Abstract
Isopentenyltransferase (IPT) genes, including those encoding ATP/ADP-IPTs and tRNA-IPTs, control the rate-limiting steps of the biosynthesis of N6-(Δ2-isopentenyl)adenine (iP)-type and trans-zeatin (tZ)-type cytokinins and cis-zeatin (cZ)-type cytokinins, respectively. However, the evolution and roles of these IPTs in angiosperms are not well understood. Here, we report comprehensive analyses of the origins, evolution, expression patterns, and possible roles of ATP/ADP-IPTs and tRNA-IPTs in angiosperms. We found that Class I and II tRNA-IPTs likely coexisted in the last common ancestor of eukaryotes, while ATP/ADP-IPTs likely originated from a Class II tRNA-IPT before the divergence of angiosperms. tRNA-IPTs are conservatively retained as 2–3 copies, but ATP/ADP-IPTs exhibit considerable expansion and diversification. Additionally, tRNA-IPTs are constitutively expressed throughout the plant, whereas the expression of ATP/ADP-IPTs is tissue-specific and rapidly downregulated by abiotic stresses. Furthermore, previous studies and our present study indicate that ATP/ADP-IPTs and their products, iPs/tZs, may regulate responses to environmental stresses and organ development in angiosperms. We therefore hypothesize that tRNA-IPTs and the associated cZs play a housekeeping role, whereas ATP/ADP-IPTs and the associated iP/tZ-type cytokinins play regulatory roles in organ development and stress responses in angiosperms, which echoes the conclusions and hypothesis presented in the accompanying study by Wang, X. et al Evolution and roles of cytokinin genes in angiosperms 2: Do ancient CKXs play housekeeping roles while non-ancient CKXs play regulatory roles? Hortic Reshttps://doi.org/10.1038/s41438-020-0246-z.
Highlights
Cytokinins (CKs) are a class of plant hormones that play essential roles in many aspects of plant development, including the delay of leaf senescence[1], root proliferation[2,3], apical dominance[4], shoot meristem function[5,6,7], regulation of reproductive meristem activity[8], fruit development[9], and nutritional signaling[10,11]
Putative IPPT domains were found in two archaeal species, miscellaneous Crenarchaeota group archaeon SMTZ-80 and candidate division MSBL1 archaeon SCGC-AAA382N08, indicating that IPT homologs are present in archaea
The former archaeal species containing an IPT homolog belongs to the TACK superphylum, which has recently been proposed to be the origin of eukaryotes, and the latter is a member of the unclassified euryarchaeota
Summary
Cytokinins (CKs) are a class of plant hormones that play essential roles in many aspects of plant development, including the delay of leaf senescence[1], root proliferation[2,3], apical dominance[4], shoot meristem function[5,6,7], regulation of reproductive meristem activity[8], fruit development[9], and nutritional signaling[10,11]. CKs play important and complex roles in environmental stress responses[12,13]. Recent studies have demonstrated that cZs are the predominant CKs in some plants, such as rice and maize, or in certain developmental stages associated with limited growth[16]. The presence of iP-type and tZ-type CKs can vary greatly between tissues, developmental stages, and environmental conditions[17,18,19]
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