Plants can detect neighbouring plants through a reduction in the ratio between red and far-red light (R:FR). This provides a signal of plant-plant competition and induces rapid plant growth while inhibiting defence against biotic stress, two interlinked responses designated as the shade avoidance syndrome (SAS). Consequently, the SAS can influence plant-herbivore interactions that could cascade to higher trophic levels. However, little is known on how the expression of the SAS can influence tritrophic interactions. We investigated whether changes in R:FR affect the emission of herbivore-induced plant volatiles (HIPVs), and whether these changes influence the attraction of the zoophytophagous predator Macrolophus pygmaeus. We also studied how the expression of the SAS and subsequent inhibition of plant defences affects the reproduction of M. pygmaeus in both the presence and absence of the greenhouse whitefly ( Trialeurodes vaporariorum) as arthropod prey. The results show that changes in R:FR have little effect on HIPV emissions and predator attraction. However, a reduction in R:FR leads to increased reproduction of both the predator and the whiteflies. We conclude that shade avoidance responses can increase the population development of M. pygmaeus directly by reducing plant defences, and indirectly by supporting higher herbivore densities.

While the response of Arabidopsis thaliana to either drought, herbivory or fungal infection has been well-examined, the consequences of exposure to a series of such (a)biotic stresses are not well studied. This work reports on the genetic mechanisms underlying Arabidopsis response to single osmotic stress, and to combinatorial stress, either fungal infection using Botrytis cinerea or herbivory using Pieris rapae caterpillars followed by an osmotic stress treatment. Several small effect genetic loci associated with rosette dry weight (DW), rosette water content (WC), and the projected rosette leaf area (PLA) in response to combinatorial stress were mapped using univariate and multi-environment genome wide association approaches. A SNP associated with DROUGHT INDUCED 19 (DI19) was identified by both approaches, supporting its potential involvement in the response to combinatorial stress. Several SNPs were found to be in linkage disequilibrium with known stress-responsive genes such as PEROXIDASE 34 ( PRX34), BASIC LEUCINE ZIPPER 25 ( bZIP25), RESISTANCE METHYLATED GENE 1 ( RMG1), and WHITE RUST RESISTANCE 4 (WRR4). An antagonistic effect between biotic and osmotic stress was found for the prx34 and arf4 mutants, which suggests PRX34 and ARF4 to play an important role in the response to the combinatorial stress.