The role of phytohormone biosynthesis in the Ergot fungus Claviceps purpurea
Final Report Abstract
Plant-pathogenic fungi are known to produce all major phytohormones; their function in hostpathogen interaction has been postulated, but unequivocal proof for this role is still lacking. In most cases where the biosynthetic genes were identified, it was shown that the fungal pathways are special and different from plants. Cytokinins (CK) were implied in several hostpathogen systems, but sofar the fungal pathways and the respective genes had not been identified. We could show that the Ergot fungus Claviceps purpurea produces CKs in axenic culture, via two different pathways, both of which are known from higher plants: a de novo synthesis, starting from prenylation of AMP, and a second one based on degradation of prenylated tRNAs . We identified the first fungal CK biosynthesis genes for the de novo pathway, encoding a novel bifunctional IPT-LOG enzyme (these activities are separated in plants) and - as in plants - a cytochrome P450 monoxygenase, respectively. In addition, we characterized the first tRNA-IPT of a filamentous fungus and its involvement in CK biosynthesis. The tRNA degradation pathway revealed to be the sole cZeatin type CK source in C. purpurea, while the fungus evolved a highly redundant biosynthesis mechanism for iP and tZeatin formation, using a link between both pathways. We suspected that in this system CKs might be involved in virulence. Indeed infected plants show a modified CK spectrum and a typical CK response. In contrast to preliminary results indicating a role of the de novo CK synthesis on virulence, this could not be substantiated by a thorough analysis. However, strains without functional de novo pathway still produce significant amounts of CKs. Interestingly, ΔcptRNA-IPT mutants lacking the degradation pathway, have a modestly reduced virulence phenotype. But a drastic effect was observed with strains lacking both pathways: they do not produce any CKs (confirming that there is no third pathway) and are nearly apathogenic. Thus, we for the first time provide evidence for an important role of CKs for the successful establishment of a plant-fungus interaction.
Publications
- (2015) De novo biosynthesis of cytokinins in the biotrophic fungus Claviceps purpurea. Environ. Microbiol. 17: 2935–2951
Hinsch J., Vrabka, J., Oeser, B., Novák, O., Galuszka, P., Tudzynski, P.
(See online at https://doi.org/10.1111/1462-2920.12838) - Functional characterization of the first filamentous fungal tRNA‐isopentenyltransferase and its role in the virulence of Claviceps purpurea. New Phytologist Vol 211 Issue 3, Aug 2016, Pages 980-992
Hinsch J, Galuszka P, Tudzynski P
(See online at https://doi.org/10.1111/nph.13960)
