Petals

Abstract

Petals are often the most obvious organ within a flower because they provide visually arresting targets for varied pollinators and show corresponding adaptions in form, colour and scent. When present, they are located in the outer layers of a flower (the perianth) surrounding the male and female reproductive organs, the stamens and carpels. Their development is a consequence of a regulatory hierarchy of meristem determining genes, organ identity genes and downstream target genes. The gene hierarchy involves transcription factors from varied families and is regulated at transcriptional and posttranscriptional levels, involving microRNAs (ribonucleic acids) and targeted protein degradation. The nature of the gene and protein interactions determining and controlling many aspects of petal development have been teased apart in dicotyledonous flowers of Arabidopsis, Antirrhinum and Petunia and in recent years, this knowledge has been used to explore the evolution of these developmental genetic systems across the angiosperms.

Key concepts:

  • Molecular genetics has identified key regulators of petal development in model species.

  • Petal initiation and subsequent development is regulated by a range of transcription factor types.

  • Petal characteristics such as colour, shape and scent are related to pollination mechanisms.

  • Ethylene is important in petal senescence.

Keywords: flower; whorls; pollination; evolution; regulatory genes; developmental genetics

Figure 1.

Variation in petal and floral structure and arrangement. (a) Typical petal consisting of blade, B, and claw, C, regions as found in Arabidopsis thaliana. (b) Zygomorphic flower from Antirrhinum majus observed in dorsoventral view, showing the dorsal, D, lateral, L and ventral, V, petals. (c) Rice flower showing the presence of lodicules, Lo, in a position corresponding to petals in other flowers.

Figure 2.

The development and specification of petals. (a) Floral meristem showing sepal primordia, Se, and the position where the petals, P, stamens, St, and carpel, C, will emerge. (b) Flower at later stage of development showing the developing organs in all whorls. (c) Mature flower and a schematic of how the combinatorial activities of the A, B and E genes specify petal identity. A and E factors are represented by green and grey circles and B‐class factors by red and yellow circles.

Figure 3.

Specialized cells of the petal epidermis. (a) Conical cells in the petal epidermis (viewed here in a scheme of a petal transverse section) enhance the colour of flowers by increasing the amount of incidental light (indicated by arrows) absorbed by the cells and reducing the amount of white light that is reflected by the cells. At a visual level, this translates into a more vibrant colour in the petals. The conical cells also provide a more attractive landing platform for pollinators. (b) Flat epidermal cells will make the petal appear paler.

Figure 4.

The evolution of petals. (a) Scheme summarizing the pattern of AP3/PI duplication in the angiosperms. Arrows indicate gene duplication events. (b) Silencing of both AP3 duplicates in opium poppy results in homeotic conversion of petals and stamens (on the left). Reproduced with permission from Drea et al..

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Further Reading

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Drea, Sinéad(Apr 2010) Petals. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0002065.pub2]