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  1. Cytokinin is an important phytohormone that employs a multistep phosphorelay to transduce the signal from receptors to the nucleus, culminating in activation of type-B response regulators which function as transcription factors. Recent chromatin immunoprecipitation-sequencing (ChIP-seq) studies have identified targets of type-B ARABIDOPSIS RESPONSE REGULATORs (ARRs) and integrated these into the cytokinin-activated transcriptional network. Primary targets of the type-B ARRs are enriched for genes involved in hormonal regulation, emphasizing the extensive crosstalk that can occur between cytokinin, auxin, abscisic acid, brassinosteroids, gibberellic acid, ethylene, jasmonic acid, and salicylic acid. Examination of hormone-related targets reveals multiple regulatory points including biosynthesis, degradation/inactivation, transport, and signal transduction. Here, we consider this early response to cytokinin in terms of the hormones involved, points of regulatory crosstalk, and physiological significance. 
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  3. The phytohormone cytokinin influences many aspects of plant growth and development, several of which also involve the cellular process of autophagy, including leaf senescence, nutrient remobilization, and developmental transitions. TheArabidopsistype-A response regulators (type-A ARR) are negative regulators of cytokinin signaling that are transcriptionally induced in response to cytokinin. Here, we describe a mechanistic link between cytokinin signaling and autophagy, demonstrating that plants modulate cytokinin sensitivity through autophagic regulation of type-A ARR proteins. Type-A ARR proteins were degraded by autophagy in an AUTOPHAGY-RELATED (ATG)5-dependent manner, and this degradation is promoted by phosphorylation on a conserved aspartate in the receiver domain of the type-A ARRs. EXO70D family members interacted with type-A ARR proteins, likely in a phosphorylation-dependent manner, and recruited them to autophagosomes via interaction of the EXO70D AIM with the core autophagy protein, ATG8. Consistently, loss-of-functionexo70D1,2,3mutants exhibited compromised targeting of type-A ARRs to autophagic vesicles, have elevated levels of type-A ARR proteins, and are hyposensitive to cytokinin. Disruption of both type-AARRsandEXO70D1,2,3compromised survival in carbon-deficient conditions, suggesting interaction between autophagy and cytokinin responsiveness in response to stress. These results indicate that the EXO70D proteins act as selective autophagy receptors to target type-A ARR cargos for autophagic degradation, demonstrating modulation of cytokinin signaling by selective autophagy.

     
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  4. Abstract

    Plants are exquisitely sensitive to the ethylene signal and also respond to a much wider range of ethylene concentrations than would seem possible based on the simple circuitry of its primary signal transduction pathway, suggesting the existence of mechanisms for amplification and adaptation to ethylene signals. Here, such regulatory systems are considered within the context of what is known about the plant ethylene signaling pathway as well as signaling by the animal G‐protein coupled receptors, and the bacterial methyl‐accepting chemotaxis proteins. Magnitude amplification and sensitivity amplification mechanisms are considered as strategies for amplification of the ethylene signal. Several families of negative feedback regulators that desensitize plants to ethylene and thereby facilitate the ethylene adaptation response of plants are described. These negative feedback regulators include the ethylene receptors themselves, the RTE1/GR family, and the ARGOS family, all of which function at the level of the ethylene receptors to desensitize plants to ethylene. These negative regulators also include the EBF family of F‐box proteins, which target the EIN3/EIL family of transcription factors for degradation. Ethylene signal amplification and adaptation employ both transcriptional and post‐transcriptional regulation.

     
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