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

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.

Comparative Genome Analysis of ‘Candidatus Phytoplasma luffae’ Reveals the Influential Roles of Potential Mobile Units in Phytoplasma Evolution

“…For example, two putative effectors were reported recently for ‘ Ca . P. mali’ ( Mittelberger et al, 2019 ; Strohmayer et al, 2019 ). The functions and evolution of these and other novel effectors warrant further studies.…”

Microbial Virulence Factors

“…The plant pathogen Candidatus Phytoplasma mali is the causal agent of apple proliferation disease, that affects apple production in Northern Italy [6]. Phytoplasma are biotrophic, obligate plant and insect bacterial symbionts, with a biphasic life cycle comprising reproduction in phloem-feeding insects and in plants [6]. The paper of Mittelberger et al focused on the effector protein PME2 (Protein in Malus Expressed 2), expressed by P. mali when infecting apples [6].…”

“…Phytoplasma are biotrophic, obligate plant and insect bacterial symbionts, with a biphasic life cycle comprising reproduction in phloem-feeding insects and in plants [6]. The paper of Mittelberger et al focused on the effector protein PME2 (Protein in Malus Expressed 2), expressed by P. mali when infecting apples [6]. The in silico analysis of the PME2 protein sequence performed revealed that the protein has features of effector proteins of Gram-positive bacteria, with a predicted final localization at the cytoplasm or nucleus of the host [6].…”

“…The paper of Mittelberger et al focused on the effector protein PME2 (Protein in Malus Expressed 2), expressed by P. mali when infecting apples [6]. The in silico analysis of the PME2 protein sequence performed revealed that the protein has features of effector proteins of Gram-positive bacteria, with a predicted final localization at the cytoplasm or nucleus of the host [6]. Two main protein variants, PME2ST PME2AT, were found associated in infected apple trees from Italy and Germany.…”

“…protoplasts. A better understanding of the molecular mechanisms used by P. mali to manipulate its host will rely on genomics analysis, since no genetic manipulation is presently available for these organisms [6].…”