Changes in peroxidase activity and isoperoxidase pattern during strawberry (Fragaria × ananassa) callus development

Changes in peroxidase (EC. 1.11.1.7) activity and isoperoxidase pattern were analysed during strawberry (Fragaria × ananassa) callus development. Peroxidase activity per g fresh weight using 4-methoxy-α-naphthol, 3,3′,5,5′-tetramethyl bencidine, coniferyl alcohol or ferulic acid as substrates was highest at 40 days of culture, which coincided with the exponential phase of growth. While the specific peroxidase activity with the first three substrates was highest during the exponential growth phase, the highest specific enzymatic activity with ferulic acid as substrate was reached after 80 days of culture when callus growth had ceased. Several isoperoxidases were constitutively expressed during the complete callus growth cycle, unlike a group of acidic isoperoxidases, which were only present when callus had ceased growing. The high reactivity of the acidic isoperoxidases with ferulic acid as substrate, as judged by the k1 and k3 values of the peroxidase catalytic cycle, strongly suggests that these acidic isoenzymes are mainly involved in the formation of diferuloyl bridges between non-cellulosic components of the cell wall, thus contributing to cell wall rigidification, which, in turn, is partly responsible for the cessation of growth.