Systemic response to aphid infestation by <Emphasis Type="Italic">Myzus persicae</Emphasis> in the phloem of <Emphasis Type="Italic">Apium graveolens</Emphasis>

Little is known about the molecular processes involved in the phloem response to aphid feeding. We investigated molecular responses to aphid feeding on celery (Apium graveolenscv. Dulce) plants infested with the aphid Myzus persicae, as a means of identifying changes in phloem function. We used celery as our model species as it is easy to separate the phloem from the surrounding tissues in the petioles of mature leaves of this species. We generated a total of 1187 expressed sequence tags (ESTs), corresponding to 891 non-redundant genes. We analysed these ESTs in silico after cDNA macroarray hybridisation. Aphid feeding led to significant increase in RNA accumulation for 126 different genes. Different patterns of deregulation were observed, including transitory or stable induction 3 or 7days after infestation. The genes affected belonged to various functional categories and were induced systemically in the phloem after infestation. In particular, genes involved in cell wall modification, water transport, vitamin biosynthesis, photosynthesis, carbon assimilation and nitrogen and carbon mobilisation were up-regulated in the phloem. Further analysis of the response in the phloem or xylem suggested that a component of the response was developed more specifically in the phloem. However, this component was different from the stress responses in the phloem driven by pathogen infection. Our results indicate that the phloem is actively involved in multiple adjustments, recruiting metabolic pathways and in structural changes far from aphid feeding sites. However, they also suggest that the phloem displays specific mechanisms that may not be induced in other tissues.EST, macroarray and clustering data are available from our website [http://www-biocel.versailles.inra.fr/phloem]. Data deposition: The sequences reported in this paper have been deposited in the Genbank database (Accession nos.: AY607692-AY607700, AY611007, CN253939-CN255151, CV512445-CV512447 and CV651120-CV651121).     

Aphid genotypes vary in their response to the presence of fungal endosymbionts in host plants

Genetic variation for fitness‐relevant traits may be maintained in natural populations by fitness differences that depend on environmental conditions. For herbivores, plant quality and variation in chemical plant defences can maintain genetic variation in performance. Apart from plant secondary compounds, symbiosis between plants and endosymbiotic fungi (endophytes) can produce herbivore‐toxic compounds. We show that there is significant variation among aphid genotypes in response to endophytes by comparing life‐history traits of 37 clones of the bird cherry‐oat aphid Rhopalosiphum padi feeding on endophyte‐free and endophyte‐infected tall fescue Lolium arundinaceum. Clonal variation for life‐history traits was large, and most clones performed better on endophyte‐free plants. However, the clones differed in the relative performance across the two environments, resulting in significant genotype × environment interactions for all reproductive traits. These findings suggest that natural variation in prevalence of endophyte infection can contribute to the maintenance of genetic diversity in aphid populations.     

What plant functional traits can reduce nitrous oxide emissions from intensively managed grasslands?

Plant species exert a dominant control over the nitrogen (N) cycle of natural and managed grasslands. Although in intensively managed systems that receive large external N inputs the emission of the potent greenhouse gas nitrous oxide (N₂O) is a crucial component of this cycle, a mechanistic relationship between plant species and N₂O emissions has not yet been established. Here we use a plant functional trait approach to study the relation between plant species strategies and N₂O emissions from soils. Compared to species with conservative strategies, species with acquisitive strategies have higher N uptake when there is ample N in the soil, but also trigger N mineralization when soil N is limiting. Therefore, we hypothesized that (1) compared to conservative species, species with acquisitive traits reduce N₂O emissions after a high N addition; and (2) species with conservative traits have lower N₂O emissions than acquisitive plants if there is no high N addition. This was tested in a greenhouse experiment using monocultures of six grass species with differing above‐ and below‐ground traits, growing across a gradient of soil N availability. We found that acquisitive species reduced N₂O emissions at all levels of N availability, produced higher biomass and showed larger N uptake. As such, acquisitive species had 87% lower N₂O emissions per unit of N uptake than conservative species (p < .05). Structural equation modelling revealed that specific leaf area and root length density were key traits regulating the effects of plants on N₂O emission and biomass productivity. These results provide the first framework to understand the mechanisms through which plants modulate N₂O emissions, pointing the way to develop productive grasslands that contribute optimally to climate change mitigation.