Development 112 1 20 1991 Printed in Great Britain The Company of Biologists Limited 1991 Genetic interactions among floral homeotic genes of Arabidopsis JOHN L BOWMAN DAVID R SMYTH and ELLIOT M MEYEROWITZt Division of Biology 156 29 California Institute of Technology Pasadena California 91125 USA Permanent address Department of Genetics and Developmental Biology Monash University Clayton Victoria 3168 Australia t Author for correspondence Summary We describe allelic series for three loci mutations in which result in homeotic conversions in two adjacent whorls in the Arabidopsis thaliana flower Both the structure of the mature flower and its development from the initial primordium are described by scanning electron microscopy New mutations at the APETALA2 locus ap2 2 ap2 8 and ap2 9 cause homeotic conversions in the outer two whorls sepals to carpels or leaves and petals to stamens Two new mutations of PISTILLATA pi 2 and pi 3 cause second and third whorl organs to differentiate incorrectly Homeotic conversions are petals to sepals and stamens to carpels a pattern similar to that previously described for the apetala3 l mutation The AGAMOUS mutations ag 2 and ag 3 affect the third and fourth whorls and cause petals to develop instead of stamens and another flower to arise in place of the gynoecium In addition to homeotic changes mutations at the APETALA2 APETALA3 and PISTILLATA loci may lead to reduced numbers of organs or even their absence in specific whorls The bud and flower phenotypes of doubly and triply mutant strains constructed with these and previously described alleles are also described Based on these results a model is proposed that suggests that the products of these homeotic genes are each active in fields occupying two adjacent whorls AP2 in the two outer whorls PI and AP3 in whorls two and three and AG in the two inner whorls In combination therefore the gene products in these three concentric overlapping fields specify the four types of organs in the wild type flower Further the phenotypes of multiple mutant lines indicate that the wild type products of the AGAMOUS and APETALA2 genes interact antagonistically AP2 seems to keep the AG gene inactive in the two outer whorls while the converse is likely in the two inner whorls This field model successfully predicts the phenotypes of all the singly doubly and triply mutant flowers described Introduction Komaki et al 1988 Haughn and Somerville 1988 Hill and Lord 1989 Meyerowitz et al 1989 Kunst et al 1989 Yanofsky et al 1990 The best studied of them are a group of four genes whose mutant phenotypes include homeotic conversions of floral organs These are the AGAMOUS AG gene mutants of which have petals where stamens are found in wild type the third whorl and internal flowers in the place of the ovary the fourth whorl in wild type the APETALA2 AP2 gene different mutant alleles of which cause different conversions in the outer two of the four whorls of the flower and the APETALA3 AP3 and PISTILLATA PI genes Mutations in the AP3 and PI genes cause sepals to develop in the positions occupied by petals in wild type flowers the second whorl and have variable effects in the third in wild type stamen whorl To date there are published descriptions of two mutant ag alleles ag 1 Pruitt et al 1987 Bowman et al 1988 1989 and ag 2 Yanofsky etal 1990 seven mutant ap2 alleles that show a broad range of related phenotypes Flowers of Arabidopsis thaliana originate as small outgrowths of cells on the flanks of the florally induced shoot apical meristem These cells divide and differentiate eventually producing a flower with a precisely defined pattern of four types of floral organs with each type found in one of the four concentric whorls of the wild type flower Bowman et al 1989 Smyth et al 1990 During the developmental process the cells in each flower primordium must in some way assess their positions either globally or in reference to nearby cells and they or their descendents must subsequently differentiate to the appropriate cell types As an approach to finding the molecular mechanisms by which cells in developing flowers recognize and realize their fates we study genes whose products are necessary for proper pattern formation in Arabidopsis flowers Several such genes have been described Pruitt et al 1987 Meyerowitz 1987 Bowman et al 1988 1989 Key words flower development Arabidopsis homeotic genes J L Bowman D R Smyth and E M Meyerowitz ap2 l through ap2 7 Pruitt et al 1987 Bowman et al 1988 1989 Komaki et al 1988 Meyerowitz et al 1989 Kunst et al 1989 and one mutant allele each of ap3 Bowman et al 1989 and pi Bowman et al 1989 Hill and Lord 1989 Given the range of phenotypes seen in ap2 mutants in which first whorl organs can be leaves or carpels second whorl organs petaloid stamens stamens or absent and third whorl organs normal or absent it seemed worthwhile to extend the allelic series for each of the homeotic loci In this paper we report the phenotypes of one new ag allele three alleles of ap2 each with phenotypes different from those described before and two new alleles of pi We also describe the mature phenotype and early development of a range of doubly and triply mutant combinations made with these new and previously described alleles These new data along with those already published have led us to a general and testable model of organ specification in Arabidopsis flowers This model details the roles of the wild type products of each of the homeotic genes in specifying organ identity in different regions of the developing flower These data also provide new information on the roles of the wild type products of the homeotic genes in establishing organ number and pattern in the flower Materials and methods The mutant alleles studied are listed in Table 1 All are recessive The new mutations are in the Landsberg ecotype homozygous for the erecta mutation and were generated by mutagenesis of seeds with ethylmethane sulfonate EMS ag1 ap2 l pi 1 and ap3 l were obtained from Maarten Koornneef Department of Genetics Wageningen Agricultural University The Netherlands Wild type alleles are symbolized in block capitals and italics mutant alleles in lower case italics Individual mutant alleles are designated by a number that follows the mutant symbol and a hyphen Doubly and triply mutant strains were constructed by manual cross pollination using as parents strains homozygous for individual mutations except in the strains involving