The Plant Journal

Schommer, Carla; Beven, Ali; Lawrenson, Tom; Shaw, Peter; Sablowski, Robert

doi: 10.1046/j.1365-313X.2003.01850.xpmid: 12974806

A new Arabidopsis meiotic mutant has been isolated. Homozygous ahp2‐1 (Arabidopsis homologue pairing 2) plants were sterile because of failure of both male and female gametophyte development. Fluorescent in situ hybridisation showed that in ahp2‐1 male meiocytes, chromosomes did not form bivalents during prophase I and instead seemed to associate indiscriminately. Chromosome fragmentation, chromatin bridges and unbalanced segregation were seen in anaphase I and anaphase II. The ahp2‐1 mutation was caused by a T‐DNA insertion in an Arabidopsis homologue of meu13+, which has been implicated in homologous chromosome pairing during meiosis in Schizosaccharomyces pombe. Our results suggest that meu13+ function is conserved in higher eukaryotes and support the idea that Arabidopsis, yeast and mouse share a pairing pathway that is not present in Drosophila melanogaster and Caenorhabditis elegans.

Holmberg, Niklas; Harker, Mark; Wallace, Andrew D.; Clayton, John C.; Gibbard, Carl L.; Safford, Richard

doi: 10.1046/j.1365-313X.2003.01851.xpmid: 12974807

The enzymes 3‐hydroxy‐3‐methylglutaryl CoA reductase (HMGR) and C24‐sterol methyltransferase type 1 (SMT1) have been proposed to be key steps regulating carbon flux through the sterol biosynthesis pathway. To further examine this hypothesis, we co‐expressed the catalytic domain of Hevea brasiliensis HMGR (tHMGR) and Nicotiana tabacum SMT1 in tobacco, under control of both constitutive and seed‐specific promoters, resulting in increased accumulation of total sterol in seed tissue by 2.5‐ and 2.1‐fold, respectively. This enhancement is greater than when tHMGR and SMT1 were expressed singularly where, for example, seed‐specific expression enhanced total sterols by 1.6‐fold. Significantly, the relative level of 4‐desmethyl sterols (end‐product sterols) was higher in seed co‐expressing tHMGR and SMT1 from seed‐specific promoters (79% of total sterols) than when co‐expressed from constitutive promoters (59% of total sterols) and similar to wild‐type seed (80% of total sterols). These results demonstrate that HMGR and SMT1 work in concert to control carbon flux into end‐product sterols and that the sterol composition can be controlled by the temporal activity of the promoters driving transgene expression. In addition, constitutive expression of the transgenes resulted in elevated accumulation of substrates for C4‐demethylation reactions, which indicates that one or several enzymes catalysing such reactions limit carbon flow to end‐product sterols, at least in a physiological situation when the carbon flow is upregulated.

Martinez‐Perez, Enrique; Shaw, Peter; Aragon‐Alcaide, Luis; Moore, Graham

doi: 10.1046/j.1365-313X.2003.01853.xpmid: 12974808

Hexaploid wheat possesses 42 chromosomes derived from its three ancestral genomes. The 21 pairs of chromosomes can be further divided into seven groups of six chromosomes (one chromosome pair being derived from each of the three ancestral genomes), based on the similarity of their gene order. Previous studies have revealed that, during anther development, the chromosomes associate in 21 pairs via their centromeres. The present study reveals that, as a prelude to meiosis, these 21 chromosome pairs in hexaploid (and tetraploid) wheat associate via the centromeres into seven groups as the telomeres begin to cluster. This results in the association of multiple chromosomes, which then need to be resolved as meiosis progresses. The formation of the seven chromosome clusters now explains the occasional occurrence of remnants of multiple associations, which have been reported at later stages of meiosis in hexaploid (and tetraploid) wheat. Importantly, the chromosomes have the opportunity to be resorted via these multiple interactions. As meiosis progresses, such interactions are resolved through the action of loci such as Ph1, leaving chromosomes as homologous pairs.

