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- Title
Siliceous and non-nutritious: Nitrogen limitation increases anti-herbivore silicon defences in a model grass.
- Authors
Johnson, Scott N.; Waterman, Jamie M.; Wuhrer, Richard; Rowe, Rhiannon C.; Hall, Casey R.; Cibils-Stewart, Ximena
- Abstract
1. Silicon (Si) accumulation alleviates a diverse array of environmental stresses in many plants, including conferring physical resistance against insect herbivores. It has been hypothesised that grasses, in particular, utilise 'low metabolic cost' Si for structural and defensive roles under nutrient limitation. While carbon (C) concentrations often negatively correlate with Si concentrations, the relationship between nitrogen (N) status and Si is more variable. Moreover, the impacts of N limitation on constitutive physical Si defences (e.g. silica and prickle cells) against herbivores are unknown. 2. We determined how N limitation affected Si deposition in the model grass Brachypodium distachyon and how changes in these constitutive defences impacted insect herbivore Helicoverpa armigera growth rates. We used scanning electron microscopy (SEM) and energy dispersive X-ray spectrometry in conjunction with X-ray mapping (XRM) to quantify physical structures on leaves and determine Si deposition patterns. We also determined how N limitation and Si supply impacted the jasmonic acid (JA) pathway, the master regulator of induced defences against arthropod herbivores. 3. N limitation reduced shoot growth by over 40%, but increased root mass (+21%), leaf Si concentrations (+50%) and the density of silica (+28%) and flattened prickle (+76%) cells. Energy-dispersive X-ray spectroscopy and XRM established that Si was being deposited in these structures, together with hooked prickle cells and macrohairs. Herbivore relative growth rates (RGR) were more than 115% lower in Sisupplied plants compared to plants without Si supply and negatively correlated with leaf Si concentration and silica cell density. RGR was further reduced by N limitation and positively correlated with leaf N concentrations. Increases in JA concentrations following induction of the JA pathway were at least doubled by N limitation. 4. Synthesis. Si accumulation and deposition were highly regulated by N availability, with N limitation promoting both constitutive Si physical defences and induction of the JA defensive pathway, in line with the resource availability hypothesis. These results indicate that grasses use 'low-cost Si' when resources are limited and suggest that plant productivity may benefit from optimising conventional fertilisers and Si fertilisation.
- Subjects
BRACHYPODIUM; ATMOSPHERIC nitrogen; SILICON; HELICOVERPA armigera; SCANNING electron microscopy; PLANT productivity; JASMONIC acid; X-ray spectroscopy
- Publication
Journal of Ecology, 2021, Vol 109, Issue 11, p3767
- ISSN
0022-0477
- Publication type
Article
- DOI
10.1111/1365-2745.13755