Regulation and function of dynamic thylakoid stacking

The photosynthetic chloroplast thylakoid membrane of higher plants is a complex 3-dimensional structure that is divided into the stacked grana membranes and interconnecting unstacked stromal lamellae membranes. Recently using structured illumination microscopy (SIM), a super-resolution fluorescence technique, we have obtained remarkable new images showing that the thylakoid is morphologically dynamic on a timescale of just a few minutes. The images reveal how grana diameter and the number of membrane layers per grana are decreased, while the number of grana per chloroplast is increased, in low light compared to in darkness and high light. We show that dynamic thylakoid stacking is regulated by LHCII phosphorylation, a process normally connected with the regulation of the relative photosystem I and II antenna sizes by state transitions (1). However, unlike unlike state transitions dynamic thylakoid stacking does not require phospho-LHCII binding sites on photosystem I (1).  The functional significance of the thylakoid stacking dynamics appears to be their ability to regulate the redox poise of the electron transfer chain to optimise the balance between linear and cyclic electron flow (2).

References:

1. Wood WHJ, Barnett S, Flannery S, Hunter CN, Johnson MP (2019) LHCII phosphorylation regulates dynamic thylakoid stacking independent of photosystem I. Plant Physiology. 180, 2152-2166.

2. Wood WHJ, MacGregor-Chatwin C, Barnett S, Mayneord G, Huang X, Hobbs J, Hunter CN, Johnson MP (2018) Dynamic thylakoid stacking regulates the balance between linear and cyclic photosynthetic electron transfer. Nature Plants. 4, 116–127.

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