Comparison of mitochondrial ascorbate peroxidase in the cultivated tomato, Lycopersicon esculentum , and its wild, salt-tolerant relative, L. pennellii - a role for matrix isoforms in protection against oxidative damage

Mitochondria require robust antioxidant defences to prevent lipid peroxidation and to protect tricarboxylic acid cycle enzymes from oxidative damage. Mitochondria from wild, salt-tolerant tomato, Lycopersicon pennellii (Lpa) did not exhibit lipid peroxidation in response to high salinity (100 mM NaCl), whereas those isolated from cultivated tomato, L. esculentum (Lem), accumulated malondialdehyde. The activity, intraorganellar distribution and salt response of mitochondrial ascorbate peroxidase (mAPX) differed dramatically in the two species. In Lem mitochondria, the majority (84%) of mAPX was associated with membranes, being located either on the inner membrane, facing the intermembrane space, or on the outer membrane. Total mAPX activity did not increase substantially in response to salt, although the proportion of matrix APX increased. In contrast, 61% of Lpa mAPX activity was soluble in the matrix, the remainder being bound to the matrix face of the inner membrane. Salt treatment increased the activity of all mAPX isoforms in Lpa, without altering their intramitochondrial distribution. The membrane-bound isoforms were detected in mitochondria of both species by western blotting and found to be induced by salt in Lpa. These observations suggest that matrix-associated APX isoforms could act in concert with other mitochondrial antioxidants to protect against salt-induced oxidative stress.