8.4.1  Phytochromes — multi-functional light sensors

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Figure 8.31 Phytochromes consist of a chromophore linked through the sulphur atom of a cysteine amino acid residue to a protein ('peptide' on diagram). Absorption of light causes a reversible conformational change in the chromophore (a cis-trans isomerisation centred on carbon 15) which alters the absorption spectrum. The two forms are referred to as Pr (left) and Pfr (right). Most phytochrome responses are activated when molecules are in the Pfr form.

(Based on Salisbury and Ross 1992; reproduced with permission of Wadsworth Publishing Co.)


Table 8.12

Early studies of plant developmental responses to light were some of the most fascinating and elegant, and led to the conclusion that not only was light quantity important but different wavelengths caused different reactions (Borthwick et al. 1954). In particular, several processes (e.g. seed germination, floral induction) responded to red (R; around 660nm) and far-red (FR; around 730nm) wavelengths in quite opposite ways. This turned out to be a manifestation of the operation of one set of morphogenetic pigments, the phytochromes. We now know, from isolation of phytochrome in a test tube, and later discovery of several phytochrome genes, that phytochromes are complex molecules consisting of a protein linked to a chromophore (Figure 8.31). Photon absorption by the latter causes a conformational change which alters the absorption spectrum (Figure 8.32a). In most types of phytochrome, these changes can occur repeatedly, a phenomenon known as photoreversibility. The two states are termed the Pr form and Pfr form, because of their optimum absorbances in the R and FR regions, respectively. Note that Pfr absorbs to some extent in the red region, which means that irradiation with pure red (660nm) will lead to absorption by both forms and so interconversion will continue indefinitely. Eventually, however, a stable state is reached, called the photostationary equilibrium, in this case with about 15% of molecules as Pr and 85% as Pfr. Because Pr absorbs very little far-red, pure far-red leads to about 97% Pr and 3% Pfr. Normally, of course, plants are exposed to sunlight which contains red and far-red wavelengths (Table 8.12). The link to the physiological responses — from experiments done under lots of different wavelengths leading to graphs known as action spectra (Figure 8.32b) — is now a lot easier to understand. Conversion of Pr to Pfr by red light is the basis of red-promoted processes. Although the classic photoreversible phytochrome responses show that Pfr is the active form, there is also evidence that Pr is important, for example in maintaining shoot gravitropism in the dark (Liscum and Hangarter 1993). Surprisingly, phytochrome is also present in roots, with Pr having a role in regulating elongation growth.


Figure 8.32 Phytochrome can be characterised chemically by its light absorption specturm, and biologically by its action specturm. (a) Absorption spectra of Pr and Pfr. Although Pr and Pfr both absorb in the blue and ultra-violet regions, their biological importance relaters mainly to the difference in the red and far-red regions. Conventionally, Pr and Pfr maximum absorbances are taken as 660 nm and 730 nm, respectively. (b) Action spectrum of inhibition of hypocotyl elongation in dark-grown lettuce seedlings. The maximum effect is at 720 nm, in the far-red zone. The effectiveness of wavelengths <500 nm is due to blue-light receptors, discussed later in the text.

(Based on Vierstra and Quail 1983 and Hartmann 1967)