Organelle-nucleus cross-talk regulates plant intercellular communication via plasmodesmata.

We use Arabidopsis thaliana embryogenesis as a model system for studying intercellular transport via plasmodesmata (PD). A forward genetic screen for altered PD transport identified increased size exclusion limit (ise) 1 and ise2 mutants with increased intercellular transport of fluorescent 10-kDa tracers. Both ise1 and ise2 exhibit increased formation of twinned and branched PD. ISE1 encodes a mitochondrial DEAD-box RNA helicase, whereas ISE2 encodes a DEVH-type RNA helicase. Here, we show that ISE2 foci are localized to the chloroplast stroma. Surprisingly, plastid development is defective in both ise1 and ise2 mutant embryos. In an effort to understand how RNA helicases that localize to different organelles have similar impacts on plastid and PD development/function, we performed whole-genome expression analyses. The most significantly affected class of transcripts in both mutants encode products that target to and enable plastid function. These results reinforce the importance of plastid-mitochondria-nucleus cross-talk, add PD as a critical player in the plant cell communication network, and thereby illuminate a previously undescribed signaling pathway dubbed organelle-nucleus-plasmodesmata signaling. Several genes with roles in cell wall synthesis and modification are also differentially expressed in both mutants, providing new targets for investigating PD development and function.