9.3 Light stress and photoprotection mechanisms

Plants face challenges from excessive light, including UV-B radiation. They've developed clever ways to protect themselves, like moving chloroplasts and using antioxidants. These mechanisms help plants stay healthy and keep photosynthesizing even when the sun's too bright.

Light stress can harm plants, but they're not defenseless. From non-photochemical quenching to UV-screening compounds, plants have a toolkit to handle bright days. Understanding these processes helps us grasp how plants adapt to their environment.

Photoinhibition and Photoprotection

Photoinhibition and its consequences

Top images from around the web for Photoinhibition and its consequences

• 

  Frontiers | Photoinhibition of Photosystem I Provides Oxidative Protection During Imbalanced ... View original

  Is this image relevant?

• 

  Frontiers | Quality Control of Photosystem II: The Mechanisms for Avoidance and Tolerance of ... View original

  Is this image relevant?

• 

  Frontiers | Quality Control of Photosystem II: The Mechanisms for Avoidance and Tolerance of ... View original

  Is this image relevant?

• 

  Frontiers | Photoinhibition of Photosystem I Provides Oxidative Protection During Imbalanced ... View original

  Is this image relevant?

• 

  Frontiers | Quality Control of Photosystem II: The Mechanisms for Avoidance and Tolerance of ... View original

  Is this image relevant?

1 of 3

Top images from around the web for Photoinhibition and its consequences

• 

  Frontiers | Photoinhibition of Photosystem I Provides Oxidative Protection During Imbalanced ... View original

  Is this image relevant?

• 

  Frontiers | Quality Control of Photosystem II: The Mechanisms for Avoidance and Tolerance of ... View original

  Is this image relevant?

• 

  Frontiers | Quality Control of Photosystem II: The Mechanisms for Avoidance and Tolerance of ... View original

  Is this image relevant?

• 

  Frontiers | Photoinhibition of Photosystem I Provides Oxidative Protection During Imbalanced ... View original

  Is this image relevant?

• 

  Frontiers | Quality Control of Photosystem II: The Mechanisms for Avoidance and Tolerance of ... View original

  Is this image relevant?

1 of 3

• Photoinhibition occurs when the photosynthetic apparatus absorbs excess light energy, leading to a decrease in photosynthetic efficiency
• Excess light energy can cause damage to the photosynthetic machinery, particularly to the D1 protein in the photosystem II (PSII) reaction center
• Photoinhibition can lead to the formation of reactive oxygen species (ROS) such as singlet oxygen ($^1O_2$), which can further damage cellular components (lipids, proteins, and DNA)
• Prolonged photoinhibition can result in a reduction of plant growth and productivity

Photoprotective mechanisms

• Non-photochemical quenching (NPQ) is a photoprotective mechanism that dissipates excess light energy as heat, preventing damage to the photosynthetic apparatus
• The xanthophyll cycle is a key component of NPQ, involving the interconversion of violaxanthin, antheraxanthin, and zeaxanthin pigments
• Under high light conditions, violaxanthin is converted to zeaxanthin via antheraxanthin, which facilitates the dissipation of excess energy as heat
• Zeaxanthin plays a crucial role in NPQ by accepting excess energy from excited chlorophyll molecules and dissipating it as heat, thus protecting the photosynthetic apparatus from damage
• The photosystem II repair cycle is another important photoprotective mechanism that replaces damaged D1 proteins in the PSII reaction center, maintaining photosynthetic efficiency (PSII turnover)

Cellular Responses to Light Stress

Chloroplast movement and its role in light stress response

• Chloroplasts can move within the cell in response to changes in light intensity, a process called chloroplast movement or chloroplast photorelocation
• Under low light conditions, chloroplasts tend to accumulate along the cell walls perpendicular to the incident light (accumulation response) to maximize light capture
• In high light conditions, chloroplasts move to the cell walls parallel to the incident light (avoidance response) to minimize exposure to excess light and reduce the risk of photodamage
• Chloroplast movement is mediated by the actin cytoskeleton and is triggered by blue light receptors called phototropins (phot1 and phot2)

Antioxidant systems and their role in mitigating light stress

• Antioxidant systems play a crucial role in neutralizing reactive oxygen species (ROS) generated during light stress, protecting the cell from oxidative damage
• Enzymatic antioxidants such as superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) scavenge ROS and convert them to less harmful compounds
• Non-enzymatic antioxidants such as ascorbate (vitamin C), glutathione, and tocopherols (vitamin E) also contribute to ROS detoxification
• The ascorbate-glutathione cycle, also known as the Halliwell-Asada cycle, is a key antioxidant pathway that uses ascorbate and glutathione to neutralize hydrogen peroxide ($H_2O_2$)

State transitions and their role in balancing light energy distribution

• State transitions are a mechanism that balances the distribution of light energy between photosystem I (PSI) and photosystem II (PSII) to optimize photosynthetic efficiency
• When PSII is overexcited relative to PSI, the light-harvesting complex II (LHCII) can dissociate from PSII and associate with PSI (state 2), redistributing energy to PSI
• Conversely, when PSI is overexcited, LHCII can dissociate from PSI and associate with PSII (state 1), redirecting energy to PSII
• State transitions are regulated by the phosphorylation and dephosphorylation of LHCII, which is controlled by the redox state of the plastoquinone pool and the cytochrome b6f complex

UV-B Stress

UV-B radiation and its effects on plants

• UV-B stress refers to the damage caused by ultraviolet-B (UV-B) radiation, which has wavelengths between 280 and 315 nm
• UV-B radiation can cause direct damage to DNA by inducing the formation of pyrimidine dimers, which can lead to mutations and impair cellular functions
• UV-B can also generate reactive oxygen species (ROS), leading to oxidative stress and damage to cellular components (proteins, lipids, and DNA)
• Exposure to UV-B radiation can reduce plant growth, photosynthetic efficiency, and crop yield

Protective mechanisms against UV-B stress

• Plants have evolved various protective mechanisms to mitigate the harmful effects of UV-B radiation, including the production of UV-screening compounds
• Flavonoids are a class of phenolic compounds that accumulate in the epidermis and act as natural sunscreens, absorbing UV-B radiation and protecting the underlying tissues
• Common flavonoids with UV-screening properties include quercetin, kaempferol, and anthocyanins, which are often responsible for the red, blue, and purple colors in plants
• Other UV-screening compounds include sinapate esters (such as sinapoyl malate) and hydroxycinnamic acid derivatives, which are found in the epidermis of many plant species
• The accumulation of UV-screening compounds is regulated by UV-B photoreceptors called UVR8 (UV RESISTANCE LOCUS 8), which trigger signaling cascades leading to the activation of genes involved in flavonoid biosynthesis