organ identity are well studied, but much less is known about how the flower is partitioned into four developmentally distinct whorls. The SUPERMAN gene is required for proper specification of the boundary between stamens in whorl 3 and carpels in whorl 4, as superman mutants exhibit supernumerary stamens but usually lack carpels. However, it has remained unclear whether extra stamens in superman mutants originate from an organ identity change in whorl 4 or the overproliferation of whorl 3. Using live confocal imaging, we show that the extra stamens in superman mutants arise from cells in whorl 4, which change their fate from female to male, while floral stem cells proliferate longer, allowing for the production of additional stamens

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Proper floral patterning, including the number and position of floral organs in most plant species, is tightly controlled by the precise regulation of the persistence and size of floral meristems (FMs). In , two known feedback pathways, one composed of WUSCHEL (WUS) and CLAVATA3 (CLV3) and the other composed of AGAMOUS (AG) and WUS, spatially and temporally control floral stem cells, respectively. However, mounting evidence suggests that other factors, including phytohormones, are also involved in floral meristem regulation. Here, we show that the boundary gene () bridges floral organogenesis and floral meristem determinacy in another pathway that involves auxin signaling. SUP interacts with components of polycomb repressive complex 2 (PRC2) and fine-tunes local auxin signaling by negatively regulating the expression of the auxin biosynthesis genes (). In mutants, derepressed local activity elevates auxin levels at the boundary between whorls 3 and 4, which leads to an increase in the number and the prolonged maintenance of floral stem cells, and consequently an increase in the number of reproductive organs. Our work presents a new floral meristem regulatory mechanism, in which , a boundary gene, coordinates floral organogenesis and floral meristem size through fine-tuning auxin biosynthesis.

The link between gene regulation and morphogenesis of multicellular organisms is a fundamental problem in biology. We address this question in the floral meristem of Arabidopsis, which generates new tissues and organs through complex changes in growth patterns. Starting from high-resolution time-lapse images, we generated a comprehensive 4-D atlas of early flower development including cell lineage, cellular growth rates and the expression patterns of 28 regulatory genes. This information was introduced in MorphoNet, a web-based open-access platform.The application of mechanistic computational models indicated that the molecular network based on the literature only explained a minority of the expression patterns. This was substantially improved by adding single regulatory hypotheses for individual genes. We next used the integrated information to correlate growth with the combinatorial expression of multiple genes. This led us to propose a set of hypotheses for the action of individual genes in morphogenesis, not visible by simply correlating gene expression and growth. This identified the central transcription factor LEAFY as a potential regulator of heterogeneous growth, which was supported by quantifying growth patterns in a leafy mutant. By providing an integrated, multiscale view of flower development, this atlas should represent a fundamental step towards mechanistic multiscale-scale models of flower development.

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Recent advances in confocal microscopy, coupled with the development of numerous fluorescent reporters, provide us with a powerful tool to study the development of plants. Live confocal imaging has been used extensively to further our understanding of the mechanisms underlying the formation of roots, shoots and leaves. However, it has not been widely applied to flowers, partly because of specific challenges associated with the imaging of flower buds. Here, we describe how to prepare and grow shoot apices of Arabidopsis in vitro, to perform both single-point and time-lapse imaging of live, developing flower buds with either an upright or an inverted confocal microscope.

The field of Arabidopsis flower development began in the early 1980s with the initial description of several mutants including apetala1, apetala2, and agamous that altered floral organ identity (Koornneef and van der Veen, Theor Appl Genet 58:257-263, 1980; Koornneef et al., J Hered 74:265-272, 1983). By the end of the 1980s, these mutants were receiving more focused attention to determine precisely how they affected flower development (Komaki et al., Development 104:195-203, 1988; Bowman et al., Plant Cell 1:37-52, 1989). In the last quarter century, impressive progress has been made in characterizing the gene products and molecular mechanisms that control the key events in flower development. In this review, we briefly summarize the highlights of work from the past 25 years but focus on advances in the field in the last several years.