Philippe Giordanengo

Identification of aphid salivary proteins: a proteomic investigation of Myzus persicae

The role of insect saliva in the first contact between an insect and a plant is crucial during feeding. Some elicitors, particularly in insect regurgitants, have been identified as inducing plant defence reactions. Here, we focused on the salivary proteome of the green peach aphid, Myzus persicae. Proteins were either directly in‐solution digested or were separated by 2D SDS‐PAGE before trypsin digestion. Resulting peptides were then identified by mass spectrometry coupled with database investigations. A homemade database was constituted of expressed sequence tags from the pea aphid Acyrtosiphon pisum and M. persicae. The databases were used to identify proteins related to M. persicae with a nonsequenced genome. This procedure enabled us to discover glucose oxidase, glucose dehydrogenase, NADH dehydrogenase, α‐glucosidase and α‐amylase in M. persicae saliva. The presence of these enzymes is discussed in terms of plant–aphid interactions.

Compatible plant-aphid interactions: How aphids manipulate plant responses

To access phloem sap, aphids have developed a furtive strategy, their stylets progressing towards sieve tubes mainly through the apoplasmic compartment. Aphid feeding requires that they overcome a number of plant responses, ranging from sieve tube occlusion and activation of phytohormone-signalling pathways to expression of anti-insect molecules. In addition to bypassing plant defences, aphids have been shown to affect plant primary metabolism, which could be a strategy to improve phloem sap composition in nutrients required for their growth. During compatible interactions, leading to successful feeding and reproduction, aphids cause alterations in their host plant, including morphological changes, modified resource allocation and various local as well as systemic symptoms. Repeated salivary secretions injected from the first probe in the epidermal tissue up to ingestion of sieve-tube sap may play a crucial role in the compatibility between the aphid and the plant.

A phloem-sap feeder mixes phloem and xylem sap to regulate osmotic potential

Phloem-sap feeders (Hemiptera) occasionally consume the dilute sap of xylem, a behaviour that has previously been associated with replenishing water balance following dehydration. However, a recent study reported that non-dehydrated aphids ingested xylem sap. Here, we tested the hypothesis that the consumption of xylem sap, which has a low osmolality, is a general response to osmotic stresses other than dehydration. Alate aphids were subjected to different treatments and subsequently transferred onto a plant, where electrical penetration graph (EPG) was used to estimate durations of passive phloem sap consumption and active sucking of xylem sap. The proportion of time aphids fed on xylem sap (i.e., time spent feeding on xylem sap/total time spent feeding on phloem plus xylem sap) was used as a proxy of the solute concentration of the uptake. The proportion of time alate aphids fed on xylem sap increased: (1) with the time spent imbibing an artificial diet containing a solution of sucrose, which is highly concentrated in phloem sap and is mainly responsible for the high osmotic potential of phloem sap; (2) with the osmotic potential of the artificial diet, when osmotic potential excess was not related to sucrose concentration; and (3) when aphids were deprived of primary symbionts, a condition previously shown to lead to a higher haemolymph osmotic potential. All our results converge to support the hypothesis that xylem sap consumption contributes to the regulation of the osmotic potential in phloem-sap feeders.

Local and systemic responses induced by aphids in Solanum tuberosum plants

The aphids Macrosiphum euphorbiae (Thomas) and Myzus persicae (Sulzer) (Homoptera: Aphididae) are serious pests of potato (Solanum tuberosum L.) (Solanaceae), notably in transmitting several plant viruses. Heterospecific interactions may occur between these two species as they are often seen at the same time on the same potato plant in the field. As aphid infestation is known to induce both local and systemic changes, we conducted experiments to determine the effect of previous infestation on probing behaviour and feeding‐related parameters. We used the DC electrical penetration graph technique to characterize the influence of previous infestation by conspecific M. persicae or by heterospecific Ma. euphorbiae on M. persicae feeding behaviour at both local and systemic levels, i.e., on previously infested leaves and on non‐previously infested leaves of infested plants, respectively. Conspecific and heterospecific infestation led to similar modification of M. persicae feeding activities. However, the effects of previous infestation occurring at the local level were opposite to those observed at the systemic level. Myzus persicae food acceptance was slightly enhanced on previously infested leaves, whereas it was inhibited on non‐infested leaves of infested plants, which indicated an induced resistance mechanism. Our results advance the understanding of the mechanisms involved in aphid–host plant acceptance and colonization processes on potato plants in conspecific and heterospecific situations.

