A Link among DNA Replication, Recombination, and Gene Expression Revealed by Genetic and Genomic Analysis of TEBICHI Gene of Arabidopsis thaliana

Spatio-temporal regulation of gene expression during development depends on many factors. Mutations in Arabidopsis thaliana TEBICHI (TEB) gene encoding putative helicase and DNA polymerase domains-containing protein result in defects in meristem maintenance and correct organ formation, as well as constitutive DNA damage response and a defect in cell cycle progression; but the molecular link between these phenotypes of teb mutants is unknown. Here, we show that mutations in the DNA replication checkpoint pathway gene, ATR, but not in ATM gene, enhance developmental phenotypes of teb mutants, although atr suppresses cell cycle defect of teb mutants. Developmental phenotypes of teb mutants are also enhanced by mutations in RAD51D and XRCC2 gene, which are involved in homologous recombination. teb and teb atr double mutants exhibit defects in adaxial-abaxial polarity of leaves, which is caused in part by the upregulation of ETTIN (ETT)/AUXIN RESPONSIVE FACTOR 3 (ARF3) and ARF4 genes. The Helitron transposon in the upstream of ETT/ARF3 gene is likely to be involved in the upregulation of ETT/ARF3 in teb. Microarray analysis indicated that teb and teb atr causes preferential upregulation of genes nearby the Helitron transposons. Furthermore, interestingly, duplicated genes, especially tandemly arrayed homologous genes, are highly upregulated in teb or teb atr. We conclude that TEB is required for normal progression of DNA replication and for correct expression of genes during development. Interplay between these two functions and possible mechanism leading to altered expression of specific genes will be discussed

MYB30 regulates root cell elongation under abscisic acid signaling

Reactive oxygen species (ROS) and plant hormones play important roles in regulating plant growth and stress responses as signaling molecules. Abscisic acid (ABA) is known as the key regulator of both abiotic and biotic stress responses. During stress responses, ABA is known to regulate ROS production, indicating that important crosstalk occurs between ROS and ABA signaling. We recently reported that MYB30, an MYB-type transcription factor, regulates root cell elongation under ROS signaling. In this study, we analyzed the molecular interaction between ROS and ABA signal during for root development, which is mediated through MYB30 transcriptional regulation. We showed that MYB30-regulated root cell elongation was mediated by ROS production under ABA signaling. Our findings will provide one piece of evidence of the complex cross talk between ROS and hormone signaling that regulates root development

Arabidopsis TEBICHI, with Helicase and DNA Polymerase Domains, Is Required for Regulated Cell Division and Differentiation in Meristems

In plant meristems, each cell divides and differentiates in a spatially and temporally regulated manner, and continuous organogenesis occurs using cells derived from the meristem. We report the identification of the Arabidopsis thaliana TEBICHI (TEB) gene, which is required for regulated cell division and differentiation in meristems. The teb mutants show morphological defects, such as short roots, serrated leaves, and fasciation, as well as defective patterns of cell division and differentiation in the meristem. The TEB gene encodes a homolog of Drosophila MUS308 and mammalian DNA polymerase θ, which prevent spontaneous or DNA damage–induced production of DNA double strand breaks. As expected from the function of animal homologs, teb mutants show constitutively activated DNA damage responses. Unlike other fasciation mutants with activated DNA damage responses, however, teb mutants do not activate transcriptionally silenced genes. teb shows an accumulation of cells expressing cyclinB1;1:GUS in meristems, suggesting that constitutively activated DNA damage responses in teb lead to a defect in G2/M cell cycle progression. Furthermore, other fasciation mutants, such as fasciata2 and tonsoku/mgoun3/brushy1, also show an accumulation of cells expressing cyclinB1;1:GUS in meristems. These results suggest that cell cycle progression at G2/M is important for the regulation of the pattern of cell division and of differentiation during plant development

COBRA encodes a putative GPI-anchored protein, which is polarly localized and necessary for oriented cell expansion in Arabidopsis

To control organ shape, plant cells expand differentially. The organization of the cellulose microfibrils in the cell wall is a key determinant of differential expansion. Mutations in the COBRA (COB) gene of Arabidopsis, known to affect the orientation of cell expansion in the root, are reported here to reduce the amount of crystalline cellulose in cell walls in the root growth zone. The COB gene, identified by map-based cloning, contains a sequence motif found in proteins that are anchored to the extracellular surface of the plasma membrane through a glycosylphosphatidylinositol (GPI) linkage. In animal cells, this lipid linkage is known to confer polar localization to proteins. The COB protein was detected predominately on the longitudinal sides of root cells in the zone of rapid elongation. Moreover, COB RNA levels are dramatically upregulated in cells entering the zone of rapid elongation. Based on these results, models are proposed for the role of COB as a regulator of oriented cell expansion