The third alpha helix of plant homeoproteins are generally cell-penetrating to plant cells

Homeoproteins are transcription factors involved in developmental regulation. Animal homeoproteins are generally cell-penetrating to animal cells, capable of crossing membranes through a conserved 3rd alpha helix "penetration" domain. While plants also possess homeoproteins, the cell-penetrating ability of plant homeoproteins has not been investigated. We catalog the 3rd alpha helixes of plant homeoproteins across 851 species and screen a subset for the ability to penetrate walled-plant cells. We identify plant 3rd alpha helixes which are cell-penetrating to plants, the efficiency of which is amino acid sequence dependent, and use these peptides for cytosolic delivery of recombinant protein cargoes. We discover plant homeoproteins are generally cell-penetrating to plants, mirroring the behavior of animal homeoproteins, with applications in plant biotechnology and implications for fundamental plant biology.