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Light: an indicator of time and place.

To grow and develop optimally, all organisms need to perceive and process information from both their biotic and abiotic surroundings. A particularly important environmental cue is light, to which organisms respond in many different ways. The responses can be simple, as in phototactic single-celled organisms, or complex, as in higher animals, which use visual inputs to modify their behavior. Many organisms can also detect cycles of light and darkness, which are exploited for seasonal and timeof-day responses. Because they are both photosynthetic and sessile, plants have to be especially plastic in response to their light environment. In addition to utilizing light as a time-keeping mechanism, plants are unique in that they use light as a source of energy and they analyze light to control such developmental decisions as when to germinate and flower. The diverse responses of plants require sophisticated sensing of intensity, direction, duration, and wavelength of light. The action spectra of light responses have provided assays to identify three photoreceptors absorbing in the ultraviolet, blue/near ultraviolet, and red/far-red (R/FR)-spectral ranges (Kendrick and Kronenberg 1994). The best characterized group of these photoreceptors is the red/far-red (R/FR) absorbing phytochromes. Phytochromes are photochromic proteins composed of a large protein (∼125 kD) covalently attached to a linear tetrapyrrole chromophore. Phytochromes are synthesized in a red-light-absorbing form, Pr (lmax = 660 nm) which, upon exposure to red light, can be phototransformed into a far-red-light-absorbing form, Pfr (lmax = 730 nm). Upon exposure to far-red light, Pfr is photoconverted to Pr (Kendrick and Kronenberg 1994). Both Pfr and Pr that has been photocycled have been shown to induce developmental responses (Shinomura et al 2000). Thus, phytochrome acts as a light-controlled developmental switch. Phytochromes have been found in all taxa of lower and higher plants examined (Mathews and Sharrock 1997; Mathews and Donoghue 1999), as well as cyanobacteria (Hughes et al. 1997; Lamparter et al. 1997; Yeh and Lagarias 1997). Phytochromes control development throughout the plant life cycle beginning with seed germination and seedling deetiolation (the transition from growth in the dark to growth in the light). They also control cotyledon/ leaf expansion and stem elongation by regulating cell division and expansion. Phytochromes enable the perception of neighboring plants or shade, and influence the transition to flowering. Because of these profound effects, phytochromes have been studied intensively for >50 years. The impact this family of photoreceptors has had on the history of Botany is extensively covered in the book, Pigment of the Imagination (Sage 1992). Many excellent reviews on phytochrome’s role in photomorphogenesis have been written recently (Mancinelli 1994; Smith 1995; Fankhauser and Chory 1997; Mustilli and Bowler 1997; Batschauer 1999; Deng and Quail 1999, and references within). In this review we will describe the complexities of phytochrome response pathways and highlight some of the recent accomplishments in elucidating the mechanisms by which phytochromes regulate so many downstream responses. Finally, we will examine the interactions between phytochrome and endogenous developmental programs.