4035 Introduction Flowers share a common underlying architecture. In general, reproductive organs occur centrally, with the female carpels internal to the male stamens. Surrounding these are the perianth organs, which in many species are differentiated into inner, showy petals and outer, protective sepals. The number of floral organs in a flower, their spatial relationships and their degree of fusion are relatively conserved properties, presumably under genetic control. A range of genes that regulate floral architecture has been identified in Arabidopsis thaliana. The PERIANTHIA (PAN) bZIP transcription factor gene ensures that flowers arise with the appropriate numbers of organs in the outer three whorls (four sepals, four petals and six stamens) (Chuang et al., 1999). In pan mutants there are usually five in each case. By contrast, the architecture of the second and third whorls is supported by the UNUSUAL FLORAL ORGANS (UFO) F-box gene (Durfee et al., 2003; Laufs et al., 2003). Another gene, PRESSED FLOWER (PRS), encoding a homeodomain protein, has roles in defining the lateral regions of the flower primordium rather than the radial regions (Matsumoto and Okada, 2001). In prs mutants, lateral sepals are frequently absent. Other genes function to define boundaries. For example, the SUPERMAN (SUP) zinc finger gene acts to control cell proliferation in the boundary between the stamens and carpels (Sakai et al., 1995). In addition, boundaries between individual organs are controlled by CUP-SHAPED COTYLEDON (CUC) genes encoding NAC transcription factors. These act to keep the primordia of adjacent floral organs, especially the sepals, separate (Aida et al., 1997). Genes required for the development of specific organ types have also been described. For example, another function of UFO is to promote petal outgrowth, perhaps by targeting an inhibitor of this process for degradation (Durfee et al., 2003; Laufs et al., 2003). Another gene that specifically promotes petal growth, RABBIT EARS (RBE), encodes a zinc finger protein (Takeda et al., 2004). In null rbe mutant plants, petals are mostly filamentous or absent. The PETAL LOSS (PTL) gene of Arabidopsis plays a unique role in controlling perianth development (Griffith et al., 1999). In mutant plants, sepals are mis-shapen and sometimes fused with an adjacent sepal. Petals are often absent, and their mean number per flower falls progressively so that later-formed flowers usually have none. Those petals that do arise are often smaller than normal and are sometimes trumpet-shaped. Petal primordia occupy the same regions of the mutant flower primordium as in the wild type (internal to each of the inter- sepal zones), although the four regions are somewhat enlarged. Perianth development is specifically disrupted in mutants of the PETAL LOSS (PTL) gene, particularly petal initiation and orientation. We have cloned PTL and show that it encodes a plant-specific trihelix transcription factor, one of a family previously known only as regulators of light-controlled genes. PTL transcripts were detected in the early-developing flower, in four zones between the initiating sepals and in their developing margins. Strong misexpression of PTL in a range of tissues universally results in inhibition of growth, indicating that its normal role is to suppress growth between initiating sepals, ensuring that they remain separate. Consistent with this, sepals are sometimes fused in ptl single mutants, but much more frequently in double mutants with either of the organ boundary genes cup-shaped cotyledon1 or 2. Expression of PTL within the newly arising sepals is apparently prevented by the PINOID auxin-response gene. Surprisingly, PTL expression could not be detected in petals during the early stages of their development, so petal defects associated with PTL loss of function may be indirect, perhaps involving disruption to signalling processes caused by overgrowth in the region. PTL-driven reporter gene expression was also detected at later stages in the margins of expanding sepals, petals and stamens, and in the leaf margins; thus, PTL may redundantly dampen lateral outgrowth of these organs, helping define their final shape. Key words: PETAL LOSS (PTL), Arabidopsis, Trihelix, GT-factor, Flower development, Perianth Summary PETAL LOSS, a trihelix transcription factor gene, regulates perianth architecture in the Arabidopsis flower Philip B. Brewer, Paul A. Howles*, Kristen Dorian, Megan E. Griffith † , Tetsuya Ishida ‡ , Ruth N. Kaplan-Levy, Aydin Kilinc and David R. Smyth § School of Biological Sciences, Monash University, Melbourne, VIC 3800, Australia *Present address: Research School of Biological Sciences, ANU, Canberra, ACT 0200, Australia † Present address: Institute of Molecular and Cell Biology, 30 Medical Drive, Singapore 117609, Republic of Singapore ‡ Present address: Plant Science Center, RIKEN, Yokohama, Kanagawa 230-0045, Japan § Author for correspondence (e-mail: david.smyth@sci.monash.edu.au) Accepted 11 May 2004 Development 131, 4035-4045 Published by The Company of Biologists 2004 doi:10.1242/dev.01279 Research article