R1R2R3-Myb proteins positively regulate cytokinesis through activation of KNOLLE transcription in Arabidopsis thaliana

Abstract

*G2/M phase-specific gene transcription in tobacco cells is mediated by R1R2R3-Myb transcriptional activators, NtmybA1 and NtmybA2, which bind to mitosis-specific activator (MSA) elements. We show here that two structurally related genes, MYB3R1 and MYB3R4, which encode homologs of NtmybA1 and NtmybA2, play a partially redundant role in positively regulating cytokinesis in Arabidopsis thaliana. The myb3r1 myb3r4 double mutant often fails to complete cytokinesis, resulting in multinucleate cells with gapped walls and cell wall stubs in diverse tissues. These defects correlate with the selective reduction of transcript levels of several G2/M phase-specific genes, which include B2-type cyclin (CYCB2), CDC20.1 and KNOLLE (KN). These genes contain MSA-like motifs in their promoters and were activated by MYB3R4 in transient expression assays in tobacco cells. The KN gene encodes a cytokinesisspecific syntaxin that is essential for cell plate formation. The cytokinesis defects of myb3r1 myb3r4 double mutants were partially rescued by KN gene expression from heterologous promoters. In addition, a kn heterozygous mutation enhanced cytokinesis defects resulting from heterozygous or homozygous mutations in the MYB3R1 and MYB3R4 genes. Our results suggest that a pair of structurally related R1R2R3-Myb transcription factors may positively regulate cytokinesis mainly through transcriptional activation of the KN gene.

Laser microdissection of Arabidopsis cells at the powdery mildew infection site reveals site-specific processes and regulators

“…Like mammalian c-MYB proteins, MYB3R4 regulates the cell cycle. It is a known transcriptional activator of cytokinesis that binds the G 2 /M-phase-specific MSA element (26). Orthologous MYBs, such as tobacco NtMYBA2 (30), contain a C-terminal domain that allows for posttranslational control of NtMYBA2 activity.…”

“…For NtMYBA2, full activation requires phosphorylation of the C-terminal domain of the protein by specific cyclin (CYC)/ cyclin-dependent kinase (CDK) complexes (30). This also appears to be the case for MYB3R4 because it contains this C-terminal domain with multiple CDK consensus phosphorylation sites, and coexpression of CYCB1 with MYB3R4 enhanced its transactivation of specific target genes (26).…”

“…These genes exclusively exhibited decreased expression at the PM infection site and include the known MYB3R4 target CYCB1:2 (26). MYB3R4 activator vs. repressor function is likely controlled by localized concentrations of specific CYCs (e.g., CYCBs) and CDKs (CDKA or CDKB) required for the C-terminal phosphorylation and mitosis-activating function of MYB3R4 (26,30). Therefore, localized regulated proteolysis and/or reduced expression of these CDKs and/or CYCs could modulate MYB3R4 function.…”

A new role for plant R2R3-MYB transcription factors in cell cycle regulation

“…Conversely the corresponding tobacco MSA element in the CYCB1;1 promoter is bound by the R1R2R3-MYB NtmybA1 and NtmybA2 factors that act as positive regulators and by NtmybB working as a repressor [3]. AtMYB59 might control the expression of CYCB1;1 and maybe other G2/M phase-specific genes, through its binding to the elements identified by Mu et al in a cooperative way with one of the R1R2R3-MYB proteins able to bind to the MSA element [4]. Another possibility is that AtMYB59 acts in a partially redundant fasion with another R2R3-MYB not yet characterized, or with the putative At2g03470 MYB transcription factor, identified as a member of a complex binding to promoter elements of the CYCB1;1 gene, in combination with transcription factors belonging to other families, such as the HYP protein, which contains a leucine zipper and MYC-type dimerization domains [10].…”

“…From an analysis of the complete Arabidopsis genome sequence 198 genes in the MYB superfamily were identified, among them 126 are R2R3-MYB, 5 are R1R2R3-MYB, 64 are MYB-related and 3 are atypical MYB genes [1]. Plant R1R2R3-MYB proteins, whose repeats are more closely related to the repeats present in vertebrate MYB proteins than to the repeats from plant R2R3-MYB proteins, play a role in the regulation of genes involved in the cell cycle G2/M phase, as shown in tobacco and Arabidopsis [3,4]. They bind to mitosis-specific activator (MSA) elements, present in most promoter sequences of genes specifically expressed in the G2/M phase in Arabidopsis [5].…”