Role of BASIC PENTACYSTEINE transcription factors in a subset of cytokinin signaling responses
PLANT JOURNAL, 95 (3):458-473
Shanks CM, Hecker A, Cheng CY, Brand L, Collani S, Schmid M, Schaller GE, Wanke D, Harter K, Kieber JJ

Abstract
Cytokinin plays diverse roles in plant growth and development, generally acting by modulating gene transcription in target tissues. The type‐B Arabidopsis response regulators (ARR) transcription factors have emerged as primary targets of cytokinin signaling and are required for essentially all cytokinin‐mediated changes in gene expression. The diversity of cytokinin function is likely imparted by the activity of various transcription factors working with the type‐B ARRs to alter specific sets of target genes. One potential set of co‐regulators modulating the cytokinin response are the BARLEY B‐RECOMBINANT/BASIC PENTACYSTEINE (BBR/BPC) family of plant‐specific transcription factors. Here, we show that disruption of multiple BPCs results in reduced sensitivity to cytokinin. Further, the BPCs are necessary for the induction of a subset of genes in response to cytokinin. We identified direct in vivo targets of BPC6 using ChIP‐Seq and found an enrichment of promoters of genes differentially expressed in response to cytokinin. Further, a significant number of BPC6 regulated genes are also direct targets of the type‐B ARRs. Potential cis‐binding elements for a number of other transcription factors linked to cytokinin action are enriched in the BPC binding fragments, including those for the cytokinin response factors (CRFs). In addition, several BPCs interact with a subset of type‐A ARRs. Consistent with these results, a significant number of genes whose expression is altered in bpc mutant roots are also mis‐expressed in crf1,3,5,6 and type‐A arr3,4,5,6,7,8,9,15 mutant roots. These results suggest that the BPCs are part of a complex network of transcription factors that are involved in the response to cytokinin.

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