Granal thylakoid structure and function: explaining an enduring mystery of higher plants

Area of Science:

• Plant Biology
• Photosynthesis Research
• Structural Biology

Background:

• Photosystems II and I in higher plants are spatially separated in grana stacks and stroma lamellae.
• Electron transport between photosystems involves complex pathways prone to macromolecular blocking.
• The evolutionary advantage of this inefficient bipartition has remained unclear.

Purpose of the Study:

• To explain the evolutionary selection of the bipartite photosystem organization in higher plants.
• To propose a novel theory linking thylakoid ultrastructure to photosynthetic regulation and environmental adaptation.
• To unify disparate observations on thylakoid structure and function.

Main Methods:

• Systematic theoretical explanation of the proposed phenomenon.
• Analysis of thylakoid swelling/shrinking dynamics.
• Modeling of osmotic water fluxes and their impact on ultrastructure and function.

Main Results:

• Grana stacks act as "bellows," enabling ultrastructural control of photosynthesis via thylakoid osmotic water fluxes.
• This control regulates macromolecular blocking, diffusion paths, Cytochrome b6f complex function, and luminal pH.
• The system balances electron transport with gas exchange, providing homeostasis in fluctuating light and drought.

Conclusions:

• Grana stacks are a dry/high irradiance adaptation, improving plant fitness in challenging terrestrial environments.
• Thylakoid ultrastructural dynamics are crucial for optimizing photosynthesis under environmental stress.
• The proposed theory integrates multiple aspects of thylakoid structure and function.