Sub-cellular responses of wheat epidermal cells to Zymoseptoria tritici

Abstract

Zymoseptoria tritici (formerly Mycosphaerella graminicola), an Ascomycete fungus, is the main causal agent of Septoria tritici blotch, one of the main devastating wheat (Triticum aestivum) foliar diseases worldwide. The infection cycle of the hemi-biotrophic Z. tritici is divided into a symptom-less biotrophic phase followed by a necrotrophic phase characterised by pycnidia development. Little is known about molecular and cellular strategies of wheat defence during the first intimate contact with Z. tritici. Furthermore, information about immune responses in the wheat epidermal cells in pre-invasion resistance is lacking. In order to address the questions outlined above, we designed a series of novel assays on this plant patho-system to analyse the role of wheat focal immunity in response to Z. tritici hyphae. We tested the hypothesis that these wheat sub-cellular responses differed between Z. tritici virulent and avirulent strains during the fungal biotrophic symptom-less phase. High-resolution microscopy approaches combined with automated object detection recognition for organelle movement suggested a response from subsidiary cells flanking guard cells in the Z. tritici pre-invasion defence with regards to early changes of cell wall architecture and organelle re-distribution at the site of fungal interaction. These responses showed only subtle differences between compatible and incompatible strain-cultivar combinations. Our preliminary evidence suggests that the presence or absence of focal responses in subsidiary cells does not depend upon the AvrStb6 / Stb6 gene-for-gene relationship. Thus, novel applications in biology combined with live-cell imaging provided us with a promising tool to analyse the spatiotemporal dynamics of Z. tritici hyphae on infected wheat tissues. To the extent of our knowledge, this is the first report that analyses wheat subsidiary cells in response to fungal pathogens at the sub-cellular scale. This may constitute the object of future studies of wheat immunity and drive to the discovery of key factors linked to the strategies that lead to fungal invasion and circumvention of the wheat immune system.