Biosynthesis of cyanogenic glucosides in and the evolution of oxime-based defenses.

Lima bean, , is a crop legume that produces the cyanogenic glucosides linamarin and lotaustralin. In the legumes and , the biosynthesis of these two α-hydroxynitrile glucosides involves cytochrome P450 enzymes of the CYP79 and CYP736 families and a UDP-glucosyltransferase. Here, we identify CYP79D71 as the first enzyme of the pathway in , producing oximes from valine and isoleucine.

The biosynthetic gene cluster for the cyanogenic glucoside dhurrin in Sorghum bicolor contains its co-expressed vacuolar MATE transporter.

Genomic gene clusters for the biosynthesis of chemical defence compounds are increasingly identified in plant genomes. We previously reported the independent evolution of biosynthetic gene clusters for cyanogenic glucoside biosynthesis in three plant lineages. Here we report that the gene cluster for the cyanogenic glucoside dhurrin in Sorghum bicolor additionally contains a gene, SbMATE2, encoding a transporter of the multidrug and toxic compound extrusion (MATE) family, which is co-expressed with the biosynthetic genes.

Lotus japonicus flowers are defended by a cyanogenic β-glucosidase with highly restricted expression to essential reproductive organs.

Flowers and leaves of Lotus japonicus contain α-, β-, and γ-hydroxynitrile glucoside (HNG) defense compounds, which are bioactivated by β-glucosidase enzymes (BGDs). The α-HNGs are referred to as cyanogenic glucosides because their hydrolysis upon tissue disruption leads to release of toxic hydrogen cyanide gas, which can deter herbivore feeding. BGD2 and BGD4 are HNG metabolizing BGD enzymes expressed in leaves.

How insects overcome two-component plant chemical defence: plant β-glucosidases as the main target for herbivore adaptation.

Insect herbivory is often restricted by glucosylated plant chemical defence compounds that are activated by plant β-glucosidases to release toxic aglucones upon plant tissue damage. Such two-component plant defences are widespread in the plant kingdom and examples of these classes of compounds are alkaloid, benzoxazinoid, cyanogenic and iridoid glucosides as well as glucosinolates and salicinoids. Conversely, many insects have evolved a diversity of counteradaptations to overcome this type of constitutive chemical defence.

Towards a molecular understanding of the biosynthesis of amaryllidaceae alkaloids in support of their expanding medical use.

The alkaloids characteristically produced by the subfamily Amaryllidoideae of the Amaryllidaceae, bulbous plant species that include well know genera such as Narcissus (daffodils) and Galanthus (snowdrops), are a source of new pharmaceutical compounds. Presently, only the Amaryllidaceae alkaloid galanthamine, an acetylcholinesterase inhibitor used to treat symptoms of Alzheimer's disease, is produced commercially as a drug from cultivated plants. However, several Amaryllidaceae alkaloids have shown great promise as anti-cancer drugs, but their further clinical development is restricted by their limited commercial availability.

Visualizing metabolite distribution and enzymatic conversion in plant tissues by desorption electrospray ionization mass spectrometry imaging.

In comparison with the technology platforms developed to localize transcripts and proteins, imaging tools for visualization of metabolite distributions in plant tissues are less well developed and lack versatility. This hampers our understanding of plant metabolism and dynamics. In this study, we demonstrate that desorption electrospray ionization mass spectrometry imaging (DESI-MSI) of tissue imprints on porous Teflon may be used to accurately image the distribution of even labile plant metabolites such as hydroxynitrile glucosides, which normally undergo enzymatic hydrolysis by specific β-glucosidases upon cell disruption.

Why biosynthetic genes for chemical defense compounds cluster.

In plants, the genomic clustering of non-homologous genes for the biosynthesis of chemical defense compounds is an emerging theme. Gene clustering is also observed for polymorphic sexual traits under balancing selection, and examples in plants are self-incompatibility and floral dimorphy. The chemical defense pathways organized as gene clusters are self-contained biosynthetic modules under opposing selection pressures and adaptive polymorphisms, often the presence or absence of a functional pathway, are observed in nature.

Genomic clustering of cyanogenic glucoside biosynthetic genes aids their identification in Lotus japonicus and suggests the repeated evolution of this chemical defence pathway.

Cyanogenic glucosides are amino acid-derived defence compounds found in a large number of vascular plants. Their hydrolysis by specific β-glucosidases following tissue damage results in the release of hydrogen cyanide. The cyanogenesis deficient1 (cyd1) mutant of Lotus japonicus carries a partial deletion of the CYP79D3 gene, which encodes a cytochrome P450 enzyme that is responsible for the first step in cyanogenic glucoside biosynthesis.

Genetic screening identifies cyanogenesis-deficient mutants of Lotus japonicus and reveals enzymatic specificity in hydroxynitrile glucoside metabolism.

Cyanogenesis, the release of hydrogen cyanide from damaged plant tissues, involves the enzymatic degradation of amino acid-derived cyanogenic glucosides (alpha-hydroxynitrile glucosides) by specific beta-glucosidases. Release of cyanide functions as a defense mechanism against generalist herbivores. We developed a high-throughput screening method and used it to identify cyanogenesis deficient (cyd) mutants in the model legume Lotus japonicus.

Diversification of an ancient theme: hydroxynitrile glucosides.

Many plants produce cyanogenic glucosides as part of their chemical defense. They are alpha-hydroxynitrile glucosides, which release toxic hydrogen cyanide (HCN) upon cleavage by endogenous plant beta-glucosidases. In addition to cyanogenic glucosides, several plant species produce beta- and gamma-hydroxynitrile glucosides.

Sugar and ABA response pathways and the control of gene expression.

Sugars are essential to plant growth and metabolism, both as energy source and as structural components. Sugar production and use are in part controlled at the level of gene expression by the sugars themselves. Responses to sugar are closely integrated with response pathways that indicate environmental conditions such as light and water availability.

Impaired sucrose induction1 encodes a conserved plant-specific protein that couples carbohydrate availability to gene expression and plant growth.

To identify the molecular mechanisms underlying carbohydrate allocation to storage processes, we have isolated mutants in which the sugar induction of starch biosynthetic gene expression was impaired. Here we describe the IMPAIRED SUCROSE INDUCTION1 (ISI1) gene, which encodes a highly conserved plant-specific protein with structural similarities to Arm repeat proteins. ISI1 is predominantly expressed in the phloem of leaves following the sink-to-source transition during leaf development, but is also sugar-inducible in mesophyll cells.

Characterization of mutants in Arabidopsis showing increased sugar-specific gene expression, growth, and developmental responses.

Sugars such as sucrose serve dual functions as transported carbohydrates in vascular plants and as signal molecules that regulate gene expression and plant development. Sugar-mediated signals indicate carbohydrate availability and regulate metabolism by co-coordinating sugar production and mobilization with sugar usage and storage. Analysis of mutants with altered responses to sucrose and glucose has shown that signaling pathways mediated by sugars and abscisic acid interact to regulate seedling development and gene expression.

Genetic approaches to understanding sugar-response pathways.

Plants as photoautotrophic organisms are able to produce the carbohydrates they require and have developed mechanisms to co-ordinate carbohydrate production and its metabolism. Carbohydrate-derived signals regulate the expression of genes involved in both photosynthesis and metabolism, and control carbohydrate partitioning. A number of genetic approaches have been initiated to understand sugar-response pathways in plants and identify the components involved.