The role ofcis-zeatin-type cytokinins in plant growth regulation and mediating responses to environmental interactions

Abstract

Cytokinins (CKs) are well-established as important phytohormonal regulators of plant growth and development. An increasing number of studies have also revealed the function of these hormones in plant responses to biotic and abiotic stresses. While the function of certain CK classes, including trans-zeatin and isopentenyladenine-type CKs, have been studied in detail, the role of cis-zeatin-type CKs (cZs) in plant development and in mediating environmental interactions is less well defined. Here we provide a comprehensive summary of the current knowledge about abundance, metabolism and activities of cZs in plants. We outline the history of their analysis and the metabolic routes comprising cZ biosynthesis and degradation. Further we provide an overview of changes in the pools of cZs during plant development and environmental interactions. We summarize studies that investigate the role of cZs in regulating plant development and defence responses to pathogen and herbivore attack and highlight their potential role as 'novel' stress-response markers. Since the functional roles of cZs remain largely based on correlative data and genetic manipulations of their biosynthesis, inactivation and degradation are few, we suggest experimental approaches using transgenic plants altered in cZ levels to further uncover their roles in plant growth and environmental interactions and their potential for crop improvement.

Dynamics and origin of cytokinins involved in plant manipulation by a leaf‐mining insect

“…Very little data are available on c Z‐type CKs and experimental proof of their possible functions in plant–biotic interactions remains lacking. In addition to their potential role in abiotic stress responses and pathogen resistance, c Z‐type CKs presumably also play a role in defence metabolite accumulations after herbivore attack (Schäffer et al ., ). However, the ability to produce c Z‐type CKs has been identified in several pathogens and very high levels have been found in larvae of the galling aphid Pachypsylla celtidis (e.g., Pertry et al ., ; Straka et al ., ) suggesting that they might be also involved in bacteria‐ or insect‐induced gall formation (Schäffer et al ., ).…”

“…In addition to their potential role in abiotic stress responses and pathogen resistance, c Z‐type CKs presumably also play a role in defence metabolite accumulations after herbivore attack (Schäffer et al ., ). However, the ability to produce c Z‐type CKs has been identified in several pathogens and very high levels have been found in larvae of the galling aphid Pachypsylla celtidis (e.g., Pertry et al ., ; Straka et al ., ) suggesting that they might be also involved in bacteria‐ or insect‐induced gall formation (Schäffer et al ., ). Contrasting with the hypothesis that c Z‐type CKs could be key factors in the strategy developed by plant‐manipulating insects to control their host–plant (Kaiser et al ., ; Schäfer et al ., ), P. blancardella larvae completely lack these types of compounds both on green and yellow leaves.…”

“…However, the ability to produce c Z‐type CKs has been identified in several pathogens and very high levels have been found in larvae of the galling aphid Pachypsylla celtidis (e.g., Pertry et al ., ; Straka et al ., ) suggesting that they might be also involved in bacteria‐ or insect‐induced gall formation (Schäffer et al ., ). Contrasting with the hypothesis that c Z‐type CKs could be key factors in the strategy developed by plant‐manipulating insects to control their host–plant (Kaiser et al ., ; Schäfer et al ., ), P. blancardella larvae completely lack these types of compounds both on green and yellow leaves. This result, combined with the strong decrease of c ZR in mined areas, call for a deeper experimental investigation of the functional role played by c Z‐type CKs in mediating plant‐insect interactions.…”

“…Our extensive CK profiling includes cis ‐type CKs and 2‐Methylthio (2‐MeS)‐derivatives. Cis ‐type CKs are a group of CKs that have often been ignored compared to trans ‐type CKs mainly due to the lack of appropriate analytical methods (Schäffer et al ., ). The ability to produce cis ‐CKs has been identified in plant species but also in several pathogens including bacteria (Scarbrough et al ., ) and fungi (Strzelczyk et al ., ).…”

Tissue‐specific hormone profiles from woody poplar roots under bending stress

“…In detail, the conjugation occurs with a sugar moiety, mostly glucose, at the O ‐ and N ‐position (Šmehilová et al ) through the activity of CKs oxidase/dehydrogenases (CKXs) or uridine diphosphate glycosyltransferases (UGTs). While the O ‐glucosylation is reversible and generates CKs storage products, the N ‐glucosylation represents an irreversible inactivation of CKs (Kieber and Schaller , Schäfer et al ). Similarly, local active ABA levels can be regulated through a balance between its biosynthesis and inactivation by hydroxylation or compartmentalization (Finkelstein ).…”