A model of chloroplast growth regulation in mesophyll cells

TL;DR

This paper presents a mathematical model explaining how chloroplast growth is regulated in mesophyll cells, accounting for observed constant ratios and shape-dependent variability across species and mutants.

Contribution

It introduces a novel switch-like signaling model inspired by bacterial quorum sensing to explain chloroplast size regulation and shape-dependent growth variability.

Findings

Model reproduces constant chloroplast-to-cell area ratio across species.

Shape influences growth variability, with disks and cylinders showing less scatter.

Spheres exhibit more scatter due to disproportionate area and volume growth.

Abstract

Chloroplasts regulate their growth to optimize photosynthesis. Quantitative data shows that the ratio of total chloroplast area to mesophyll cell area is constant across different cells within a single species, and also across species. Wild-type chloroplasts exhibit little scatter around this trend; highly irregularly-shaped mutant chloroplasts exhibit more scatter. Here we propose a model motivated by a bacterial quorum-sensing model consisting of a switch-like signalling network that turns off chloroplast growth. We calculated the dependence of the location of the relevant saddle-node bifurcation on the geometry of the chloroplasts. Our model exhibits a linear trend, with linearly growing scatter dependent on chloroplast shape, consistent with the data. When modelled chloroplasts are of a shape that grows with a constant area to volume ratio (disks, cylinders) we find a linear trend with minimal scatter. Chloroplasts with area and volume that do not grow proportionally (spheres) exhibit a linear trend with additional scatter.