Evolution of cytokinin biosynthesis and degradation

Abstract

Cytokinin hormones are important regulators of development and environmental responses of plants that execute their action via the molecular machinery of signal perception and transduction. The limiting step of the whole process is the availability of the hormone in suitable concentrations in the right place and at the right time to interact with the specific receptor. Hence, the hormone concentrations in individual tissues, cells, and organelles must be properly maintained by biosynthetic and metabolic enzymes. Although there are merely two active cytokinins, isopentenyladenine and its hydroxylated derivative zeatin, a variety of conjugates they may form and the number of enzymes/isozymes with varying substrate specificity involved in their biosynthesis and conversion gives the plant a variety of tools for fine tuning of the hormone level. Recent genome-wide studies revealed the existence of the respective coding genes and gene families in plants and in some bacteria. This review summarizes present knowledge on the enzymes that synthesize cytokinins, form cytokinin conjugates, and carry out irreversible elimination of the hormones, including their phylogenetic analysis and possible variations in different organisms.

Cytokinins - recent news and views of evolutionally old molecules

“…Decomposition of tRNA was thus originaly thought to be a source of free CKs. Although, this has been considered to be insufficient to establish the significant amount of CKs required by seed plants (Barnes et al 1980), multiplication of genes coding for tRNA-IPT in basal land plants (see Frébort et al 2011) seems to be the first evolutionary step to increase the source of free CKs as described later.…”

“…HGT (combined with gene duplication) played a role in the evolution of Rubisco (Delwiche and Palmer 1996), the key enzyme employed by green plants. Recent genome-wide studies identified CK-related genes coding for the key enzymes of CK biosynthesis (isopentenyltransferase, IPT) and degradation (CK oxidase-dehydrogenase, CKX), respectively, in cyanobacteria and other bacteria (Frébort et al 2011). Although the function of cyanobacterial IPT and CKX in CK metabolism has not been confirmed yet (Frébort et al 2011), CKs were identified in several cyanobacteria species such as Synechocystis, Chroococcidiopsis, Anabaena, Phormidium and Oscillatoria (Hussain et al 2010).…”

“…Recent genome-wide studies identified CK-related genes coding for the key enzymes of CK biosynthesis (isopentenyltransferase, IPT) and degradation (CK oxidase-dehydrogenase, CKX), respectively, in cyanobacteria and other bacteria (Frébort et al 2011). Although the function of cyanobacterial IPT and CKX in CK metabolism has not been confirmed yet (Frébort et al 2011), CKs were identified in several cyanobacteria species such as Synechocystis, Chroococcidiopsis, Anabaena, Phormidium and Oscillatoria (Hussain et al 2010). This evidence implies that CK metabolism in plants evolved through HGT from bacteria to plants via the chloroplast, which is of cyanobacterial origin ( Fig.…”

“…TCS is shared among bacteria, plants and origin and paths of evolution of CK biosynthesis, degradation and signalling in the context of general evolutionary events (indicated by red arrows; the relative order is unknown). Phylogenetic trees were modified from Simpson (2006) and the CK evolution compiled from Pils and Heyl (2009) and Frébort et al (2011). Stars indicate secondary endosymbiosis.…”

“…However, neither appears to be a cytokinin receptor and CKs that are essential inducers of Dictyostelium sporulation act through another pathway that is as yet unknown (Anjard and Loomis 2008). No other gene with significant homology to the CK-sensing CHASE domain has been found anywhere apart from land plants, including in algal or cyanobacterial genomes, except for the genome of the virus Ectocarpus siliculosus virus 1 (Pils and Heyl 2009;Frébort et al 2011). This virus encoding a CHASE domain-containing histidine (His) kinase integrates itself into the genome of the brown algae Ectocarpus siliculosus (which produces CKs) and thus may have acted as a possible vector (Pils and Heyl 2009).…”