engleski

SECRETS OF THE PHYTOPLASMA GENOME: FROM VIRULENCE FACTORS TO HOST ADAPTATION

Plant diseases showing virus-like symptoms, but of unknown etiology, were intriguing scientists worldwide for a long time. Only 50 years ago, by examining electron micrographs of phloem sections from diseased plants, the mystery has been solved when mycoplasma-like organisms were discovered as the etiological agents. Afterwards, they were named phytoplasmas and assigned to the genus ‘Candidatus Phytoplasma’ within the class Mollicutes. Nevertheless, these peculiar bacteria share many common features with plant viruses regarding their life-style, host range, transmission and pathogenicity strategies. In nature, phytoplasmas are transmitted and hosted by phloem-feeding insects. Their genomes are small and reduced, but repeat-rich, lacking many metabolic genes as a consequence of their life in a nutrient rich environment and the host- dependent life cycles. In spite of the fact that phytoplasma cultivation is still challenging, the development and improvement of sequencing technologies enabled successful sequencing and assembly of five complete phytoplasma genomes and several genome drafts. The comparative genome analyses have revealed many unique features as well as the presence of effector/virulence factor genes ; however, some of their survival strategies and mechanisms of adaptation to parasitism need to be further elucidated. One of the potential mechanisms importamt for phytoplasma evolution and genome plasticity is the presence of repeats termed potential mobile units (PMUs) that have characteristics of replicative transposones. So far, several phytoplasma effectors have been well characterized: TENGU, a unique virulence factor from ‘Ca. P. asteris’ strain OY-M ; SAP- 11 and SAP-54 from Ca. P. asteris’ strain AYWB, and SAP-11 from ’Ca. P. mali’. It was discovered that those small molecules interfere with the plant developmental processes and defense responses by inhibiting auxin-related pathways and destabilizing different transcription factors. Nevertheless, these findings suggest that future studies are needed to elucidate the function of many other putative effectors as well as their interactions with phytoplasma hosts.

effector, pathogenicity, phytopathogens

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