Cytokinin Perception in Ancient Plants beyond Angiospermae

Area of Science:

• Plant biology
• Molecular evolution
• Biochemistry

Background:

• Cytokinins (CKs) are crucial plant hormones regulating development and environmental responses.
• CK signaling pathways are well-characterized, but their evolutionary history is less understood.

Purpose of the Study:

• To investigate the evolution of cytokinin perception in early-divergent plant species.
• To characterize the ligand binding properties and pH dependence of CHASE-domain containing histidine kinases (CHKs) in *Physcomitrium patens*, *Selaginella moellendorffii*, and *Picea abies*.

Main Methods:

• Biochemical assays were used to examine cytokinin binding affinity and specificity of CHK receptors.
• pH dependence of CK binding was analyzed to infer potential subcellular localization.

Main Results:

• Two of eight examined CHKs in *Physcomitrium patens* did not bind CKs.
• Other CHK receptors exhibited high-affinity CK binding (nM range).
• Moss CHKs showed a preference for isopentenyladenine, while the gymnosperm showed a preference for *trans*-zeatin over *cis*-zeatin.

Conclusions:

• The study demonstrates the properties of cytokinin perception apparatuses in early land plant lineages.
• Evolutionary diversification of CHK ligand specificity occurred, with a notable increase in preference for *trans*-zeatin over *cis*-zeatin.
• This specialization culminated in vascular plants, particularly angiosperms.