On the role of phloem protein in plant-pathogen interaction

Diseases caused by phytoplasmas, prokaryotic plant pathogens restricted to phloem tissue, result in a huge impact on agriculture and, at the present day, no effective control strategy has been developed. The individuation of resistance or tolerance sources, need to understand the mechanisms existing at the basis of plant responses to phytoplasma infection. Investigations in natural plant hosts are limited by the impossibility to control the numerous environmental conditions that could affect plant-pathogen relationship. The reliability of Arabidopsis thaliana as model plant for studying phytoplasma-plant interaction was proved in this work, comparing macroscopic, histological and ultrastructural modifications induced by phytoplasma infection in A. thaliana with those reported in natural host plants. The agglutination of phloem protein filaments in the sieve elements following phytoplasma infection has been hypothesised to limit phloem mass flow and, consequently, to prevent pathogen spread. To elucidate the interaction between phytoplasmas and SE-protein filaments, Arabidopsis mutant lines lacking genes related to SE-protein filament (AtSEOR1, AtSEOR2 AtPP2-A1) were used both in healthy and in infected conditions. Various microscopic techniques were used, in order to combine fresh and embedded tissue observations, gaining information on both ultrastructural and physiological sieve element modifications. The analysis of healthy and infected plants suggested that SE-protein filaments affect phloem flow only even if agglutinated SE-protein masses are present in the sieve elements, and this occurs only in infected tissues. Moreover, even if filament presence was observed in mutant lines (indicating that in case of stressful condition SE-protein filament formation can overcome AtSEOR1 and AtSEOR2 absence), only filaments proper of wild-type plants are able to block the phloem mass flow in case of phytoplasma infection. No correlation between phloem impairment and pathogen concertation was found, but the low phytoplasma titre in AtSEOR1ko lines could indicate the involvement of this gene in plant defence mechanism, probably related to jasmonic acid and cis-12-oxo-phytodienoic acid pathway.