Lipid metabolism is differentially modulated by salicylic acid and heptanoyl salicylic acid during the induction of resistance in wheat against powdery mildew

Heptanoyl salicylic acid (HSA) is a salicylic acid (SA) derivative obtained by esterification of 2-OH benzoic acid with heptanoic acid. In wheat, the protection levels obtained against Blumeria graminis f. sp. tritici (Bgt) increased from 50% with SA to 95% with HSA. Using molecular, biochemical and cytological approaches, we investigated here how wheat lipid metabolism is differentially activated by SA and HSA in both infectious and non-infectious conditions, and how Bgt infectious process is altered by both inducers. First, in the absence of Bgt, continuous lipoxygenase (LOX)-encoding gene expression and corresponding activity were specifically induced by HSA. Moreover, compared to SA, HSA treatment resulted in earlier up-regulations of the phospholipase C2-encoding gene expression and it specifically affected the expression of a lipid transfer protein-encoding gene. In infectious context, both HSA and SA sprayings impaired penetration events and therefore haustorium formation, leading to less frequent fungal colonies. While this alteration only slowed down the evolution of Bgt infectious process in SA-sprayed leaves, it completely impaired the establishment of successful infectious events in HSA-sprayed leaves. In addition, HSA induced continuous increases of a LOX-encoding gene expression and of the corresponding LOX activity when compared to SA-sprayed leaves. Lipid metabolism is therefore overall highly responsive to HSA spraying and could represent effective defence mechanism triggered during the induction of resistance in wheat toward Bgt. The concepts of priming and energy costs of the defences induced by SA and HSA are also discussed.

Transcriptomics of induced defense responses to greenbug aphid feeding in near isogenic wheat lines

The greenbug aphid, Schizaphis graminum (Rondani) is an important cereal pest, periodically threatening wheat yields in the United States and around the world. The single dominant gene, Gb3-based resistance is highly durable against prevailing greenbug biotypes under field conditions; however, the molecular mechanisms of Gb3-mediated defense responses remain unknown. We used Affymetrix GeneChip Wheat Genome Arrays to investigate the transcriptomics of host defense responses upon greenbug feeding on resistant and susceptible bulks (RB and SB, respectively) derived from two near-isogenic lines. The study identified 692 differentially expressed transcripts and further functional classification recognized 122 transcripts that are putatively associated to mediate biotic stress responses. In RB, Gb3-mediated resistance resulted in activation of transmembrane receptor kinases and signaling-related transcripts involved in early signal transduction cascades. While in SB, transcripts mediating final steps in jasmonic acid biosynthesis, redox homeostasis, peroxidases, glutathione S-transferases, and notable defense-related secondary metabolites were induced. Also transcripts involved in callose and cell wall decomposition were elevated SB, plausibly to facilitate uninterrupted feeding operations. These results suggest that Gb3-mediated resistance is less vulnerable to cell wall modification and the data provides ample tools for further investigations concerning R gene based model of resistance.