Multicellularity is central to the stunning diversity of biological forms on earth today. In multicellular species, individual cells become dependent on each other, differentiate to specialize their functions, and may even undergo cell death as the whole organism develops. This developmental process requires intense co-ordination of genetic programs and physiology across the organism, relying on communication between cells. There are only a handful of lineages of obligate multicellular eukaryotes – animals, a few groups of fungi, certain algal lineages, and land plants – but each arose independently, and each employs a distinct mechanism of intercellular communication1. Direct physical cell–cell communication between animal cells occurs via gap junctions, which transport only very small molecules, and via tunnelling nanotubes, which permit exchange of larger molecules. Fungal cells never fully separate after cell division, in a sense, because they leave behind septal pores that connect adjacent cytoplasts. In plants, intercellular communication is primarily facilitated by plasmodesmata (PD).

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