Guo, Mei; Rupe, Mary A.; Danilevskaya, Olga N.; Yang, Xiaofeng; Hu, Zihua

doi: 10.1046/j.1365-313X.2003.01852.xpmid: 12974809

We have taken a genomic approach to examine global gene expression in the maize endosperm in relation to dosage and parental effects. Endosperm of eight hybrids generated by reciprocal crosses and their seven inbred parents were sampled at three developmental stages: 10, 14, and 21 days after pollination (DAP). These samples were subjected to GeneCalling, an open‐ended mRNA‐profiling technology, which simultaneously analyzes thousands of genes. Results indicated that the overall level of gene expression in the maize endosperm was dosage‐dependent, that is, the gene expression was proportional to the parental genome contribution of 2n maternal : 1n paternal. However, approximately 8% of the genes deviated from such allelic additive expression and exhibited differential expression in hybrids of reciprocal crosses, resembling either maternally or paternally expressed genes. There were more genes with maternal‐like expression (MLE) than those with paternal‐like expression (PLE). Allele‐specific expression analysis of four selected genes using the WAVE denaturing HPLC (dHPLC) system revealed several mechanisms responsible for the deviation from the allelic additive expression in the hybrid endosperm: heterochronic allelic variation, allelic variation in the level of expression, and genomic imprinting. We discovered a novel imprinted gene no‐apical‐meristem (NAM) related protein1 (nrp1) that was expressed only in the endosperm and regulated by gene‐specific imprinting. The nrp1 gene, a putative transcriptional factor, may play an important role in endosperm development.

Kamiya, Noriko; Itoh, Jun‐Ichi; Morikami, Atsushi; Nagato, Yasuo; Matsuoka, Makoto

doi: 10.1046/j.1365-313X.2003.01856.xpmid: 12974810

Asymmetric cell division is one of the most important mechanisms in the diversification of cell function and fate. In Arabidopsis, SCARECROW (SCR) is essential for the asymmetric division of the cortex/endodermis progenitor cell in the root. To learn more about how SCR is involved in asymmetric division, we analyzed the rice SCR (OsSCR) expression. In the root tip, OsSCR expression was observed in the endodermal cell layer and downregulated in the daughter cortex cell after asymmetric division, just as with Arabidopsis SCR. In leaf primordia, expression of OsSCR was observed in stomatal and ligule formation. In stomatal development, OsSCR was specifically expressed in the stomatal cell files before formation of guard mother cells (GMCs), and then, its expression was localized in GMCs, when the first asymmetric division occurred to generate the GMCs. Before the second asymmetric division of subsidiary mother cells (SMCs), localized OsSCR expression was observed in SMCs in the area close to the GMCs. Before these asymmetric divisions, the localization of OsSCR mRNA in GMC‐forming cells and SMCs was observed in the area of the daughter GMC and subsidiary cells. OsSCR expression was also observed in the initiation area of ligule formation, and its downregulation occurred in the inner L2 cells generated by asymmetric division. Based on these observations, we proposed that OsSCR is involved not only in the asymmetric division of the cortex/endodermis progenitor cell but also during stomata and ligule formation by establishing the polarization of cytoplasm.

Kurata, Tetsuya; Kawabata‐Awai, Chie; Sakuradani, Eiji; Shimizu, Sakayu; Okada, Kiyotaka; Wada, Takuji

doi: 10.1046/j.1365-313X.2003.01854.xpmid: 12974811

We have identified a new Arabidopsis mutant, yore‐yore (yre), which has small trichomes and glossy stems. Adhesion between epidermal cells was observed in the organs of the yre shoot. The cloned YRE had high homology to plant genes involved in epicuticular wax synthesis, such as ECERIFERUM1 (CER1) and maize GLOSSY1. The phenotype of transgenic plants harboring double‐stranded RNA interference (dsRNAi) YRE was quite similar to that of the yre mutant. The amount of epicuticular wax extracted from leaves and stems of yre‐1 was approximately one‐sixth of that from the wild type. YRE promoter::GUS and in situ hybridization revealed that YRE was specifically expressed in cells of the L1 layer of the shoot apical meristem and young leaves, stems, siliques, and lateral root primordia. Strong expression was detected in developing trichomes. The trichome structure of cer1 was normal, whereas that of the yre cer1 double mutant was heavily deformed, indicating that epicuticular wax is required for normal growth of trichomes. Double mutants of yre and trichome‐morphology mutants, glabra 2 (gl2) and transparent testa glabra1 (ttg1), showed that the phenotype of the trichome structure was additive, suggesting that the wax‐requiring pathway is distinct from the trichome development pathway controlled by GL2 and TTG1.

Vilaine, F.; Palauqui, J.‐C.; Amselem, J.; Kusiak, C.; Lemoine, R.; Dinant, S.

