Phenotypic and Molecular Characterisation of Silicon Uptake and Deposition in Festuca arundinacea

Silicon (Si) is taken up from the soil as monosilicilic acid by plant roots, transported to leaves and deposited as phytoliths, amorphous silica (SiO2) bodies, which are a key component of anti-herbivore defense in grasses. Silicon transporters have been identified in important crop species such as rice and barley, but the mechanisms behind the transport of Si remain poorly characterised in many non-crop grasses. Specifically, the extent to which Si uptake and deposition is driven by Si transporter expression remains disputed. Induction of Si defenses in response to herbivory suggests plants exhibit control over Si uptake and distribution. This thesis investigated the effects of wounding and Si addition on foliar Si concentration and deposition, and on Si transporter gene expression, in different genotypes of the economically important forage grass Festuca arundinacea, which exhibits variation in Si uptake and deposition. Following Si addition and damage, varieties differed in Si concentration, the numbers of leaf spines, and in the magnitude of the increase in Si uptake induced by damage. Some previous studies suggest trade-offs may exist between Si and carbon, but thus far such potential trade-offs have not been investigated intra-specifically, nor have there been any tests of the existence of trade-offs between different types of Si-based defenses. Trade-offs between leaf spines and phytoliths, and between Si and the key structural component, lignin, were found. This thesis presents novel findings on how Si defenses are mobilised in response to damage, how they are regulated at the level of gene expression, and how Si is deposited in different structures on the leaf surface and within cells. These findings have implications for improved understanding of plant defense and for the targeted selection of traits during breeding for sustainable crop protection.