Sieve-element specific responses to phytoplasma infection: new insights about the role of occlusion proteins and callose

Phytoplasma are phloem-limited, wall-less, uncultivable prokaryotes. They affect several economically important crops and fruit trees. Actually, no efficient treatments are available to directly control the diseases associated to these pathogens. The study of the molecular interaction between plant and phytoplasma could provide, in a close future, knowledge about the mechanism of host colonization and plant counteract, which is important to actuate new control approaches. Plants face mechanically phytoplasma infection by sealing the sieve elements by phloem protein and/or callose accumulation at the sieve pores, and activating several pathways involved in defense process. We used different mutant lines of Arabidopsis thaliana and a strain of the ‘Candidatus Phytoplasma asteris’ (the Chrysanthemum Yellows phytoplasma), as pathosystem, to give insight on the possible link occurring between mechanical site-specific plant responses and systemic defense signaling. The first part of the study, dealing with the interaction between phytoplasmas and the sieve-element occlusion related (SEOR) proteins, underlined the involvement of AtSEOR2 protein with the pathway of phytohormones. Phytohormone accumulation (in particular jasmonic acid, indol-acetic acid and abscisic acid) was early activated at infection site in Atseor1ko mutants, an Arabidopsis line expressing the sole AtSEOR2 protein in the sieve tubes. The possible mechanism involved in the reduction of phytoplasma titre in Atseor1ko mutants was discussed. The second part of the study regarded the role of phloem callose in the interaction. Also in this case, an important relation between the synthesis of callose at the sieve plate (by the phloem specific callose synthase 7) and the early activation of systemic defense responses, was shown. In particular, plant lacking the production of phloem callose, resulted able to unbalance sugar transport (at both the symplasmic and apoplasmic level) and metabolism, favoring the priming of sugar-related signaling processes. Taken together these results drive us to the conclusion that, in Arabidopsis following phytoplasma infection, site-specific mechanical responses trigger systemic signals, mediated by SEOR proteins and callose.