Glyoxylate cycle enzymes in seedlings and in mature plants of tomato (Lycopersicon esculentum Mill.)

The presence of the two glyoxylate cycle marker enzymes, isocitrate lyase (ICL, EC 4.1.3.1) and malate synthase (MS, EC 4.1.3.2), was investigated in cotyledons during the post-germinative growth of tomato (Lycopersicon esculentum Mill., cv Chico III) seedlings. Both ICL and MS increased in the dark and declined after illumination. Cotyledons from seedlings grown in the dark for 7 days were employed for sucrose density gradient fractionation of cell organelles. It was shown that ICL and MS are localized in glyoxysomal fractions. The presence of the two key enzymes of the glyoxylate cycle in tomato cotyledons was confirmed by immunoblotting both in crude extracts and in glyoxysomal fractions. The occurrence of such enzymes was also investigated in different tissues of tomato plants (leaves, fruits, roots and flowers). Although it was difficult to detect ICL and MS activities by spectrophotometric assays, immunoblot analysis showed the presence of both enzymes in senescent leaves and, surprisingly, in young expanding leaves, but not in mature leaves. The role of glyoxylate cycle enzymes in leaves and the value of further studies with tomato are discussed.