Redox control of aphid resistance through altered cell wall composition and nutritional quality.

The mechanisms underpinning plants perception of phloem feeding insects, particularly aphids, remain poorly characterized. The role of apoplastic redox state in controlling aphid infestation was therefore explored using transgenic tobacco plants that have either high (PAO) or low (TAO) ascorbate oxidase (AO) activities relative to the wild type. Only a small number of leaf transcripts and metabolites were changed in response to genotype and cell wall composition was largely unaffected. Aphid fecundity was significantly decreased in TAO plants compared to other lines. Leaf sugar levels were increased and maximum extractable AO activities were decreased in response to aphids in all genotypes. Transcripts encoding the Respiratory Burst Oxidase Homologue F, signaling components involved in ethylene and other hormone-mediated pathways, photosynthetic electron transport components, sugar, amino acid and cell wall metabolism were significantly increased in the TAO plants in response to aphid perception relative to other lines. The levels of (1-4)-β-D-galactan were significantly decreased in response to aphid feeding in all the lines, the effect being greatest in the TAO plants, which also showed higher levels of partially methylesterified epitopes of homogalacturonan and un-esterified homogalacturonan than other lines. Taken together, these findings identify AO-dependent mechanisms that limit aphid infestation.