title: Degradation of Glucan Primers in the Absence of Starch Synthase 4 Disrupts Starch Granule Initiation in Arabidopsis.
abstract: Arabidopsis leaf chloroplasts typically contain five to seven semicrystalline starch granules. It is not understood how the synthesis of each granule is initiated or how starch granule number is determined within each chloroplast. An Arabidopsis mutant lacking the glucosyl-transferase, STARCH SYNTHASE 4 (SS4) is impaired in its ability to initiate starch granules; its chloroplasts rarely contain more than one large granule, and the plants have a pale appearance and reduced growth. Here we report that the chloroplastic α-amylase AMY3, a starch-degrading enzyme, interferes with granule initiation in the ss4 mutant background. The amy3 single mutant is similar in phenotype to the wild type under normal growth conditions, with comparable numbers of starch granules per chloroplast. Interestingly, the ss4 mutant displays a pleiotropic reduction in the activity of AMY3. Remarkably, complete abolition of AMY3 (in the amy3 ss4 double mutant) increases the number of starch granules produced in each chloroplast, suppresses the pale phenotype of ss4, and nearly restores normal growth. The amy3 mutation also restores starch synthesis in the ss3 ss4 double mutant, which lacks STARCH SYNTHASE 3 (SS3) in addition to SS4. The ss3 ss4 line is unable to initiate any starch granules and is thus starchless. We suggest that SS4 plays a key role in granule initiation, allowing it to proceed in a way that avoids premature degradation of primers by starch hydrolases, such as AMY3.
authors.affiliation: From the Institute for Agricultural Sciences, ETH Zurich, 8092 Zürich, Switzerland.
authors.forename: David
authors.identifier: 0000-0003-3905-3647
authors.initials: D
authors.lastname: Seung
authors.affiliation: From the Institute for Agricultural Sciences, ETH Zurich, 8092 Zürich, Switzerland.
authors.forename: Kuan-Jen
authors.initials: KJ
authors.lastname: Lu
authors.affiliation: From the Institute for Agricultural Sciences, ETH Zurich, 8092 Zürich, Switzerland.
authors.forename: Michaela
authors.initials: M
authors.lastname: Stettler
authors.affiliation: From the Institute for Agricultural Sciences, ETH Zurich, 8092 Zürich, Switzerland.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Streb
authors.affiliation: From the Institute for Agricultural Sciences, ETH Zurich, 8092 Zürich, Switzerland szeeman@ethz.ch.
authors.forename: Samuel C
authors.identifier: 0000-0002-2791-0915
authors.initials: SC
authors.lastname: Zeeman
mesh_terms: D017360:Arabidopsis / Q000235:genetics / Q000378:metabolism*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism; D005964:Glucosyltransferases / Q000235:genetics / Q000378:metabolism; D009154:Mutation; D013213:Starch / Q000096:biosynthesis* / Q000235:genetics; D013214:Starch Synthase / Q000235:genetics / Q000378:metabolism*; D000516:alpha-Amylases / Q000235:genetics / Q000378:metabolism*
chemical_list: D029681:Arabidopsis Proteins; D013213:Starch; D005964:Glucosyltransferases; C504530:starch synthase III, Arabidopsis; D013214:Starch Synthase; C520418:starch synthase IV, Arabidopsis; D000516:alpha-Amylases
keywords: Arabidopsis thaliana; alpha-amylase; carbohydrate metabolism; chloroplast; photosynthesis; plant biochemistry; starch biosynthesis; starch granule initiation; starch synthase