Cytokinin biosynthesis in Hexapoda and Insecta: a bioinformatic analysis

Abstract

Cytokinins (CKs) are widespread in a variety of organisms from bacteria to humans, and are particularly abundant in insects and hexapods. However, how organisms other than bacteria and plants obtain CKs has not been thoroughly studied. We examined the transcriptomes of 670 species of Hexapoda (predominantly Insecta) to determine if transcripts that encode proteins homologous to any of the known enzymes involved in CK biosynthesis and metabolism are widespread in these groups (occur in > 80% of species). We found that transcripts encoding proteins homologous to the enzymes tRNA-dimethylallyltransferase (EC: 2.5.1.75) and tRNA-2-methylthio-N6-dimethylallyladenosine synthase (EC: 2.8.4.3) are widespread in insects and hexapods. These enzymes could allow insects and hexapods to synthesize iP-based CKs and methylthiolated iP-based CKs via a tRNA-degradation pathway whereby tRNA is first prenylated and possibly methylthiolated prior to releasing CKs or methylthiolated CKs upon degradation. We also found widespread occurrence in insects and hexapods of transcripts encoding proteins that are homologous to five enzymes in the adenine salvage pathway: 5’- nucleotidase (EC: 3.1.3.5), adenosine kinase (EC:2.7.1.20), purine-nucleoside phosphorylase (EC: 2.4.2.1), purine nucleosidase (EC: 3.2.2.1), and adenine phosphoribosyltransferase (EC: 2.4.2.7). These enzymes could allow insects and hexapods to convert CK nucleotides to nucleosides and free base CKs. We found few transcripts encoding proteins homologous to enzymes that would convert CKs to storage forms such as their O-glucosides and no transcripts encoding proteins homologous to enzymes that would degrade CKs such as CK oxidases. We suggest that insects and hexapods have the enzymatic pathways necessary to synthesize and metabolize CKs, in contrast to the presumption that CKs are merely obtained via consumption and sequestration from plants or via microbial symbiosis.