Mathematical modelling of aliphatic glucosinolate chain length distribution in Arabidopsis thaliana leaves

Abstract

Aliphatic glucosinolates are a major class of defensive secondary metabolites in plants that are mostly derived from methionine. Occurring in different chain lengths, they show a structural diversity arising from the variable number of chain elongation cycles taking place during their biosynthesis. The key enzymes in determining glucosinolate chain length are the methylthioalkylmalate (MAM) synthases, MAM1 and MAM3, with MAM3 showing a broader substrate specificity than MAM1. A comparison of the measurements of wild type and MAM1 knockout mutant plants shows the following distinct changes in glucosinolate chain length profiles:(1) a reversal of the relative proportions of the two shortest glucosinolates, (2) a significant increase in the concentration of the longest glucosinolate, (3) an increase in total glucosinolate content in the mutant.MAM3 knockout mutants on the contrary differ from wild type plants by a pronounced abundance of the second shortest glucosinolate and the depletion of the two longest glucosinolates. To clarify the contribution of the multifunctional enzymes MAM1 and MAM3 to the glucosinolate profile of Arabidopsis thaliana leaves, we simulated glucosinolate biosynthesis in a kinetic model, taking into account the structure of the pathway and measured enzymatic properties. The predicted glucosinolate profiles show all characteristics of the actual differences between wild-type and MAM1 mutants or MAM3 mutants, respectively. The model strongly supports experimental indications that the two MAM activities are not independent of each other. In particular, it showed that an elevated expression of MAM3 in the MAM1 mutant is critical in determining the glucosinolate profile of this plant line. The simulation was critical for this finding since it allowed us to assess the individual effects of two processes-the knocking out of MAM1 and the overexpression of MAM3-that are difficult to separate experimentally.

Simulating Plant Metabolic Pathways with Enzyme-Kinetic Models

“…Another complex kinetic model in Arabidopsis simulates the synthesis of aliphatic glucosinolates (Knoke et al, 2009), which are important defensive metabolites in plants. The model compared the wild-type Arabidopsis leaves with knockout mutants of MAM1 and MAM3 (methylthiolalkylmalate).…”

Methylthioalkylmalate synthases: genetics, ecology and evolution

“…MAMa, MAMb and MAMc have originated by gene duplication events, and are functionally diversified. MAMa controls short-chain aliphatic glucosinolates (Benderoth et al 2006) and MAMb is, like its A. thaliana ortholog MAM3 Knoke et al 2008), presumably involved in the biosynthesis of long-chain aliphatic glucosinolates. However, the function of MAMc is not yet known.…”

“…MAM substrate specificity (Table 1) determines whether the reaction products of a given cycle enter the biosynthetic pathway generating the glucosinolate core structure or whether they undergo additional cycles of carbon chain elongation. A. thaliana MAM1 and MAM2 are involved in the formation of aliphatic glucosinolates with short carbon chains (Kroymann et al 2001;Textor et al 2004;Benderoth et al 2006), while MAM3 catalyzes condensation reactions in the biosynthesis of aliphatic glucosinolates with long carbon chains (Field et al 2004;Textor et al 2007; Knoke et al 2008). MAM synthases determine variability during the earliest stage of aliphatic glucosinolate biosynthesis and play a central role in glucosinolate diversity.…”

Mathematical Modeling of the Central Carbohydrate Metabolism in Arabidopsis Reveals a Substantial Regulatory Influence of Vacuolar Invertase on Whole Plant Carbon Metabolism

“…For example, in aliphatic glucosinolate synthesis, knockouts in one of several methylthioalkylmalate synthases generate varying patterns of glucosinolate chain lengths. Simulations using kinetic models revealed that chain length distribution is a function of the ratio, not of absolute activities, of different methylthioalkylmalate synthases (Knoke et al, 2009).…”