Phytochrome

EFB530 Plant Physiology

Phytochrome and photomorphogenesis

Light is vital for photosynthesis, but is necessary to direct plant growth and development as well - can act as a signal in photomorphogenesis

light triggers phototropism

light also triggers a switch from the etiolated phase of growth (skotomorphogenesis=seedling grown in complete darkness) to a de-etiolated phase

etiolated seedlings

de-etiolated seedlings

an action spectrum of the light needed for de-etiolation indicates that red light is required (peak at about 660 nm)

this effect can be reversed by an application of far-red light (peak at 730 nm) soon after the red treatment
a spectrophotometer was designed to measure the color absorbance of seedlings - can measure a decrease in absorbance at 660 nm and in increase in absorbance at 730 nm when the seedling is exposed to red light
due to the interconversion of a single photoreceptor molecule

Phytochrome is the photoreceptor that activates the de-etiolation (and many other) responses to red light

phytochrome exists in two forms Pr = absorbs at a peak of 666 nm and Pfr = peak absorbance at 730 nm
Pr is the form synthesized in dark-grown seedlings
when Pr absorbs red light, it is converted to the Pfr form
the Pfr form is the active form that initiates biological response
when Pfr absorbs far red light, it is converted to the Pr form
Pfr can also spontaneously revert to the Pr form in the dark over time = dark reversion; Pfr is also susceptible to proteinases
Pfr absorbs a little bit of red light, so in red light, there is a balance of 85% Pfr and 15% Pr
Pr absorbs a very small amount of far red light, so in far red light, there is a balance of 97% Pr to 3% Pfr

Phytochrome is a protein with a pigment molecule covalently bound

the pigment molecule is called the chromophore = linear tetrapyrrole
the chromophore absorbs light, then there is a slight change in structure - activates the protein portion - protein is active to initiate the response
phytochrome levels are much higher (about 50X) in dark-grown seedlings than in light-grown plants
levels are highest near the apex
in Arabidopsis there are at least 5 different phytochrome proteins (and genes that encode them), but all use the same chromophore
different phytochromes are involved in different phenomena
mutants which don't produce phytochrome have tall hypocotyls even in the light and are called hy mutants

In Arabidopsis, PHY A is a Type I phytochrome

It accumulates to high levels in dark-grown seedlings
when it receives RED light and is converted to Pfr - which induces de-etiolation, but then it is rapidly degraded
PHY A in the Pfr form also inhibits its own expression, so levels of PHY A are much lower in seedlings once they receive light

In Arabidopsis, PHY B, C, D, and E are Type II phytochromes

The Pfr forms of these are stable in the light and they control reversible responses to shading, day length perception for example

Phytochrome allows the plant to detect properties of light "color" related to growth conditions

sunlight has a R:FR ratio of 1.2
light under a canopy of leaves has a R:FR ratio of 0.13
light under 5 mm of soil has a R:FR ratio of 0.88
the higher proportion of FR light allows plants to detect when they are shaded
plants adapted for growth in full sun will display greater stem elongation when shaded due to greater proportion of Pr to Pfr, at the cost of smaller leaves and less branching
many small seeds (low amounts of storage reserves) require red light for germination; FR light inhibits germination
if they are buried below the level of light penetration, won't germinate
if they are shaded by a canopy, causing a high proportion of FR, germination is inhibited, Pfr is required for germination