Dramatic losses of tidal wetlands in the Mississippi Delta and a few areas along the U.S. Atlantic coast have raised concerns about whether these marshes will survive if global sea level continues to rise due to greenhouse warming [Stevenson et al., 1986]. Original greenhouse warming sea‐level scenarios projected global sea levels several meters or more higher than present by 2100 [Barth and Titus, 1984], which would result in the disappearance of all coastal marshes, as the scarcity of marsh deposits from the rapid transgression during the middle Holocene testifies [Rampino and Sanders, 1981]. However, more recent estimates of global sealevel change suggest that some coastal marshes could survive [Douglas et al., 2000].

Most plants form mutualistic associations with arbuscular mycorrhizal (AM) fungi that are ubiquitous in soils. Through this symbiosis, plants can withstand abiotic and biotic stresses. The underlying molecular mechanisms involved in mediating mycorrhiza-induced resistance against insects needs further research, and this is particularly true for potato (Solanum tuberosum L. (Solanales: Solanaceae)), which is the fourth most important crop worldwide. In this study, the tripartite interaction between potato, the AM fungus Rhizophagus irregularis (Glomerales: Glomeraceae), and cabbage looper (Trichoplusia ni Hübner) (Lepidoptera: Noctuidae) was examined to determine whether potato exhibits mycorrhiza-induced resistance against this insect. Plant growth, insect fitness, AM fungal colonization of roots, and transcript levels of defense-related genes were measured in shoots and roots after 5 and 8 d of herbivory on mycorrhizal and nonmycorrhizal plants. AM fungal colonization of roots did not have an effect on potato growth, but root colonization levels increased by herbivory. Larval weight gain was reduced after 8 d of feeding on mycorrhizal plants compared with nonmycorrhizal plants. Systemic upregulation of Allene Oxide Synthase 1 (AOS1), 12-Oxo-Phytodienoate Reductase 3 (OPR3) (jasmonic acid pathway), Protease Inhibitor Type I (PI-I) (anti-herbivore defense), and Phenylalanine Ammonia Lyase (PAL) transcripts (phenylpropanoid pathway) was found during the tripartite interaction. Together, these findings suggest that potato may exhibit mycorrhiza-induced resistance to cabbage looper by priming anti-herbivore defenses aboveground. This study illustrates how mycorrhizal potato responds to herbivory by a generalist-chewing insect and serves as the basis for future studies involving tripartite interactions with other pests.

This study sheds light on a poorly understood area in insect-plant-microbe interactions, focusing on aphid probing and feeding behavior on plants with varying levels of arbuscular mycorrhizal (AM) fungus root colonization. It investigates a commonly occurring interaction of three species: pea aphid Acyrthosiphon pisum, barrel medic Medicago truncatula, and the AM fungus Rhizophagus irregularis, examining whether aphid-feeding behavior changes when insects feed on plants at different levels of AM fungus colonization (42% and 84% root length colonized). Aphid probing and feeding behavior was monitored throughout 8 h of recording using the electrical penetration graph (EPG) technique, also, foliar nutrient content and plant growth were measured. Summarizing, aphids took longer to reach their 1st sustained phloem ingestion on the 84% AM plants than on the 42% AM plants or on controls. Less aphids showed phloem ingestion on the 84% AM plants relative to the 42% AM plants. Shoots of the 84% AM plants had higher percent carbon (43.7%) relative to controls (40.5%), and the 84% AM plants had reduced percent nitrogen (5.3%) relative to the 42% AM plants (6%). In conclusion, EPG and foliar nutrient data support the hypothesis that modifications in plant anatomy (e.g., thicker leaves), and poor food quality (reduced nitrogen) in the 84% AM plants contribute to reduced aphid success in locating phloem and ultimately to differences in phloem sap ingestion. This work suggests that M. truncatula plants benefit from AM symbiosis not only because of increased nutrient uptake but also because of reduced susceptibility to aphids.

This research examined aphid and plant responses to distinct levels (none, low, and high) of arbuscular mycorrhizal (AM) fungal root colonization by studying the association between potato aphids (Macrosiphum euphorbiae), potatoes (Solanum tuberosum), and AM fungi (Rhizophagus intraradices). It extends knowledge on gene expression changes, assessed by RT–qPCR, of ten defense-related genes at two time-points post-herbivory (24 h and 10 days), focusing on aphid-infested local leaves, non-infested systemic leaves, and roots. The results showed that aphid fitness was not altered by AM symbiosis. At 24 h, ETHYLENE RECEPTOR 1 gene expression was repressed in roots of aphid-infested non-mycorrhizal plants and aphid-infested plants with a high level of AM fungal root colonization, but not on aphid-infested plants with a low level of AM fungal root colonization. At 10 days, ALLENE OXIDE CYCLASE and POTATO TYPE I PROTEASE INHIBITOR were upregulated exclusively in local leaves of aphid-infested plants with a low level of AM fungal root colonization. In addition, local and systemic changes in plant gene expression appeared to be regulated exclusively by AM status and aphid herbivory. In summary, the gene expression data provide insights on mycorrhizal potato responses to aphid herbivory and serve as a starting point for future studies using this system.