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Universitaet Wien (1996)

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

Wanek, W.

Titre : Mechanisms of accumulation of D-ononitol in Vigna umbellata in response to drought stress

Auteur : Wanek, W.

Akad. Grad : Thesis.Doctoral 1996

Université de soutenance : Institut fuer Pflanzenphysiologie ; Formal- und Naturwissenschaftliche Fakultaet ; Universitaet Wien

Résumé
Cyclitols, and in particular O-methyl-inositols accumulate in certain legumes in response to abiotic stresses. In the rice bean (Vigna umbellata Ohwi et Ohashi) imposition of drought stress by withholding water from the soil for nine days led to the accumulation of D-ononitol (1D-4-O-methyl-myo-inositol) which was most pronounced in leaves (increase from 33 to 88 #mu#mol g"-"1 dry weight). S-adenosyl-L- methionine:myo-inositol 6-O-methyltranferase (m6OMT), the enzyme which catalyzes the synthesis of D-ononitol, was extracted from stems of V. umbellata and purified to apparent homogeneity by a combination of conventional chromatographic techniques and by affinity chromatography on immobilized S-adenosyl-L-homocysteine (SAH). The native molecular weight was determined to be 106 kDa, with a subunit molecular weight of 40 kDa. Substrate-saturation kinetics of m6OMT for myoinositol and S-adenosyl-L- methionine (SAM) were Michaelis-Menten type with Km values of 2.92 mM and 63 #mu#M, respectively. SAH competitively inhibited the enzyme with respect to SAM (K_i of 1.63 #mu#M). The enzyme did not require divalent cations for activity, but was strongly inhibited by sulfhydryl group inhibitors. The purified m6OMT was found to be highly specific for the 6- hydroxyl group of myo-inositol and showed no activity on other naturally occurring miositols and inositol O-methyl-ethers. Neither D-ononitol, nor D-1-O-methyl-muco-inositol or D-3-O-methyl-chiro-inositol and D-chiro- inositol (end products of the biosynthetic pathway in which m6OMT catalyzes the first step), inhibited the activity of the enzyme. However, the activity of m6OMT decreased in leaves during the drought stress experiment. Detailed analysis of whole plants revealed that the accumulation of ononitol in leaves was correlated with an enhancement of m6OMT activity in steins of the rice bean. On the whole plant basis, ononitol accumulation was found to be linearly related to m6OMT activity during drought stress, indicating that m6OMT may control the in vivo biosynthetic rate of this cyclitol. Furthermore, the conditions for enhanced rates of ononitol synthesis by m6OMT (increased myo-inositol availability and high levels of enzymes involved in methyl group transfer, i.e. SAM synthase, SAH hydrolase) were coordinately improved during the development of drought stress.

Mots clé : Trockenstress / Myo-Inosit-6-O-Methyltransferase / D-Ononit / Akkumulation / aktivierter Methylgruppen-Zyklus / Kompartimentierung / Vigna umbellata — drought stress / myo-inositol 6-O-methyltransferase / D-ononitol / accumulation / activated methyl-group cycle / compartmentation / rice bean

Présentation : Österreichische Dissertationsdatenbank

Page publiée le 11 juin 2008, mise à jour le 14 mars 2019