Plants cleverly adapt their photosynthesis to changes in light

Plants use the process of photosynthesis to convert light into energy for growth. However, the amount of sunlight they receive can fluctuate throughout the day. Too much or too little sunlight can negatively impact their energy production.

Recent research sheds light on how plants manage these shifts, revealing a communication system that lets them adjust their photosynthesis in real-time.

Stages in photosynthesis

Photosynthesis can be broken down into two main interconnected stages:

Stage 1: The light-driven reaction

This stage takes place within the chloroplasts, specialized structures found in plant cells. Chloroplasts are packed with membranes called thylakoids, which is where the light-driven reactions occur.

• The power of chlorophyll: Chlorophyll, the green pigment that gives plants their color, is located within the thylakoid membranes. Its job is to capture light energy from the sun, acting like a tiny solar panel.
  
• Energy conversion: When sunlight hits chlorophyll, it excites electrons within the molecules. This newfound energy is passed through a series of proteins within the thylakoids. Think of this like a relay race where the energy gets passed along.
  
• Generating chemical energy: This electron transfer chain creates a sort of ‘electrical current’ within the chloroplast. The plant cell harnesses this energy flow to produce ATP (adenosine triphosphate) and NADPH (nicotinamide adenine dinucleotide phosphate). Consider ATP and NADPH as energy-carrying molecules, like rechargeable batteries that power the next stage of photosynthesis.

Stage 2: The carbon-fixing reaction

This stage occurs in the stroma, the fluid-filled area of the chloroplast surrounding the thylakoids.

• Building with carbon dioxide: The carbon-fixing reaction, also known as the Calvin Cycle, uses the ATP and NADPH generated in the light-driven reaction to capture carbon dioxide (CO2) from the air.
  
• Sugar production: Through a series of enzyme-driven steps, CO2 is combined with other molecules and rearranged, using the energy stored in ATP and NADPH. The ultimate output of this process is sugar, primarily in the form of glucose.
  
• Fuel for the plant: These glucose molecules form the building blocks for the plant’s growth and development. They can be used immediately for energy, combined into more complex carbohydrates like starches for storage, or even used to build the plant’s physical structure in the form of cellulose.

Fluctuating light and photosynthesis

Light availability for plants is in a state of near-constant change. Several factors contribute to this:

• Clouds: Cloud cover can dramatically reduce the amount of sunlight reaching a plant, often switching between bright sun and shade within minutes.
  
• Shade: Plants growing beneath trees, within dense foliage, or in the shadows of buildings experience less direct sunlight and may need to adapt accordingly.
  
• Day and night: The natural cycle of day and night presents the most predictable change in light conditions. Plants often adjust their internal processes in preparation for darkness.

Hence, plants require a delicate balance within their photosynthetic machinery to ensure survival and efficient growth. In low-light conditions, plants need to focus on capturing as much sunlight as possible to produce the chemical energy that fuels their growth. They need to ramp up their light-harvesting processes to make the most of the limited light.

Too much sunlight can be harmful. The intense energy can overload the plant’s photosynthetic machinery, leading to the production of damaging molecules called reactive oxygen species. To counter this, plants need mechanisms to quickly dissipate excess energy and protect themselves.

Due to dynamic light conditions, plants have evolved sophisticated ways to fine-tune their photosynthesis on a moment-to-moment basis, ensuring they either make the most of available sunlight or protect themselves from its excess.

“qE”: Plant’s protection mechanism

Plants activate a protective mechanism called “energy-dependent quenching” (qE) when there’s varying sunlight. This mechanism gets triggered when sensors inside the plant’s chloroplasts signal an energy overload.

Once active, qE acts like a safety valve – excess light energy is released as heat, preventing damage to the plant’s photosynthetic machinery. While qE is crucial for survival in bright light, it reduces the plant’s ability to harvest energy, especially when light levels change quickly.

Understanding this balance between protection and energy production is an important area of research for scientists aiming to improve plant growth and agricultural yields.

KEA3: The master switch of qE

Scientists have discovered that a protein called KEA3 acts as a control switch for photosynthesis.

“The research team has decoded the molecular mechanism used by the plant to synchronize the two sub-processes of photosynthesis with each other,” explained Professor Dr. Ute Armbruster from the Institute of Molecular Photosynthesis, the study’s lead author.

KEA3 functions by monitoring conditions inside the chloroplast:

• Sensor 1 (pH levels): The acidity (or pH level) of the area around chloroplasts changes in response to light – it becomes more acidic in bright light and less acidic in the shade.
  
• Sensor 2 (Energy availability): KEA3 also gauges the plant’s energy supply by directly sensing the levels of the ATP and NADPH molecules.

KEA3 uses the information from these two sensors to regulate photosynthesis. As the light-driven reactions ramp up, they pump protons (positively charged hydrogen ions) into the interior of the thylakoid membranes. This leads to a rapid drop in pH within the chloroplast, making it more acidic.

KEA3 role in photosynthesis during varying light

Increased light also means a surge in the production of the energy-carrying molecules ATP and NADPH. The chloroplast becomes flush with chemical fuel.

KEA3, the master switch protein, directly senses both the pH change and the high levels of ATP and NADPH. This combination of signals causes a change in the protein’s structure that renders it inactive.

With KEA3 inactive, the protective qE mechanism is allowed to fully engage. Excess energy is safely released as heat, preventing damage caused by the overload of sunlight. Lots of sunlight leads to a rapid drop in pH and ample fuel supply. This inactivates KEA3, allowing qE to engage (protecting the plant).

When shifting to shade, the pH rises again, and fuel supplies decrease. This activates KEA3, which then signals to other parts of the plant to boost the light-harvesting reactions and maximize energy production under lower light conditions.

Understanding photosynthesis in changing light

“Through our work, we now understand for the first time how the two functional modules of photosynthesis communicate with each other via KEA3,” said Professor Armbruster.

“It is important to know this with a view to developing strategies to improve photosynthesis in the field, in order to increase crop yields in the long term.”

The study is published in the journal Nature Communications.

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