Transcriptome and metabolite profiling of the infection cycle of Zymoseptoria tritici on wheat reveals a biphasic interaction with plant immunity involving differential pathogen chromosomal contributions and a variation on the hemibiotrophic lifestyle definition.

Jason J Rudd,Keywan Hassani-Pak,Kim E Hammond-Kosack,Arvind Bharti,Mark Derbyshire,Mikael Courbot,Mansoor Saqi,Linda Ambroso,Nalini M Desai,Andrew Farmer,Juliet Hooper,Stephen J Powers,Jean Devonshire,Robert A Dietrich,Artem Lysenko,Ambrose Andongabo,Kostya Kanyuka

doi:10.1104/pp.114.255927

Abstract

The hemibiotrophic fungus Zymoseptoria tritici causes Septoria tritici blotch disease of wheat (Triticum aestivum). Pathogen reproduction on wheat occurs without cell penetration, suggesting that dynamic and intimate intercellular communication occurs between fungus and plant throughout the disease cycle. We used deep RNA sequencing and metabolomics to investigate the physiology of plant and pathogen throughout an asexual reproductive cycle of Z. tritici on wheat leaves. Over 3,000 pathogen genes, more than 7,000 wheat genes, and more than 300 metabolites were differentially regulated. Intriguingly, individual fungal chromosomes contributed unequally to the overall gene expression changes. Early transcriptional down-regulation of putative host defense genes was detected in inoculated leaves. There was little evidence for fungal nutrient acquisition from the plant throughout symptomless colonization by Z. tritici, which may instead be utilizing lipid and fatty acid stores for growth. However, the fungus then subsequently manipulated specific plant carbohydrates, including fructan metabolites, during the switch to necrotrophic growth and reproduction. This switch coincided with increased expression of jasmonic acid biosynthesis genes and large-scale activation of other plant defense responses. Fungal genes encoding putative secondary metabolite clusters and secreted effector proteins were identified with distinct infection phase-specific expression patterns, although functional analysis suggested that many have overlapping/redundant functions in virulence. The pathogenic lifestyle of Z. tritici on wheat revealed through this study, involving initial defense suppression by a slow-growing extracellular and nutritionally limited pathogen followed by defense (hyper) activation during reproduction, reveals a subtle modification of the conceptual definition of hemibiotrophic plant infection.

Highlights

The hemibiotrophic fungus Zymoseptoria tritici causes Septoria tritici blotch disease of wheat (Triticum aestivum)
One of the best studied Mycosphaerella spp. fungi is Zymoseptoria tritici, which is exclusively pathogenic toward leaves of wheat (Triticum aestivum), causing Septoria tritici blotch disease
Five time points of leaf infection were selected to span key phases of the host-pathogen interaction (Fig. 1). These included 1 dpi, where fungal spores had just germinated on the leaf surface; 4 dpi, which represents early poststomatal penetration and slow symptomless intercellular growth inside leaves; 9 dpi, representing the transition phase associated with the onset of more rapid fungal intercellular growth and the first appearance of macroscopic disease symptoms on leaves; and 14 dpi, representing accelerating plant cell death with rapid fungal necrotrophic growth and the onset of sporulation

Summary

Introduction

The hemibiotrophic fungus Zymoseptoria tritici causes Septoria tritici blotch disease of wheat (Triticum aestivum). Typical of most Mycosphaerella spp. plant pathogens, Z. tritici enters leaves via their natural openings, stomata, and completes its full asexual reproductive cycle without physically penetrating host cells (Kema et al, 1996; Pnini-Cohen et al, 2000; Deller et al, 2011; Dean et al, 2012) This strictly intercellular (or apoplastic) mode of colonization and reproduction differentiates Z. tritici from almost all other well-studied plant pathogenic fungi with available genomic resources, including all species of Magnaporthe, Fusarium, Ustilago, Colletotrichum, Blumeria, and Puccinia, all of which have at least one period of intracellular (invasive) growth within plant cells during infection (Brown and Hammond-Kosack, 2014). This lifestyle suggests that communication between the Z. tritici hyphae and the neighboring wheat cells must rely heavily on the exchange of secreted or cell surface-localized molecules but raises questions regarding the exact nature of the dialogue throughout infection

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Conclusion