Gomez, Susana Karen
Burns, Patrick D.
University of Northern Colorado
Type of Resources
Place of Publication
University of Northern Colorado
Arbuscular mycorrhizal fungi (AMF) are obligate biotrophs known for forming mutualistic associations with over 80% of terrestrial plant species. Plants forming symbiosis with AMF, in general, exhibit enhanced nutritional status as well as abiotic and biotic stress tolerance. In addition, plants benefit from mycorrhiza-induced resistance (MIR), which “primes” plant defenses, enabling a faster and stronger response against subsequent attacks by pathogens and insect herbivores. Nevertheless, MIR against insect herbivores is primarily dependent on the feeding guild of the attacking insect. Herbivory by leaf-chewing insects and generalist species on mycorrhizal plants often results in decreased insect weight gain and reproductive rates, whereas the effect of AM symbiosis on the performance of phloem-feeding insects such as aphids is variable. The present study addresses the effects of simultaneous herbivory by the potato aphid (Macrosiphum euphorbiae) and root colonization by the AM fungus Glomus intraradices on potato (Solanum tuberosum) gene expression and physiology. The objectives of this study were to: 1) determine whether an optimal level of AM fungus colonization exists that triggers changes in gene expression leading to MIR against potato aphids, 2) profile the early changes in gene expression during tripartite potato aphid-potato-AM fungus interactions, and 3) analyze the effect of the tripartite interaction on plant physiology and potato aphid fitness. To assess objectives 1 and 2, a three-way interaction experiment that included two mycorrhizal stages was carried out to measure changes in gene expression at 24 hours of aphid feeding. Additionally, the differential effects of aphid herbivory and AM fungus colonization were compared between local, aphid-infested leaves and systemic (non-infested) leaves of the same plant. To assess objective 3, a three-way interaction experiment was conducted to determine the impact of 7 and 14 days of aphid infestation and an established AM symbiosis on photosynthetic rate, chlorophyll content, and water status, with a focus on the differential effects on physiology between locally infested and systemic non-infested leaves. The results indicate that after 24 hours of aphid feeding, the leaves of aphid-infested non-mycorrhizal plants showed higher expression of the jasmonic acid (JA) transcription factor MYC2 gene compared to non-infested plants with a high level of AM fungus colonization. In potato roots, 24 hours of aphid herbivory resulted in decreased expression of the ethylene response sensor 1 (ERS1) gene in aphidinfested non-mycorrhizal and aphid-infested low-colonized plants compared to noninfested high-colonized plants. Aphid number and weight increased on high-colonized plants compared to their non-mycorrhizal counterparts. Shoot tissue of mycorrhizal plants contained higher levels of phosphorous than non-mycorrhizal plants. After 14 days of aphid herbivory, non-infested mycorrhizal plants showed increased shoot fresh weight compared to aphid-infested and non-infested non-mycorrhizal plants. Photosynthetic rates increased in non-infested mycorrhizal plants compared to aphid-infested nonmycorrhizal plants. Additionally, photosynthetic rates increased in undamaged leaves on aphid-infested mycorrhizal plants compared to aphid-damaged leaves of the same plant. Additionally, aphid herbivory increased chlorophyll content in the leaves of non-infested mycorrhizal plants compared to non-infested non-mycorrhizal plants. Across all mycorrhizal and non-mycorrhizal plants, aphid herbivory decreased the chlorophyll content of aphid-damaged leaves compared to undamaged leaves of the same plant. Overall, these results reveal enhanced performance of aphids on mycorrhizal plants, likely resulting from the enhanced nutritional status of mycorrhizal plants compared to non-mycorrhizal plants. While aphid herbivory and low levels of AMF colonization increased the expression of the MYC2 gene, further research into the timing of defenserelated gene expression may yield a more complete picture of the effects of both species. Further, the enhanced photosynthetic rates observed in mycorrhizal plants may indicate the first steps toward the enhanced tolerance of mycorrhizal plants against aphid herbivory. The results from this research serve as foundation for a thorough characterization of the interactions between herbivorous pests, beneficial AMF, and the agriculturally important crop species, the potato.
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