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Published on May 12, 2006; 10.1104/pp.106.079186

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Received February 14, 2006
Returned for revision February 28, 2006
Accepted May 2, 2006

REDOX REGULATION OF A NOVEL PLASTID-TARGETED {beta}-AMYLASE OF Arabidopsis thaliana

Francesca Sparla , Alex Costa , Fiorella Lo Schiavo , Paolo Pupillo , and Paolo Trost *

Laboratory of Molecular Plant Physiology, Department of Experimental Evolutionary Biology, University of Bologna, Via Irnerio 42, I-40126 Bologna, Italy
Dipartimento di Biologia, Università degli Studi di Padova, Via U. Bassi 58/B, I-35131 Padova, Italy

* Corresponding author; email: trost{at}alma.unibo.it.

Nine genes of Arabidopsis thaliana encode for {beta}-amylase isozymes. Six members of the family are predicted to be extrachloroplastic isozymes and three contain predicted plastid transit peptides. Among the latter, chloroplast-targeted {beta}-amylase (CT-BMY = At4g17090; Lao et al., 1999) and thioredoxin-regulated {beta}-amylase (TR-BAMY = At3g23920; this work) are experimentally demonstrated to be targeted to plastids. Recombinant TR-BAMY was catalytically active only when expressed as a mature protein, i.e. with no transit peptide. Mature TR-BAMY was a monomer of 60 kDa, hydrolyzing soluble starch with optimal activity between pH 6.0 and 8.0. The activity of recombinant TR-BAMY was strictly dependent on redox potential with an Em,7.0 of -302 ± 14 mV. Thioredoxins f1, m1 and y1 of Arabidopsis were all able to mediate the reductive activation of oxidized TR-BAMY. Site-specific mutants showed that TR-BAMY oxidative inhibition depended on the formation of a disulfide bridge between Cys-32 and Cys-470. Consistent with TR-BAMY redox dependency, total {beta}-amylase activity in Arabidopsis chloroplasts was partially redox-regulated and required reducing conditions for full activation.

In Arabidopsis, TR-BAMY transcripts were detected in leaves, roots, flowers, pollen and seeds. TR-BAMY may be the only {beta}-amylase of nonphotosynthetic plastids suggesting a redox regulation of starch metabolism in these organelles. In leaves, where CT-BMY is involved in physiological degradation of starch in the dark, TR-BAMY is proposed to participate to a redox-regulated pathway of starch degradation under specific stress conditions.




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