Nine genes of Arabidopsis (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 (At4g17090) 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 kD, 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 chloroplast-targeted beta-amylase 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.