Abstract
It has been known for quite some time that cytokinins, hormones typical of plants, are also produced and metabolized in bacteria. Most bacteria can only form the tRNA-bound cytokinins, but there are examples of plant-associated bacteria, both pathogenic and beneficial, that actively synthesize cytokinins to interact with their host. Similar to plants, bacteria produce diverse cytokinin metabolites, employing corresponding metabolic pathways. The identification of genes encoding the enzymes involved in cytokinin biosynthesis and metabolism facilitated their detailed characterization based on both classical enzyme assays and structural approaches. This review summarizes the present knowledge on key enzymes involved in cytokinin biosynthesis, modifications, and degradation in bacteria, and discusses their catalytic properties in relation to the presence of specific amino acid residues and protein structure.
Highlights
Cytokinins (CKs) are a group of plant hormones that, together with other plant hormones, promote cytokinesis and affect many other aspects of plant development [1].Despite the classification as plant hormones, CKs are evolutionally older than plants, constituting a class of highly conserved low-molecular-mass molecules present in many organisms, including bacteria, amoebae, filamentous fungi, algae, nematodes, insects, and humans
ZmCKX1 and many other plant CKXs, uses oxygen rather efficiently, the activity is higher in the dehydrogenase mode [22]. It has been known for several decades that bacteria can synthesize CKs [91], but most of them only in a form bound to tRNA
CKs are produced by plant-associated bacteria, both pathogenic and beneficial [92]
Summary
Cytokinins (CKs) are a group of plant hormones that, together with other plant hormones, promote cytokinesis and affect many other aspects of plant development [1]. The spectrum of CK molecules further extends to various conjugates with sugars, sugar phosphates, and amino acids (such as ribosides, ribonucleotides, N-glucosides, O-glucosides, O-xylosides, lupinic acid, or discadenine), or other compounds substituted either at the C2 position of adenine ring (2-methylthiolated CKs) or on the isopentenyl side-chain (monoand dimethylated iP) [3,4] These structural variations affect the biological activity of CKs and determine the function and compartmentalization of the respective CK metabolites [2]. In addition to the LOG enzyme, enzymes of common purine metabolism, such as phosphatases, nucleotidases, nucleosidases, kinases, and phosporibosyl transferases, seem to be involved in a step-wise conversion of CK nucleotides to ribosides and free bases, as well as in the conversions in the opposite direction, as many of the enzymes catalyze reverse reactions. This review will focus on the structural and biochemical characterization of key proteins of CK metabolism present in bacteria
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