Light has profound effects on the development of plants. The light-mediated changes in plant growth and development are called photomorphogenesis. The most striking effects of light are observed when a germinating seedling emerges from the soil and is exposed to light for the first time.

Normally the seedling radicle (root) emerges first from the seed, and the shoot appears as the root becomes established. Later, with growth of the shoot (particularly when it merges into the light) there is increased secondary root formation and branching. This coordinated progression of developmental responses are early manifestations of correlative growth phenomena where the root affects the growth of the shoot and vice versa. To a large degree, these coordinated differential growth responses are hormone mediated.

In the absence of light, plants develop an etiolated growth pattern. Etiolation of the seedling adapts it to emerging from the soil.

Comparison of dark-grown (etiolated) and light-grown (de-etiolated) seedlings

Etiolated characteristics De-etiolated characteristics

Distinct "apical hook" (dicot) or coleoptile (monocot)

No leaf growth

No chlorophyll

Rapid stem elongation

Limited radial expansion of stem

Limited root elongation

Limited production of lateral roots

Apical hook opens or coleoptile splits open

Leaf growth promoted

Chlorophyll produced

Stem elongation suppressed

Radial expansion of stem

Root elongation promoted

Lateral root development accelerated

The developmental changes characteristic of photomorphogenesis shown by de-etiolated seedlings, are induced by light. Typically, plants are responsive to wavelengths of light in the blue, red and far-red regions of the spectrum through the action of several different photosensory systems. The photoreceptors for red and far-red wavelengths are know as phytochromes. There are at least 5 members of the phytochrome family of photoreceptors. There are several blue light photoreceptors.

Photoreceptor systems in plants

Red/far-red systems: Phytochrome

Plants use phytochrome to detect and respond to red and far-red wavelengths.

Phytochromes are proteins with a light absorbing pigment attached (chromophore).

The chromophore is a linear tetrapyrrole called phytochromobilin.

The phytochrome apoprotein is synthesized in the Pr form. Upon binding the chromophore, the holoprotein becomes sensitive to light. If it absorbs red light it will change conformation to the biologically active Pfr form. The Pfr form can absorb red light and switch back to the Pr form.

Schematic diagram of phytochrome protein:

Most plants have multiple phytochromes encoded by different genes. The different forms of phytochrome control different responses but there is also a lot of redundancy so that in the absence of one phytochrome, another may take on the missing functions.

Arabidopsis has 5 phytochromes - PHYA, PHYB, PHYC, PHYD, PHYE

Molecular analyses of phytochrome and phytochrome-like genes in higher plants, ferns, mosses, algae, and photosynthetic bacteria have shown that phytochromes evolved from prokaryotic photoreceptors that predated the origin of plants.

Click here to read a brief account of the history of phytochrome research

Blue light systems:

As for the red/far-red system, plants contain multiple blue light photoreceptors which have different functions.

Based on studies with action spectra, mutants and molecular analyses, it has been determined that higher plants contain at least 4, and probably 5, different blue light photoreceptors.

Cryptochromes were the first blue light receptors to be isolated and characterized from any organism. The proteins use a flavin as a chromophore. The cryptochromes have evolved from microbial DNA-photolyase, an enzyme that carries out light-dependent repair of UV damaged DNA.

Two cryptochromes have been identified in plants.

Cryptochromes control stem elongation, leaf expansion, circadian rhythms and flowering time.

In addition to blue light, cryptochromes also perceive long wavelength UV irradiation (UV-A).

Phototropin is the blue light photoreceptor that controls phototropism. It also uses flavin as chromophore. Only one phototropin has been identified so far (NPH1). Phototropin also perceives long wavelength UV irradiation (UV-A) in addition to blue light.

Recent experiments indicate that a 4th blue light receptor exists that uses a carotenoid as a chromophore. This new photoreceptor controls blue light induction of stomatal opening. However, the gene and protein have not yet been found.

Other blue light responses exist that seem to function in plants that are missing the cryptochrome, phototropin and carotenoid photoreceptors suggesting that at least one more will be found.

Since the cryptochromes were discovered in plants, several labs have identified homologous genes and photoreceptors in a number of other organisms, including humans, mice and flys. It appears that in mammals and flys, the cryptochromes function in entrainment of the biological clock. Indeed, in flys, a cryptochrome may be a functional part of the clock mechanism.

UV systems:

Based on various responses to UV light, it is assumed that there are UV-specific photoreceptors.

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