Arabidopsis thaliana encodes a bacterial-type heterodimeric isopropylmalate isomerase involved in both Leu biosynthesis and the Met chain elongation pathway of glucosinolate formation

The last steps of the Leu biosynthetic pathway and the Met chain elongation cycle for glucosinolate formation share identical reaction types suggesting a close evolutionary relationship of these pathways. Both pathways involve the condensation of acetyl-CoA and a 2-oxo acid, isomerization of the resulting 2-malate derivative to form a 3-malate derivative, the oxidation-decarboxylation of the 3-malate derivative to give an elongated 2-oxo acid, and transamination to generate the corresponding amino acid. We have now analyzed the genes encoding the isomerization reaction, the second step of this sequence, in Arabidopsis thaliana. One gene encodes the large subunit and three encode small subunits of this enzyme, referred to as isopropylmalate isomerase (IPMI) with respect to the Leu pathway. Metabolic profiling of large subunit mutants revealed accumulation of intermediates of both Leu biosynthesis and Met chain elongation, and an altered composition of aliphatic glucosinolates demonstrating the function of this gene in both pathways. In contrast, the small subunits appear to be specialized to either Leu biosynthesis or Met chain elongation. Green fluorescent protein tagging experiments confirms the import of one of the IPMI small subunits into the chloroplast, the localization of the Met chain elongation pathway in these organelles. These results suggest the presence of different heterodimeric IPMIs in Arabidopsis chloroplasts with distinct substrate specificities for Leu or glucosinolate metabolism determined by the nature of the different small subunit

Specific and coordinated control of indolic and aliphatic glucosinolate biosynthesis by R2R3-MYB transcription factors in Arabidopsis thaliana

Five members of subgroup 12 R2R3-MYB transcription factors, namely MYB51, MYB122, MYB28, MYB29 and MYB76, are novel regulators of glucosinolate biosynthesis in Arabidopsis thaliana. Overexpression of MYB51 and MYB122 led to an increased accumulation of tryptophan-derived indolic glucosinolates whereas MYB28, MYB29 and MYB76 overexpression lines showed an increase in methionine-derived aliphatic glucosinolates. Likewise, disruption of the corresponding genes caused a significant downregulation of indolic and aliphatic glucosinolates, respectively. Expression analysis of promoter-GUS fusions revealed promoter activities at the sites of glucosinolate synthesis and accumulation. Indolic glucosinolate regulators were mainly found in vegetative organs and roots, whereas aliphatic glucosinolate regulators were preferentially expressed in generative organs. Mechanical stimuli such as touch or wounding induced a transient expression of the regulators and overexpression of MYB28 and MYB51 reduced insect performance demonstrating the role of these transcription factors in plant biotic responses. The subgroup 12 R2R3-MYB transcription factors interdependently control the response to biotic challenges. For the regulation of methionine-derived glucosinolates, the coordinated activation of MYB28, MYB76 and MYB29 is required, whereas MYB51, MYB122 and the sixth member of subgroup 12 R2R3-MYB transcription factors, the previously described ATR1/MYB34, are involved in the regulation of tryptophan-derived glucosinolates. Because these two pathways are reciprocally inhibiting each other, a metabolic balance between both biosynthetic pathways can be accomplished in plants exposed to continuous biotic challenges. © 2008 Springer Science+Business Media B.V.Tamara Gigolashvili, Bettina Berger, Ulf-Ingo Flügg