Although curvature of biological surfaces has been considered from mathematical and biophysical perspectives, its molecular and developmental basis is unclear. We have studied the cin mutant of Antirrhinum , which has crinkly rather than flat leaves. Leaves of cin display excess growth in marginal regions, resulting in a gradual introduction of negative curvature during development. This reflects a change in the shape and the progression of a cell-cycle arrest front moving from the leaf tip toward the base. CIN encodes a TCP protein and is expressed downstream of the arrest front. We propose that CIN promotes zero curvature (flatness) by making cells more sensitive to an arrest signal, particularly in marginal regions.

This is the first report of a gene that is specifically required for transmitting-tract development in Arabidopsis. Mutations in NTT cause reduced fertility by severely inhibiting pollen-tube movement. The data support the idea that the function of the transmitting tract is to increase fertilization efficiency, particularly in the lower half of the ovary. This occurs by facilitating pollen-tube growth through differentiation and then death of transmitting-tract cells.

To understand how differentiation and growth may be coordinated during development, we have studied the action of the CINCINNATA (CIN) gene of Antirrhinum. We show that in addition to affecting leaf lamina growth, CIN affects epidermal cell differentiation and growth of petal lobes. Strong alleles of cin give smaller petal lobes with flat instead of conical cells, correlating with lobe-specific expression of CIN in the wild type. Moreover, conical cells at the leaf margins are replaced by flatter cells, indicating that CIN has a role in cell differentiation of leaves as well as petals. A weak semidominant cin allele affects cell types mainly in the petal but does not affect leaf development, indicating these two effects can be separated. Expression of CIN correlates with expression of cell division markers, suggesting that CIN may influence petal growth, directly or indirectly, through effects on cell proliferation. For both leaves and petals, CIN affects growth and differentiation of the more distal and broadly extended domains (leaf lamina and petal lobe). However, while CIN promotes growth in petals, it promotes growth arrest in leaves, possibly because of different patterns of growth control in these systems.Development involves coordination of two interconnected processes: growth and cellular differentiation. The genetic control of each of these processes has been studied extensively in plants. For example, genes affecting leaf growth or epidermal cell fate have been isolated and analyzed (Masucci et al., 1996;Tsuge et al., 1996;Gu et al., 1998;Kim et al., 1998;Mizukami and Fischer, 2000;Kim et al., 2002). However, the way that growth and differentiation are coupled through the action of genes has been less well studied. To help address this, we have investigated the effects of the CINCINNATA (CIN) gene on growth and cell differentiation in Antirrhinum.CIN encodes a TCP transcription factor that promotes growth arrest, particularly in leaf margins (Nath et al., 2003). In cin mutants, leaves are larger and have an undulating edge due to excessive growth in marginal regions. In addition to these effects on leaf growth, cin mutants also show alterations in petal shape and cell types. This suggests that the CIN transcription factor may have targets involved in both growth and differentiation pathways. To determine how these pathways may be affected, we have analyzed the role of CIN in petal growth and differentiation.Flowers of Antirrhinum have five petals, which are united in their proximal region to form a corolla tube.The distal region of each petal forms a lobe, which itself can be subdivided into a more proximal region, termed the lip, and a more distal region ( Fig. 1A; Keck et al., 2003). The flowers exhibit dorsoventral asymmetry and comprise two dorsal petals, two lateral petals, and one ventral petal. The lateral and ventral petal lobes form a platform for bees to land on and pry open the flower. A variety of cell types can be recognized in the flower, forming characteristic patterns along the proximo...

SUMMARYSuccessful fertilization in angiosperms requires the growth of pollen tubes through the female reproductive tract as they seek out unfertilized ovules. In Arabidopsis, the reproductive tract begins with the stigma, where pollen grains initially adhere, and extends through the transmitting tract of the style and ovary. In wild-type plants, cells within the transmitting tract produce a rich extracellular matrix and undergo programmed cell death to facilitate pollen movement. Here, we show that the HAF, BEE1 and BEE3 genes encode closely related bHLH transcription factors that act redundantly to specify reproductive tract tissues. These three genes are expressed in distinct but overlapping patterns within the reproductive tract, and in haf bee1 bee3 triple mutants extracellular matrix formation and cell death fail to occur within the transmitting tract. We used a minimal pollination assay to show that HAF is necessary and sufficient to promote fertilization efficiency. Our studies further show that HAF expression depends on the NTT gene and on an auxin signaling pathway mediated by the ARF6, ARF8 and HEC genes.