A classical role of the hormone auxin is in the formation of flowers at the periphery of the reproductive shoot apex. Mutants in regulators of polar auxin transport or in the auxin-responsive transcription factor MONOPTEROS (MP) form naked inflorescence "pins" lacking flowers. How auxin maxima and MP direct initiation of flower primordia is poorly understood. Here, we identify three genes whose expression is directly induced by auxin-activated MP that furthermore jointly regulate flower primordium initiation. These three genes encode known regulators of flower development: LEAFY (LFY), which specifies floral fate, and two AINTEGUMENTA-LIKE/PLETHORA transcription factors, key regulators of floral growth. Our study thus reveals a mechanistic link between flower primordium initiation and subsequent steps in flower morphogenesis. Finally, we uncover direct positive feedback from LFY to the auxin pathway. The auxin LFY module we describe may have been recruited during evolution to pattern other plant organ systems.

Although several members of the AP2/ERF family of transcription factors are important developmental regulators in plants, many genes in this large protein family remain uncharacterized. Here, we present a phylogenetic analysis of the 18 genes that make up the AP2 subgroup of this family. We report expression analyses of seven Arabidopsis genes most closely related to the floral development gene AINTEGUMENTA (ANT) and show that all AINTEGUMENTA-like (AIL) genes are transcribed in multiple tissues during development. They are expressed primarily in young actively dividing tissues of a plant and not in mature leaves or stems. The spatial distribution of AIL5, AIL6, and AIL7 mRNA in inflorescences was characterized by in situ hybridization. Each of these genes is expressed in a spatially and temporally distinct pattern within inflorescence meristems and flowers. Ectopic expression of AIL5 resulted in a larger floral organ phenotype, similar to that resulting from ectopic expression of ANT. Our results are consistent with AIL genes having roles in specification of meristematic or division-competent states.

The Arabidopsis protein AINTEGUMENTA (ANT) is a member of a plant-specific family of transcription factors (AP2/EREBP) that share either one or two copies of an approximately 70 amino acid region called the AP2 repeat. DNA binding activity has been demonstrated previously for members of this family containing a single AP2 repeat. Using an in vitro selection procedure, the DNA binding specificity of the two AP2 repeat containing protein ANT was found to be 5'-gCAC(A/G)N(A/T)TcCC(a/g)ANG(c/t)-3'. This consensus site is much longer than sites recognized by proteins containing a single AP2 repeat and neither AP2 repeat of ANT was alone capable of binding to the selected sequences, suggesting that both AP2 repeats make DNA contacts. ANT binds to these DNA sequences as a monomer but a higher order complex is also observed at high protein concentrations. The ANT consensus site shows some similarity to the C-repeat/DRE elements bound by proteins that contain a single AP2 repeat, and we find that ANT binds weakly to such sites. We propose a model in which each AP2 repeat of ANT contacts adjacent sites within the consensus sequence. Our results suggest that the AP2 repeat can be utilized in different ways for DNA binding.

Lateral organs in flowering plants display polarity along their adaxial-abaxial axis with distinct cell types forming at different positions along this axis. Members of three classes of transcription factors in Arabidopsis (Arabidopsis thaliana; the Class III homeodomain/leucine zipper [HD-ZIP] proteins, KANADI proteins, and YABBY proteins) are expressed in either the adaxial or abaxial domain of organ primordia where they confer these respective identities. Little is known about the factors that act upstream of these polarity-determining genes to regulate their expression. We have investigated the relationship between AINTEGUMENTA (ANT), a gene that promotes initiation and growth of lateral organ primordia, and polarity genes. Although ant single mutants do not display any obvious defects in organ polarity, loss of ANT activity in combination with mutations in one or more YABBY genes results in polarity defects greater than those observed in the yabby mutants alone. Our results suggest that ANT acts in combination with the YABBY gene FILAMENTOUS FLOWER (FIL) to promote organ polarity by upregulating the expression of the adaxial-specifying HD-ZIP gene PHABULOSA. Furthermore, we show that ANT acts with FIL to up-regulate expression of the floral homeotic gene APETALA3. Our work defines new roles for ANT in the development of lateral organs.

The Arabidopsis (Arabidopsis thaliana) gynoecium, the female floral reproductive structure, requires the action of genes that specify positional identities during its development to generate an organ competent for seed development and dispersal. Early in gynoecial development, patterning events divide the primordium into distinct domains that will give rise to specific tissues and organs. The medial domain of the gynoecium gives rise to the ovules, and several other structures critical for reproductive competence. Here we report a synergistic genetic interaction between seuss and aintegumenta mutants resulting in a complete loss of ovule initiation and a reduction of the structures derived from the medial domain. We show that patterning events are disrupted early in the development of the seuss aintegumenta gynoecia and we identify PHABULOSA (PHB), REVOLUTA, and CRABS CLAW (CRC) as potential downstream targets of SEUSS (SEU) and AINTEGUMENTA (ANT) regulation. Our genetic data suggest that SEU additionally functions in pathways that are partially redundant and parallel to PHB, CRC, and ANT. Thus, SEU and ANT are part of a complex and robust molecular system that coordinates patterning cues and cellular proliferation along the three positional axes of the developing gynoecium.During organ development, positional identity information must be coordinated with cellular proliferation to achieve proper organ shape and function. In the Arabidopsis (Arabidopsis thaliana) gynoecium, the female reproductive floral structure, early patterning events divide the gynoecial primordium into distinct zones that distinguish adaxial (inner) versus abaxial (outer), medial versus lateral, and apical versus basal