Flower differentiation

Flower differentiation is the developmental process by which a floral meristem which has committed to flowering produces the distinct organs of a flower, typically sepals, petals, stamens, and carpels which are arranged in concentric whorls [1]. The process is controlled by a small set of transcription factors which act in overlapping domains to specify the identity of each organ [1]. The combinatorial logic behind floral organ specification is governed by the ABCDE model of flower development, which was derived from the study of homeotic mutants of Arabidopsis thaliana and the snapdragon Antirrhinum majus [1][2].

The ABCDE Model

The ABCDE model proposes five classes of gene activity: A, B, C, D, and E, the different combinations of which determine the identity and formation of floral organs [3].

  • Whorl 1 = sepals: A + E
  • Whorl 2 = petals: A + B + E
  • Whorl 3 = stamens: B + C + E
  • Whorl 4 = carpels: C + E
  • Inside whorl 4 = ovules: C + D + E

The A-class and C-class activity is mutually antagonistic [3]

  • If A is lost, C expands to all whorls of A
  • If C is lost, A expands to all whorls of C

These genes encode MADS-box proteins forming quarterly complexes (quartets) to activate specific organ-building programs: building a sepal, petal, stamen, or carpel out of the same basic leafy starting materials [4].

The Floral Quartet Model

Key Protein Classes in Arabidopsis

The homeotic genes generally encode MIKC-type MADS-domain transcription factors [4][5]:

  • Class A: APETALA1 (AP1) and APETALA2 (AP2)
  • Class B: APETALA3 (AP3) and PISTILLATA (PI)
  • Class C: AGAMOUS (AG)
  • Class D: SEEDSTICK (STK), SHATTERPROOF1 (SHP1), and SHP2
  • Class E: SEPALLATA1-4 (SEP1-SEP4). Class E proteins are structural components necessary to bridge the A, B, C, and D complexes into functioning tetramers [6].

Floral Quartet Model

These A-, B-, C-, D-, and E-class genes encode MADS-box proteins forming quarterly complexes (quartets) to activate specific organ-building programs: building a sepal, petal, stamen, or carpel out of the same basic leafy starting materials [4][5]:

  • Whorl 1 = sepals: A + E (e.g.,: AP1 - SEP - SEP - AP1)
  • Whorl 2 = petals: A + B + E (e.g.,: AP1 - AP3 - PI - SEP)
  • Whorl 3 = stamens: B + C + E (e.g.,: SEP - AP3 - PI - AG)
  • Whorl 4 = carpels: C + E (e.g.,: AG - SEP - SEP - AG)
  • Inside whorl 4 = ovules: C + D + E (e.g.,: AG - STK - SHP - SEP)

The Regulation of The ABCDE Gene Expression

The ABCDE system is regulated by several overlapping layers. LFY and AP1 which are identity activators start the program.[7] Then the spatial cofactors, UNUSUAL FLORAL ORGANS (UFO) and WUSCHEL (WUS), tell LFY where to act, generating whorl-specific patterns from a uniform signal [8]. Next, the boundary genes such as SUPERMAN and RABBIT EARS sharpen the borders between domains, while miR172 reinforces the inner/outer split post-transcriptionally.[9] The temporal gate is set by the chromatin regulators PRC2, LHP1, and the opposing Trithorax group, keeping the program off until the right moment.[1]

Furthermore, two feedback loops close the circuit: AG terminates stem-cell activity by repressing WUS, giving the flower its fixed whorl number,[10] and the broadly-expressed SEPALLATA (E-class) proteins are obligate partners without which the A, B, C, and D activities cannot function.[11] Once the quartets form and the auto-regulatory loop closes, organ identity is locked in and no longer needs continuous input from the upstream signal.[11]

Even though the ABCDE factors specify floral organ identity, the programs which build each organ are only partly understood. Genome-wide binding studies identify thousands of potential targets of the MADS-box quartets, and selectivity arises from a combination of DNA-sequence preference and co-operative binding [1][12]. For example, the gene CRABS CLAW which is a member of the YABBY family is a direct target of AGAMOUS (AG) and is required for carpel and nectary development in Arabidopsis [13][14][15].

