Does silicon supplementation affect plant traits that impact the performance and feeding behaviour of cereal aphids?

Abstract

Plants have a range of natural defences that they deploy to protect themselves when faced with attacks from insect pests, many of which can be enhanced and exploited to protect commercially important crops. Plant biophysical properties (e.g. abrasiveness, toughness) can be manipulated with supplementation of bio-available silicon (Si), enabling plants to withstand attacks from herbivorous pests. Moreover, Si is reported to augment other forms of plant defence involving allelochemicals. Si is the second most abundant element in soil; however, for plants to take it up Si must be in the bio-available form of silicic acid. Bio-available Si can, however, become depleted in agricultural regions that are subjected to recurrent cropping which limits Si uptake. The extent of Si defence against herbivores is variable and may be dependent on the feeding guild to which they belong. Numerous studies report that high Si concentrations have strong negative effects on chewing folivores, but results for phloem feeders are mixed. Phloem feeding aphids are an important pest in Australian grain crops causing damage to foliage and grain while reducing yield with an average annual economic loss of A$241-482 million. In this study, two invasive, oligophagous aphids Diuraphis noxia and Rhopalosiphum maidis are examined to investigate the impact of foliage Si accumulation on feeding behaviour and reproductive success. This glasshouse study uses two grass systems 1) wheat (Triticum aestivum) a globally important cereal crop; and 2) purple false brome (Brachypodium distachyon) a model plant system. We investigate the effect of Si supplementation on a) plant chemistry (including Si accumulation in foliage); b) plant biomass; c) aphid performance; and d) aphid feeding behaviour. Wheat and Brachypodium are thought to accumulate high concentrations of Si and both possess the Lsi1 and Lsi2 transporters necessary for active uptake and deposition of Si. The presence of these transporters, and the grass-focused oligophagous feeding behaviour of the study aphids make these grasses ideal for this research. We found that Si supplementation altered plant chemistry in both plant systems. Foliage Si content increased by 170% for wheat and 102% for B. distachyon. Furthermore, carbon content was decreased by 5% wheat and 4% in B. distachyon. In B. distachyon, trichome density increased by 45%. Aphid feeding behaviour, performance and plant biomass were largely unaffected. Although the overall performance and feeding behaviour of the aphids was not significantly impacted by changes to foliage caused by Si supplementation, this research does contribute to addressing some research gaps. No previous work has been conducted on Si supplementation with D. noxia and R. maidis in either wheat or Brachypodium systems. To gain a better understanding of how Si can be used as a defence against herbivores, it is necessary to publish studies that report no significant effects. A meta-analysis on Si defences against herbivores would help to address the current research gaps.

Date of Award	2018
Original language	English