Control of starch granule numbers in Arabidopsis chloroplasts.

Matilda Crumpton-Taylor,Scott Grandison,Alison M Smith,Kenneth M Y Png,Andrew J Bushby

doi:10.1104/pp.111.186957

Abstract

The aim of this work was to investigate starch granule numbers in Arabidopsis (Arabidopsis thaliana) leaves. Lack of quantitative information on the extent of genetic, temporal, developmental, and environmental variation in granule numbers is an important limitation in understanding control of starch degradation and the mechanism of granule initiation. Two methods were developed for reliable estimation of numbers of granules per chloroplast. First, direct measurements were made on large series of consecutive sections of mesophyll tissue obtained by focused ion beam-scanning electron microscopy. Second, average numbers were calculated from the starch contents of leaves and chloroplasts and estimates of granule mass based on granule dimensions. Examination of wild-type plants and accumulation and regulation of chloroplast (arc) mutants with few, large chloroplasts provided the following new insights. There is wide variation in chloroplast volumes in cells of wild-type leaves. Granule numbers per chloroplast are correlated with chloroplast volume, i.e. large chloroplasts have more granules than small chloroplasts. Mature leaves of wild-type plants and arc mutants have approximately the same number of granules per unit volume of stroma, regardless of the size and number of chloroplasts per cell. Granule numbers per unit volume of stroma are also relatively constant in immature leaves but are greater than in mature leaves. Granule initiation occurs as chloroplasts divide in immature leaves, but relatively little initiation occurs in mature leaves. Changes in leaf starch content over the diurnal cycle are largely brought about by changes in the volume of a fixed number of granules.

Highlights

The aim of this work was to investigate starch granule numbers in Arabidopsis (Arabidopsis thaliana) leaves
When Arabidopsis plants grown in 12 h of light and 12 h of dark were subjected to darkness after only 8 h of light, the rate of starch degradation was one-half that on previous nights, so that starch reserves were not exhausted until dawn (Graf et al, 2010)
Most observations and measurements of starch granules in leaves have been made on a transmission electron microscope (TEM) or light microscope images of sections through mesophyll cells

Summary

Introduction

The aim of this work was to investigate starch granule numbers in Arabidopsis (Arabidopsis thaliana) leaves. Chloroplasts in Arabidopsis (Arabidopsis thaliana) mesophyll cells are generally stated to contain about five starch granules at the end of the light period (Zeeman et al, 2002, 2007), but nothing is known about how this number is determined or the extent of genetic, temporal, developmental, and environmental variation in the number This is an important limitation in understanding control of starch degradation at night, a process essential for the normal growth of the plant (Gibon et al, 2006; Smith and Stitt, 2007; Stitt et al, 2007; Usadel et al, 2008). We applied these methods to leaves at different points in the daynight cycle, of different ages, under different growth conditions, and with different chloroplast sizes and numbers per cell

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