AtFZL is required for correct starch granule morphology in Arabidopsis chloroplasts

The control of starch granule number and morphology in plastids is poorly understood. Here, we demonstrate that AtFZL, a protein involved in thylakoid membrane organisation, is required for correct starch granule morphology in Arabidopsis. Leaves of mutants lacking AtFZL had the same starch content as wild-type leaves, but their starch granules were smaller and had a distinct, uneven surface morphology. Most chloroplasts in the mutant were larger than those of the wild type. However, the difference in chloroplast size could not explain the difference in granule size and shape in the Atfzl mutants, since other mutants with larger chloroplasts than the wild type (arc mutants) had granules that were similar in size and shape to wild-type granules. As reported previously, the Atfzl mutant had aberrant thylakoid organisation. We found that this phenomenon was particularly pronounced in regions surrounding starch granules. The location of the thylakoid-bound granule initiation protein MFP1 was unaffected in the Atfzl mutant. We propose that AtFZL affects starch granule size and shape by influencing thylakoid organisation at the periphery of starch granules. Our results are consistent with an important role for thylakoid architecture in determining granule morphology.