Photomorphogenesis in Plants and Light Regulated Development

Plants have several uses for the light that goes far beyond their ability to photosynthesise low-molecular-weight sugars using only CO2, light, and water. Photomorphogenesis in plants could be described as the growth and development of plants in response to light. It allows plants to optimise their use of light and space. 

Photoperiodism is the ability of a plant to use light to track time. Plants can tell the time of day and year by sensing and using various wavelengths of sunlight. Phototropism is a directional response that allows plants to grow towards, or even away from sunlight.

The sensing of light in the environment is important to plants; it is highly important for competition and survival. A plant’s response to light is mediated by different photoreceptors. These photoreceptors are comprised of a protein covalently bonded to a light-absorbing pigment called a chromophore. Together, the two are called a chromoprotein.

The red/far-red and violet-blue regions of the visible light trigger structural development in plants. Sensory photoreceptors can absorb light in these particular regions of the visible light spectrum. This is due to the quality of light available in the daylight spectrum. In terrestrial habitats, light absorption by chlorophyll in plant leaves reach the peak in the blue and red regions of the spectrum.

As light filters through the canopy and the blue and red light wavelengths are absorbed. The spectrum shifts to the far-red end region of light, shifting the plant community to those plants better adapted to respond to far-red light.

Blue-light receptors allow plants in moving towards the direction of sunlight, which is rich in blue-green emissions. It should be noted water absorbs red light, which makes the detection of blue light important for algae and aquatic plants.

Stages of Photomorphogenesis

According to Hans Mohr (1983) the two important stages of Photomorphogenesis are:

1. Pattern specification, in which plant cells and tissues develop specific ability or competence to respond to light during certain developmental stages.

2. Pattern realisation, during which time the photo-response occurs.

There Are Two Different Types of Plant Responses of Light Signals

1. Phytochrome mediated photoresponses and

2. Blue-light responses or cryptochrome mediated photo-responses.

Phytochrome-Mediated Photoresponse

There are several  photomorphogenic responses in plant-mediated by phytochrome which is a proteinaceous pigment acting as a photoreceptor and absorbs red and far-red light. It can also absorb blue light.

The phytochrome-mediated response can sub-divided into three categories based on the amount of light absorbed:

1. Very Low Fluence Responses (VLFR): These responses are initiated by very low fluences (0.1 to 1 nmol m-2) saturating at 50 nmol m-2 and are non-photo reversible. For example, a brief flash of red light with fluence as low as 0.1 nmol m-2 can stimulate the growth of coleoptile and inhibit the growth of mesocotyl in oat seedlings that have been grown in dark.

Similarly, red light with a fluence of only 1-100 nmol m-2 is enough to stimulate seed germination in Arabidopsis.

2. Low Fluence Responses (LFRs): These responses require fluence of at least 1.0 nmol m-2 saturating at 1000 nmol m-2 and are photo-reversible. Most of the red/far-red photo-responses including lettuce seed germination belong to this category.

3. High Irradiance Responses (HIRs): These responses require continuous or prolonged exposure to light of relatively high irradiance saturating at much higher fluences (at least 100 times more) than LFRs and are non- photo-reversible.

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Blue-Light Responses or Cryptochrome Mediated Photoresponses

Apart from phytochrome mediated photoresponses, a large number of photoresponses in plants are known which are regulated by blue light and mediated through pigments called cryptochrome (crypto from cryptogams), the latter acting as a photoreceptor in such responses. Blue light responses have been reported in algae, fungi, ferns and higher plants.

Some of the typical and most commonly known blue-light responses in plants are:

• Phototropism

• Stomatal opening

• Phototaxis

• Inhibition of hypocotyl elongation

• Movements of chloroplasts within the cells 

• Sun tracking by leaves

• Stimulation of synthesis of carotenoids and chlorophylls etc.

Cryptochrome absorbs light rays mostly in the violet-blue region of the spectrum (400 – 500 nm). It also absorbs long-wave ultraviolet rays in the UV-A region (320 to 400 nm). However, most photo-responses of plants caused by cryptochrome result from absorption in the violet-blue region of the spectrum but they are simply called blue-light responses.

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Importance of Photomorphogenesis

Growth and development of plants are influenced by several environmental factors including light. However, light causes several responses in the plant body other than photosynthesis. These responses greatly influence the course of plant growth and the final plant appearance. They are photomorphogenic responses.

For example, the seeds of many plants do not germinate unless they are exposed to light. Germination of seeds in light shows that the seedlings require light to grow. Phototropic responses of seedlings and leaves of mature plants are also beneficial photomorphogenic processes.

Photomorphogenesis responses are also important to older plants. Many such responses respond to the relative lengths of day and night by forming reproductive structures or by forming dormant buds that can resist a cold winter (i.e., the phenomenon of photoperiodism and vernalisation.)