Expression of the Arabidopsis floral homeotic gene AGAMOUS is restricted to specific cell types late in flower development.

Abstract

Mutations in the AGAMOUS (AG) gene cause transformations in two adjacent whorls of the Arabidopsis flower. Petals develop in the third floral whorl rather than the normal stamens, and the cells that would normally develop into the fourth whorl gynoecium behave as if they constituted an ag flower primordium. Early in flower development, AG RNA is evenly distributed throughout third and fourth whorl organ primordia but is not present in the organ primordia of whorls one and two. In contrast to the early expression pattern, later in flower development, AG RNA is restricted to specific cell types within the stamens and carpels as cellular differentiation occurs in those organs. Ectopic AG expression patterns in flowers mutant for the floral homeotic gene APETELA2 (AP2), which regulates early AG expression, suggest that the late AG expression is not directly dependent on AP2 activity. INTRODUCTIONJust as each flower consists of a precise pattern of organ types, each individual floral organ consists of severa1 cell types in stereotyped positions. The organs of the flower begin their development as small outgrowths of cells from the floral meristem. At about the time these floral organ primordia arise, their identity is thought to be determined in accordance with their position within the flower, causing them to follow organ type-specific developmental programs. Subsequently, the cells of each of the determined organ primordia must then asses or know their relative positions within the primordia and differentiate into the appropriate cell types. It is this precise pattern of cellular differentiation that results in the different morphological characteristics of each floral organ type. Thus, during flower development, cells must know their position relative to others not only during the specification of organ primordium identity, but also later, during the cellular differentiation of individual floral organs.Severa1 homeotic mutations have been isolated in Arabidopsis. They cause cells to misinterpret their positions in early flower development. As a consequence, they differentiate into inappropriate cell types (Pruitt et al., 1987;Komaki et al., 1988;Bowman et al., 1988Bowman et al., , 1989Bowman et al., , 1991Meyerowitz et al., 1989Meyerowitz et al., , 1991Hill and Lord, 1989;Kunst ' To whom correspondence should be addressed. lrish and Sussex, 1990). The result is a flower with morphologically normal organ types found in abnormal positions. A model has been proposed, based on a series of genetic experiments, to explain how four homeotic genes, AGAMOUS (AG), APf TALA2 (AP2), A f f TALA3 (AP3), and PlSTILLATA, specify the identity of the floral organs by establishing positional cues in the early floral primordium . However, genes involved in specifying positional information later in development, within the individual floral organs, have not yet been identified.Mutations in one of the homeotic genes, AG, cause alterations in the third and fourth whorls of the flower. In the third whorl, six petals develop in the posit...

BASIC PENTACYSTEINE Proteins Mediate MADS Domain Complex Binding to the DNA for Tissue-Specific Expression of Target Genes in Arabidopsis

“…The STK promoter was divided into three parts (regions A, B, and C; Figure 4A) spanning all putative SVP promoter binding sites. This ChIP analysis revealed that the highest enrichment in all three replicates was in region B ( Figures 4A and 4B STK is closely related to AG, a gene regulating ovule, stamen, and carpel identity and floral meristem determinacy (Bowman et al, 1991). Recently, we showed that in the agl24 svp ap1-12 triple mutant, AG is ectopically expressed in floral meristems of stage 1 and 2 (Gregis et al, 2008), evidencing the redundant role of these three genes in the repression of AG at these very early stages of flower development.…”

Ovule Development in Wild-Type Arabidopsis and Two Female-Sterile Mutants.

“…This additional curvature is diagnostic of the amphitropous ovule. The innermost layer of the inner integument stains intensely, indicating the differentiation of a tissue referred to as the integumentary tapeturn or endothelium (Kapil and Tiwari, 1978;Bowman et al, 1991a).…”

“…a Event marking beginning of stage as described (Smyth et al, 1990;Bowman et al, 1991a an egg cell closely associated with two synergids at the micropylar end of the embryo sac, a large central cell with a diploid secondary nucleus, and three ephemeral antipodal cells located opposite the micropyle at the chalazal end of the embryo sac ( Figure 2A). Most of the embryo sac lies in direct contact with the inner integument, the nucellar cells lying closest to the micropyle having degenerated during embryo sac formation.…”

Photo and hormonal control of meristem identity in the Arabidopsis flower mutants apetala2 and apetala1.

“…AP2 is best known for its role in the specification of flower organ identity and the negative regulation of AG homeotic gene expression (Komaki et al, 1988;Kunst et al, 1989;Bowman et al, 1991aCoen and Meyerowitz, 1991; l To whom correspondence should be addressed. Drews et al, 1991;Meyerowitz et al, 1991).…”