| Popis: |
Cytokinin is a phytohormone that regulates numerous processes in plant growth and development, including cell division, meristem maintenance, sink/source relationships, nutrient up-take, vascular development, and biotic and abiotic stress responses. The cytokinin signal is relayed through a two-component signaling system and ultimately leads to changes in gene expression. The type-A Arabidopsis response regulators (ARRs) are transcriptionally up-regulated in response to cytokinin and are stabilized by phosphorylation of their receiver domain. The ten type-A ARRs act as redundant negative regulators of cytokinin signaling and participate in a negative feedback loop to reduce cytokinin responsiveness. Previous studies have suggested that the type-A ARRs interact with other target proteins to negatively regulate the pathway, however, the mechanism has remained unclear. Here we explore how the type-A ARRs regulate cytokinin signaling. In this study, the type-A ARRs are implicated in multiple plant processes, including nematode infection, transcription factor regulation, and interaction with the exocyst complex. For example, we find defense response genes are basally up-regulated in the type-A arr3,4,5,6,7,8,9,15 loss-of-function mutant, and upon nematode infection these genes are hyper-induced, which leads to decreased pathogen success. To further examine type-A ARR function, we conducted a yeast two-hybrid screen for type-A ARR binding partners and found that the type-A ARRs interact with a member of the BASIC PENTACYSTEINE (BPC6) transcription factor family and a subunit of the exocyst complex, Exo70D3. Our research suggests that the BPC proteins are part of a network of transcription factors that regulates cytokinin response genes, and the type-A ARRs interact with BPC proteins to modify their activity. Furthermore, we find that the Exo70D proteins are positive regulators of cytokinin signaling and our data suggest that the Exo70D proteins regulate type-A ARR protein levels. Overall, we provide some mechanistic insight into the multiple roles of the type-A ARRs and how they regulate cytokinin responsiveness. |
