The powdery mildew-resistant Arabidopsis mlo2 mlo6 mlo12 triple mutant displays altered infection phenotypes with diverse types of phytopathogens

Lingxue Cao,Ralph Panstruga,Stefan Kusch,Ariane Kemen,Anja Reinstädler,Katrin Gruner,Richard J. O’Connell,Eric Kemen,Marco U. Reitz,Hannah Kuhn,Patrick Schäfer,Johanna Acevedo-Garcia,Frank L. W. Takken

doi:10.1038/s41598-017-07188-7

Abstract

Arabidopsis thaliana mlo2 mlo6 mlo12 triple mutant plants exhibit complete immunity against infection by otherwise virulent obligate biotrophic powdery mildew fungi such as Golovinomyces orontii. While this phenotype is well documented, the interaction profile of the triple mutant with other microbes is underexplored and incomplete. Here, we thoroughly assessed and quantified the infection phenotypes of two independent powdery mildew-resistant triple mutant lines with a range of microbes. These microorganisms belong to three kingdoms of life, engage in diverse trophic lifestyles, and deploy different infection strategies. We found that interactions with microbes that do not directly enter leaf epidermal cells were seemingly unaltered or showed even enhanced microbial growth or symptom formation in the mlo2 mlo6 mlo12 triple mutants, as shown for Pseudomonas syringae and Fusarium oxysporum. By contrast, the mlo2 mlo6 mlo12 triple mutants exhibited reduced host cell entry rates by Colletotrichum higginsianum, a fungal pathogen showing direct penetration of leaf epidermal cells comparable to G. orontii. Together with previous findings, the results of this study strengthen the notion that mutations in genes MLO2, MLO6 and MLO12 not only restrict powdery mildew colonization, but also affect interactions with a number of other phytopathogens.

Highlights

Powdery mildew is a widespread fungal disease of many angiosperm plants[1]
Mutations in MLO6 (At1g61560) and MLO12 (At2g39200) do not affect powdery mildew interactions on their own. They co-operatively enhance mlo2-conditioned resistance, and in combination with a mutation in MLO2 cause a complete lack of host cell penetration by fungal sporelings, leading to complete immunity (“pre-penetration resistance”). This type of powdery mildew resistance is best known from barley, where natural and induced mlo mutants have been discovered more than 70 years ago and have been successfully employed in agriculture for over 35 years[12, 13]
The panel of tested microorganisms comprised fungal (G. orontii, F. oxysporum, C. higginsianum, S. indica), oomycete (H. arabidopsidis, A. laibachii) and bacterial (P. syringae) species, which engage in obligate biotrophic (G. orontii, H. arabidopsidis, A. laibachii), and hemibiotrophic (P. syringae, C. higginsianum and F. oxysporum) interactions, respectively

Summary

Introduction

Powdery mildew is a widespread fungal disease of many angiosperm plants[1]. It is caused by ascomycetes of the order Erysiphales. They co-operatively enhance mlo2-conditioned resistance, and in combination with a mutation in MLO2 cause a complete lack of host cell penetration by fungal sporelings, leading to complete immunity (“pre-penetration resistance”) This type of powdery mildew resistance is best known from barley, where natural and induced mlo mutants have been discovered more than 70 years ago and have been successfully employed in agriculture for over 35 years[12, 13]. The mlo[2] mlo[6] mlo[12] mutant shows a spectacular level of resistance against different powdery mildews[11, 15] It revealed slightly enhanced disease symptoms– and in part pathogen proliferation–in interactions with some hemibiotrophic/necrotrophic pathogens such as Alternaria alternata, A. brassicicola and Phytophthora infestans[11]. Similar to the barley mlo mutant, leaves of the triple mutant are subject to spontaneous deposition of callose-containing cell wall appositions and premature senescence[11, 20,21,22]

Methods

Results

Conclusion