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Gene Review

AP3  -  Floral homeotic protein APETALA 3

Arabidopsis thaliana

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Disease relevance of AP3

  • To explore the significance of this regulation, we expressed KNOLLE protein under the control of two constitutive promoters, the flower-specific AP3 and the cauliflower mosaic virus 35S promoter [1].

High impact information on AP3

  • Here, AP3/PI function was put under posttranslational control to analyze its immediate effect on the floral mRNA population, with indirect effects blocked by cycloheximide [2].
  • Ectopic expression of AP3 causes a second floral homeotic gene, PISTILLATA (PI), to function in the fourth whorl of 35S-AP3 flowers [3].
  • AP3 and PI also activate an AP3 promoter-reporter gene fusion, demonstrating that AP3 positively autoregulates [3].
  • Like several other plant homeotic genes, the AP3 gene contains a MADS box and likely acts as a transcription factor [4].
  • Since DEFA and AP3 have very similar protein products, mutant phenotypes, and spatial expression patterns, it is likely that these genes are cognate homologs [4].

Biological context of AP3

  • Experiments using a steroid-inducible form of LFY show that, in contrast to its direct transcriptional activation of other floral homeotic genes, LFY acts in both a direct and an indirect manner to regulate AP3 expression [5].
  • The distribution of replacement and synonymous changes in AP3 and PI core and noncore functional domains also indicates differences in the patterns of molecular evolution between these interacting floral regulatory genes [6].
  • This finding suggests that AP3 and PI act relatively directly to regulate the genes required for the basic cellular processes responsible for petal and stamen morphogenesis [7].
  • DNA microarrays consisting of 9216 Arabidopsis ESTs were screened with probes corresponding to mRNAs from different mutant and transgenic lines that misexpress AP3 and/or PI [7].
  • Surprisingly, DEF could not replace the AP3 protein in properly maintaining AP3 transcripts (autoregulation) [8].

Associations of AP3 with chemical compounds

  • Specially, K1 of AP3 and PI resembles a leucine zipper motif [9].

Regulatory relationships of AP3

  • Kinetic studies, using transgenic Arabidopsis plants in which both AP3 and PI are under post-translational control, show that AP1 transcript levels are downregulated within 2 h of AP3/PI activation [10].
  • Furthermore, our results from RNA in situ hybridizations indicate that ASK1 regulates early AP3 and PI expression [11].
  • Transgenic constructs were used to provide evidence that UFO regulates floral organ identity by activating or maintaining transcription of the class B organ-identity gene APETALA 3, but not PISTILLATA [12].
  • In this study, we blocked the expression of Arabidopsis U1-70K in petals and stamens by expressing U1-70K antisense transcript using the AP3 (APETALA3) promoter specific to these floral organs [13].
  • Microscopic observations revealed that the innermost-whorl carpels had been replaced by stamen-like organs, which resembled the flowers of the previously described Arabidopsis thaliana (L.) Heynh. mutation superman as well as those ectopically expressing the AP3 gene [14].

Other interactions of AP3

  • Proper development of petals and stamens in Arabidopsis flowers requires the activities of APETALA3 (AP3) and PISTILLATA (PI), whose transcripts can be detected in the petal and stamen primordia [15].
  • Both the AP3/PI heterodimer and the AP1 or AG homodimers are formed when the three corresponding proteins are present together [16].
  • 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 [17].
  • It has been proposed that UFO provides spatial cues and that LFY specifies competence for AP3 and PI expression in the developing flower [15].
  • The floral meristem identity gene LEAFY (LFY) plays a role in the initiation phase through at least two pathways, which differ from each other in the involvement of two homeotic genes, APETALA3 (AP3) and PISTILLATA (PI) [18].

Analytical, diagnostic and therapeutic context of AP3

  • We have used microarray analysis to conduct a broad survey of genes whose expression is affected by AP3 and PI activity [7].
  • MdMADS13 is an AP3-like B class MADS box gene, and was mainly expressed in petals and stamens as demonstrated by Northern blot analysis [19].
  • Transcription analysis of B-class genes in Eranthis hyemalis using reverse transcribed in situ PCR revealed that both AP3 and PI are expressed in developing vascular bundles in the tuberous rhizome, flowering stem and floral primordia [20].
  • Using a reverse transcriptase-polymerase chain reaction assay, we demonstrate that the percentage of full-length exon 5-containing AP3 RNAs correlates with the phenotype of the flowers in both ap3-1 and ap3-11 [21].


