No abstract

Although it is generally accepted that auxin is important for the patterning of the female reproductive organ, the gynoecium, the flow as well as the temporal and spatial actions of auxin have been difficult to show during early gynoecial development. The primordium of the Arabidopsis (Arabidopsis thaliana) gynoecium is composed of two congenitally fused, laterally positioned carpel primordia bisected by two medially positioned meristematic regions that give rise to apical and internal tissues, including the ovules. This organization makes the gynoecium one of the most complex plant structures, and as such, the regulation of its development has remained largely elusive. By determining the spatiotemporal expression of auxin response reporters and localization of PINFORMED (PIN) auxin efflux carriers, we have been able to create a map of the auxin flow during the earliest stages of gynoecial primordium initiation and outgrowth. We show that transient disruption of polar auxin transport (PAT) results in ectopic auxin responses, broadened expression domains of medial tissue markers, and disturbed lateral preprocambium initiation. Based on these results, we propose a new model of auxin-mediated gynoecial patterning, suggesting that valve outgrowth depends on PIN1-mediated lateral auxin maxima as well as subsequent internal auxin drainage and provascular formation, whereas the growth of the medial domains is less dependent on correct PAT. In addition, PAT is required to prevent the lateral domains, at least in the apical portion of the gynoecial primordium, from obtaining medial fates.

A common aspect of gene regulation in all developmental systems is the sustained repression of key regulatory genes in inappropriate spatial or temporal domains. To understand the mechanism of transcriptional repression of the floral homeotic gene AGAMOUS (AG), we identified two mutations in the BELLRINGER (BLR) gene based on a striking floral phenotype, in which homeotic transformations from sepals to carpels are found in flowers derived from old terminating shoots. Furthermore, this phenotype is drastically enhanced by growth at a high temperature and by combining blr with mutants of LEUNIG and SEUSS, two putative transcriptional corepressors of AG. We showed that the floral phenotype of blr mutants is caused by derepression of AG, suggesting that BLR functions as a transcription repressor. Because BLR encodes a BELL1-like (BELL) homeobox protein, direct binding of BLR to AG cis-regulatory elements was tested by gel-shift assays, and putative BLR binding motifs were identified. In addition, these putative BLR binding motifs were shown to be conserved in 17 of the 29 Brassicaceae species by phylogenetic footprinting. Because BELL homeobox proteins are a family of plant-specific transcription factors with 12 members in Arabidopsis thaliana, our findings will facilitate the identification of regulatory targets of other BELL proteins and help determine their biological functions. The age-dependent and high temperature-enhanced derepression of AG in blr mutants led us to propose that AG expression might be regulated by a thermal time-dependent molecular mechanism.

Summary Divergence of developmental mechanisms within populations may lead to hybrid developmental failure, and may be a factor driving speciation in angiosperms. We investigate patterns of endosperm and embryo development in Mimulus guttatus and the closely related, serpentine endemic M. nudatus, and compare them to those of reciprocal hybrid seed. We address whether disruption in hybrid seed development is the primary source of reproductive isolation between these sympatric taxa. M. guttatus and M. nudatus differ in the pattern and timing of endosperm and embryo development. Some hybrid seed exhibit early disruption of endosperm development and are completely inviable, while others develop relatively normally at first, but later exhibit impaired endosperm proliferation and low germination success. These developmental patterns are reflected in mature hybrid seed, which are either small and flat (indicating little to no endosperm), or shriveled (indicating reduced endosperm volume). Hybrid seed inviability forms a potent reproductive barrier between M. guttatus and M. nudatus. We shed light on the extent of developmental variation between closely related species within the M. guttatus species complex, an important ecological model system, and provide a partial mechanism for the hybrid barrier between M. guttatus and M. nudatus.