Positive selection driving diversification in plant secondary metabolism

Abstract

In Arabidopsis thaliana and related plants, glucosinolates are a major component in the blend of secondary metabolites and contribute to resistance against herbivorous insects. Methylthioalkylmalate synthases (MAM) encoded at the MAM gene cluster control an early step in the biosynthesis of glucosinolates and, therefore, are central to the diversification of glucosinolate metabolism. We sequenced bacterial artificial chromosomes containing the MAM cluster from several Arabidopsis relatives, conducted enzyme assays with heterologously expressed MAM genes, and analyzed MAM nucleotide variation patterns. Our results show that gene duplication, neofunctionalization, and positive selection provide the mechanism for biochemical adaptation in plant defense. These processes occur repeatedly in the history of the MAM gene family, indicating their fundamental importance for the evolution of plant metabolic diversity both within and among species.biochemical neofunctionalization ͉ glucosinolate metabolism ͉ methylthioalkylmalate synthase ͉ plant-enemy interactions P lants synthesize an immense number of secondary compounds, so called because their significance for processes of basic growth and development is not immediately evident. More than 200,000 known secondary metabolites provide an increasingly exploited reservoir for the generation of pharmaceutically active agents (1), and many more await discovery. Classic hypotheses that seek to explain this vast metabolic diversity propose a stepwise and reciprocal process of adaptation and counteradaptation between plants and their natural enemies, molded by mutual selection (2). In Arabidopsis thaliana and other crucifers, glucosinolates are a major component in the mélange of secondary metabolites. More than 120 glucosinolates are known, which share a chemical core structure but differ in their amino acid-derived side chain. Glucosinolate composition and quantity varies among and within species (3-5). Upon tissue disruption, myrosinase-catalyzed hydrolysis of glucosinolates generates biologically active compounds, which play an important ecological role in plant defense against herbivorous insects (4, 6-9). However, insects can adapt to glucosinolate profiles or evade deleterious effects from glucosinolate hydrolysis by counteradaptation (9-12). In A. thaliana, a modular genetic system regulates diversity in glucosinolate profiles among accessions and may permit the rapid generation of new glucosinolate combinations in response to challenges imposed by the biotic environment (3). Several genetic loci (3, 9), each present in several alleles, interact epistatically and, together with modifying proteins (7), determine the blend of bioactive products that emerges during glucosinolate hydrolysis (9). One genetic locus central for glucosinolate diversity, the methylthioalkylmalate synthase gene (MAM) cluster, controls an early step in the biosynthesis of aliphatic glucosinolates, the predominant glucosinolate class in A. thaliana. This locus comprises a small family of MAM genes, ...

Understanding the Evolution of Defense Metabolites in Arabidopsis thaliana Using Genome-wide Association Mapping

“…traits was unexpected, given its role in aliphatic GSL metabolism (Figure 4) (Benderoth et al 2006;Hansen et al 2007;Li et al 2008). Interestingly, 11 of the 16 known causal genes showed no significant association with any GSL trait.…”

“…This enrichment of significant GWA gene-trait associations extended hundreds of genes and several hundred kilobases around both loci ( Figure 5). Gene-specific studies have shown that both loci have normal or elevated levels of diversity suggesting this nonrandom GWA clustering is not an artifact of a loss of diversity around either locus (Kliebenstein et al 2001a;Wright et al 2002;Benderoth et al 2006).…”

“…Interestingly, the relationship between fitness and GSL is not linear and displays hallmarks of fluctuating and/ or balancing selection. These selection pressures are dependent upon both the insect and neighboring plant species within the plant's environment (Benderoth et al 2006;Lankau 2007;Lankau and Strauss 2007;Bakker et al 2008;Lankau and Strauss 2008).…”

Facing the Future of Plant-Insect Interaction Research: Le Retour à la “Raison d'Être”

“…Substantial progress has been made, based on the model species Arabidopsis, in understanding this pathway, including, notably, documentation of the roles of gene duplication, neofunctionalization, and positive selection on methylthioalkylmalate synthases encoded at the methylthioalkylmalate synthase gene cluster in generating glucosinolate diversity (Benderoth et al, 2006).…”

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

“…In the Columbia ecotype, the first reaction of the three-step chain elongation cycle, the condensation of a 2-oxo acid with acetyl-CoA, is catalyzed by one of two methylthioalkylmalate synthase enzymes: MAM1 and MAM-L. MAM1 acts principally on the 2-oxoacids with the two shortest chain lengths and with a very weak activity on the third one Benderoth et al 2006), while MAM-L acts on all six 2-oxo-acids present (Textor et al 2007). Textor et al (2007) proposed renaming MAM-L as MAM3, based on the biochemical properties of the encoded enzyme and the earlier naming of MAM1 and MAM2 (Kroymann et al 2003).…”