Optimal disk packing of chloroplasts in plant cells

TL;DR

This study investigates how chloroplasts in plant cells are optimally packed to balance light absorption and protection, revealing that cell shape and organelle size are tuned for efficient photosynthesis and light avoidance.

Contribution

The paper introduces a model linking cell shape and chloroplast size to optimal packing strategies, supported by experimental measurements and simulations, highlighting a universal principle in biological packing systems.

Findings

Chloroplast packing shows signatures of optimality for light absorption and avoidance.

Optimal cell shape closely matches model predictions based on packing constraints.

Universal principles of packing and jamming are applicable across biological and physical systems.

Abstract

Photosynthesis is vital for the survival of entire ecosystems on Earth. While light is fundamental to this process, excessive exposure can be detrimental to plant cells. Chloroplasts, the photosynthetic organelles, actively move in response to light and self-organize within the cell to tune light absorption. These disk-shaped motile organelles must balance dense packing for enhanced light absorption under dim conditions with spatial rearrangements to avoid damage from excessive light exposure. Here, we reveal that the packing characteristics of chloroplasts within plant cells show signatures of optimality. Combining measurements of chloroplast densities and three-dimensional cell shape in the water plant Elodea densa, we construct an argument for optimal cell shape versus chloroplast size to achieve two targets: dense packing into a two-dimensional monolayer for optimal absorption under dim light conditions and packing at the sidewalls for optimal light avoidance. We formalize these constraints using a model for random close packing matched with packing simulations of polydisperse hard disks confined within rectangular boxes. The optimal cell shape resulting from these models corresponds closely to that measured in the box-like plant cells, highlighting the importance of particle packing in the light adaptation of plants. Understanding the interplay between structure and function sheds light on how plants achieve efficient photo adaptation. It also highlights a broader principle: how cell shape relates to the optimization of packing finite and relatively small numbers of organelles under confinement. This universal challenge in biological systems shares fundamental features with the mechanics of confined granular media and the jamming transitions in dense active and passive systems across various scales and contexts.