Genomic organization and regulation of the LeIMP-1 and LeIMP-2 genes encoding myo-inositol monophosphatase in tomato.

Myo-inositol (inositol) monophosphatase (IMP), an enzyme which catalyzes the synthesis of free inositol from various inositol monophosphates, is encoded by a small multigene family in many organisms. The tomato IMP gene family encodes three IMP isoforms with identical in vitro biochemical properties. To determine the role of each tomato LeIMP gene in plant growth, we isolated the genomic DNA copies of the LeIMP-1 and LeIMP-2 genes. The LeIMP-1 gene spans approximately 5.8 kb and consists of 12 exons, whereas the LeIMP-2 gene consists of an uninterrupted, single open reading frame (ORF). We have previously shown that steady-state levels of LeIMP-2 mRNA were very low in comparison to LeIMP-1 and LeIMP-3 mRNA levels. To determine whether LeIMP-2 gene expression was spatially restricted to a discreet domain within the plant we constructed transgenic plants containing an LeIMP-2 promoter::uidA gene fusion. Analysis of transgenic seedlings revealed that the LeIMP-2 promoter directed gene expression within epidermal and cortex cells of specific stem/leaf junctions in an abaxial-specific pattern and in the shoot apical meristem. Further, inositol, the product of IMP catalysis, and Li+, an inhibitor of IMP catalysis, decreased expression of the LeIMP-2 promoter as measured by a decrease in beta-glucuronidase activity after treatment.