Consisting of only four morphological parts, the Arabidopsis ovule is a relatively simple structure that lends itself to the study of genetic regulation of pattern formation and organogenesis in plants (for review, see Gasser and Robinson-Beers 1993;Reiser and Fischer 1993;Angenent and Colombo 1996;Gasser et al. 1998;Schneitz et al. 1998b). Although initiating as a radially symmetrical primordium, the developing ovule subsequently exhibits differences between the side toward the apex of the carpel (adaxial) and the side toward the base of the carpel (abaxial)-resulting in a bilaterally symmetrical structure. Asymmetric development is most apparent in the outer integument (one of two sheathing structures) and in the funiculus (supporting stalk). The outer integument grows extensively only on the abaxial side of the ovule, and the funiculus curves in the abaxial direction. Two genetic loci involved in asymmetric development of the outer integument have been described. In addition to effects on flower development (Schultz et al. 1991;Bowman et al. 1992), the SUPERMAN (SUP) gene is essential for suppressing adaxial growth of the outer integument (Gaiser et al. 1995), and mutations in this gene lead to nearly equal outer integument growth on both sides of the ovule. In contrast, inner no outer (ino) mutations can lead to an absence of outer integument growth on both sides of the ovule primordium, implicating INO as a positive regulator of integument growth or a determinant of polarity (Gaiser et al. 1995;Baker et al. 1997;Schneitz et al. 1997).In angiosperms, bilateral symmetry is also a common characteristic of leaves, floral organs, and often whole flowers. In these structures, bilateral symmetry results from significant differences in development between the adaxial (toward the shoot apex) and abaxial (away from the shoot apex) sides. Recently, several genes involved in establishing these abaxial-adaxial patterns have been identified. The cycloidea (cyc) and dichotoma (dich) genes of Antirrhinum majus are expressed on the adaxial sides of flowers, where they specify adaxial floral development (Luo et al. 1996). In contrast, phantastica (phan), which appears to be essential for identity of the adaxial leaf surface in this same species, is expressed throughout the leaf (Waites et al. 1998), and must, therefore, require additional asymmetrically distributed factors for its activity.In Arabidopsis thaliana, vegetative structures of the phabulosa-1d mutant are radially symmetrical, apparently as a result of adaxialization, and PHABULOSA may be one determinant of abaxial cell fate in this species (McConnell and Barton 1998). A recently described family of Arabidopsis genes, the YABBY genes, encoding putative transcription factors (Bowman and Smyth 1999;Kumaran et al. 1999;Sawa et al. 1999;Siegfried et al. 1999), participate in determination of abaxial identity in a variety of organs.

SummaryThe Arabidopsis aberrant testa shape (ats) mutant produces a single integument instead of the two integuments seen in wild-type ovules. Cellular anatomy and patterns of marker gene expression indicate that the single integument results from congenital fusion of the two integuments of the wild type. Isolation of the ATS locus showed it to encode a member of the KANADI (KAN) family of putative transcription factors, previously referred to as KAN4. ATS was expressed at the border between the two integuments at the time of their initiation, with expression later confined to the abaxial layer of the inner integument. In an inner no outer (ino) mutant background, where an outer integument does not form, the ats mutation led to amorphous inner integument growth. The kan1 kan2 double mutant exhibits a similar amorphous growth of the outer integument without affecting inner integument growth. We hypothesize that ATS and KAN1/KAN2 play similar roles in the specification of polarity in the inner and outer integuments, respectively, that parallel the known roles of KAN proteins in promoting abaxial identity during leaf development. INO and other members of the YABBY gene family have been hypothesized to have similar parallel roles in outer integument and leaf development. Together, these two hypotheses lead us to propose a model for normal integument growth that also explains the described mutant phenotypes.

SummaryINNER NO OUTER (INO) expression is limited to the abaxial cell layer of the incipient and developing outer integument in Arabidopsis ovules. Using deletion analysis of the previously de®ned INO promoter (P-INO), at least three distinct regions that contribute to the endogenous INO expression pattern were identi®ed. One such positive element, designated POS9, which comprises at least three distinct subelements, was found to include suf®cient information to duplicate the INO expression pattern when four or more copies were used in conjunction with a heterologous minimal promoter. While known regulators of INO, including INO, SUPERMAN, BELL1, and AINTEGUMENTA, did not detectably interact with POS9 in yeast one-hybrid assays, two groups of proteins that interact speci®cally with POS9 were identi®ed in one-hybrid library screens. Members of one group include C2H2 zinc ®nger motifs. Members of the second group contain a novel, conserved DNA-binding region and were designated the BASIC PENTACYSTEINE (BPC) proteins on the basis of conserved features of this region. The BPC proteins are nuclear localized and speci®cally bind in vitro to GA dinucleotide repeats located within POS9. The widespread expression patterns of the BPCs and the large number of GA repeat potential target sequences in the Arabidopsis genome indicate that BPC proteins may affect expression of genes involved in a variety of plant processes.

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SUMMARYThe BASIC PENTACYSTEINE (BPC) proteins are a plant-specific transcription factor family that is present throughout land plants. The Arabidopsis BPC proteins have been categorized into three classes based on sequence similarity, and we demonstrate that there is functional overlap between classes. Single gene mutations produce no visible phenotypic effects, and severe morphological phenotypes occur only in higher order mutants between members of classes I and II, with the most severe phenotype observed in bpc1-1 bpc2 bpc4 bpc6 plants. These quadruple mutants are dwarfed and display small curled leaves, aberrant ovules, altered epidermal cells and reduced numbers of lateral roots. Affected processes include coordinated growth of cell layers, cell shape determination and timing of senescence. Disruption of BPC3 function rescues some aspects of the bpc1-1 bpc2 bpc4 bpc6 phenotype, indicating that BPC3 function may be antagonistic to other members of the family. Ethylene response is diminished in bpc1-1 bpc2 bpc4 bpc6 plants, although not all aspects of the phenotype can be explained by reduced ethylene sensitivity. Our data indicate that the BPC transcription factor family is integral for a wide range of processes that support normal growth and development.