| NADP-malic enzyme [malate dehydrogenase (NADP+) EC 1.1.1.40] catalyzes the oxidative decarboxylation of malate to yield pyruvate, CO2 and NADPH. In C3 plants such as tomato, the cytoplasmic isoform has anabolic and catabolic functions, providing NADPH and pyruvate for biosynthesis and respiration. Other proposed roles include cytoplasmic pH regulation and malate metabolism during fruit ripening.; In order to understand the spatial and temporal expression of the cytoplasmic tomato malic enzyme, LeME2, during development, fruit ripening, wound and defense responses, its 5' regulatory region was fused to the beta- glucuronidase (GUS) gene, and transformed into tomato and tobacco. LeME2 promoter-driven GUS expression was found to be constitutive, developmentally controlled and tissue specific. In mature tomato and tobacco plants, GUS expression occurred at higher levels in stems, roots and petioles compared to leaves. GUS expression was also observed in various tissues within flowers, fruits, seeds and seedlings of both tobacco and tomato. Furthermore, LeME2 promoter activity was predominantly associated with the vasculature throughout the plant. In tomato fruits, high levels of GUS expression were observed within the vascular bundles present throughout the fruit and radiating to each seed. GUS levels dramatically increased up to the mature green phase and decreased during transition to red ripe phase of ripening. In tobacco, GUS expression occurred in the walls and vasculature of the ovary and the outer walls of ovules until the formation of seed coat and dry capsule fruit.; The activity of LeME2 promoter during wound and defense responses was also investigated. In response to wounding alone, leaves displayed a 10-fold average increase over basal levels of GUS expression. Elicitors such as cellulase, xylanase, jasmonic acid and polygalacturonase further enhanced the wound-induced response of LeME2 promoter.; These results suggest that LeME2 is a ubiquitous housekeeping enzyme involved in several metabolic processes occurring throughout plant development. The predominance of LeME2 promoter activity in the vasculature suggests its involvement in lignin biosynthesis. Furthermore, LeME2 maybe be activated as a component of early defense response, to provide carbon skeletons and reducing power for the biosynthesis of secondary metabolites used in cell wall modifications or defense against invading pathogens. |
