The Characteristic Polyhedral, Sharp-edged Shape of Compound-type Starch Granules in Rice Endosperm is Achieved via the Septum-like Structure of the Amyloplast

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  • Kawagoe Yasushi
    Division of Plant Sciences, National Institute of Agrobiological Sciences

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The starch granules in rice endosperm are of the compound-type, which comprises up to several dozen granules per amyloplast. In contrast, the granules in the endosperm of maize and wheat are of the simple-type, which comprises only a single granule. The molecular mechanisms that control granule-type have long been an open question. Laser scanning confocal microscopy (LSCM) is advantageous over conventional microscopy when combined with the expression of fluorescent proteins (FPs) in the tissue under investigation. We have analyzed numerous transgenic rice expressing FPs fused to amyloplast-targeted proteins in endosperm. The fluorescent signals of these fusion proteins provided high-quality images of the internal structures of amyloplasts. Interestingly, changing expression levels of plastid-division proteins or isoamylase3 alter both amyloplast division and granule synthesis. On the basis of these results, a model for the synthesis of compound-type granules in rice has been proposed. In this model, the septum-like structure (SLS) is synthesized beneath the outer limit envelope prior to de novo granule synthesis, where it functions like a partition-wall in the stroma, casting the growing granules into their characteristic polyhedral, sharp-edged shape. LSCM analyses using FPs fused to starch synthase I (SSI-GFP) and the transit peptide of granule-bound starch synthase I (tpGBSSICherry) revealed that SSI-GFP and tpGBSSI Cherry are respectively targeted to the granule surface in the stroma and the SLS sandwiched between the inner envelope membranes. This review examines the technical issues of LSCM and transgenic analysis, summarizes models for compound-type granule synthesis, and presents future questions.

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