Mutations in MYB3R1 and MYB3R4 Cause Pleiotropic Developmental Defects and Preferential Down-Regulation of Multiple G2/M-Specific Genes in Arabidopsis    

Abstract

R1R2R3-Myb proteins represent an evolutionarily conserved class of Myb family proteins important for cell cycle regulation and differentiation in eukaryotic cells. In plants, this class of Myb proteins are believed to regulate the transcription of G2/M phase-specific genes by binding to common cis-elements, called mitosis-specific activator (MSA) elements. In Arabidopsis (Arabidopsis thaliana), MYB3R1 and MYB3R4 act as transcriptional activators and positively regulate cytokinesis by activating the transcription of KNOLLE, which encodes a cytokinesis-specific syntaxin. Here, we show that the double mutation myb3r1 myb3r4 causes pleiotropic developmental defects, some of which are due to deficiency of KNOLLE whereas other are not, suggesting that multiple target genes are involved. Consistently, microarray analysis of the double mutant revealed altered expression of many genes, among which G2/M-specific genes showed significant overrepresentation of the MSA motif and a strong tendency to be down-regulated by the double mutation. Our results demonstrate, on a genome-wide level, the importance of the MYB3R-MSA pathway for regulating G2/M-specific transcription. In addition, MYB3R1 and MYB3R4 may have diverse roles during plant development by regulating G2/M-specific genes with various functions as well as genes possibly unrelated to the cell cycle.

Genetic interaction between G2/M phase-specific transcription factor MYB3R4 and MAPKKK ANP3 for execution of cytokinesis inArabidopsis thaliana

“…Leaf samples were used for ploidy analysis, as described previously. 11 Higher levels of DNA ploidy correspond to nuclei that underwent endoreplication during the leaf development, as typically observed in this analysis. The gANP3 construct was introduced into the EDC plants, and both hemizygous and heterozygous plants in the T2 generation were used for phenotypic analysis.…”

“…The transcript levels of these genes were reduced significantly upon myb3r4 mutation, as reported previously. 10,11 However, an additional anp3 mutation did not further decrease their transcript levels (Fig. 5A), suggesting that the phenotypic effect of the anp3 mutation is not due to decreased MYB3R1/4 activity.…”

“…1B). 11 To compare quantitatively the cytokinesis defects in myb3r4 and EDC plants, we determined the proportion of single-celled stomata in silique valves, and showed that this value was much higher in EDC plants than in myb3r4 plants, despite the fact that these plants have identical genotypes at the MYB3R1, MYB3R4, and KN loci (Fig. 1C).…”

“…The two R1R2R3-type Myb transcription factors, MYB3R1 and MYB3R4 (hereafter MYB3R1/ 4), act redundantly and bind to the cis-regulatory element called mitosis-specific activator (MSA), for transcriptional activation. 10,11 Through binding to the MSA elements, MYB3R1/4 activates a variety of genes that are expressed specifically during G2 and M phases in the cell cycle. 10,11 Loss of these Myb proteins by mutation results in the frequent occurrence of incomplete cytokinesis, mainly because of the reduced expression of the critical target gene, KNOLLE (KN), which encodes a syntaxinlike protein that has an essential function for vesicle fusion at the site of cell-plate formation.…”

“…10,11 Through binding to the MSA elements, MYB3R1/4 activates a variety of genes that are expressed specifically during G2 and M phases in the cell cycle. 10,11 Loss of these Myb proteins by mutation results in the frequent occurrence of incomplete cytokinesis, mainly because of the reduced expression of the critical target gene, KNOLLE (KN), which encodes a syntaxinlike protein that has an essential function for vesicle fusion at the site of cell-plate formation. 12 Although NACK1, a tobacco homolog of AtNACK1/HIK, is known as a downstream target of an R1R2R3-type Myb in tobacco, 13,14 it remains unclear if there is a functional interconnection between MYB3R-mediated transcriptional regulation and the NACK-PQR pathway for the promotion of cytokinesis.…”