Tetrapyrrole biosynthetic intermediates act as chloroplast-to-nucleus retrograde signals to regulate intercellular trafficking via plasmodesmata

Intercellular communication via plasmodesmata (PD) is essential for plant growth, development, and defense, yet its regulation remains poorly understood. We set out to identify the chloroplast retrograde signals that regulate intercellular trafficking via PD. Using a combination of Arabidopsis thaliana mutants and gene silencing in Nicotiana benthamiana, we found that the metabolites of the tetrapyrrole biosynthetic pathway, most likely heme, can act to modulate PD-mediated intercellular trafficking. Knockout or knockdown of GUN5, encoding the catalytic subunit of the magnesium chelatase, revealed a correlation between reduced levels of total heme and severely decreased intercellular trafficking. Further, plants with reduced expression of genes involved in chloroplast RNA processing were shown to have decreased total heme levels and decreased intercellular trafficking. Like with GUN5, reduced expression of FC2, but not FC1, led to reduced levels of total heme and drastically reduced intercellular trafficking. Thus, the results support a model where a specific pool of heme regulates plasmodesmata and intercellular trafficking. We also identified genes that are potentially regulated by the heme signal to modify plasmodesmal function. Together, these findings strengthen the link between chloroplasts and PD in coordinating intercellular communication for optimal plant development and resource allocation.