In plants, the first step in myo-inositol synthesis is the cyclization of d-G1c-6-P to myo — inositol-1-P (Ino-1P) by lL-myo-inositol 1-phosphate synthase. In Arabidopsis, two func­tional isoforms were reported, At4g39800 and At2g22240 (454). The second step involves dephosphorylation of Inol-P to myo-inositol by myo-inositol monophosphatase (IMPase; EC 3.1.3.25) (455). Distinct multiple but highly conserved IMPase isoforms are found in each plant species. Three IMPases were identified in tomato (456). In Arabidopsis, a con­served IMPase-like-protein (At3g02870) was proposed by Glilaspi to act as IMPase; however, biochemical and genetic data indicate that At3g02870 encodes L-Gal-1-P phosphatase (457, 458). It is possible that other IMPase-like proteins in Arabidopsis (for example, At1g31190, At4g39120) encode the Ino-1P phosphatase activity to form myo-inositol. The identification of the true IMPase gene product is critical to evaluate what controls the pathway to shunt Ino-1P to the myo-inositol oxidation pathway. Free myo-inositol is oxidized by inositol oxy­genase (MIOX; E. C. 1.13.99.1) to D-GlcA. Arabidopsis contains four Miox isoforms (453).

It would be interesting to determine if the myo-inositol oxidation pathway operates inde­pendently of the pathway leading to synthesis of UDP-GlcA from UDP-Glc. This knowledge could aid in determining which flux of sugars the plant uses to facilitate wall synthesis in specific tissues. For example, myo-inositol in seed is stored as phytic acid (inositol hexaphos — phate). During germination, phosphatases provide a rapid source of inositol which is con­verted, in part, to GlcA. Hence, this would provide a source of UDP-GlcA for wall pectin synthesis. However, during germination, rapid synthesis of L-ascorbate from myo-inositol also occurs. The relationship and coordination of the supply of sugars to wall glycans and to ascorbate synthesis must be better understood at all stages of growth.