The epidermal cell layer of plants has important functions in regulating plant growth and development. We have studied the impact of an altered epidermal cytokinin metabolism on Arabidopsis shoot development. Increased epidermal cytokinin synthesis or breakdown was achieved through expression of the cytokinin synthesis gene LOG4 and the cytokinin-degrading CKX1 gene, respectively, under the control of the epidermis-specific AtML1 promoter. During vegetative growth, increased epidermal cytokinin production caused an increased size of the shoot apical meristem and promoted earlier flowering. Leaves became larger and the shoots showed an earlier juvenile-to-adult transition. An increased cytokinin breakdown had the opposite effect on these phenotypic traits indicating that epidermal cytokinin metabolism can be a factor regulating these aspects of shoot development. The phenotypic consequences of abbreviated cytokinin signaling in the epidermis achieved through expression of the ARR1-SRDX repressor were generally milder or even absent indicating that the epidermal cytokinin acts, at least in part, cell non-autonomously. Enhanced epidermal cytokinin synthesis delayed cell differentiation during leaf development leading to an increased cell proliferation and leaf growth. Genetic analysis showed that this cytokinin activity was mediated mainly by the AHK3 receptor and the transcription factor ARR1. We also demonstrate that epidermal cytokinin promotes leaf growth in a largely cell-autonomous fashion. Increased cytokinin synthesis in the outer layer of reproductive tissues and in the placenta enhanced ovule formation by the placenta and caused the formation of larger siliques. This led to a higher number of seeds in larger pods resulting in an increased seed yield per plant. Collectively, the results provide evidence that the cytokinin metabolism in the epidermis is a relevant parameter determining vegetative and reproductive plant growth and development.

• Klíčová slova
• Arabidopsis, cytokinin, developmental transitions, epidermis, seed yield, shoot growth,
• Publikační typ
• časopisecké články MeSH

Degradation of the plant hormone cytokinin is controlled by cytokinin oxidase/dehydrogenase (CKX) enzymes. The molecular and cellular behavior of these proteins is still largely unknown. In this study, we show that CKX1 is a type II single-pass membrane protein that localizes predominantly to the endoplasmic reticulum (ER) in Arabidopsis (Arabidopsis thaliana). This indicates that this CKX isoform is a bona fide ER protein directly controlling the cytokinin, which triggers the signaling from the ER. By using various approaches, we demonstrate that CKX1 forms homodimers and homooligomers in vivo. The amino-terminal part of CKX1 was necessary and sufficient for the protein oligomerization as well as for targeting and retention in the ER. Moreover, we show that protein-protein interaction is largely facilitated by transmembrane helices and depends on a functional GxxxG-like interaction motif. Importantly, mutations rendering CKX1 monomeric interfere with its steady-state localization in the ER and cause a loss of the CKX1 biological activity by increasing its ER-associated degradation. Therefore, our study provides evidence that oligomerization is a crucial parameter regulating CKX1 biological activity and the cytokinin concentration in the ER. The work also lends strong support for the cytokinin signaling from the ER and for the functional relevance of the cytokinin pool in this compartment.

• MeSH
• Arabidopsis metabolismus   MeSH
• endoplazmatické retikulum metabolismus   MeSH
• membránové proteiny chemie   metabolismus   MeSH
• multimerizace proteinu * MeSH
• oxidoreduktasy chemie   metabolismus   MeSH
• proteinové domény MeSH
• proteiny - lokalizační signály MeSH
• proteiny huseníčku chemie   metabolismus   MeSH
• rekombinantní fúzní proteiny metabolismus   MeSH
• sekvence aminokyselin MeSH
• stabilita proteinů MeSH
• zelené fluorescenční proteiny metabolismus   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Research Support, N.I.H., Extramural MeSH
• Názvy látek
• cytokinin oxidase MeSH Prohlížeč
• membránové proteiny MeSH
• oxidoreduktasy MeSH
• proteiny - lokalizační signály MeSH
• proteiny huseníčku MeSH
• rekombinantní fúzní proteiny MeSH
• zelené fluorescenční proteiny MeSH

The phytohormone cytokinin is a regulator of numerous processes in plants. In Arabidopsis (Arabidopsis thaliana), the cytokinin signal is perceived by three membrane-located receptors named ARABIDOPSIS HISTIDINE KINASE2 (AHK2), AHK3, and AHK4/CRE1. How the signal is transmitted across the membrane is an entirely unknown process. The three receptors have been shown to operate mostly in a redundant fashion, and very few specific roles have been attributed to single receptors. Using a forward genetic approach, we isolated constitutively active gain-of-function variants of the AHK2 and AHK3 genes, named repressor of cytokinin deficiency2 (rock2) and rock3, respectively. It is hypothesized that the structural changes caused by these mutations in the sensory and adjacent transmembrane domains emulate the structural changes caused by cytokinin binding, resulting in domain motion propagating the signal across the membrane. Detailed analysis of lines carrying rock2 and rock3 alleles revealed how plants respond to locally enhanced cytokinin signaling. Early flowering time, a prolonged reproductive growth phase, and, thereby, increased seed yield suggest that cytokinin regulates various aspects of reproductive growth. In particular, it counteracts the global proliferative arrest, a correlative inhibition of maternal growth by seeds, an as yet unknown activity of the hormone.

• MeSH
• Arabidopsis anatomie a histologie   genetika   fyziologie   MeSH
• časové faktory MeSH
• cytokininy metabolismus   farmakologie   MeSH
• geneticky modifikované rostliny MeSH
• histidinkinasa genetika   MeSH
• květy genetika   fyziologie   ultrastruktura   MeSH
• meristém genetika   fyziologie   ultrastruktura   MeSH
• mikroskopie elektronová rastrovací MeSH
• mutace * MeSH
• polymerázová řetězová reakce s reverzní transkripcí MeSH
• proteiny huseníčku genetika   MeSH
• regulace genové exprese u rostlin MeSH
• regulátory růstu rostlin metabolismus   farmakologie   MeSH
• rozmnožování genetika   fyziologie   MeSH
• sekvence aminokyselin MeSH
• sekvenční homologie aminokyselin MeSH
• signální transdukce účinky léků   genetika   MeSH
• Publikační typ
• časopisecké články MeSH
• práce podpořená grantem MeSH
• Názvy látek
• AHK2 protein, Arabidopsis MeSH Prohlížeč
• AHK3 protein, Arabidopsis MeSH Prohlížeč
• cytokininy MeSH
• histidinkinasa MeSH
• proteiny huseníčku MeSH
• regulátory růstu rostlin MeSH