Plant responses to multiple antagonists are mediated by order of attack and phytohormone crosstalk
Plants are often attacked by multiple antagonists, and traits of the attacking organisms, and their order of arrival onto hosts, may affect plant defenses. However, few studies have assessed how multiple antagonists, and varying attack order, affect plant defense or nutrition. To address this, we assessed defensive and nutritional responses of Pisum sativum plants after attack by a vector herbivore (Acrythosiphon pisum), a non-vector herbivore (Sitona lineatus), and a pathogen (Pea enation mosaic virus, PEMV). We show PEMV-infectious A. pisum induced several pathogen-specific plant defense signals, but these defenses were inhibited when S. lineatus was present in peas infected with PEMV. In contrast, feeding by S. lineatus induced anti-herbivore defense signals, but these defenses were enhanced by PEMV. Sitona lineatus also increased abundance of plant amino acids, but only when they attacked after PEMV-infectious A. pisum. Our results suggest that diverse communities of biotic antagonists alter defense and nutritional traits of plants through complex pathways that depend on the identity of attackers and their order of arrival onto hosts. Moreover, we show interactions among a group of biotic stressors can vary along a spectrum from antagonism to enhancement/synergism based on the identity and order of attackers, and these interactions are mediated by a multitude of phytohormone pathways.