Flower colour intensity depends on specialized cell shape controlled by a Myb-related transcription factor

Abstract

Flower colour is determined primarily by the production of pigments, usually anthocyanins or carotenoids, but the shade and intensity of the colour are often changed by other factors such as vacuolar compounds, pH and metal ions. Pigmentation can also be affected by the shape of epidermal cells, especially those facing prospective pollinators. A conical shape is believed to increase the proportion of incident light that enters the epidermal cells, enhancing light absorption by the floral pigments, and thus the intensity of their colour. We have identified a gene (mixta) that affects the intensity of pigmentation of epidermal cells in Antirrhinum majus petals. The cells of the corolla lobes fail to differentiate into their normal conical form in mixta mutants. We have cloned the mixta gene by transposon tagging; its sequence reveals that it encodes a Myb-related protein that probably participates in the transcriptional control of epidermal cell shape.

Ultrastructure and optics of the prism‐like petal epidermal cells of Eschscholzia californica (California poppy)

“…This observed prism‐like shape is highly unusual; petal surfaces are most commonly composed of either elongated flat cells or isotropically structured conical‐papillate cells (Kay et al ., 1981; Whitney et al ., 2011; Vignolini et al ., 2012a). The circular or hexagonally based conical‐papillate cells that characterize petals of many other eudicots differ markedly in that the wall is relatively evenly thickened around the entire cell as the conical outgrowth is formed by turgor‐driven expansion of the entire cell and is therefore vacuolated in cross‐section (Noda et al ., 1994). The petal epidermal cells of E. californica apparently represent a unique structure.…”

Effects of Epidermal Cell Shape and Pigmentation on Optical Properties of Antirrhinum Petals at Visible and Ultraviolet Wavelengths

“…An earlier investigation (Noda et al, 1994) of the Mixta+ and mixta-lines of Antirrkinum reported no significant difference in pigmentation between the two, but under our growth and assay conditions we found clear differences in the amount of flavonoid extractable from the two lines (Fig. 4) and tested the significance of differences with Student's t tests on data from each of the wavelengths marked with an error bar.…”

“…Although previous studies (Brehm and Krell, 1975;Noda et al, 1994) have implicated epidermal cell shape in influencing absorption of both visible and UV light, this effect has not been quantified. The separate effects of epidermal cell shape and pigmentation have been difficult to identify because these parameters often vary together (Baagae, 1977;Levy, 1978;Rieseberg and Schilling, 1985;Steiner, 1990).…”

“…Similar to leaves, flower petals often have large, lens-like, conical-papillate epidermal cells, which protrude from the peta1 surface (Kay et al, 1981). In flowers, the intense absorption of both UV (Brehm and Krell, 1975) and visible light (Noda et al, 1994) has been linked to the shape of these strongly domed cells, and in both cases the flattened cells are reported to absorb the light less strongly. This difference in absorption between conical-papillate and flattened cells has several possible sources.…”

“…This gene, expressed only in flower lobes, controls epidermal cell shape. In the normal, Mixta+ line, the adaxial epidermis of the corolla contains strongly conical-papillate cells; the mixtrz-line carries a single-gene mutation, which causes these epidermal cells to be only slightly domed (Noda et al, 1994). To alter pigment content we used supplemental UV light, which stimulates synthesis of flavonoid pigments in epidermal cells of Antirrkinum corollas (Fritze et al, 1991) and leaf and flower epidermises of a variety of other plants Flint et al, 1985;Lois, 1994;Liu et al, 1995).…”