Compromised Photosynthetic Electron Flow and H2O2 Generation Correlate with Genotype-Specific Stomatal Dysfunctions during Resistance against Powdery Mildew in Oats.

Javier Sánchez-Martín,Diego Rubiales,Elena Prats,Gracia Montilla-Bascón,Luis A J Mur

doi:10.3389/fpls.2016.01660

Abstract

Stomatal dysfunction known as “locking” has been linked to the elicitation of a hypersensitive response (HR) following attack of fungal pathogens in cereals. We here assess how spatial and temporal patterns of different resistance mechanisms, such as HR and penetration resistance influence stomatal and photosynthetic parameters in oat (Avena sativa) and the possible involvement of hydrogen peroxide (H2O2) in the dysfunctions observed. Four oat cultivars with differential resistance responses (i.e., penetration resistance, early and late HR) to powdery mildew (Blumeria graminis f. sp. avenae, Bga) were used. Results demonstrated that stomatal dysfunctions were genotype but not response-type dependent since genotypes with similar resistance responses when assessed histologically showed very different locking patterns. Maximum quantum yield (Fv/Fm) of photosystem II were compromised in most Bga–oat interactions and photoinhibition increased. However, the extent of the photosynthetic alterations was not directly related to the extent of HR. H2O2 generation is triggered during the execution of resistance responses and can influence stomatal function. Artificially increasing H2O2 by exposing plants to increased light intensity further reduced Fv/Fm ratios and augmented the patterns of stomatal dysfunctions previously observed. The latter results suggest that the observed dysfunctions and hence a cost of resistance may be linked with oxidative stress occurring during defense induced photosynthetic disruption.

Highlights

Powdery mildew is an important foliar disease of cultivated oat, Avena sativa L. caused by the biotrophic fungus Blumeria graminis (DC.) E.O
One of the most important stomatal dysfunctions observed in the resistant oat cvs to powdery mildew was the loss of stomatal ability to close in darkness or lock-up
Since opening and close of stomata provide the necessary balance to maintain the relative concentration of CO2 for photosynthesis while preventing water losses (Roelfsema and Hedrich, 2005), one expected consequence of this altered stomatal behavior would be an alteration of the electron transport fluxes and of the carbohydrate balance

Summary

Introduction

Powdery mildew is an important foliar disease of cultivated oat, Avena sativa L. caused by the biotrophic fungus Blumeria graminis (DC.) E.O. Resistance to powdery mildew is mainly determined by 7 major resistance (R) genes (Pm1 to Pm7; Hsam et al, 2014; Okori, 2015). These trigger a hypersensitive response (HR) when matches the corresponding avirulence gene in the pathogen, (Jones and Jones, 1979). Resistance have been described, care should be taken to avoid undesired effects such as rapid emergence of new virulent strains, unforeseen susceptibility to non-target diseases or pests, and yield penalties through unexpected impacts on plant physiology and crop performance (SmedegaardPetersen and Tolstrup, 1985) Given the current drive to improve food security through sustainable means it becomes imperative to establish the possible sources of resistance costs in order to circumvent them

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Conclusion