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, ...

A Sebacinales species was recovered from a clone library made from a pooled rhizosphere sample of Nicotiana attenuata plants from 14 native populations. Axenic cultures of the related species, Piriformospora indica and Sebacina vermifera, were used to examine their effects on plant performance. Inoculation of N. attenuata seeds with either fungus species stimulated seed germination and increased growth and stalk elongation. S. vermifera inoculated plants flowered earlier, produced more flowers and matured more seed capsules than did non-inoculated plants. Jasmonate treatment during rosette-stage growth, which slows growth and elicits herbivore resistance traits, erased differences in vegetative, but not reproductive performance resulting from S. vermifera inoculation. Total nitrogen and phosphorous contents did not differ between inoculated and control plants, suggesting that the performance benefits of fungal inoculation did not result from improvements in nutritional status. Since the expression of trypsin proteinase inhibitors (TPI), defensive proteins which confer resistance to attack from Manduca sexta larvae, incur significant growth and fitness costs for the plant, we examined the effect of S. vermifera inoculation on herbivore resistance and TPI activity. After 10 days of feeding on S. vermifera-inoculated plants, larval mass was 46% higher and TPI activity was 48% lower than that on non-inoculated plants. These results suggest that Sebacina spp. may interfere with defense signaling and allow plants to increase growth rates at the expense of herbivore resistance mediated by TPIs.

Glucosinolates display an enormous amount of structural variation, both within and between species. This diversity is thought to have evolved in response to challenges imposed on plants by their biotic environment. During the past decade, glucosinolates and myrosinase-catalyzed glucosinolate hydrolysis have become excellent examples for understanding functional diversification in plant secondary metabolism and plant defence. Methylthioalkylmalate (MAM) synthase genes and enzymes are central to the diversification of aliphatic glucosinolate structures in Arabidopsis thaliana and related plants. This review summarizes efforts to elucidate how MAM-mediated diversity in aliphatic glucosinolate structures is generated and maintained. It also attempts to put variability in methionine carbon chain elongation during glucosinolate biosynthesis into an ecological and evolutionary context.

A method of modifying the roughness of soda-lime glass spheres is presented, with the purpose of tuning inter-particle friction. The effect of chemical etching on the surface topography and the bulk frictional properties of grains is systematically investigated. The surface roughness of the grains is measured using white light interferometry and characterised by the lateral and vertical roughness length scales. The underwater angle of repose is measured to characterise the bulk frictional behaviour. We observe that the coefficient of friction depends on the vertical roughness length scale.