Understanding the Impact of Phytochrome B (PHYB) on Plant Life Cycle

Phytochrome is predominantly found in plant leaves and consists of a protein with a bilin chromophore. It’s present in most plants and even some bacteria, with a related bacterial version having a solved protein structure.

Arabidopsis thaliana, a model plant, possesses genes for five phytochromes. Researchers are exploring whether these phytochromes serve unique or overlapping purposes. They compared phyA and phyB mutants to the wild type in their tests. Intriguingly, both mutants displayed germination issues: phyA in far-red light and phyB in the dark. Interestingly, the inhibitory and stimulatory effects on germination were observed when the phyA mutation in far-red light was suppressed by a phyB mutation. 

In red light, the phyA phyB double mutant showed underdeveloped cotyledons, reduced CAB gene expression, and weaker chlorophyll induction. The role of PHYA in sensing day length for flowering response was highlighted, while PHYB played a role in early flowering under certain conditions. Thus, PHYA and PHYB work in harmony to regulate germination, seedling growth, and flowering. 

In this article, we will understand Phytochrome B (PhyB) as a specific type of phytochrome.

What is Phytochrome B (phyB)?

Phytochrome B (phyB) is a light-sensing molecule in plants that creates a specialized structure called a photobody, even though its exact makeup is not completely understood. Phytochrome B is like a light-sensitive superhero in plants. Based on the sort of light they get, it assists them in determining when and how to grow.

When this protein detects red or far-red light, which are two different colors of light, it can alter its structure. So, think of phytochrome B as a plant’s way of reading and responding to the light around it, almost like a plant’s light detector and growth manager rolled into one.

Difference between phyA and phyB

The absence of phyA led to a lack of ability in detecting the optimal photoperiod for flowering, indicating that PHYA is involved in perceiving day length. On the other hand, the phyB mutation caused the plant to flower earlier than both the normal plants and the phyA mutant across different photoperiods. However, the phyB mutant still responded to an inducing photoperiod for flowering. This is the main difference between phyA and phyB.

Role of Phytochrome B (phyB) in Seed Germination

Researchers explain the impact of phytochrome B on seed germination responses to light pulses. They compared normal (WT) and phyB-1-mutant seeds of Arabidopsis thaliana, stored for a month in dry conditions. Notably, dry-stored WT and phyB mutant seeds showed similar rates of germination in the dark. Light pulse experiments revealed a two-phase response in WT seeds. 

The first phase, the very-low-fluence response, displayed similar slopes in both WT and phyB-1 seeds. However, in the second phase, the low-fluence response, the WT slope was significantly higher, varying based on factors like storage time and temperature pre-treatments. Both genotypes exhibited enhanced germination with hourly and continuous far-red (FR) light pulses. This comparison of both WT and phyB-1 seeds tells that phytochrome B (phyB) plays an in seed germination. 

Phytochrome B (phyB) Function During Plant Adaptation


Plants face challenges when exposed to temperatures beyond their comfort range, affecting growth and survival. To adapt, plants have evolved term adaptive mechanisms to cope with heat shock. Recent studies suggest a connection between light signaling and heat responses, with phyB playing a significant role in heat acclimation. Light priming, a pre-exposure to light, enhances thermotolerance, as phyB activates the expression of a gene that counters reactive oxygen species (ROS) accumulation at high temperatures.

Interestingly, plant thermotolerance varies based on the red-to-far-red light ratio (R/FR ratio). A low R/FR ratio enhances thermotolerance, possibly due to phyB stock and PIF transcription factors’ interplay. PIF activity is induced under shade conditions, reducing fatty acid desaturation and increasing thermotolerance. 

The balance between phyB Pr and Pfr forms critically influences thermotolerance responses, which seem paradoxical. However, these responses align with natural light environments where shade-induced low R/FR conditions require enhanced thermotolerance during high temperatures, but not necessarily at night.

Cold Tolerance

Cold temperatures can harm plants, leading to growth issues and even death. Molecular genetic research has identified key elements involved in sensing and signaling cold temperatures. The CBF-COLD-COR regulon is well-studied, where CBF transcription factors quickly induce COR gene expression. Osmoprotection, lipid metabolism, cell wall changes, and growth hormone signaling are all supported by the products encoded by COR genes.

Cold shocks often align with light-dark cycles, prompting the connection between light signaling and cold responses. Evidence shows shared signaling mediators and regulatory mechanisms between light and cold acclimation. Photoperiods affect cold tolerance development; short days (SDs) increase cold tolerance compared to long days (LDs). PIF4 and PIF7 gene expression rise under LDs, and these PIF factors suppress CBF genes. Notably, phyB deficiency disrupts this photoperiod-controlled cold tolerance.

R/FR light ratio is vital for cold tolerance; a high R/FR ratio decreases cold tolerance, especially in phyB-dependent manners. In Arabidopsis, phyB influences PIF7, which inhibits CBF gene expression. This indicates that the interplay between phyB Pr and Pfr forms acts as a molecular switch integrating light signals into cold tolerance development.


In the domain of plant science, Phytochrome B (phyB) arises as a spellbinding hero, organizing an orchestra of reactions that length the whole vegetation cycle. From the fragile phase of germination to the unpredictable cycles of blossoming and transformation, phyB uses its light-detecting powers to impact development, endurance, and strength. Its job as an essential player in seeing red and far-red light, changing its design, and in this manner directing plants through different stages is genuinely noteworthy.

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