The phytochromes are a family of photoreceptors that absorb radiation across the 600–800 nm waveband. The 600–700 nm band is conventionally referred to as red light (R) while the 700–800 nm band as far-red (FR). Each phytochrome can exist in two photoconvertible conformers: Pr has an absorption maximum at ca. 665 nm and is converted to Pfr, which absorbs maximally at ca. 730 nm, being thereby converted back to Pr. The overall scheme, first worked out by Sterling Hendricks and Harry Borthwick, with their colleagues at Beltsville in the 1940s and 1950s on the basis of supremely elegant physiological experiments, is as follows:
Both Pr and Pfr have relatively broad absorption bands and these overlap below ca. 700 nm; this means that in broad-band irradiation, such as daylight, Pr and Pfr are continually being inter-converted resulting in a dynamic equilibrium, known as the phytochrome photoequilibrium, which is a quantitative function of the relative amounts of R and FR incident upon the plant. The photoequilibrium is conventionally expressed numerically as a proportion of the total phytochrome present as Pfr, or Pfr/P. The roles and mechanisms of Pr and Pfr have been investigated almost entirely by reductionist approaches, growing plants in darkness and exposing them to brief or prolonged irradiation from narrow-beam sources. For over fifty years this research has been and continues to be a tour de force of modern biology, and the knowledge accrued can now be applied to the infinitely more complex situations where plants growing in a natural environment are subject to continuous and often rapid fluctuations in light signals over a wide wavelength range.
The phytochromes are encoded by a small multi-gene family comprising five genes in Arabidopsis but possibly only three in the cereals (Mathews et al., 1996). The five Arabidopsis phytochromes are known as phytochrome A (phyA) to phytochrome E (phyE). Prior to molecular characterisation, phytochromes were classified into two main pools, based on their biochemical and kinetic characteristics as determined from in vivo and in vitro spectrophotometry. The so-called type I phytochrome is the predominant phytochrome present in etiolated seedlings, the first to be identified and still the most completely characterised. It is described as being ‘light-labile’, although it is only the Pfr form which is labile; thus type I phytochromes are also labile in the dark as long as Pfr has been previously generated.
Degradation half-lives of Pfr are typically around 20–45 minutes, and thus Pfr is essentially absent from plants exposed to more than ca. 24 hours of white light. Phytochrome A (phyA) is the only known representative of the type I phytochrome pool. In etiolated seedlings and dark-grown tissues phyA accumulates to relatively high levels in the Pr form. Exposure to light causes rapid loss of phyA, not only through degradation of Pfr
In contrast, type II phytochromes are present in low steady-state concentrations in both dark-grown and light-grown plants because the Pfr forms are relatively stable (Furuya and Schäfer, 1996). All other members of the phytochrome family, that is, phyB to phyE, have these characteristics and are thus regarded as type II phytochromes. PhyB to phyE are synthesised slowly and appear not to be under such strict transcriptional regulation as phyA. In most cases studied, phyB is the predominant phytochrome in light-grown plants. As phytochrome action is a function of the concentration of Pfr, regulation of the rates of synthesis and degradation of the individual phytochromes is clearly crucial in determining the physiological response. It follows that attempts to regulate development by the transgenic expression of phytochrome genes must take account of the rates of synthesis and degradation of the transgene products.
0 komentar:
Post a Comment