Disruption of a DNA Topoisomerase I Gene Affects Morphogenesis in Arabidopsis

The genesis of phyllotaxis, which often is associated with the Fibonacci series of numbers, is an old unsolved puzzle in plant morphogenesis. Here, we show that disruption of an Arabidopsis topoisomerase (topo) I gene named TOP1α affects phyllotaxis and plant architecture. The divergence angles and internode lengths between two successive flowers were more random in the top1α mutant than in the wild type. The top1α plants sporadically produced multiple flowers from one node, and the number of floral organ primordia often was different. The mutation also caused the twisting of inflorescences and individual flowers and the serration of leaf margins. These morphological abnormalities indicate that TOP1α may play a critical role in the maintenance of a regular pattern of organ initiation. The top1α mutant transformed with the RNA interference construct for TOP1β, another topo I gene arrayed tandemly with TOP1α, was found to be lethal at young seedling stages, suggesting that topo I activity is essential in plants

OBE3 and WUS interaction in shoot meristem stem cell regulation

The stem cells in the shoot apical meristem (SAM) are the origin of all above ground tissues in plants. In Arabidopsis thaliana, shoot meristem stem cells are maintained by the homeobox transcription factor gene WUS (WUSCHEL) that is expressed in cells of the organizing center underneath the stem cells. In order to identify factors that operate together with WUS in stem cell maintenance, we performed an EMS mutant screen for modifiers of the hypomorphic wus-6allele. We isolated the oberon3-2 (obe3-2)mutant that enhances stem cell defects in wus-6, but does not affect the putative null allele wus-1.TheOBE3 gene encodes a PHD (Plant Homeo Domain) protein that is thought to function in chromatin regulation. Single mutants of OBE3 or its closest homolog OBE4 do not display any defects, whereas the obe3-2 obe4-2 double mutant displays broad growth defects and developmental arrest of seedlings. Transcript levels of WUS and its target gene in the stem cells, CLAVATA3, are reduced in obe3-2. On the other hand, OBE3 and OBE4 transcripts are both indirectly upregulated by ectopic WUS expression. Our results suggest a positive feedback regulation between WUS and OBE3 that contributes to shoot meristem homeostasis

<i>OBE3</i> gene structure and mutant phenotypes.

<p>(A) Structure of the <i>OBE3</i> gene. The upstream region used for the complementation is shown in green. (B-D) Phenotypes of the denoted genotypes of 10-day-old seedlings (B), shoots (C), and flowers (D). Scale bars: 1 mm (B, D), 2 cm (C).</p

Changes of transcripts in <i>obe3-2</i> and <i>obe4-2</i>.

<p>(A) Transcript levels of 7-day-old seedlings as indicated. Error bars represent SE. (B) After induction of <i>OBE3</i> overexpression, mRNA levels of <i>WUS</i> are increased, whereas mRNA levels of <i>CLV3</i>, <i>STM</i>, and <i>ARR7</i> are unchanged in 7-day-old seedling. Error bars represent SD. (C) <i>pWUS</i>:<i>GUS</i> expression in 6-day-old <i>obe3-1 obe4-1/+</i> seedlings is confined to the OC as in the wild type. (D) <i>WUS</i> mRNA is undetectable by <i>in situ</i> hybridization in <i>obe3-1 obe4-1/+</i> floral meristems of 30-day-old plants. (E) <i>WUS</i> overexpression upregulates <i>OBE3</i> and <i>OBE4</i> mRNA levels in 7-day-old seedlings. <i>ARR7</i> expression is used as a control. Error bars represent SD. Relative mRNA levels compared to the mock control are shown.*,p<0.05, calculated from Cp’ values; ***, p<0.001, calculated from Cp’ values.</p