Cell-wall structural carbohydrates reinforcements are part of the defence mechanisms of wheat against Russian wheat aphid (Diuraphis noxia) infestation

Abstract

Russian wheat aphid (RWA) is one of the most challenging pests for wheat crops globally. In South Africa, RWA has breached the strategy of introducing resistant genes into wheat plants, and so far, five RWA biotypes with different virulence levels have been documented in the field. Our study investigated how the cell wall plays a defensive role in Tugela-Dn1 (susceptible) and-Dn5 (resistant) cultivars infested with South African RWA-biotype 2 (RWASA2). The activities of enzymes related to defense responses were measured. The cell wall's holo-cellulose content, soluble lignin and physicochemical changes were quantified in the infested susceptible and resistant cultivars. Lastly, in vitro RWASA2 saliva-associated CWDEs activity was determined on cell wall-related model substrates. The results show that apoplastic peroxidase and β-1,3-glucanase activities were significantly higher in Tugela-Dn5 relative to the control during the infestation periods. Peroxidase activity is associated with lignin cross-linking of the cell wall, which could deter RWASA2 feeding. The total phenolic and holo-cellulose contents were significantly induced in Tugela-Dn5 at 72 and 120 h post infestation (hpi). These findings were corroborated by the FTIR results, which showed that holocellulose and lignin regions of the resistant and susceptible wheat were affected by infestation at 72 hpi. However, Tugela-Dn5 reinforced cell wall content at 120 hpi. An increased crystallinity index in the resistant cultivar validated the cell wall reinforcement at 120 hpi, while Tugela-Dn1 delayed cell wall reinforcement. This study demonstrates that cell wall reinforcement's modification is part of defense responses against Russian wheat aphid infestation.

Keywords: Aphid saliva; Cell wall; Cell wall degrading enzymes; Holo-cellulose; Peroxidase; Plant defense responses.