Elliot M. Meyerowitz

Early flower development in Arabidopsis.

The early development of the flower of Arabidopsis thaliana is described from initiation until the opening of the bud. The morphogenesis, growth rate, and surface structure of floral organs were recorded in detail using scanning electron microscopy. Flower development has been divided into 12 stages using a series of landmark events. Stage 1 begins with the initiation of a floral buttress on the flank of the apical meristem. Stage 2 commences when the flower primordium becomes separate from the meristem. Sepal primordia then arise (stage 3) and grow to overlie the primordium (stage 4). Petal and stamen primordia appear next (stage 5) and are soon enclosed by the sepals (stage 6). During stage 6, petal primordia grow slowly, whereas stamen primordia enlarge more rapidly. Stage 7 begins when the medial stamens become stalked. These soon develop locules (stage 8). A long stage 9 then commences with the petal primordia becoming stalked. During this stage all organs lengthen rapidly. This includes the gynoecium, which commences growth as an open-ended tube during stage 6. When the petals reach the length of the lateral stamens, stage 10 begins. Stigmatic papillae appear soon after (stage 11), and the petals rapidly reach the height of the medial stamens (stage 12). This final stage ends when the 1-millimeter-long bud opens. Under our growing conditions 1.9 buds were initiated per day on average, and they took 13.25 days to progress through the 12 stages from initiation until opening.

LEAFY controls floral meristem identity in Arabidopsis

The first step in flower development is the generation of a floral meristem by the inflorescence meristem. We have analyzed how this process is affected by mutant alleles of the Arabidopsis gene LEAFY. We show that LEAFY interacts with another floral control gene, APETALA1, to promote the transition from inflorescence to floral meristem. We have cloned the LEAFY gene, and, consistent with the mutant phenotype, we find that LEAFY RNA is expressed strongly in young flower primordia. LEAFY expression procedes expression of the homeotic genes AGAMOUS and APETALA3, which specify organ identify within the flower. Furthermore, we demonstrate that LEAFY is the Arabidopsis homolog of the FLORICAULA gene, which controls floral meristem identity in the distantly related species Antirrhinum majus.

The CLAVATA1 gene encodes a putative receptor kinase that controls shoot and floral meristem size in arabidopsis

The shoot apical meristem is responsible for above-ground organ initiation in higher plants, accomplishing continuous organogenesis by maintaining a pool of undifferentiated cells and directing descendant cells toward organ formation. Normally, proliferation and differentiation are balanced, so that the structure and size of the shoot meristem is maintained. However, Arabidopsis plants homozygous for mutations at the CLAVATA1 (CLV1) locus accumulate excess undifferentiated cells. We describe the molecular cloning and expression pattern of the CLV1 gene. It encodes a putative receptor kinase, suggesting a role in signal transduction. The extracellular domain is composed of 21 tandem leucine-rich repeats that resemble leucine-rich repeats found in animal hormone receptors. We provide evidence that CLV1 expression in the inflorescence is specifically associated with meristematic activity.

Genes directing flower development in Arabidopsis.

We describe the effects of four recessive homeotic mutations that specifically disrupt the development of flowers in Arabidopsis thaliana. Each of the recessive mutations affects the outcome of organ development, but not the location of organ primordia. Homeotic transformations observed are as follows. In agamous-1, stamens to petals; in apetala2-1, sepals to leaves and petals to staminoid petals; in apetala3-1, petals to sepals and stamens to carpels; in pistillata-1, petals to sepals. In addition, two of these mutations (ap2-1 and pi-1) result in loss of organs, and ag-1 causes the cells that would ordinarily form the gynoecium to differentiate as a flower. Two of the mutations are temperature-sensitive. Temperature shift experiments indicate that the wild-type AP2 gene product acts at the time of primordium initiation; the AP3 product is active later. It seems that the wild-type alleles of these four genes allow cells to determine their place in the developing flower and thus to differentiate appropriately. We propose that these genes may be involved in setting up or responding to concentric, overlapping fields within the flower primordium.

Ethylene Responses Are Negatively Regulated by a Receptor Gene Family in Arabidopsis thaliana

A family of genes including ETR1, ETR2, EIN4, ERS1, and ERS2 is implicated in ethylene perception in Arabidopsis thaliana. As only dominant mutations were previously available for these genes, it was unclear whether all of them are components in the ethylene signaling pathway and whether they code for positive or negative regulators of ethylene responses. In this study, we have isolated loss-of-function mutations of four of these genes (ETR1, ETR2, EIN4, and ERS2) and identified an ethylene-independent role of ETR1 in promoting cell elongation. Quadruple mutants had constitutive ethylene responses, revealing that these proteins negatively regulate ethylene responses and that the induction of ethylene response in Arabidopsis is through inactivation rather than activation of these proteins.

