Photomorphogenesis
In developmental biology, photomorphogenesis is light-mediated development, where plant growth patterns respond to the light spectrum. This is a completely separate process from photosynthesis where light is used as a source of energy. Phytochromes, cryptochromes, and phototropins are photochromic sensory receptors that restrict the photomorphogenic effect of light to the UV-A, UV-B, blue, and red portions of the electromagnetic spectrum.[1]
The photomorphogenesis of
Most research on photomorphogenesis is derived from plants studies involving several kingdoms:
History
Theophrastus of Eresus (371 to 287 BC) may have been the first to write about photomorphogenesis. He described the different wood qualities of fir trees grown in different levels of light, likely the result of the photomorphogenic "shade-avoidance" effect. In 1686, John Ray wrote "Historia Plantarum" which mentioned the effects of etiolation (grow in the absence of light). Charles Bonnet introduced the term "etiolement" to the scientific literature in 1754 when describing his experiments, commenting that the term was already in use by gardeners.[4]
Developmental stages affected
Seed germination
Light has profound effects on the development of plants. The most striking effects of light are observed when a
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 emerges into the light) there is increased secondary root formation and branching. In 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, the growth responses are hormone mediated.
Seedling development
In the absence of light, plants develop an etiolated growth pattern. Etiolation of the seedling causes it to become elongated, which may facilitate it emerging from the soil.
A seedling that emerges in darkness follows a developmental program known as skotomorphogenesis (dark development), which is characterized by etiolation. Upon exposure to light, the seedling switches rapidly to photomorphogenesis (light development).[5]
There are differences when comparing dark-grown (etiolated) and light-grown (de-etiolated) seedlings
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
De-etiolated characteristics:
- 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.
Photoperiodism
Some plants rely on light signals to determine when to switch from the vegetative to the flowering stage of plant development. This type of photomorphogenesis is known as photoperiodism and involves using red photoreceptors (phytochromes) to determine the daylength. As a result, photoperiodic plants only start making flowers when the days have reached a "critical daylength," allowing these plants to initiate their flowering period according to the time of year. For example, "long day" plants need long days to start flowering, and "short day" plants need to experience short days before they will start making flowers.[2]
Photoperiodism also has an effect on vegetative growth, including on bud dormancy in perennial plants, though this is not as well-documented as the effect of photoperiodism on the switch to the flowering stage.[2]
Light receptors for photomorphogenesis
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 known as phytochromes. There are at least 5 members of the phytochrome family of photoreceptors. There are several blue light photoreceptors known as cryptochromes. The combination of phytochromes and cryptochromes mediate growth and the flowering of plants in response to red light, far-red light, and blue light.
Red/far-red light
Plants use
There are two forms of phytochromes: red light absorbing, Pr, and far-red light absorbing, Pfr. Pfr, which is the active form of phytochromes, can be reverted to Pr, which is the inactive form, slowly by inducing darkness or more rapidly by irradiation by far-red light.
Most plants have multiple phytochromes encoded by different
Molecular analyses of phytochrome and phytochrome-like genes in
Takuma Tanada observed that the root tips of barley adhered to the sides of a beaker with a negatively charged surface after being treated with red light, yet released after being exposed to far-red light.[8] For mung bean it was the opposite, where far-red light exposure caused the root tips to adhere, and red light caused the roots to detach.[9] This effect of red and far-red light on root tips is now known as the Tanada effect.
Blue light
Plants contain multiple blue light
There are blue light photoreceptors that are not a part of photomorphogenesis. For example, phototropin is the blue light photoreceptor that controls phototropism.
UV light
Plants show various responses to UV light. UVR8 has been shown to be a UV-B receptor.[11] Plants undergo distinct photomorphogenic changes as a result of UV-B radiation. They have photoreceptors that initiate morphogenetic changes in the plant embryo (hypocotyl, epicotyl, radicle)[12] Exposure to UV- light in plants mediates biochemical pathways, photosynthesis, plant growth and many other processes essential to plant development. The UV-B photoreceptor, UV Resistance Locus8 (UVR8) detects UV-B rays and elicits photomorphogenic responses. These response are important for initiating hypocotyl elongation, leaf expansion, biosynthesis of flavonoids and many other important processes that affect the root-shoot system.[13] Exposure to UV-B rays can be damaging to DNA inside of the plant cells, however, UVR8 induces genes required to acclimate plants to UV-B radiation, these genes are responsible for many biosynthesis pathways that involve protection from UV damage, oxidative stress, and photorepair of DNA damage.[14]
There is still much to be discovered about the mechanisms involved in UV-B radiation and UVR8. Scientists are working to understand the pathways responsible for plant UV receptors response to solar radiation in natural environments.[14]
References
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- ^ ISBN 978-3-642-65418-3.
- ^ "Photomorphogenesis". photobiology.info. Retrieved 2018-12-07.
- ^ ISBN 978-1-4020-3809-9.
- PMC 1464706.
- ^ PMID 22303272.
- ^ a b c d Taiz, Lincoln; Zeiger, Eduardo; Møller, Ian Max (2015). Plant Physiology and Development (Sixth ed.). Sunderland, MA: Sinauer Associates, Inc.
- PMID 16591610.
- PMID 16658001.
- ^ PMID 21841916.
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- ^ PMID 23250626.