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

Bharti Sharma,Elena M Kramer,Damien Wolfe,Cristina Walcher-Chevillet,Clara Meaders,Lynn Holappa

doi:10.3389/fpls.2019.01218

Abstract

Homologs of the transcription factor LEAFY (LFY) and the F-box family member UNUSUAL FLORAL ORGANS (UFO) have been found to promote floral meristem identity across diverse dicot model systems. The lower eudicot model Aquilegia produces cymose inflorescences that are independently evolved from the well-studied cymose models Petunia and tomato. We have previously characterized the expression pattern of the Aquilegia homolog AqLFY but in the current study, we add expression data on the two UFO homologs, AqUFO1 and 2, and conduct virus-induced gene silencing of all the loci. Down-regulation of AqLFY or AqUFO1 and 2 does not eliminate floral meristem identity but, instead, causes the transition from inflorescence to floral identity to become gradual rather than discrete. Inflorescences in down-regulated plants generate several nodes of bract/sepal chimeras and, once floral development does commence, flowers initiate several whorls of sepals before finally producing the wildtype floral whorls. In addition, silencing of AqUFO1/2 appears to specifically impact petal identity and/or the initiation of petal and stamen whorls. In general, however, there is no evidence for an essential role of AqLFY or AqUFO1/2 in transcriptional activation of the B or C gene homologs. These findings highlight differences between deeply divergent dicot lineages in the functional conservation of the floral meristem identity program.

Highlights

The generation of inflorescence architecture reflects a complex interplay between two meristem identity programs
As a point of reference, dicot lfy mutants characterized to date typically show partial to complete loss of floral meristem (FM) identity, and when flowers are produced, they show conversion of floral organs towards leaf identity, of the outer whorls, as well as shifts towards spiral phyllotaxy
The architecture of cymose inflorescences is fundamentally about timing— How long is the indeterminate inflorescence meristem (IM) identity program expressed in a given meristem before it transitions to determinate FM identity? In the previously studied cymose models of the Solanaceae, this timing is primarily controlled by differential expression of UNUSUAL FLORAL ORGANS (UFO) homologs (Park et al, 2014)

Summary

Introduction

The generation of inflorescence architecture reflects a complex interplay between two meristem identity programs (reviewed Bartlett and Thompson, 2014). The genetic basis of these two identity programs was first investigated in several model systems that produce racemes. In this monopodial architecture, the terminal IM retains indeterminacy while lateral meristems may immediately express determinate FM identity or, in the case of branched inflorescences, the expression of FM. LFY is further required to activate all of the genes that confer floral organ identity (Weigel and Meyerowitz, 1993), as described by the ABC model (Coen and Meyerowitz, 1991). The B class gene APETALA3, which confers petal and stamen identity, requires the presence of a co-factor, the F-box protein UNUSUAL FLORAL ORGANS (Lee et al, 1997). The stamen and carpel identity C class gene AGAMOUS is activated by LFY together with the homeodomain protein WUSCHEL (Lohmann et al, 2001)

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Conclusion