Antagonistic regulation of PIN phosphorylation by PP2A and PINOID directs auxin flux

In plants, cell polarity and tissue patterning are connected by intercellular flow of the phytohormone auxin, whose directional signaling depends on polar subcellular localization of PIN auxin transport proteins. The mechanism of polar targeting of PINs or other cargos in plants is largely unidentified, with the PINOID kinase being the only known molecular component. Here, we identify PP2A phosphatase as an important regulator of PIN apical-basal targeting and auxin distribution. Genetic analysis, localization, and phosphorylation studies demonstrate that PP2A and PINOID both partially colocalize with PINs and act antagonistically on the phosphorylation state of their central hydrophilic loop, hence mediating PIN apical-basal polar targeting. Thus, in plants, polar sorting by the reversible phosphorylation of cargos allows for their conditional delivery to specific intracellular destinations. In the case of PIN proteins, this mechanism enables switches in the direction of intercellular auxin fluxes, which mediate differential growth, tissue patterning, and organogenesis.

The ABCs of floral homeotic genes

Homeotic mutants, that is, mutants with a normal organ in a place where an organ of another type is typically found, were first recognized in plants. The earliest descriptions of mutants in which petals replace stamens, giving double flowers, go back to ancient Greece and Rome. Similar accounts can be found in the botanical literature of China more than a thousand years ago, and in the books of the herbalists of Renaissance Europe (Meyerowitz et al., 1989). The use of such mutants (and similar but noninherited developmental abnormalities) to understand developmental processes in plants is more recent, dating from Linnaeus in the mid-eighteenth century (see Cullen and Stevens, 1990), and from Goethe (1790), who derived the ideas of organ homology and homological comparison from plants showing what was then called abnormal metamorphosis. Our term homeosis dates from Bateson’s work (1894) on organismal variation, in which he expanded Masters’treatment (1889) of abnormal metamorphosis in plants to animals and introduced the term homoeosis as a replacement for the older term. Goethe (1790) used homeotic variation in plants as the basis for a specific model explaining the developmental origin of different organ types in flowers. In his view, the four types of floral organs (sepals, petals, stamens, and carpels) are all modified leaves. As sap rises through developing flowers it is progressively refined, thereby inducing different organ types in different positions.

The homeotic gene APETALA3 of Arabidopsis thaliana encodes a MADS box and is expressed in petals and stamens

Mutations in the APETALA3 (AP3) gene of A. thaliana result in homeotic transformations of petals to sepals and stamens to carpels. We have cloned the AP3 gene from Arabidopsis based on its homology to the homeotic flower gene deficiens (DEFA) from the distantly related plant Antirrhinum majus. The sequence of four ap3 mutant alleles and genetic mapping analysis prove that the DEFA homolog is AP3. Like several other plant homeotic genes, the AP3 gene contains a MADS box and likely acts as a transcription factor. The region-specific spatial expression pattern of AP3 rules out certain types of sequential models of flower development and argues in favor of a spatial model based on positional information. Since DEFA and AP3 have very similar protein products, mutant phenotypes, and spatial expression patterns, it is likely that these genes are cognate homologs.

Patterns of Auxin Transport and Gene Expression during Primordium Development Revealed by Live Imaging of the Arabidopsis Inflorescence Meristem

BACKGROUND Plants produce leaf and flower primordia from a specialized tissue called the shoot apical meristem (SAM). Genetic studies have identified a large number of genes that affect various aspects of primordium development including positioning, growth, and differentiation. So far, however, a detailed understanding of the spatio-temporal sequence of events leading to primordium development has not been established. RESULTS We use confocal imaging of green fluorescent protein (GFP) reporter genes in living plants to monitor the expression patterns of multiple proteins and genes involved in flower primordial developmental processes. By monitoring the expression and polarity of PINFORMED1 (PIN1), the auxin efflux facilitator, and the expression of the auxin-responsive reporter DR5, we reveal stereotypical PIN1 polarity changes which, together with auxin induction experiments, suggest that cycles of auxin build-up and depletion accompany, and may direct, different stages of primordium development. Imaging of multiple GFP-protein fusions shows that these dynamics also correlate with the specification of primordial boundary domains, organ polarity axes, and the sites of floral meristem initiation. CONCLUSIONS These results provide new insight into auxin transport dynamics during primordial positioning and suggest a role for auxin transport in influencing primordial cell type.

Negative regulation of the Arabidopsis homeotic gene AGAMOUS by the APETALA2 product

We characterized the distribution of AGAMOUS (AG) RNA during early flower development in Arabidopsis. Mutations in this homeotic gene cause the transformation of stamens to petals in floral whorl 3 and of carpels to another ag flower in floral whorl 4. We found that AG RNA is present in the stamen and carpel primordia but is undetectable in sepal and petal primordia throughout early wild-type flower development, consistent with the mutant phenotype. We also analyzed the distribution of AG RNA in apetela2 (ap2) mutant flowers. AP2 is a floral homeotic gene that is necessary for the normal development of sepals and petals in floral whorls 1 and 2. In ap2 mutant flowers, AG RNA is present in the organ primordia of all floral whorls. These observations show that the expression patterns of the Arabidopsis floral homeotic genes are in part established by regulatory interactions between these genes.