The influence of light and leaf antioxidant status on plant responses to aphids

Cross-tolerance to environmental stresses results from the synergistic co-activation of defence pathways that cross biotic-abiotic stress boundaries. However, the signalling mechanisms that underpin such responses remain poorly characterised. The effects of an abiotic stress (high light; HL) on the responses of Arabidopsis thaliana and tobacco (Nicotiana tabacum) plants to a biotic stress (infestation by the green peach aphid, Myzus persicae) were therefore analysed. Particular focus was placed on the role of cellular redox state as a regulator of cross-tolerance phenomena and the identification of signalling pathways that underpin aphid resistance. Aphid fecundity was measured in a range of A. thaliana mutants that have defects in non-enzymatic antioxidants (ascorbate and glutathione), enzymatic antioxidants (catalase) or downstream kinase/phosphatase signalling cascades, and in transgenic tobacco lines that have either increased or decreased levels of ascorbate oxidase. A pre-treatment with HL increased the resistance of transgenic tobacco plants with low ascorbate oxidase to aphid infestation. In contrast, the A. thaliana ascorbate oxidase knockout mutants did not show the HL-dependent decrease in aphid infestation. Aphid fecundity was decreased on A. thaliana mutants that have altered antioxidant (ascorbate, glutathione, catalase) status, or that lack the gamma (γ) subunit of protein phosphatase (PP2A). A pre-treatment with HL increased the resistance of A. thaliana plants to aphid infestation in all of the genotypes, except for the cat2 mutants that lack the photorespiratory form of leaf catalase and glutathione defective mutants. Taken together these findings demonstrate that redox processes and oxidative signalling are important modulators of aphid resistance and the light-aphid interaction. Moreover, the analysis of aphid fecundity on these A. thaliana mutants, which also have different levels of leaf camalexin, suggests that the levels of this secondary metabolite alone do not influence aphid infestation. A transcriptome and metabolome profiling analysis of the responses of the different tobacco lines highlights the central role of cell wall modifications/signalling as key components in plant responses to aphid infestation

Effects of light and the regulatory B-subunit composition of protein phosphatase 2A on the susceptibility of Arabidopsis thaliana to aphid (Myzus persicae) infestation

The interactions between biotic and abiotic stress signaling pathways are complex and poorly understood but protein kinase/phosphatase cascades are potentially important components. Aphid fecundity and susceptibility to Pseudomonas syringae infection were determined in the low light-grown Arabidopsis thaliana wild type and in mutant lines defective in either the protein phosphatase (PP)2A regulatory subunit B'γ (gamma; pp2a-b'γ) or B'ζ (zeta; pp2a-b'ζ1-1 and pp2a-b'ζ 1-2) and in gamma zeta double mutants (pp2a-b'γζ) lacking both subunits. All the mutants except for pp2a-b'ζ 1-1 had significantly lower leaf areas than the wild type. Susceptibility to P. syringae was similar in all genotypes. In contrast, aphid fecundity was significantly decreased in the pp2a-b'γ mutant relative to the wild type but not in the pp2a-b'γζ double mutant. A high light pre-treatment, which led to a significant increase in rosette growth in all mutant lines but not in the wild type, led to a significant decrease in aphid fecundity in all genotypes. The high light pre-treatment abolished the differences in aphid resistance observed in the pp2a-b'γ mutant relative to the wild type. The light and CO2 response curves for photosynthesis were changed in response to the high light pre-treatment, but the high light effects were similar in all genotypes. These data demonstrate that a pre-exposure to high light and the composition of B-subunits on the trimeric PP2A holoenzymes are important in regulating plant resistance to aphids. The functional specificity for the individual regulatory B-subunits may therefore limit aphid colonization, depending on the prevailing abiotic stress environment.</p

Nitrogen deficiency in barley (<i>Hordeum vulgare)</i> seedlings induces molecular and metabolic adjustments that trigger aphid resistance

Agricultural N2O pollution resulting from the use of synthetic fertilisers represents a significant contribution to anthropogenic greenhouse gas emissions, providing a rationale for reduced use of nitrogen fertilisers. Nitrogen limitation results in extensive systems rebalancing that remodels metabolism and defence processes. To analyse the regulation underpinning these responses, barley (Horedeum vulgare) seedlings were grown for seven days under nitrogen-deficient conditions until net photosynthesis was 50% lower than in nitrogen-replete controls. Although shoot growth was decreased there was no evidence for the induction of oxidative stress despite lower total concentrations of nitrogen containing antioxidants. Nitrogen deficient barley leaves were rich in amino acids, sugars and tricarboxylic acid cycle intermediates. In contrast to N-replete leaves one day old nymphs of the green peach aphid (Myzus persicae) failed to reach adulthood when transferred to N-deficient barley leaves. Transcripts encoding cell, sugar and nutrient signalling, protein degradation and secondary metabolism were over-represented in nitrogen-deficient leaves while those associated with hormone metabolism were similar under both nutrient regimes with the exception of mRNAs encoding proteins involved in auxin metabolism and responses. Significant similarities were observed between the N-limited barley leaf transcriptome and that of aphid infested Arabidopsis leaves. These findings not only highlight significant similarities between biotic and abiotic stress signalling cascades but also identify potential targets for increasing aphid resistance with implications for the development of sustainable agriculture

Ascorbate-mediated regulation of growth, photoprotection, and photoinhibition in Arabidopsis thaliana.

The requirements for ascorbate for growth and photosynthesis were assessed under low (LL; 250 µmol m-2 s-1) or high (HL; 1600 µmol m-2 s-1) irradiance in wild-type Arabidopsis thaliana and two ascorbate synthesis mutants (vtc2-1 and vtc2-4) that have 30% wild-type ascorbate levels. The low ascorbate mutants had the same numbers of leaves but lower rosette area and biomass than the wild type under LL. Wild-type plants experiencing HL had higher leaf ascorbate, anthocyanin, and xanthophyll pigments than under LL. In contrast, leaf ascorbate levels were not increased under HL in the mutant lines. While the degree of oxidation measured using an in vivo redox reporter in the nuclei and cytosol of the leaf epidermal and stomatal cells was similar under both irradiances in all lines, anthocyanin levels were significantly lower in the low ascorbate mutants than in the wild type under HL. Differences in the photosynthetic responses of vtc2-1 and vtc2-4 mutants were observed. Unlike vtc2-1, the vtc2-4 mutants had wild-type zeaxanthin contents. While both low ascorbate mutants had lower levels of non-photochemical quenching of chlorophyll a fluorescence (NPQ) than the wild type under HL, qPd values were greater only in vtc2-1 leaves. Ascorbate is therefore essential for growth but not for photoprotection