Genetic identification of a second site modifier of ctr1-1 that controls ethylene-responsive and gravitropic root growth in Arabidopsis thaliana

ORESARA 15, a PLATZ transcription factor, controls root meristem size through auxin and cytokinin signalling-related pathways

An optimal size of post-embryonic root apical meristem (RAM) is achieved by a balance between cell division and differentiation. Despite extensive research, molecular mechanisms underlying the coordination of cell division and differentiation are still fragmentary. Here, we report that ORESARA 15 (ORE15), an Arabidopsis PLANT A/T-RICH SEQUENCE-AND ZINC-BINDING PROTEIN (PLATZ) transcription factor preferentially expressed in the RAM, determines RAM size. Primary root length, RAM size, cell division rate, and stem cell niche activity were reduced in an ore15 loss-of-function mutant but enhanced in an activation-tagged line overexpressing ORE15, compared with wild type. ORE15 forms mutually positive and negative feedback loops with auxin and cytokinin signalling, respectively. Collectively, our findings imply that ORE15 controls RAM size by mediating the antagonistic interaction between auxin and cytokinin signalling-related pathways.

Temporal changes in cell division rate and genotoxic stress tolerance in quiescent center cells of Arabidopsis primary root apical meristem

Plant roots provide structural support and absorb nutrients and water; therefore, their proper development and function are critical for plant survival. Extensive studies on the early stage of ontogenesis of the primary root have revealed that the root apical meristem (RAM) undergoes dynamic structural and organizational changes during early germination. Quiescent center (QC) cells, a group of slowly dividing cells at the center of the stem-cell niche, are vital for proper function and maintenance of the RAM. However, temporal aspects of molecular and cellular changes in QC cells and their regulatory mechanisms have not been well studied. In the present study, we investigated temporal changes in QC cell size, expression of QC cell-specific markers (WOX5 and QC25), and genotoxic tolerance and division rate of QC cells in the Arabidopsis primary root. Our data revealed the decreased size of QC cells and the decreased expression of the QC cell-specific markers with root age. We also found that QC cell division frequency increased with root age. Furthermore, our study provides evidence supporting the link between the transition of QC cells from a mitotically quiescent state to the frequently dividing state and the decrease in tolerance to genotoxic stress.

Temporal changes in cell division rate and genotoxic stress tolerance in quiescent center cells of Arabidopsis primary root apical meristem

Plant roots provide structural support and absorb nutrients and water; therefore, their proper development and function are critical for plant survival. Extensive studies on the early stage of ontogenesis of the primary root have revealed that the root apical meristem (RAM) undergoes dynamic structural and organizational changes during early germination. Quiescent center (QC) cells, a group of slowly dividing cells at the center of the stem-cell niche, are vital for proper function and maintenance of the RAM. However, temporal aspects of molecular and cellular changes in QC cells and their regulatory mechanisms have not been well studied. In the present study, we investigated temporal changes in QC cell size, expression of QC cell-specific markers (WOX5 and QC25), and genotoxic tolerance and division rate of QC cells in the Arabidopsis primary root. Our data revealed the decreased size of QC cells and the decreased expression of the QC cell-specific markers with root age. We also found that QC cell division frequency increased with root age. Furthermore, our study provides evidence supporting the link between the transition of QC cells from a mitotically quiescent state to the frequently dividing state and the decrease in tolerance to genotoxic stress.

The Triple Response Assay and Its Use to Characterize Ethylene Mutants in Arabidopsis

Exposure of plants to ethylene results in drastic morphological changes. Seedlings germinated in the dark in the presence of saturating concentrations of ethylene display a characteristic phenotype known as the triple response. This phenotype is robust and easy to score. In Arabidopsis the triple response is usually evaluated at 3 days post germination in seedlings grown in the dark in rich media supplemented with 10 μM of the ethylene precursor ACC in air or in unsupplemented media in the presence of 10 ppm ethylene. The triple response in Arabidopsis consists of shortening and thickening of hypocotyls and roots and exaggeration of the curvature of apical hooks. The search for Arabidopsis mutants that fail to show this phenotype in ethylene or, vice versa, display the triple response in the absence of exogenously supplied hormone has allowed the identification of the key components of the ethylene biosynthesis and signaling pathways. Herein, we describe a simple protocol for assaying the triple response in Arabidopsis. The method can also be employed in many other dicot species, with minor modifications to account for species-specific differences in germination. We also compiled a comprehensive table of ethylene-related mutants of Arabidopsis, including many lines with auxin-related defects, as wild-type levels of auxin biosynthesis, transport, signaling, and response are necessary for the normal response of plants to ethylene.

Genome Sequencing of Arabidopsis abp1-5 Reveals Second-Site Mutations That May Affect Phenotypes

Auxin regulates numerous aspects of plant growth and development. For many years, investigating roles for AUXIN BINDING PROTEIN1 (ABP1) in auxin response was impeded by the reported embryo lethality of mutants defective in ABP1. However, identification of a viable Arabidopsis thaliana TILLING mutant defective in the ABP1 auxin binding pocket (abp1-5) allowed inroads into understanding ABP1 function. During our own studies with abp1-5, we observed growth phenotypes segregating independently of the ABP1 lesion, leading us to sequence the genome of the abp1-5 line described previously. We found that the abp1-5 line we sequenced contains over 8000 single nucleotide polymorphisms in addition to the ABP1 mutation and that at least some of these mutations may originate from the Arabidopsis Wassilewskija accession. Furthermore, a phyB null allele in the abp1-5 background is likely causative for the long hypocotyl phenotype previously attributed to disrupted ABP1 function. Our findings complicate the interpretation of abp1-5 phenotypes for which no complementation test was conducted. Our findings on abp1-5 also provide a cautionary tale illustrating the need to use multiple alleles or complementation lines when attributing roles to a gene product.

Rootin, a compound that inhibits root development through modulating PIN-mediated auxin distribution

Plant roots anchor the plant to the soil and absorb water and nutrients for growth. Understanding the molecular mechanisms regulating root development is essential for improving plant survival and agricultural productivity. Extensive molecular genetic studies have provided important information on crucial components for the root development control over the last few decades. However, it is becoming difficult to identify new regulatory components in root development due to the functional redundancy and lethality of genes involved in root development. In this study, we performed a chemical genetic screen to identify novel synthetic compounds that regulate root development in Arabidopsis seedlings. The screen yielded a root growth inhibitor designated as 'rootin', which inhibited Arabidopsis root development by modulating cell division and elongation, but did not significantly affect shoot development. Transcript analysis of phytohormone marker genes revealed that rootin preferentially altered the expression of auxin-regulated genes. Furthermore, rootin reduced the accumulation of PIN1, PIN3, and PIN7 proteins, and affected the auxin distribution in roots, which consequently may lead to the observed defects in root development. Our results suggest that rootin could be utilized to unravel the mechanisms underlying root development and to investigate dynamic changes in PIN-mediated auxin distribution.