Two distantly related genes encoding 1âdeoxyâdâxylulose 5âphosphate synthases: differential regulation in shoots and apocarotenoidâaccumulating mycorrhizal roots
2002
Walter, Michael H. | Hans, Joachim | Strack, Dieter
Isopentenyl diphosphate, the universal precursor of isoprenoids, is synthesized by two separate routes, one in the cytosol and the other in plastids. The initial step of the plastidial pathway is catalysed by 1âdeoxyâdâxylulose 5âphosphate synthase (DXS), which was previously thought to be encoded by a singleâcopy gene. We have identified two distinct classes of DXSâlike cDNAs from the model legume Medicago truncatula. The deduced mature MtDXS1 and MtDXS2 proteins, excluding the predicted plastidâtargeting peptides, are similar in size (72.7 and 71.2âkDa) yet share only 70% identity in their amino acid sequences, and both encode functional DXS proteins as shown by heterologous expression in Escherichia coli. Available DXS sequences from other plants can easily be assigned to either classâ1 or classâ2. Partial sequences of multiple DXS genes in a single genome may be found in the databases of several monocot and dicot plants. Blot analyses of RNA from M. truncatula, maize, tomato and tobacco demonstrate preferential expression of DXS1 genes in many developing plant tissues except roots. By contrast, DXS2 transcript levels are low in most tissues but are strongly stimulated in roots upon colonization by mycorrhizal fungi, correlated with accumulation of carotenoids and apocarotenoids. Monoterpeneâsynthesizing gland cells of leaf trichomes appear to be another site of DXS2 gene activity. The potential importance of DXS1 in many housekeeping functions and a still hypothetical role of DXS2 in the biosynthesis of secondary isoprenoids is discussed.
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