{Free} Short Notes on Phytochromes - Download PDF!!

By Neetesh Tiwari|Updated : September 20th, 2021

Are you looking for some short and reliable notes during your CSIR-NET preparations? Then, you have come to a perfect place!

Candidates preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at Byjus Exam Prep come up with short notes on Phytochromeswhich comes under the Plant Physiology section of the Life Science syllabus

Our experienced exam experts have meticulously designed this set of short notes on the short notes on Phytochromes to give you the most standard set of study materials to be focused upon. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them learn and for their future. So, here we are offering the best study notes that are reliable and can be used by the students during their preparations for the upcoming CSIR-NET 2021 exam.

Are you looking for some short and reliable notes during your CSIR-NET preparations? Then, you have come to a perfect place!

Candidates preparing for their CSIR NET exam might need to get some short study notes and strategies to apply while preparing for the key exam of their life. At this point, We at Byjus Exam Prep come up with short notes on Phytochromes, which comes under the Plant Physiology section of the Life Science syllabus

Our experienced exam experts have meticulously designed this set of short notes on the short notes on Phytochromes to give you the most standard set of study materials to be focused upon. In this cut-throat competitive world, students need to prepare themselves with the best study materials to help them learn and for their future. So, here we are offering the best study notes that are reliable and can be used by the students during their preparations for the upcoming CSIR-NET 2021 exam.

Study Notes on Phytochromes

Phytochrome

Phytochrome is a photomorphogenic pigment that absorbs red and far-red light and causes photomorphogenesis. It also absorbs blue light. Phytochromes have Been found in most plants where It regulates many growths and developmental processes such as photoperiodic Induction of flowering, chloroplast development (not including chlorophyll synthesis), leaf senescence, leaf abscission, seed germination, stem elongation etc. Phytochrome is a family of chromo proteins with small covalently bound pigment molecules. Phytochrome proteins occur as a dimer of two ~125 kDa polypeptides, each with a covalently attached pigment molecule. The pigment is called the chromophore. It is a linear tetrapyrrole termed phytochromobilin and similar in structure to mammalian bile pigments, bilirubin. Phytochromobilin is synthesized in the plastids and its precursor is δ-aminolaevulinic acid. Together, the apoprotein (polypeptide chain) and its chromophore make up the holoprotein. Assembly of a proprotein with its chromophore is autocatalytic and occurs spontaneously.

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Fig: A structural domain is a phytochrome. The N-terminal half contains a PAS domain, GAF domain and PHY domain. The PAS-GAF-PHY domains comprise the photosensory region of phytochrome. The C-terminal half contains two PAS related domains (PRD) that mediate phytochrome dimerization and histidine kinase-related domain (HKRD). B. The structure of phytochrome, the regulatory domain contains the dimerization domain and the histidine kinase-related domain.

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The phytochrome pigment is found to be present in two photo reversible forms:

  1. Red light-absorbing (666 nm) Pr form
  2. Far-red light-absorbing (730- nm) Pfr form

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The Pr form absorbs red light (at a peak of 660 nm bright blue in colour and inactive form. When it absorbs red light it is converted to the Pr form. The Pfr form absorbs far-red light (maximum at 730 nm), green (olive) in colour and active form. When it 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).

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  • When the chromophore absorbs light, there is a slight change in its structure. This causes a change in the conformation of the protein portion to the form that initiates a response. The action spectrum of light needed for phytochrome-mediated responses shows a peak in the red at about 666nm. These responses can be reversed by an application of far red-light (peak at 730 nm) soon after the red treatment.
  • The change in absorbance is caused by the conversion of the photoreceptor from one structural form to another. The red-absorbing form changes to the far-red absorbing form when it absorbs red light (666 nm) and back again when it absorbs far-red light (730 nm). Upon absorption of light, the Pr chromophore undergoes a cis-trans isomerization in the C-15 and C-16 double bond. Very dim light will work if the irradiation time is enough. This reciprocal between fluence rate and time is known as the law of reciprocity. Both VLFRs and LFRs obey the law of reciprocity.
  • The HIRs require prolonged exposure to the light of a relatively high photon family. The HIR does not show red and far-red Photoreversibility and does not obey the law of reciprocity.

 

VLFR

LFR

HIR

Photoreversibility

No

Yes

No

Reciprocity

Yes

Yes

No

Fluence requirement

<1 m mol m-2

1-1000 mmol m-2

>1000 mmol m-2

Photoreceptor

PHYA

PHYB, PHYD and PHYE

Dark grown -phy A; Light brown – phy B

Active wavelength

Red and Blue

Red and Fag-red

Dark Brown -far-red and blue; Light grown-red

Between C and D rings of the linear tetrapyrrole. However, a recent NMR analysis showed that the A pyrrole ring around C4-C5 double bond rotates during photoconversion.

  • Phytochrome in all plants is synthesized in dark entirely as Pr. Absorption of red light by Pr converts it to Pfr and absorption of far-red light by Pfr converts it to Pr. However, the absorption spectra of the two forms overlap in the red region of the spectrum.
  • When Pr molecules are exposed to red light, most of them absorb it and are converted to Pfr, but some of the Pfr also absorb the red light and is converted back to Pr because both Pr and Pfr absorb red light.
  • Thus, the proportion of phytochrome in the Pfr form after saturating irradiation by red light is only about 85%.
  • Similarly, a very small amount of far-red light also absorbed by Pr makes it impossible to convert Pfr entirely to Pr by broad-spectrum far-red light. Instead, an equilibrium of 97% Pr and 3% Pfr is achieved.
  • This equilibrium is termed the photostationary state. Both Pr and Pfr forms also absorb blue light. Hence phytochrome effects can also be mediated by blue light.
  • There are two different cases of phytochrome with distinct properties.

These have been termed Type I and Type II phytochromes.

Phytochromes proteins are enclosed by the phytochrome gene family termed PHY.

Its five members are PHYA, PHYB, PHYC, PHYD and PHYE.

  • Type I phytochrome, which is encoded by the PHYA gene, is abundant in dark-grown plants (etiolated plants). Its concentration decreases rapidly upon exposure to light as a result of transcription inhibition, mRNA degradation and proteolysis.
  • Type II phytochrome (encoded by the PHYB, PHYC and PHYD and PHYE genes) is light stable and present in both light-grown and dark grown plants. In light grown plants, PHYB is the most abundant phytochrome, whereas PHYC-PHYE is less abundant.

Phytochrome response

Phytochrome responses can be distinguished by the amount of light required. The number of photons impinging on the unit surface area is termed fluence. Its unit is mol m-1 Fluence rate, that is fluence per unit time, is termed as irradiance. Its unit is mol m-2 sec-1. Different phytochrome’s responses occur at different light furnaces. These responses (LFRs) and high-irradiance responses (HIRs).

  • VLFR can be initiated by fluences below I mmol m- Because the amount of light which is needed to induce a response in a minute, so a very small amount (less than 0.02%) of the total phytochrome is converted to Pfr. Thus far-red light cannot reverse VLFRs.
  • LFRs are initiated by fluences of 1 m molm-2 and they are saturated at 1000 mmolm-2.
  • The classical red and far-red photoreversible responses of phytochromes which were discovered by Hendricks and Borthwick are examples of LRFs.
  • These responses show phytochromes which were discovered by Hendricks and Borthwick's are examples of LFRs. These responses show photo reversibility because the far-red light reverses the responses.
  • Both VLFRs and LFRs can be induced by low fluence. The total fluence is a function of two factors: the fluence rate and the irradiation time.  Thus a brief pulse of red light will induce a response.

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