Changes in apoplastic peroxidases during auxin-induced in vitro rooting of cultured grapevine (Vitis vinifera L. cv. Touriga) stems have been studied. The largest increase in peroxidase activity (EC 1.11.1.7) was associated with the early stages of root initiation and could be attributed to an increase in activity of an apoplastic 36 kDa cationic peroxidase (PxB2). Relative to other peroxidases, PxB2 demonstrated high indole-3-acetic acid (IAA) oxidase activity and apparently contributed the majority of potential IAA oxidase activity in rooting tissues. The distribution of this peroxidase in developing roots additionally associates it with early phases of growth restriction. PxB2 was purified from cell wall extracts prepared from the basal 1 cm of rooting stems. Microsequencing and subsequent cloning of its corresponding 3' truncated cDNA (encoding 255 amino acids of the mature protein) revealed it to have a typical class III peroxidase structure. The results suggest that this class III peroxidase with IAA oxidase activity is important for the control of IAA levels during root initiation and development.
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