COS 85-4 - Silicon supplementation and simulated tissue damage result in non-plastic extrafloral nectar production

Abstract: While plants have evolved multiple strategies to defend themselves against herbivores, the expression of these defense traits may come at the cost to other plant functions. For example, plants can reduce herbivory through the production of secondary metabolites, which may trade-off with plant growth or other physical structures, such as thorns or lignified cells. Currently, it has been shown that silicon uptake in Brachypodium distachyon, a model grass, interrupts the jasmonic acid (JA) pathway, a pathway implicated in the induction of anti-herbivory responses. This interference with the JA pathway comes at a cost to phenolic production, a class of compounds that can reduce herbivore feeding. Plants can uptake silicon from the soil and deposit it as phytoliths within their tissues, which can reduce herbivory by interfering with herbivore digestion, increasing leaf toughness, and increasing wear on herbivore mouthparts. Because silicon uptake impacts the JA pathway, it is possible silicon uptake may interfere with other JA related defenses. For example, extra-floral nectar (EFN) is a sugar- rich reward that can attract predacious insects which reduce herbivores. The JA pathway upregulates EFN production after herbivory. We hypothesized that silicon uptake would reduce JA-related carbon-based defenses, like EFN. The Fabaceae family is an ideal candidate to study this potential trade-off due to its global distribution, and frequency of EFN producing plants. We chose faba bean (Vicia faba L.) as our model due to the long cultivation history and breeding programs which target tannin content. We grew four genotypes of V. faba in growth chambers in a randomized complete block design. Plants grew in either potting soil (Si-), or potting soil supplemented with silic acid (Si+). Once plants reached the seven-leaf stage, we treated plants with either dilute soap/ethanol solution (control) or methyl-JA (to simulate an herbivore response). We measured EFN production (volume, brix) at 24 and 48 hours and then harvested leaves to quantify silicon and phenolic content. Overall, EFN production decreased between 24 and 48 hours, but was not dependent on silicon supplementation or methyl-JA application. Plant silica content increased with silicon supplementation, however we observed no effect of methyl-JA application on silica content. This research is the first to show that silicon supplementation can increase plant silica content without impacting EFN production in V. faba.