Date Published: June 1, 2018
Publisher: Public Library of Science
Author(s): Victoria Florencio-Ortiz, Susana Sellés-Marchart, José Zubcoff-Vallejo, Georg Jander, José L. Casas, Daniel Doucet.
Amino acids play a central role in aphid-plant interactions. They are essential components of plant primary metabolism, function as precursors for the synthesis of defense-related specialized metabolites, and are major growth-limiting nutrients for aphids. To quantify changes in the free amino acid content of pepper (Capsicum annuum L.) leaves in response to green peach aphid (Myzus persicae Sulzer) feeding, plants were infested with a low (20 aphids/plant) or a high (200 aphids/plant) aphid density in time-course experiments ranging from 3 hours to 7 days. A parallel experiment was conducted with pepper plants that had been subjected to water stress. Factor Analysis of Mixed Data revealed a significant interaction of time x density in the free amino acid response of aphid-infested leaves. At low aphid density, M. persicae did not trigger a strong response in pepper leaves. Conversely, at high density, a large increase in total free amino acid content was observed and specific amino acids peaked at different times post-infestation. Comparing aphid-infested with water-stressed plants, most of the observed differences were quantitative. In particular, proline and hydroxyproline accumulated dramatically in response to water stress, but not in response to aphid infestation. Some additional differences and commonalities between the two stress treatments are discussed.
Although they are best known as constituents of proteins, amino acids also play a central role in a wide variety of other plant physiological processes . They act as osmolytes, regulate ion transport, modulate stomatal opening, participate in detoxification of heavy metals, contribute to redox-homeostasis, influence gene expression, and affect the synthesis and activity of some enzymes . Moreover, amino acids serve as precursors for numerous plant secondary metabolites that fulfill critical functions such as signaling, defense, interactions with other organisms, and photoprotection [1, 3]. Many plant studies have demonstrated accumulation of free amino acids (FAA), especially proline, in response to both abiotic and biotic stresses (reviewed in ), including the two explored in the present study: water stress and aphid herbivory.
FAA analysis of leaves was performed on pepper plants under high aphid density, low aphid density, or water stress, as well as on the corresponding control plants. In all cases, 19 amino acids, but not Cys and Gly, were detected. Fig 1 shows the concentration of each amino acid, as well as EAA and non-essential amino acids (NEAA) content, found in control leaves.
The two densities of infestation assayed gave very different results in our study. Low aphid density provoked minor variations in the FAA composition of pepper leaves, but there was a significant decrease in total FAA content at the end of the study period (7 dpi). These results are in line with previous studies with different asymptomatic aphids, which showed little or no effect on the FAA content of their host plants [11, 15, 16], especially when compared to symptomatic aphid species causing chlorotic lesions or galls in their plant hosts [7, 10–13]. Thus, it seems that at low aphid density M. persicae either remain undetected or do not trigger a strong response in pepper plants. Conversely, high aphid density triggered a large increase in total FAA content. Specific amino acids, including the aromatics Phe, Tyr, Trp; the branched-chain amino acids (BCAA), Val, Ile, and Leu; and a miscellaneous group with Arg, Lys, Met, Thr, Ala, Asn, and His, accumulated and peaked at different times post-infestation. This again coincides with previously published works describing strong effects of asymptomatic aphids in plant FAA content, when using aphid densities similar  or even higher  to our high aphid density. Nevertheless, we have observed a significant interaction of time x density. Therefore, not only the density of infestation is important to define plant amino acid responses, but also the time of exposure.