Scott N. Johnson, Susan E. Hartley, James M.W. Ryalls, Adam Frew, Jane L. DeGabriel, Michael Duncan and Andrew N. Gherlenda
In ‘Lord Jim’, one of Joseph Conrad’s characters says ‘Each blade of grass has its spot on earth whence it draws its life, its strength’. This could be more accurate than he thought. Grasses take up large amounts of silicon from the soil and use it to cope with a diverse range of stresses, including defence against insect herbivores. Silicon also often kick-starts plant growth, possibly because plants can use it as a cheaper alternative to structural components based on carbon (e.g. lignin and cellulose).
Most researchers focus on grasses because they often contain the highest levels of silicon. Other plants may, however, use silicon for defence and growth but we know very little about this. In this study, we investigated silicon supplementation in a nitrogen-fixing legume, lucerne (Medico sativa), also known as alfalfa. Legumes capture nitrogen from the atmosphere and turn it into a plant-usable form by associating with nitrogen-fixing bacteria housed in root nodules. We predicted that silicon supplementation might either make lucerne better defended against a herbivore (a phloem-feeding aphid), or it might stimulate root nodulation and increase nitrogen production. Aphids thrive when feeding on plants that have increased fluxes of nitrogen (amino acids) in the phloem sap, so they could indirectly benefit from silicon supplementation.
Our second prediction turned out to be true. Silicon supplemented plants grew three times faster and had 44% more root nodules than those without supplementation. Linked to this, silicon supplemented plants synthesised 65% more essential amino acids and had twice the number of aphids as non-supplemented plants. What about the defensive role of silicon? It seems that silicon-supplemented plants were growing so fast that they didn’t have the chance to build up silicon concentrations in their leaves – a kind of dilution effect. We show that while silicon often increases resistance to herbivores in grasses (e.g. cereals), this can’t be assumed for other plant functional groups. In fact, silicon may indirectly benefit herbivores where it stimulates plant growth and nutritional quality.
Image provided by authors.