Phytochrome Response Modes

Decades of physiological investigation have resulted in the identification of several distinct ‘response modes’, based on photobiological criteria (for review see Smith, 1995). The classical phytochrome-mediated response, first demonstrated by the pioneering studies of Sterling Hendricks, Harry Borthwick and their colleagues in the 1950s (Borthwick et al., 1952) is the low fluence response (LFR). LFRs (such as the stimulation of seed germination, the inhibition of seedling elongation and the control of flowering by night breaks) are saturated by low fluences of R and reversed by similarly low fluences of FR.

This R/FR reversibility became, and remains, the classical criterion of phytochrome action. Decades later, a ‘very low fluence response’ (VLFR) was recognised, in which minute amounts of light saturate response by establishing very low concentrations of Pfr. Because Pr has a tail of absorption stretching up to 730 nm, even FR radiation will establish low levels of Pfr and therefore the VLFR is not FR-reversible. The most important VLFR in nature is the stimulation of germination of small seeds buried beneath the ground and briefly exposed to daylight by disturbance. Both the LFR and the VLFR require only brief periods of irradiation, and when the light is kept on for several hours a different mode of action becomes apparent. This ‘high irradiance reaction’ (HIR) requires continued irradiation for several hours and the response is usually a diminishing logarithmic function of irradiance (or fluence rate). The HIR can be observed in etiolated seedlings grown under continuous irradiation. It is characterised by dependence on fluence rate, usually with an action maximum in the FR, and by non-conformation to the reciprocity law (for review see Mancinelli, 1994).

All three of these response modes (LFR, VLFR, HIR) can, and often are, involved in the control of germination and de-etiolation. Also, the role of the phytochromes in photoperiodism involves interaction with the circadian rhythms, and can take the form of either an LFR or HIR. Finally, phytochromes regulate growth and flowering in mature plants in the natural environment via a R:FR ratio response. A wide range of phenomena, including elongation growth and the rate of flowering (separately from the induction of flowering), exhibit a direct linear relationship to the proportion of Pfr established by the incident radiation (Smith, 1983, 1995, 2000). These R:FR ratio responses, which are the basis of proximity perception and shade avoidance, may indicate a quite separate response mode, or may represent a sub-set of LFR responses conditioned by acting within the environment of light-grown tissues.

Relating the different response modes to the individual phytochromes became possible with the generation of null mutants that lack functional phytochromes (for review, see Whitelam and Devlin, 1997). The conclusion from many mutant studies is that the VLFR and the FR–HIR are both mediated by phyA, whereas the LFR is mediated predominantly by phyB. Furthermore, the R:FR ratio responses are mediated predominantly by phyB, with supplementary action by phyD and phyE. These discoveries opened the way to exploiting our knowledge of the individual functions of the members of the phytochrome family by transgenic over-expression of the PHY genes. Over-expression has been important for fundamental objectives, to analyse the molecular actions of the phytochromes and to help elucidate signal transduction pathways, but increasingly transgenic methods are being applied towards the improvement of crop plant performance.

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