Biosynthesis and accumulation of D-ononitol in Vigna umbellata in response to drought stress

Metabolic responses to water deficit that lead to an accumulation of cyclitols, have been examined in rice bean (Vigna umbellata [Thunb.] Ohwi et Ohashi). Imposition of drought stress by withholding water from the soil for 9 days led to an accumulation of D-ononitol (1D-4-O-methyl-myo-inositol) which was most pronounced in leaves (from 33 to 88 mu mol g(-1) dry mass). However, the activity of the enzyme myo-inositol 6-O-methyltransferase (m60MT, EC 2.1.1.X), which catalyzes the synthesis of ononitol from myo-inositol and S-adenosyl-L-methionine (AdoMet). increased in stems but not in leaves during the drought stress experiment. Detailed analysis of different plant parts revealed that the accumulation of ononitol in leaves was linearly related to stem m60MT activity during drought stress, indicating that m6OMT may control the in vivo biosynthetic rate of this cyclitol. The availability of myo-inositol, required for enhanced rates of ononitol synthesis by m60MT, increased during the stress experiment, while the capacity to synthezise AdoMet by S-adenosyl-L-methionine synthetase (SMS, EC 2.5.1.6) decreased. However the high capacity for degradation of S-adenosyl-L-homocysteine (AdoHcy; a potent competitive inhibitor of m60MT) by the enzyme S-adenosyl-L-homocysteine hydrolase (SHH, EC 3.3.1.1) provided favourable conditions for ononitol biosynthesis during the whole stress treatment.