Ectoparasitic Growth of Magnaporthe on Barley Triggers Expression of the Putative Barley Wax Biosynthesis Gene CYP96B22 Which is Involved in Penetration Resistance

Background: Head blast caused by the fungal plant pathogen Magnaporthe oryzae is an upcoming threat for wheat and barley cultivation. We investigated the nonhost response of barley to an isolate of the Magnaporthe species complex which is pathogenic on Pennisetum spp. as a potential source for novel resistance traits.

Results: Array experiments identified a barley gene encoding a putative cytochrome P450 monooxygenase whose transcripts accumulate to a higher concentration in the nonhost as compared to the host interaction. The gene clusters within the CYP96 clade of the P450 plant gene family and is designated as CYP96B22. Expression of CYP96B22 was triggered during the ectoparasitic growth of the pathogen on the outside of the leaf. Usage of a fungicidal treatment and a Magnaporthe mutant confirmed that penetration was not necessary for this early activation of CYP96B22. Transcriptional silencing of CYP96B22 using Barley stripe mosaic virus led to a decrease in penetration resistance of barley plants to Magnaporthe host and nonhost isolates. This phenotype seems to be specific for the barley-Magnaporthe interaction, since penetration of the adapted barley powdery mildew fungus was not altered in similarly treated plants.

Conclusion: Taken together our results suggest a cross-talk between barley and Magnaporthe isolates across the plant surface. Since members of the plant CYP96 family are known to be involved in synthesis of epicuticular waxes, these substances or their derivatives might act as signal components. We propose a functional overlap of CYP96B22 in the execution of penetration resistance during basal and nonhost resistance of barley against different Magnaporthe species.