The inositol oxygenase gene family ofArabidopsisis involved in the biosynthesis of nucleotide sugar precursors for cell-wall matrix polysaccharides.

The nucleotide sugar UDP-glucuronic acid (UDP-GlcA) is the principal precursor for galacturonic acid, xylose, apiose and arabinose residues of the plant cell-wall polymers. UDP-GlcA can be synthesized by two different functional pathways inArabidopsisinvolving either UDP-glucose dehydrogenase or inositol oxygenase as the initial enzyme reaction to channel carbohydrates into a pool of UDP sugars used for cell-wall biosynthesis. The genes for the enzyme myo-inositol oxygenase (MIOX) were analyzed inArabidopsis. They represent a small gene family containing four members. The transcription of all those members indicates a transient and organ-specific gene expression pattern in growing plant tissues as analyzed by RT-PCR and in promoter::GUS reporter gene lines. Two isoforms (MIOX1, MIOX2) are expressed in almost all tissues of the plant, whereas the expression ofMIOX4andMIOX5is largely restricted to flowers, particularly maturing pollen. T-DNA insertion lines inMIOXgenes were isolated; however, single knock-outs show growth phenotypes similar to the wild type. The monosaccharide composition of the cell wall in these mutants is not significantly changed compared to wild type plants. However, the incorporation of³H-inositol into wall polymers of seedlings is greatly impaired in the mutant lines ?MIOX1and ?MIOX2, which are the only isoforms that are expressed in seedlings. [ABSTRACT FROM AUTHOR]