Plasmodesmata-located proteins regulate plasmodesmal function at specific cell interfaces in Arabidopsis

Plasmodesmata (PD) are membrane-lined channels connecting adjoining plant cells. PD control symplasmic intercellular communication by allowing molecules to move between cells. Plant polysaccharide callose ({beta}-1,3-glucan) is deposited at PD, affecting plasmodesmal function; however, the regulation of PD at different cell interfaces is largely unknown. This study discovered that two PD-located proteins, PDLP5 and PDLP6, are expressed in non-overlapping cell types. The constitutive expression of PDLP5 and PDLP6 results in the overaccumulation of PD callose at different cell interfaces and starch hyperaccumulation in different cell types within mature leaves. Using a proximity labeling approach, we identified sucrose synthase 6 (SUS6) as a functional partner of PDLP6. We further demonstrated that PDLP6 physically and genetically interacts with SUS6. In addition, callose synthase 7 (CalS7) interacts with both SUS6 and PDLP6 and is required for PDLP6s function. We propose that PDLP6-SUS6-CalS7 forms a callose synthase complex in the vasculature to regulate the plasmodesmal function.