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During Arabidopsis embryo development, cotyledon primordia are generated at transition stage from precursor cells that are not derived from the embryonic shoot apical meristem (SAM). To date, it is not known which genes specifically instruct these precursor cells to elaborate cotyledons, nor is the role of auxin in cotyledon development clear. In laternemutants, the cotyledons are precisely deleted, yet the hypocotyl and root are unaffected. The laterne phenotype is caused by a combination of two mutations: one in the PINOID (PID) gene and another mutation in a novel locus designated ENHANCER OF PINOID (ENP). The expression domains of shoot apex organising genes such as SHOOT MERISTEMLESS (STM) extend along the entire apical region of laterne embryos. However, analysis of pid enp stm triple mutants shows that ectopic activity of STM does not appear to cause cotyledon obliteration. This is exclusively caused by enp in concert with pid. In pinoid embryos, reversal of polarity of the PIN1 auxin transport facilitator in the apex is only occasional, explaining irregular auxin maxima in the cotyledon tips. By contrast, polarity of PIN1:GFP is completely reversed to basal position in the epidermal layer of the laterne embryo. Consequently auxin, which is believed to be essential for organ formation, fails to accumulate in the apex. This strongly suggests that ENP specifically regulates cotyledon development through control of PIN1 polarity in concert with PID.

The spatial coordination of growth is of central importance for the regulation of plant tissue architecture. Individual layers, such as the epidermis, are clonally propagated and structurally maintained by symmetric cell divisions that are oriented along the plane of the layer. The developmental control of this process is poorly understood. The simple cellular basis and sheet-like structure of Arabidopsis integuments make them an attractive model system to address planar growth. Here we report on the characterization of the Arabidopsis UNICORN (UCN) gene. Analysis of ucn integuments reveals localized distortion of planar growth, eventually resulting in an ectopic multicellular protrusion. In addition, ucn mutants exhibit ectopic growth in filaments and petals, as well as aberrant embryogenesis. We further show that UCN encodes an active AGC VIII kinase. Genetic, biochemical, and cell biological data suggest that UCN suppresses ectopic growth in integuments by directly repressing the KANADI transcription factor ABERRANT TESTA SHAPE. Our findings indicate that UCN represents a unique plant growth regulator that maintains planar growth of integuments by repressing a developmental regulator involved in the control of early integument growth and polarity.AGC protein kinase | growth suppression | floral development | ovule | signal transduction P lant tissue morphogenesis depends on the coordination of cellular behavior within tissue layers. The formation of distinct layers depends on asymmetric cell division, the developmental control of which is under intense investigation (1, 2). After initiation, individual cell layers are propagated by symmetric cell divisions (3) with division planes often oriented along the plane of the layer (planar growth). For example, the layered structure of the epidermis is maintained by anticlinal cell divisions, whereas periclinal cell divisions are usually suppressed (4). Division planes of symmetrically dividing cells can be accurately predicted by a mathematical rule linking cell geometry and cytoskeletal dynamics (5), and much progress has been made in the elucidation of the cellular machinery controlling the division plane of dividing plant cells. However, developmental controls that maintain the planar orientation of symmetrical cell divisions within a tissue layer remain poorly understood (3).Arabidopsis integuments represent an attractive model to study tissue morphogenesis. They are lateral determinate tissues of the ovule, the progenitors of the seed coat, and undergo a regular mode of development resulting in simple tissue architecture. An inner and an outer integument originate from the epidermis of the central chalaza (6, 7). They grow into laminar extensions with each integument consisting of a bilayered sheet of anticlinally dividing cells. The outer integument eventually develops into a hood-like structure that envelops the distal nucellus carrying the developing embryo sac and the inner integument.Coordination of symmetrical cell divisions along the plane of the extendi...

Arabidopsis thaliana has, in conjunction with A. arenosa, developed into a system for the molecular analysis of alloplolyploidy. However, there are very few Arabidopsis lines available to study autopolyploidy. In order to investigate polyploidy on a reliable basis, we have optimised conventional methodologies and developed a novel strategy for the rapid generation and identification of polyploids based on trichome branching patterns. The analysis of more than two dozen independently induced Arabidopsis lines has led to interesting observations concerning the relationship between cell size and ploidy levels and on the relative stability of tetraploidy in Arabidopsis over at least three consecutive generations. The most important finding of this work is that neo-tetraploid lines exhibit considerable stability through all the generations tested. The systematic generation of tetraploid collections through this strategy as well as the lines generated in this work will help to unravel the consequences of polyploidy, particularly tetraploidy, on the genome, on gene expression and on natural diversity in Arabidopsis.

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