Fig. 1. Distribution of cytokinin biosynthesis (IPT and LOG) and degradation (CKX) genes in living organisms, showing that CKX genes are uniquely restricted to land plants, Excavata and a small number of bacteria.

Cytokinin biosynthesis and degradation genes and their homologs (IPTs, green; LOGs, blue; and CKXs, red circles) are present in all (100%, solid circles), a high percentage (10–90%, multiple-slashes filled circles), a low percentage (<3%, one-slash filled circles), or none (open circles) of the lineages within each taxon. Fifty complete genomes from representative lineages of bacteria, archaea, and eukaryotes (see Table S2) were sampled to estimate the percentages of the presence of IPT and LOG genes. Branch length is not proportional to evolutionary time.