Auxin and Flower Development: A Blossoming Field.

The establishment of the species-specific floral organ body plan involves many coordinated spatiotemporal processes, which include the perception of positional information that specifies floral meristem and floral organ founder cells, coordinated organ outgrowth coupled with the generation and maintenance of inter-organ and inter-whorl boundaries, and the termination of meristem activity. Auxin is integrated within the gene regulatory networks that control these processes and plays instructive roles at the level of tissue-specific biosynthesis and polar transport to generate local maxima, perception, and signaling. Key features of auxin function in several floral contexts include cell nonautonomy, interaction with cytokinin gradients, and the central role of MONOPTEROS and ETTIN to regulate canonical and noncanonical auxin response pathways, respectively. Arabidopsis flowers are not representative of the enormous angiosperm floral diversity; therefore, comparative studies are required to understand how auxin underlies these developmental differences. It will be of great interest to compare the conservation of auxin pathways among flowering plants and to discuss the evolutionary role of auxin in floral development.

The founder-cell transcriptome in the Arabidopsis apetala1 cauliflower inflorescence meristem.

BACKGROUND: Although the pattern of lateral organ formation from apical meristems establishes species-specific plant architecture, the positional information that confers cell fate to cells as they transit to the meristem flanks where they differentiate, remains largely unknown. We have combined fluorescence-activated cell sorting and RNA-seq to characterise the cell-type-specific transcriptome at the earliest developmental time-point of lateral organ formation using DORNRÖSCHEN-LIKE::GFP to mark founder-cell populations at the periphery of the inflorescence meristem (IM) in apetala1 cauliflower double mutants, which overproliferate IMs. RESULTS: Within the lateral organ founder-cell population at the inflorescence meristem, floral primordium identity genes are upregulated and stem-cell identity markers are downregulated. Additional differentially expressed transcripts are involved in polarity generation and boundary formation, and in epigenetic and post-translational changes. However, only subtle transcriptional reprogramming within the global auxin network was observed. CONCLUSIONS: The transcriptional network of differentially expressed genes supports the hypothesis that lateral organ founder-cell specification involves the creation of polarity from the centre to the periphery of the IM and the establishment of a boundary from surrounding cells, consistent with bract initiation. However, contrary to the established paradigm that sites of auxin response maxima pre-pattern lateral organ initiation in the IM, auxin response might play a minor role in the earliest stages of lateral floral initiation.

Auxin response factors.

Auxin signalling involves the activation or repression of gene expression by a class of auxin response factor (ARF) proteins that bind to auxin response elements in auxin-responsive gene promoters. The release of ARF repression in the presence of auxin by the degradation of their cognate auxin/indole-3-acetic acid repressors forms a paradigm of transcriptional response to auxin. However, this mechanism only applies to activating ARFs, and further layers of complexity of ARF function and regulation are being revealed, which partly reflect their highly modular domain structure. This review summarizes our knowledge concerning ARF binding site specificity, homodimer and heterodimer multimeric ARF association and cooperative function and how activator ARFs activate target genes via chromatin remodelling and evolutionary information derived from phylogenetic comparisons from ARFs from diverse species. ARFs are regulated in diverse ways, and their importance in non-auxin-regulated pathways is becoming evident. They are also embedded within higher-order transcription factor complexes that integrate signalling pathways from other hormones and in response to the environment. The ways in which new information concerning ARFs on many levels is causing a revision of existing paradigms of auxin response are discussed.

Cytokinin-auxin crosstalk in cell type specification.

Auxin and cytokinin affect cell fate specification transcriptionally and non-transcriptionally, and their roles have been characterised in several founder cell specification and activation contexts. Similarly to auxin, local cytokinin synthesis and response gradients are instructive, and the roles of ARABIDOPSIS RESPONSE REGULATOR 7/15 (ARR7/15) and the negative cytokinin response regulator ARABIDOPSIS HISTIDINE PHOSPHOTRANSFER PROTEIN 6, as well as auxin signalling via MONOPTEROS/BODENLOS, are functionally conserved across different developmental processes. Auxin and cytokinin crosstalk is tissue- and context-specific, and may be synergistic in the shoot apical meristem (SAM) but antagonistic in the root. We review recent advances in understanding the interactions between auxin and cytokinin in pivotal developmental processes, and show that feedback complexity and the multistep nature of specification processes argue against a single morphogenetic signal.

Polarity in the early floral meristem of Arabidopsis.

The diversity of angiosperm flowers depends on organ meristy and position. However, the signaling pathways that establish polarity and positional information remain largely unelucidated. Use of the founder-cell marker DORNRÖSCHEN-LIKE (DRNL) in Arabidopsis has recently highlighted the importance of the abaxial-adaxial axis for early floral development. We have extended the use of DRNL::GFP to further characterize floral organogenesis in genotypes that are altered in floral organ meristy or position, including ettin (ett-3) and blade-on-petiole (bop)1-11 bop2-4 double mutants. The creation of supernumery sepals by the splitting of sepal founder-cell populations along an ab-/adaxial axis strengthens the importance of the ab-/adaxial developmental axis in early floral meristem development. Furthermore, we confirm the dependency of the wildtype sequence of sepal initiation on bract suppression and demonstrate that supernumery stamens derive from the imprecise resolution of a ring of DRNL expression. Expression of DRNL in apetala1 (ap1-1) and ap2-8 mutants reflect the altered whorl structure and show that these homeotic genes function upstream of DRNL. Analyzing the dynamism of early floral meristem ontogeny at a fine temporal and spatial resolution in Arabidopsis can reveal mechanisms of organogenesis and is applicable to other species with differing floral body plans in a comparative evolutionary context.

The role of Dornröschen-like in early floral organogenesis.

Positional signals that specify founder cells and determine where lateral organs initiate and how these signals are perceived by cells that transition to the periphery of the meristem is a challenging problem. We recently showed that expression of the AP2 ERF transcription factor Dornröschen-like (DRNL) marks all floral organ founder cells and pre-patterns lateral stamen and petal, or medial stamen founder cells by two regions of expression that we propose represent morphogenetic fields, that subsequently resolve into discrete foci. The spatio-temporal expression pattern of DRNL allows speculation concerning evolutionary aspects of plant developmental biology and the control of the floral plant body. It further paves the way to use DRNL as a tool to address fundamental questions of cell type specification.

DORNRÖSCHEN-LIKE expression marks Arabidopsis floral organ founder cells and precedes auxin response maxima.

Live imaging during floral development revealed that expression of the DORNRÖSCHEN-LIKE (DRNL) gene encoding an AP2-like transcription factor, marks all organ founder cells. Transcription precedes the perception of auxin response maxima as measured by the DR5 reporter and is unaffected in early organogenesis, by mutation of four canonical auxin response elements (AuxREs) in the DRNL promoter. DRNL expression identifies discrete modes of organ initiation in the four floral whorls, from individual or pairs of organ anlagen in the outer whorl of sepals to two morphogenetic fields pre-patterning petals and lateral stamens, or a ring-shaped field giving rise to the medial stamens before carpel primordia are specified. DRNL function only overlaps in the central stem cell zone with that of its paralogue, DORNRÖSCHEN (DRN). drnl mutants are affected in floral organ outgrowth, which functionally interplays with boundary specification as organ fusions are sensitized by loss of CUP-SHAPED COTYLEDON (CUC) gene activity, and synergistic interactions exist with mutants in local auxin biosynthesis and polar transport. DRNL apparently monitors and contributes to cellular decisions in the SAM and thus provides a novel molecular access to the interplay of founder cell specification, organ anlage and organogenesis in the SAM peripheral zone.