Plant Physiology Preview
Published on August 6, 2004; 10.1104/pp.104.045203
Received April 26, 2004
Returned for revision May 17, 2004
Accepted June 11, 2004
The lys5 Mutations of Barley Reveal the Nature and Importance of Plastidial ADP-Glc Transporters for Starch Synthesis in Cereal Endosperm
Nicola J. Patron , Boris Greber , Brendan F. Fahy , David A. Laurie , Mary L. Parker , and Kay Denyer *
John Innes Centre, Colney, Norfolk NR4 7UH, United Kingdom (N.J.P., B.G., D.A.L., B.F.F., K.D.); and Institute of Food Research, Colney, Norfolk NR4 7UA, United Kingdom (M.L.P.)
* Corresponding author; email: kay.denyer{at}bbsrc.ac.uk.
Much of the ADP-Glc required for starch synthesis in the plastids of cereal endosperm is synthesized in the cytosol and transported across the plastid envelope. To provide information on the nature and role of the plastidial ADP-Glc transporter in barley (Hordeum vulgare), we screened a collection of low-starch mutants for lines with abnormally high levels of ADP-Glc in the developing endosperm. Three independent mutants were discovered, all of which carried mutations at the lys5 locus. Plastids isolated from the lys5 mutants were able to synthesize starch at normal rates from Glc-1-P but not from ADP-Glc, suggesting a specific lesion in the transport of ADP-Glc across the plastid envelope. The major plastidial envelope protein was purified, and its sequence showed it to be homologous to the maize (Zea mays) ADP-Glc transporter BRITTLE1. The gene encoding this protein in barley, Hv.Nst1, was cloned, sequenced, and mapped. Like lys5, Hv.Nst1 lies on chromosome 6(6H), and all three of the lys5 alleles that were examined were shown to carry lesions in Hv.Nst1. Two of the identified mutations in Hv.Nst1 lead to amino acid substitutions in a domain that is conserved in all members of the family of carrier proteins to which Hv.NST1 belongs. This strongly suggests that Hv.Nst1 lies at the Lys5 locus and encodes a plastidial ADP-Glc transporter. The low-starch phenotype of the lys5 mutants shows that the ADP-Glc transporter is required for normal rates of starch synthesis. This work on Hv.NST1, together with the earlier work on BRITTLE1, suggests that homologous transporters are probably present in the endosperm of all cereals.
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