Pre-symptomatic modified phytohormone profile is associated with lower phytoplasma titres in an Arabidopsis seor1ko line

Chiara Bernardini,Nazia Loi,Alberto Loschi,Axel Mithöfer,Aart J E Van Bel,Simonetta Santi,Marta Martini,Marilia Almeida-Trapp,Valeria De Rosa,Laura Pagliari,Sara Buoso,Rita Musetti,Fiorella Chiesa

doi:10.1038/s41598-020-71660-0

Abstract

The proteins AtSEOR1 and AtSEOR2 occur as conjugates in the form of filaments in sieve elements of Arabidopsis thaliana. A reduced phytoplasma titre found in infected defective-mutant Atseor1ko plants in previous work raised the speculation that non-conjugated SEOR2 is involved in the phytohormone-mediated suppression of Chrysanthemum Yellows (CY)-phytoplasma infection transmitted by Euscelidius variegatus (Ev). This early and long-lasting SEOR2 impact was revealed in Atseor1ko plants by the lack of detectable phytoplasmas at an early stage of infection (symptomless plants) and a lower phytoplasma titre at a later stage (fully symptomatic plants). The high insect survival rate on Atseor1ko line and the proof of phytoplasma infection at the end of the acquisition access period confirmed the high transmission efficiency of CY-phytoplasma by the vectors. Transmission electron microscopy analysis ruled out a direct role of SE filament proteins in physical phytoplasma containment. Time-correlated HPLC–MS/MS-based phytohormone analyses revealed increased jasmonate levels in midribs of Atseor1ko plants at an early stage of infection and appreciably enhanced levels of indole acetic acid and abscisic acid at the early and late stages. Effects of Ev-probing on phytohormone levels was not found. The results suggest that SEOR2 interferes with phytohormonal pathways in Arabidopsis midrib tissues in order to establish early defensive responses to phytoplasma infection.

Highlights

The proteins AtSEOR1 and AtSEOR2 occur as conjugates in the form of filaments in sieve elements of Arabidopsis thaliana
Three individuals of healthy E. variegatus or Chrysanthemum Yellows (CY)-infected E. variegatus were placed on wild-type, Atseorko[1] and Atseor2ko Arabidopsis plants
AtSEOR1 and AtSEOR2 are thought to be necessary for filament formation through their heterodimeric interaction[8]

Summary

Introduction

The proteins AtSEOR1 and AtSEOR2 occur as conjugates in the form of filaments in sieve elements of Arabidopsis thaliana. Studying the role of SEOR proteins following Chrysanthemum Yellows (CY)-phytoplasma infection in Arabidopsis thaliana, Pagliari et al.[9] noted that the Atseor1ko mutant line hosted a considerably lower number of phytoplasmas, even though the phloem flow (and the pathogen spread capability) is not affected[9]. This observation led to the hypothesis that an unknown SEOR2-associated mechanism assists the plant to combat the pathogen. Conjugated with SEOR1, as is the case in Atseor[1] ko plants, may be involved in plant immune responses[10,11] or in phytohormone-mediated signalling p athways[12]

Methods

Results

Conclusion