A Novel Effector Protein of Apple Proliferation Phytoplasma Disrupts Cell Integrity of Nicotiana spp. Protoplasts.

Cecilia Mittelberger,Christine Kerschbamer,Thomas Letschka,Katrin Janik,Hagen Stellmach,Bettina Hause,Katja Schlink

doi:10.3390/ijms20184613

Abstract

Effector proteins play an important role in the virulence of plant pathogens such as phytoplasma, which are the causative agents of hundreds of different plant diseases. The plant hosts comprise economically relevant crops such as apples (Malus × domestica), which can be infected by ‘Candidatus Phytoplasma mali’ (P. mali), a highly genetically dynamic plant pathogen. As the result of the genetic and functional analyses in this study, a new putative P. mali effector protein was revealed. The so-called “Protein in Malus Expressed 2” (PME2), which is expressed in apples during P. mali infection but not in the insect vector, shows regional genetic differences. In a heterologous expression assay using Nicotiana benthamiana and Nicotiana occidentalis mesophyll protoplasts, translocation of both PME2 variants in the cell nucleus was observed. Overexpression of the effector protein affected cell integrity in Nicotiana spp. protoplasts, indicating a potential role of this protein in pathogenic virulence. Interestingly, the two genetic variants of PME2 differ regarding their potential to manipulate cell integrity. However, the exact function of PME2 during disease manifestation and symptom development remains to be further elucidated. Aside from the first description of the function of a novel effector of P. mali, the results of this study underline the necessity for a more comprehensive description and understanding of the genetic diversity of P. mali as an indispensable basis for a functional understanding of apple proliferation disease.

Highlights

Phytoplasma are small, biotrophic bacteria that cause hundreds of different plant diseases and are involved in their infection cycle in plant hosts, and in insect vectors. ’Candidatus Phytoplasma mali’ (P. mali), the causal agent of apple proliferation (AP) disease, has caused significant economic losses in apple production in Northern Italy in the last decades [1]
Since phytoplasma phylogenetically belong to Gram-positive bacteria [3], a prediction algorithm trained on this bacterial group was applied
The results show that Protein in Malus Expressed 2” (PME2) is expressed in roots and leaves of infected Malus × domestica, but not in infected individuals of its insect vector C. picta, underlining the hypothesis that PME2 plays a role as an effector protein in plant cells

Summary

Introduction

Phytoplasma are small, biotrophic bacteria that cause hundreds of different plant diseases and are involved in their infection cycle in plant hosts, and in insect vectors. ’Candidatus Phytoplasma mali’ (P. mali), the causal agent of apple proliferation (AP) disease, has caused significant economic losses in apple production in Northern Italy (one of Europe’s main production areas) in the last decades [1]. Phytoplasma are obligate plant and insect symbionts that exhibit a biphasic life cycle comprising reproduction in certain phloem-feeding insects as well as in plants [2,3]. Within their plant host, phytoplasma colonize the phloem. By ingestion of phloem sap, insect vectors acquire the phytoplasma, with the colonization of those insects enabling the transmission of the pathogen between host plants [3,4]. The immunodominant membrane protein Imp of P. mali was shown not to be involved in symptom development but is considered to play a role during plant colonization [15]. Potential effector proteins may be identified by the presence of a characteristic N-terminal secretion signal

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