Starch synthase 4 is essential for coordination of starch granule formation with chloroplast division during Arabidopsis leaf expansion

Matilda Crumpton‐Taylor,Marilyn Pike,Regina Feil,Alison M Smith,John E Lunn,Christopher M Hylton,Kuan‐Jen Lu,Simona Eicke,Samuel C Zeeman

doi:10.1111/nph.12455

Matilda Crumpton‐Taylor, Marilyn Pike + Show 7 more

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https://doi.org/10.1111/nph.12455

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Abstract

Arabidopsis thaliana mutants lacking the SS4 isoform of starch synthase have strongly reduced numbers of starch granules per chloroplast, suggesting that SS4 is necessary for the normal generation of starch granules. To establish whether it plays a direct role in this process, we investigated the circumstances in which granules are formed in ss4 mutants.Starch granule numbers and distribution and the accumulation of starch synthase substrates and products were investigated during ss4 leaf development, and in ss4 mutants carrying mutations or transgenes that affect starch turnover or chloroplast volume.We found that immature ss4 leaves have no starch granules, but accumulate high concentrations of the starch synthase substrate ADPglucose. Granule numbers are partially restored by elevating the capacity for glucan synthesis (via expression of bacterial glycogen synthase) or by increasing the volumes of individual chloroplasts (via introduction of arc mutations). However, these granules are abnormal in distribution, size and shape.SS4 is an essential component of a mechanism that coordinates granule formation with chloroplast division during leaf expansion and determines the abundance and the flattened, discoid shape of leaf starch granules.

Highlights

The process by which starch granules arise is not known
SS4 contributes little to total starch synthase activity, ss4 mutants of Arabidopsis have at most one or two starch granules per chloroplast (Roldan et al, 2007) rather than the normal five or six (Crumpton-Taylor et al, 2012)
No other starch synthase is individually necessary for normal granule numbers (Roldan et al, 2007), SS4 may have a specific function in granule formation

Summary

Introduction

The process by which starch granules arise is not known. Suggestions range from largely physico-chemical mechanisms (Doi, 1965; Geddes & Greenwood, 1969; Ziegler et al, 2005) to the existence of specific protein primers analogous to the glycogenins of fungi and animals (e.g. Rothschild & Tandecarz, 1994; Singh et al, 1995; Langeveld et al, 2002; Chatterjee et al, 2005). Recent attention has focussed on the role of one isoform of soluble starch synthase, starch synthase 4 (SS4, At4 g18240). SS4 contributes little to total starch synthase activity, ss mutants of Arabidopsis have at most one or two starch granules per chloroplast (Roldan et al, 2007) rather than the normal five or six (Crumpton-Taylor et al, 2012). No other starch synthase is individually necessary for normal granule numbers (Roldan et al, 2007), SS4 may have a specific function in granule formation. Other isoforms of starch synthase may partially substitute for this function. The additional loss of SS3 further reduces starch granule numbers in the ss mutant background (Szydlowski et al, 2009; Merida & D′Hulst, 2012)

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