Introduction to Phytochrome
Among the most important of light cues used by plants are those that indicate where they are in relation to
neighbors that might impinge on their access to photosynthetically active radiation. Phytochromes are uniquely
suited to the task of neighbor detection due to their capacity to interconvert between forms with absorption maxima
in the red (~660 nm) and far-red (~730 nm). The phytochrome apoprotein is a dimer composed of two approximately 124
kDa polypeptide monomers. Each monomer attaches a linear bilin chromophore that is susceptible to structural
rearrangement upon the absorption of red or far-red light. Absorption of red light (R) by the red-absorbing form (Pr)
induces conversion of the protein to the far-red-absorbing form (Pfr); likewise, absorption of far-red light (FR) by
Pfr induces conversion to Pr (See animation below). Thus, at any one time, Pr and Pfr are in a dynamic equilibrium
that reflects the relative proportions of R and FR in ambient light. Because plant pigments absorb most visible light
below 700 nm, this equilibrium reflects the presence or absence of neighboring vegetation, by its impact on the ratio
of red (R) to far-red (FR) light, which is about 1.05 – 1.25 above the canopy and ranges to as low as 0.05 below the
canopy. Small changes in the R:FR ratio may lead to large changes in the ratio of Pfr to Ptotal so that even very
small changes are detectable, such as those that occur in the light reflected from stems of small neighbors. (Images and animation by Ethan Levesque.)
For many responses mediated by phytochromes, a saturating fluence of R induces a response that is reversible by FR. For example, a pulse of R may induce lettuce seed to germinate but a FR pulse following the R pulse will inhibit germination. In nature, such saturating fluences are promoted by exposure to light that is rich in R, such as above the canopy, while they are inhibited by light depleted in R, such as beneath the canopy. In this way germination and seedling development of shade-intolerant species are responsive to the proximity of neighboring vegetation.
When seedlings of such species sense or are overtopped by neighbors, aspects of seedling development promoted by R are reversed; for example, under decreasing R:FR ratios, extension growth is increased, branching is suppressed, and developing leaves are thinner and have less chlorophyll. Thus, phytochromes control antagonistic activities in open and shaded environments.