Arabidopsis Membrane Contact Site protein SYNAPTOTAGMIN A maintains sieve element endomembrane morphology and function

Very little is known about the sieve element (SE) endomembrane system. In terms of surface area, the most important membrane is the SE endoplasmic reticulum, characterized by a unique cisternal structure of anchored flat and smooth stacks. Plasma membrane contact sites (MCSs) play a crucial role in anchoring the endoplasmic reticulum (ER) to the plasma membrane (PM) and in maintaining membrane intergrity. Here, we tested their role of MCSs in the endomembrane system of the sieve elements. Synaptotagmin A (SYTA) is one of the best-studied proteins known to form ER-PM MCSs in plants. We show that SYTA:RFP co-localizes with SUC2::GFP and CALS7::GFP, confirming its presence in Arabidopsis SEs. In syta-1 mutants, SER lost its discrete shape and separated from the SE wall. The export of 14C-compounds from leaves of wild type plants was about 10% higher than in syta-1 mutants. Finally, we explored SYTAs role in phytoplasma infection response. After infection, syta-1 plants displayed 50% less callose deposition and an uneven distribution pattern of the pathogen. In conclusion, our work shows that SYTA is required for maintaining the unique shape of the SE endomembrane system, and for diverse SE functions including callose deposition, carbon translocation and response to pathogens.