Impact of lepidopteran oral secretions on the transcriptome of Arabidopsis thaliana

Plants respond to attack by chewing insects through the recognition of herbivore-associated molecular patterns (HAMPs) that are present in oral secretions (OS) and released at the wound site, leading to appropriate deployment of plant immune responses. Because insect feeding is accompanied by severe wounding of the leaf tissue, the specific contribution of HAMPs to defense is not well characterized. In addition, OS contain effectors that interfere with activation of defenses, including the suppression of wound-induced genes. Here, we analyze the transcriptome of Arabidopsis thaliana leaves in response to wounding and application of OS from Spodoptera littoralis or Pieris brassicae. We report that OS amplify wound-induced responses and specifically promote the activation of hormonal pathways, as well as pathogen- and cell wall-related responses. Furthermore, application of OS on wound sites extensively suppresses the expression of genes involved in the regulation and biosynthesis of aliphatic glucosinolates. Also, OS suppress the expression of genes involved in wound-healing processes and callus formation, including ERF114, DOFs and CYCD3s, in addition to genes encoding cell-wall modifying enzymes, whose activation completes wound sealing. Altogether these results highlight the contribution of OS components to defense and unveil the potential role of conserved OS-derived effector(s) in inhibiting the production of defense compounds and interfering with wound healing processes. Overall design: Three leaves of each 5-week-old Arabidopsis thaliana plants, except for the unwounded controls, were wounded by piercing four holes of 1 mm each, avoiding the midvein. Two µl of either H2O, Pieris brassicae oral secretions (OS) or Spodoptera littoralis OS were applied to each hole. Samples were collected either 3h after wounding or 24h after wounding. Unwounded controls were collected 3h after start of the treatments. Leaf discs of 1 cm diameter around the wound site were collected, frozen in liquid nitrogen and stored at -80°C. RNA was then extracted for RNAseq analyses. Three to four our independent biological replicates were carried out using the same procedure.