Wild Solanum resistance to aphids: antixenosis or antibiosis?

Abstract The type (antixenosis or antibiosis) of resistance against the aphids Myzus persicae (Sulzer) and Macrosiphum euphorbiae (Thomas) was characterized for the wild tuber-bearing potatoes, Solanum chomatophilum Bitter and Solanum stoloniferum Schltdl. & Bouché through behavioral (olfactometry and electrical penetration graph) and physiological studies. In dual-choice assays, only S. stoloniferum exerted attraction to M. euphorbiae. This ruled out the possibility that plant volatiles of S. chomatophilum and S. stoloniferum may contribute to the high resistance expressed. In electrical penetration graph experiments, aphids feeding on S. stoloniferum showed increased salivation phases, whereas phloem ingestion was drastically reduced for both aphid species. Because reaching phloem elements was not delayed in both species, the resistance mechanism was phloem-located. The antixenosis exhibited by S. stoloniferum was similar on young and mature leaves. S. chomatophilum also showed phloem-located antixenosis against M. persicae. In contrast, M. euphorbiae had no difficulty to reach S. chomatophilum phloem tissues and to ingest sap. S. chomatophilum resistance against M. euphorbiae was antibiosis and only expressed in mature leaves, where a complete nymphal mortality was observed.

Comparative study of the strategies evolved by two parasitoids of the genus Asobara to avoid the immune response of the host, Drosophila melanogaster

Asobara tabida and Asobara citri are two braconid endoparasitoids of Drosophila melanogaster larvae. We studied and compared the strategies evolved by these two species to avoid the immune reaction of their host. A. tabida has no negative impact on host cellular defenses and its eggs avoid encapsulation by adhering to host tissues. At the opposite, we found that A. citri, whose eggs are devoid of adhesive properties, affects the host encapsulation abilities, hemolymph phenoloxidase activity and concentrations of circulating hemocytes. Some of these effects could directly rely on a severe disruption of the hematopoietic organ anterior lobes observed in parasitized larvae. This is the first report of the immune suppressive abilities of a parasitoid from the Asobara genus. Results are presented and discussed with respect to the strategies of virulence evolved by other parasitoids to counteract the D. melanogaster immune system.

Role of xylem consumption on osmoregulation in Macrosiphum euphorbiae (Thomas).

Aphids are phloem feeders that occasionally ingest xylem sap. The duration of xylem consumption by Macrosiphum euphorbiae (Hemiptera: Aphididae) was positively correlated with the level of dehydration of alate aphids of different ages after a period of starvation, supporting the hypothesis that aphids ingest xylem sap to replenish their water balance. However, the duration of xylem sap ingestion but not phloem sap consumption varied in unstarved alate adults of different ages. Furthermore, both alate and apterous aphids ingested xylem sap at the end of their life, when aphids were not dehydrated but when fecundity started to decrease. Fecundity was negatively correlated with the proportion of time spent ingesting xylem sap, and that over the entire reproductive life of alate and apterous aphids. The lower proportion of xylem ingested by apterous than by alate aphids during the first few days of adult life may be related to a higher symbiont density in apterous morphs. As previous studies have demonstrated a relationship between sucrose assimilation, which is directly influenced by fecundity and symbiont density, and osmoregulation, we suggest that xylem consumption may play a role in the osmoregulation of haemolymph of aphids.

Unexpected effects of chitinases on the peach-potato aphid (Myzus persicae Sulzer) when delivered via transgenic potato plants (Solanum tuberosum Linné) and in vitro.

