An article in the latest issue of Proceedings of the Natural Academy of Sciences (PNAS) caught my eye, which showcases quite nicely that a multitude of chemical spectroscopy techniques can be used for solving a simple observation that dates back nearly 100 years!
In short, a team of US scientists using high-resolution mass spectrometry and NMR (nuclear magnetic resonance) spectroscopy have isolated a rare excitatory amino acid from geranium flowers, known for their paralytic effect on the Japanese beetle.
The flowering plants Pelargonium include around 280 species of evergreen perennial shrubs indigenous to Southern Africa. These plants are resistant to heat, drought and can tolerate minor frosts. They are extremely popular garden plants in temperate climates such as Australia, New Zealand and the Far East. They are also popular as ornamental plants in America and Europe. Japanese beetles are a known pest and cost the ornamental plant industry an estimated $450 million each year. In the past, chemicals extracted from the Pelargonium species were used in traditional medicines for a wide range of ailments and wounds.
The species has chemical-based defences against insects and pathogens, with the most commonly known being the paralysis of the Japanese beetle. This happens when the beetle eats a couple of the petals of the flowering plant with the varying colour of plants being equally active. The paralysis affects the hind legs and progresses further until the beetle is completely paralysed. This eventually wears off and under laboratory conditions the beetles make a full recovery – but in their natural environment, the beetles, once paralysed can become prone to predation. However, over excitation of receptors to excitatory amino acids often leads to neuronal degradation and eventual death. Even though the first description of these flowers paralysing the Japanese beetle was back in 1920, the chemical cause remained unsolved.
Cracking the paralysis puzzle
The team needed to first identify a solvent that would effectively extract the chemicals from within the petals. The highest paralysis effect was seen in extracts infused into agar gels with methanol. HPLC was then used to help isolate the paralytic compound further and the extracts were again tested on the beetles to ascertain the level of paralysis. The non-paralytic fractions typically contained anthocyanidins and flavonols. Once a pure fraction was obtained, deduced from hydrophilic interaction liquid chromatography, the sample was ready for high-resolution mass spectrometry. The use of mass spectrometry established a molecular weight for the fraction, which allowed for a tentative elucidation of the paralytic compound. NMR analysis finally enabled the identification of the known, but rare, amino acid quisqualic acid. l-quisqualic acid was confirmed to be the agent causing the paralysis after comparing specific optical rotation data with synthetic l-quisqualic acid samples.
Quisqualic acid is an amino acid that occurs naturally in the seeds of Quiqualis species and is an agonist for both the ?-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPA receptor, AMPAR, or quisqualate receptor) a receptor for glutamate that mediates fast synaptic transmission in the central nervous system and also group I metabotropic glutamate receptors. The molecule can cause excitatory toxic responses and is used in neuroscience to selectively destroy neurons in the brain or spinal cord.
The finding that zonal geraniums are a source of l-quisqualic acid could produce an opportunity to pursue botanically based formulations for the control of insects. l-quisqualic acid is said to mimic l-glutamic acid, a neurotransmitter in the insect neuromuscular junction and in the mammalian central nervous system. The neurotoxic effects of l-quisqualic acid are similar to the effects produced by nicotine in aphids. An interesting find within the paper reveals that the Japanese beetle has no sense of food aversion and will preferentially eat the geranium flowers and be repeatedly paralysed. It would be interesting to see if any other parts of the plant had the same effect on the Japanese beetle to localise the sites of l-quisqualic acid and to also study the effect (if any) on other insects and pathogens.
Ranger et al., Proc. Natl. Acad. Sci., 2011, 108, (4), 1217