Use of the glucosyltransferase UGT71B6 to disturb abscisic acid homeostasis in Arabidopsis thaliana

A glucosyltransferase (GT) of Arabidopsis, UGT71B6, recognizing the naturally occurring enantiomer of abscisic acid (ABA) in vitro, has been used to disturb ABA homeostasis in planta. Transgenic plants constitutively overexpressing UGT71B6 (71B6-OE) have been analysed for changes in ABA and the related ABA metabolites abscisic acid glucose ester (ABA-GE), phaseic acid (PA), dihydrophaseic acid (DPA), 7'-hydroxyABA and neo-phaseic acid. Overexpression of the GT led to massive accumulation of ABA-GE and reduced levels of the oxidative metabolites PA and DPA, but had marginal effect on levels of free ABA. The control of ABA homeostasis, as reflected in levels of the different metabolites, differed in the 71B6-OEs whether the plants were grown under standard conditions or subjected to wilt stress. The impact of increased glucosylation of ABA on ABA-related phenotypes has also been assessed. Increased glucosylation of ABA led to phenotypic changes in post-germinative growth. The use of two structural analogues of ABA, known to have biological activity but to differ in their capacity to act as substrates for 71B6 in vitro, confirmed that the phenotypic changes arose specifically from the increased glucosylation caused by overexpression of 71B6. The phenotype and profile of ABA and related metabolites in a knockout line of 71B6, relative to wild type, has been assessed during Arabidopsis development and following stress treatments. The lack of major changes in these parameters is discussed in the context of functional redundancy of the multigene family of GTs in Arabidopsis.     

Identification of gibberellins A17, A25, A45, abscisic acid,phaseic acid,and dihydrophaseic acid in seeds of Pyrus communis

Gibberellin A₁₇, abscisic acid, and 4′-dihydrophaseic acid were identified by GC-MS of derivatized extracts from both immature and mature seeds of pear. Immature seeds also contained phaseic acid, gibberellins A₂₅ and A₄₅, and two presumed mono-hydroxylated derivatives of GA₄₅, one of which was tentatively identified as 3β-hydroxy-GA₄₅. Several presumed metabolites of abscisic acid were detected in both mature and immature seeds.     

The expression of a rab-related gene,rab18, is induced by abscisic acid during the cold acclimation process of Arabidopsis thaliana (L.) Heynh

We have isolated a rab-related (responsive to ABA) gene, rab18 from Arabidopsis thaliana. The gene encodes a hydrophilic, glycine-rich protein (18.5 kDa), which contains the conserved serine- and lysine-rich domains characteristic of similar RAB proteins in other plant species. The rab18 mRNA accumulates in plants exposed to low temperature, water stress or exogenous ABA but not in plants subjected to heat shock. This stress-related accumulation of the rab18 mRNA is markedly decreased in the ABA-synthesis mutant aba-1, the ABA-response mutant abi-1 or in wild-type plants treated with the carotenoid synthesis inhibitor, fluridone. Exogenous ABA treatment can induce the rab18 mRNA in the aba-1 mutant but not in the abi-1 mutant. These results provide direct genetic evidence for the ABA-dependent regulation of the rab18 gene in A. thaliana.     

The hydrophobic segment of Arabidopsis thaliana cluster I diacylglycerol kinases is sufficient to target the proteins to cell membranes

Diacylglycerol kinases (DGKs) catalyze the phosphorylation of diacylglycerol into phosphatidic acid. To fulfill their role in many signalling processes, DGKs must be located at, or in, membranes. Most mammalian DGKs are cytosolic and are recruited to membranes upon stimulation, except for epsilon type DGKs that are permanently membrane-associated through a hydrophobic segment. Nothing is known about the mechanism(s) involved in the membrane localization of plant DGKs. By fusion to fluorescent proteins, we show that two DGKs from cluster I in Arabidopsis thaliana possess amino-terminal hydrophobic segments that are sufficient to address them to endoplasmic reticulum membranes.