Effects of Intraspecific Density on Garlic Mustard (Alliaria Petiolata) Sinigrin Concentration

The production of secondary defense chemicals in plants represents a trade-off between defense and the primary functions of growth and reproduction, but the relative allocation to growth versus defense varies across species, types of defenses, ontogeny, and environment. Alliaria petiolata (garlic mustard) is a brassica that produces glucosinolates, a class of constituent secondary metabolites that defend against herbivores and pathogens. Sinigrin, a hydrolyzed product of glucosinolate present in garlic mustard, may aid in its success as an invasive species by disrupting native plant–mycorrhizae mutualisms and decreasing forest species diversity in North America. Here, we measured sinigrin concentration in garlic mustard populations of different field densities and in greenhouse experiments to evaluate the relationship between sinigrin concentration and growth in response to density and varying environmental conditions. We found clear evidence for growth vs. defense tradeoffs in both experimental and field settings, as well as higher levels of defense in more densely growing, smaller individual plants. However, sinigrin levels and tradeoffs were not explained by soil fertility or light, allowing us to conclude that sinigrin expression is not controlled by limitations in the measured abiotic factors. Our findings suggest sinigrin leaf concentration increases at high densities despite the pressures of intraspecific competition that demand allocation to growth.

Plant Defense Chemicals against Insect Pests

Insect pests cause significant global agricultural damage and lead to major financial and environmental costs. Crops contain intrinsic defenses to protect themselves from such pests, including a wide array of specialized secondary metabolite-based defense chemicals. These chemicals can be induced upon attack (phytoalexins) or are constitutive (phytoanticipins), and can have a direct impact on the pests or be used indirectly to attract their natural enemies. They form part of a global arms race between the crops and their insect pests, with the insects developing methods of suppression, avoidance, detoxification, or even capture of their hosts defensive chemicals. Harnessing and optimizing the chemical defense capabilities of crops has the potential to aid in the continuing struggle to enhance or improve agricultural pest management. Such strategies include breeding for the restoration of defense chemicals from ancestral varieties, or cross-species transfer of defense metabolite production.

Nature's Chemical Weapons: Beetle Defenses

The defense chemicals secreted by beetles are very diverse. They are exemplified by those of members of the families Carabidae (ground beetles) and Coccinellidae (ladybirds).

A gene horizontally transferred from bacteria protects arthropods from host plant cyanide poisoning

Cyanogenic glucosides are among the most widespread defense chemicals of plants. Upon plant tissue disruption, these glucosides are hydrolyzed to a reactive hydroxynitrile that releases toxic hydrogen cyanide (HCN). Yet many mite and lepidopteran species can thrive on plants defended by cyanogenic glucosides. The nature of the enzyme known to detoxify HCN to β-cyanoalanine in arthropods has remained enigmatic. Here we identify this enzyme by transcriptome analysis and functional expression. Phylogenetic analysis showed that the gene is a member of the cysteine synthase family horizontally transferred from bacteria to phytophagous mites and Lepidoptera. The recombinant mite enzyme had both β-cyanoalanine synthase and cysteine synthase activity but enzyme kinetics showed that cyanide detoxification activity was strongly favored. Our results therefore suggest that an ancient horizontal transfer of a gene originally involved in sulfur amino acid biosynthesis in bacteria was co-opted by herbivorous arthropods to detoxify plant produced cyanide.