With the aim of producing insect-resistant potato plants, internode explants of Solanum tuberosum L. cv. Désirée were transformed with an Agrobacterium strain C58pMP90 containing an insect (Phaedon cochleariae: Coleoptera, Chrysomelidae) chitinase gene and the neomycin phosphotransferase (nptII) gene as selectable marker, both under the control of the viral CaMV 35S promoter. Three transformed potato lines (CH3, CH5 and CH25) exhibiting the highest chitinolytic activities were selected for feeding experiments with the peach-potato aphid, Myzus persicae (Sulzer), under controlled photoperiod and temperature conditions. Aphids fed on transgenic potato plants showed a reduced pre-reproductive period and an enhanced daily fecundity. Transgenic potato lines did not affect nymphal mortality, but improved several biological parameters related to aphid population’s growth. Artificial diets were used to provide active (1, 10, 100 and 500 μg ml−1) and inactive (500 μg ml−1) bacterial (Serratia marcescens) chitinase to M. persicae. These compounds increased nymph survival at all active chitinase doses when compared to the control diet, while inactive chitinase did not. Although the pre-reproductive period was slightly shortened and the daily fecundity slightly higher, active and inactive chitinase provided as food led a reduction from 1 to 1.5 day population’s doubling time. Therefore chitinase activity was responsible for the probiotic effects on aphids. Our results question the relevance of a chitinase-based strategy in the context of potato culture protection.

Effects of parasitism by Asobara tabida (Hymenoptera: Braconidae) on the development, survival and activity of Drosophila melanogaster larvae.

The impact of parasitism by Asobara tabida on Drosophila melanogaster larval development, survival features and larval activity has been investigated using two strains of the parasitoid. The successful parasitism rate of the A1 strain was four times greater than that of the WOPV strain. Both strains induced equivalent mortality rates but hosts parasitized by A1 predominantly died as pupae. The time necessary for the host pupariation and emergence, and the larval weight at 72, 96 and 120 h post-parasitization were measured. Parasitized larvae exhibited longer periods of development and lower weights than controls, especially when parasitized by A1. These results suggest that hosts underwent physiological costs varying with respect to the outcome of the parasitic relationship. Of the parasitoid factors possibly responsible for these costs, we examined venoms for their impact on host mortality. Artificial injections of WOPV venoms induced higher mortality rates than did A1 venoms. Venoms were also found responsible for the induction of a transient paralysis, naturally occuring after parasitization. Again, the strongest effect was observed after parasitization by WOPV or injections of its venoms. This study gives new insights into the intriguing features of A. tabida and constitutes the first report of the paralysing properties of the venoms.

Host‐plant mediated interactions between two aphid species

Herbivory induces numerous defence reactions in plants, which can in turn alter the plant quality for insects. The potato aphid, Macrosiphum euphorbiae (Thomas), and the green peach aphid, Myzus persicae (Sulzer) (both Hemiptera: Aphididae), are two important sympatric potato pests in northern France. The objective of this study was to evaluate the effect of a previous infestation of a potato plant, Solanum tuberosum L. (Solanaceae), by M. persicae or M. euphorbiae on the host attractiveness, feeding behaviour, and biological performance of M. euphorbiae subsequently colonising the plant. The preference of aphids was studied with a dual‐choice olfactometer and their feeding behaviour was monitored using the electrical penetration graph technique. Their biological performance was assessed by an in planta bioassay. Non‐infested plants were significantly more attractive to M. euphorbiae than plants pre‐infested by conspecific individuals. Aphids showed a strong reduction in the time spent ingesting phloem sap when feeding on pre‐infested plants. The biological performance of M. euphorbiae was not affected by previous conspecific infestation. Conversely, M. euphorbiae feeding behaviour was not affected on plants previously infested by M. persicae but aphids were more attracted to and had a faster population build‐up on those plants. Our results show that plant response and its effect on M. euphorbiae differed depending on the aphid species previously feeding on the potato plant. This variability in plant response could lead to competition or facilitation between aphids temporally and spatially separated, and promote dispersal under field conditions.