References

  1. ^ a b c d e Bowman, John L; Moyroud, Edwige (2024-05-01). "Reflections on the ABC model of flower development". The Plant Cell. 36 (5): 1334–1357. doi:10.1093/plcell/koae044. ISSN 1040-4651. PMC 11062442. PMID 38345422.
  2. ^ Coen, Enrico S.; Meyerowitz, Elliot M. (1991). "The war of the whorls: genetic interactions controlling flower development". Nature. 353 (6339): 31–37. Bibcode:1991Natur.353...31C. doi:10.1038/353031a0. ISSN 0028-0836. PMID 1715520.
  3. ^ a b Bowman, John L.; Smyth, David R.; Meyerowitz, Elliot M. (1991-05-01). "Genetic interactions among floral homeotic genes of Arabidopsis". Development. 112 (1): 1–20. doi:10.1242/dev.112.1.1. ISSN 0950-1991. PMID 1685111.
  4. ^ a b c Theißen, Günter; Saedler, Heinz (2001). "Floral quartets". Nature. 409 (6819): 469–471. doi:10.1038/35054172. ISSN 0028-0836. PMID 11206529.
  5. ^ a b Honma, Takashi; Goto, Koji (2001). "Complexes of MADS-box proteins are sufficient to convert leaves into floral organs". Nature. 409 (6819): 525–529. Bibcode:2001Natur.409..525H. doi:10.1038/35054083. ISSN 0028-0836. PMID 11206550.
  6. ^ Theissen, G.; Melzer, R. (2007-08-08). "Molecular Mechanisms Underlying Origin and Diversification of the Angiosperm Flower". Annals of Botany. 100 (3): 603–619. doi:10.1093/aob/mcm143. ISSN 0305-7364. PMC 2533597. PMID 17670752.
  7. ^ Winter, Cara M.; Yamaguchi, Nobutoshi; Wu, Miin-Feng; Wagner, Doris (2015). "Transcriptional programs regulated by both LEAFY and APETALA1 at the time of flower formation". Physiologia Plantarum. 155 (1): 55–73. Bibcode:2015PPlan.155...55W. doi:10.1111/ppl.12357. ISSN 0031-9317. PMC 5757833. PMID 26096587.
  8. ^ Wang, Hanhui; Lu, Yanan; Zhang, Yanru; Liu, Guan; Yu, Song; Zheng, Zhimin (2024-11-15). "The overall regulatory network and contributions of ABC(D)E model genes in yellowhorn flower development". BMC Plant Biology. 24 (1) 1081. Bibcode:2024BMCPB..24.1081W. doi:10.1186/s12870-024-05796-w. ISSN 1471-2229. PMC 11566546. PMID 39543490.
  9. ^ Takeda, Seiji; Matsumoto, Noritaka; Okada, Kiyotaka (2004-01-15). "RABBIT EARS , encoding a SUPERMAN-like zinc finger protein, regulates petal development in Arabidopsis thaliana". Development. 131 (2): 425–434. doi:10.1242/dev.00938. ISSN 1477-9129. PMID 14681191.
  10. ^ Ming, Feng; Ma, Hong (2009-08-01). "A terminator of floral stem cells: Figure 1". Genes & Development. 23 (15): 1705–1708. doi:10.1101/gad.1834409. ISSN 0890-9369. PMC 2720261. PMID 19651982.
  11. ^ a b Shang, Erlei; Ito, Toshiro; Sun, Bo (2019-11-02). "Control of floral stem cell activity in Arabidopsis". Plant Signaling & Behavior. 14 (11) 1659706. Bibcode:2019PlSiB..1459706S. doi:10.1080/15592324.2019.1659706. ISSN 1559-2324. PMC 6804719. PMID 31462133.
  12. ^ Lai, Xuelei; Stigliani, Arnaud; Lucas, Jérémy; Hugouvieux, Véronique; Parcy, François; Zubieta, Chloe (2020-09-25). "Genome-wide binding of SEPALLATA3 and AGAMOUS complexes determined by sequential DNA-affinity purification sequencing". Nucleic Acids Research. 48 (17): 9637–9648. doi:10.1093/nar/gkaa729. ISSN 0305-1048. PMC 7515736. PMID 32890394.
  13. ^ Alvarez, John; Smyth, David R. (1999-06-01). "CRABS CLAW and SPATULA , two Arabidopsis genes that control carpel development in parallel with AGAMOUS". Development. 126 (11): 2377–2386. doi:10.1242/dev.126.11.2377. ISSN 0950-1991. PMID 10225997.
  14. ^ Bowman, John L.; Smyth, David R. (1999-06-01). "CRABS CLAW , a gene that regulates carpel and nectary development in Arabidopsis , encodes a novel protein with zinc finger and helix-loop-helix domains". Development. 126 (11): 2387–2396. doi:10.1242/dev.126.11.2387. ISSN 0950-1991. PMID 10225998.
  15. ^ Gómez-Mena, Concepción; de Folter, Stefan; Costa, Maria Manuela R.; Angenent, Gerco C.; Sablowski, Robert (2005-02-01). "Transcriptional program controlled by the floral homeotic gene AGAMOUS during early organogenesis". Development. 132 (3): 429–438. doi:10.1242/dev.01600. ISSN 1477-9129. PMID 15634696.


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