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Fig. 6. Ethylene and cytokinin signalling pathways. (A) At low
concentrations of the ligand, ethylene receptors are active and can therefore
stimulate the negative regulator CTR1, which in turn shuts down ethylene
signalling by allowing EIN3 degradation. (B) Ethylene binding
inactivates the receptors and therefore they are unable to stimulate
CTR1-mediated inhibition. As a result, EIN2 is active and prevents EIN3
degradation, which leads to EIN3 accumulation and activation of ethylene
responsive gene transcription. (C) Cytokinin binding initiates
autophosphorylation of the receptors, followed by transfer of the phosphoryl
group (P) to a histidine phosphotransfer protein (AHP), and further on to a
response regulator (ARR), which leads to the transcription of
cytokinin-response genes. There may be some interaction between ethylene and
cytokinin signalling through crosstalk via this phosphorelay. ER, endoplasmic
reticulum; NM, nuclear membrane; PM, plasma membrane; +ub, ubiquitination.
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