Submitted to: Journal of Experimental Botany
Publication Type: Peer reviewed journal
Publication Acceptance Date: 5/28/2004
Publication Date: 9/1/2004
Citation: Karner, U., Peterbauer, T., Raboy, V., Jones, D.A., Hedley, C.L., Richter, A. 2004. Myo-inositol and sucrose concentrations affect the accumulation of raffinose family oligosaccharides in seeds.. Journal of Experimental Botany 55: 1981-1987. Interpretive Summary: Raffinose oligosaccharides have important functions in seeds, representing nutrient reserves important for germination and conferring tolerance to dessication. However, they also may have anti-nutritional properties and affect the end-use quality of seeds when used in foods. This study showed that the amount of substrate needed for raffinose synthesis is as important to the final amount accumulated in seeds as is the activity of enzymes involved in their synthesis. In particular, the amount of myo-inositol in seeds helps to determine the final amount of raffinose. Since some types of low phytic acid seeds have high seed inositol, they may also have high raffinose. In other cases low phytic acid seeds have low inositol, and in this case have low raffinose.
Technical Abstract: Raffinose family oligosaccharides (RFOs) fulfill multiple functions in plants. In seeds, they possibly protect cellular structures during dessication and constitute carbon reserves for early germination. Their biosynthesis proceeds by transfer of galactose units from galactinol to sucrose. Galactinol synthase (GolS), which mediates the synthesis of galactinol from myo-inositol and UDP-galactose, has been proposed to be the key enzyme of the pathway. However, no significant relationship was detected between the extractable GolS activity and the amount of RFOs in seeds from seven pea (Pisum sativum L.) genotypes selected for high variation in RFO content. Instead, a highly significant correlation was found between the levels of myo-inositol and RFOs. Moderately strong relationships were also found between sucrose and RFO content as well as between myo-inositol and galactinol. These data suggest that the extent of RFO accumulation is controlled by levels of the initial substrates myo-inositol and sucrose rather than by GolS activity. Further evidence for a key role of myo-inositol for the synthesis of galactinol was obtained by feeding exogenous myo-inositol to intact pea seeds and by analysis of four barley (Hordeum vulgare L.) low phytic acid mutants. In seeds of three of these mutants, the reduced demand for myo-inositol for the synthesis of phytic acid (myo-inositol 1,2,3,4,5,6-hexakisphosphate) was associated with an increased level in myo-inositol. These seeds contained also more galactinol as wild type seeds.