Abscisic acid-induced changes in inositol metabolism in Spirodela polyrrhiza

In response to abscisic acid (ABA), the duckweed Spirodela polyrrhiza (L.) activates a developmental pathway that culminates in the formation of dormant structures known as turions. Levels of the mRNA encoding D-myo-inositol-3-phosphate synthase (EC.5.5.1.4) which converts glucose-6-phosphate to inositol-3-phosphate, increase early in response to ABA, In order to understand the role of this enzyme in turion formation, we have investigated changes in inositol metabolism in ABA-treated plants. Here, we show that ABA-treatment leads to a 3-fold increase in free inositol. which peaks 2 d after treatment. This increase is followed by sequential increases in inositol phosphates and in accumulation of inositol hexakisphosphate (InsP(6)), in particular. In addition, we observed an early increase in a novel inositol bisphosphate which is not directly on the pathway to InsP(6). In control plants, we observed synthesis and turnover of both inositol pentakisphosphate and InsP(6). Two compounds more polar than InsP(6) (diphosphoinositol polyphosphates were present in both ABA-treated and control plants. Together, this suggests that the role of InsP(6) in plants may be more complex than simply that of a storage compound during dormancy.