Metabolomics of cereals under biotic stress: current knowledge and techniques

Abstract

Prone to attacks by pathogens and pests, plants employ intricate chemical defense mechanisms consisting of metabolic adaptations. However, many plant attackers are manipulating the host metabolism to counteract defense responses and to induce favorable nutritional conditions. Advances in analytical chemistry have allowed the generation of extensive metabolic profiles during plant-pathogen and pest interactions. Thereby, metabolic processes were found to be highly specific for given tissues, species, and plant-pathogen/pest interactions. The clusters of identified compounds not only serve as base in the quest of novel defense compounds, but also as markers for the characterization of the plants' defensive state. The latter is especially useful in agronomic applications where meaningful markers are essential for crop protection. Cereals such as maize make use of their metabolic arsenal during both local and systemic defense responses, and the chemical response is highly adapted to specific attackers. Here, we summarize highlights and recent findings of metabolic patterns of cereals under pathogen and pest attack.

Keywords: chemical analytical techniques; chemical defense; metabolic profile; monocots; phytoalexins; secondary metabolites.

FIGURE 1

Main metabolic pathways involved in cereal defense.

FIGURE 2

Necrotrophs interacting with barley and wheat. The main responses of cereals during necrotrophic interactionsare focused in the activation of the phenylpropanoid pathway. The infected plant accumulates lignin, phenol-glucosides, hydroxycinnamic acid conjugated with polyamine derivatives (HCAA; Gunnaiah et al., 2012) and also flavonoids. Abundant metabolites in cereal–necrotoph interactions are represented in red. Pathways that are activated during the interaction with necrotrophs are represented in bold red. This model is based on interactions between barley/wheat and Fusarium sp. (Bollina et al., 2010)

FIGURE 3

Biotrophs interacting with rice. Biotrophic pathogens feed from living cells forcing the host to increase its primary metabolism. Plant cells over-compensate the carbon and nitrogen depletion by acting as a sink for organic compounds that are imported from plant source tissues, and also by increasing their photosynthesis, gluconeogenesis, and glycolysis (1) The main pathways activated upon fungal infection are the tricarboxylic acid cycle (TCA) and the glycolysis. On the other hand, plant defense attempts to stimulate the shikimate pathway and lignin biosynthesis (2), but the fungus hijacks this processes with the help of effectors (Mentlak et al., 2012) and by inhibiting oxidative crosslinking of phenolics, thus leading to an over-accumulation of free phenylpropanoids and lignin precursors (3) (Parker et al., 2009). Upregulated metabolic pathways are depicted in green, compounds present in high abundance upon infection in blue, and processes inhibited by the pathogen in red. This model is essentially based on rice-Magnaporthe grisea interactions.