  1. Cell cycle-independent expression of the Arabidopsis cytokinesis-specific syntaxin KNOLLE results in mistargeting to the plasma membrane and is not sufficient for cytokinesis. Völker, A., Stierhof, Y.D., Jürgens, G. J. Cell. Sci. (2001) [Pubmed]
  2. A homolog of NO APICAL MERISTEM is an immediate target of the floral homeotic genes APETALA3/PISTILLATA. Sablowski, R.W., Meyerowitz, E.M. Cell (1998) [Pubmed]
  3. Arabidopsis homeotic gene APETALA3 ectopic expression: transcriptional and posttranscriptional regulation determine floral organ identity. Jack, T., Fox, G.L., Meyerowitz, E.M. Cell (1994) [Pubmed]
  4. The homeotic gene APETALA3 of Arabidopsis thaliana encodes a MADS box and is expressed in petals and stamens. Jack, T., Brockman, L.L., Meyerowitz, E.M. Cell (1992) [Pubmed]
  5. Regulation of APETALA3 floral homeotic gene expression by meristem identity genes. Lamb, R.S., Hill, T.A., Tan, Q.K., Irish, V.F. Development (2002) [Pubmed]
  6. Molecular population genetics of floral homeotic loci. Departures from the equilibrium-neutral model at the APETALA3 and PISTILLATA genes of Arabidopsis thaliana. Purugganan, M.D., Suddith, J.I. Genetics (1999) [Pubmed]
  7. Global identification of target genes regulated by APETALA3 and PISTILLATA floral homeotic gene action. Zik, M., Irish, V.F. Plant Cell (2003) [Pubmed]
  8. Divergence of function and regulation of class B floral organ identity genes. Samach, A., Kohalmi, S.E., Motte, P., Datla, R., Haughn, G.W. Plant Cell (1997) [Pubmed]
  9. The K domain mediates heterodimerization of the Arabidopsis floral organ identity proteins, APETALA3 and PISTILLATA. Yang, Y., Fanning, L., Jack, T. Plant J. (2003) [Pubmed]
  10. Direct regulation of the floral homeotic APETALA1 gene by APETALA3 and PISTILLATA in Arabidopsis. Sundström, J.F., Nakayama, N., Glimelius, K., Irish, V.F. Plant J. (2006) [Pubmed]
  11. The ASK1 gene regulates B function gene expression in cooperation with UFO and LEAFY in Arabidopsis. Zhao, D., Yu, Q., Chen, M., Ma, H. Development (2001) [Pubmed]
  12. The UNUSUAL FLORAL ORGANS gene of Arabidopsis thaliana is an F-box protein required for normal patterning and growth in the floral meristem. Samach, A., Klenz, J.E., Kohalmi, S.E., Risseeuw, E., Haughn, G.W., Crosby, W.L. Plant J. (1999) [Pubmed]
  13. Expression of U1 small nuclear ribonucleoprotein 70K antisense transcript using APETALA3 promoter suppresses the development of sepals and petals. Golovkin, M., Reddy, A.S. Plant Physiol. (2003) [Pubmed]
  14. Alteration of floral organ identity in rice through ectopic expression of OsMADS16. Lee, S., Jeon, J.S., An, K., Moon, Y.H., Lee, S., Chung, Y.Y., An, G. Planta (2003) [Pubmed]
  15. Activation of the Arabidopsis B class homeotic genes by APETALA1. Ng, M., Yanofsky, M.F. Plant Cell (2001) [Pubmed]
  16. Dimerization specificity of Arabidopsis MADS domain homeotic proteins APETALA1, APETALA3, PISTILLATA, and AGAMOUS. Riechmann, J.L., Krizek, B.A., Meyerowitz, E.M. Proc. Natl. Acad. Sci. U.S.A. (1996) [Pubmed]
  17. Genetic interactions among floral homeotic genes of Arabidopsis. Bowman, J.L., Smyth, D.R., Meyerowitz, E.M. Development (1991) [Pubmed]
  18. Regulation of SUP expression identifies multiple regulators involved in arabidopsis floral meristem development. Sakai, H., Krizek, B.A., Jacobsen, S.E., Meyerowitz, E.M. Plant Cell (2000) [Pubmed]
  19. Cloning and characterization of four apple MADS box genes isolated from vegetative tissue. van der Linden, C.G., Vosman, B., Smulders, M.J. J. Exp. Bot. (2002) [Pubmed]
  20. Genes from the APETALA3 and PISTILLATA lineages are expressed in developing vascular bundles of the tuberous rhizome, flowering stem and flower Primordia of Eranthis hyemalis. Skipper, M. Ann. Bot. (2002) [Pubmed]
  21. An intragenic suppressor of the Arabidopsis floral organ identity mutant apetala3-1 functions by suppressing defects in splicing. Yi, Y., Jack, T. Plant Cell (1998) [Pubmed]
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