Thiol-disulfide oxidoreductase required for maintaining symplastic permeability in the meristem. Involved in redox regulation of callose deposition, plasmodesmata cell-to-cell communication and meristem maintenance. A number of isoforms are produced. According to EST sequences. Expressed during embryogenesis in the root meristem at the transition from heart to torpedo stage. At the cotyledon stage, expressed in root and shoot meristems, with weak expression in provascular tissues. Expressed later in the seedling root meristem, shoot apex, and vasculature. During flowering, expressed in inflorescence and floral meristems, and in petal, stamen, and carpel primordia. Seedling lethality when homozygous. Increased callose deposition and accumulation of reactive oxygen species in roots. Belongs to the thioredoxin family. Plant M-type subfamily.