Increasing amyloplast size in wheat endosperm through mutation of PARC6 affects starch granule morphology

The determination of starch granule morphology in plants is poorly understood. The amyloplasts of wheat endosperm contain large discoid A-type granules and small spherical B-type granules. To study the influence of amyloplast structure on these distinct morphological types, we isolated a mutant in durum wheat (Triticum turgidum) defective in the plastid division protein PARC6, which had giant plastids in both leaves and endosperm. Endosperm amyloplasts of the mutant contained more A- and B-type granules than those of the wild-type. The mutant had increased A- and B-type granule size in mature grains, and its A-type granules had a highly aberrant, lobed surface. This morphological defect was already evident at early stages of grain development and occurred without alterations in polymer structure and composition. Plant growth and grain size, number and starch content were not affected in the mutants despite the large plastid size. Interestingly, mutation of the PARC6 paralog, ARC6, did not increase plastid or starch granule size. We suggest TtPARC6 can complement disrupted TtARC6 function by interacting with PDV2, the outer plastid envelope protein that typically interacts with ARC6 to promote plastid division. We therefore reveal an important role of amyloplast structure in starch granule morphogenesis in wheat.

Automated summary

The understanding of starch granule morphology in plants is still lacking. A durum wheat mutant with giant plastids, caused by defective PARC6, showed an increase in A- and B-type granules in the endosperm amyloplasts. The mutant had abnormal lobed surface in A-type granules, but it did not affect plant growth and grain size, number, and starch content. Interestingly, mutation of the PARC6 paralog, ARC6, did not have the same effect. The study reveals the importance of amyloplast structure in starch granule morphogenesis in wheat.