doi: 10.1046/j.1365-313X.2003.01855.xpmid: 12974812

Events occurring in the phloem tissue are key to understanding a wide range of developmental and physiological processes in vascular plants. While a considerable amount of molecular information on phloem proteins has emerged in the past decade, a unified picture of the molecular mechanisms involved in phloem differentiation and function is still lacking. New models to increase our understanding of this complex tissue can be created by the development of global approaches such as genomic analysis. In order to obtain a comprehensive overview of the molecular biology of the phloem tissue, we developed a genomic approach using Apium graveolens as a model. cDNA libraries were constructed from mRNAs extracted from isolated phloem of petioles. Expression data obtained from the analysis of 989 expressed sequence tags (ESTs) and the transcript profile deduced from a cDNA macroarray of 1326 clones were combined to identify genes showing distinct expression patterns in the vascular tissues. Comparisons of expression profiles obtained from the phloem, xylem and storage parenchyma tissues uncovered tissue‐specific differential expression patterns for given sets of genes. The major classes of mRNAs predominantly found in the phloem encode proteins related to phloem structure, metal homeostasis or distribution, stress responses and degradation or turnover of proteins. Of great interest for future studies are the genes we found to be specifically expressed in the phloem but for which the function is still unknown, and also those genes described in previous reports to be up or downregulated by specific interactions. From a broader prospective, our results also clearly demonstrate that cDNA macroarray technology can be used to identify the key genes involved in various physiological and developmental processes in the phloem.

Nemoto, Yasue; Kisaka, Mayumi; Fuse, Takuichi; Yano, Masahiro; Ogihara, Yasunari

doi: 10.1046/j.1365-313X.2003.01859.xpmid: 12974813

The CONSTANS (CO) gene of Arabidopsis plays a key role in the photoperiodic flowering pathway. To investigate photoperiod responses in cereals in more detail, we isolated three kinds of CO/Hd1 (rice ortholog of CO) homolog from hexaploid wheat, derived from the A, B, and D genomes and designated as wheat ortholog of CO from A genome (TaHd1‐1), TaHd1‐2, and TaHd1‐3, respectively. They were highly similar to each other and to Hd1, and in addition harbored two conserved regions: two zinc finger motifs and CONSTANS, CONSTANS‐LIKE and TIMING OF CAB EXPRESSION 1 (CCT) domain like CO/Hd1. They were located on the long arm of the homoeologous chromosome 6. TaHd1‐2 harbored a 63‐bp deletion at the promoter region containing the GATA‐1 box, and consequently, we detected no subsequent transcript. The TaHd1‐1 genomic clone was introduced to a rice line deficient in Hd1 function. Transgenic plants complemented the functions of rice Hd1: they promoted heading under short‐day (SD) conditions and delayed it under long‐day (LD)/natural conditions, indicating that Hd1 proteins from SD and LD plants share common structures and functions.

Kim, Jeong Hoe; Choi, Dongsu; Kende, Hans

doi: 10.1046/j.1365-313X.2003.01862.xpmid: 12974814

Previously, we identified a novel rice gene, GROWTH‐REGULATING FACTOR1 (OsGRF1), which encodes a putative transcription factor that appears to play a regulatory role in stem elongation. We now describe the GRF gene family of Arabidopsis thaliana (AtGRF), which comprises nine members. The deduced AtGRF proteins contain the same characteristic regions—the QLQ (Gln, Leu, Gln) and WRC (Trp, Arg, Cys) domains—as do OsGRF1 and related proteins in rice, as well as features indicating a function in transcriptional regulation. Most of the AtGRF genes are strongly expressed in actively growing and developing tissues, such as shoot tips, flower buds, and roots, but weakly in mature stem and leaf tissues. Overexpression of AtGRF1 and AtGRF2 resulted in larger leaves and cotyledons, as well as in delayed bolting of the inflorescence stem when compared to wild‐type plants. In contrast, triple insertional null mutants of AtGRF1–AtGRF3 had smaller leaves and cotyledons, whereas single mutants displayed no changes in phenotype and double mutants displayed only minor ones. The alteration of leaf growth in overexpressors and triple mutants was based on an increase or decrease in cell size, respectively. These results indicate that AtGRF proteins play a role in the regulation of cell expansion in leaf and cotyledon tissues.

Chen, Shuangyan; Jin, Weizheng; Wang, Mingyi; Zhang, Fan; Zhou, Jie; Jia, Qiaojun; Wu, Yunrong; Liu, Feiyan; Wu, Ping

doi: 10.1046/j.1365-313X.2003.01860.xpmid: 12974815

We generated T‐DNA insertions throughout the rice genome for saturation mutagenesis. More than 1000 flanking sequences were mapped on 12 rice chromosomes. Our results showed that T‐DNA tags were not randomly spread on rice chromosomes and were preferentially inserted in gene‐rich regions. Few insertions (2.4%) were found in repetitive regions. T‐DNA insertions in genic (58.1%) and intergenic regions (41.9%) showed a good correlation with the predicted size distribution of these sequences in the rice genome. Whereas, obvious biases were found for the insertions in the 5′‐ and 3′‐regulatory regions outside the coding regions both at 500‐bp size and in introns rather than in exons. Such distribution patterns and biases for T‐DNA integration in rice are similar to that of the previous report in Arabidopsis, which may result from T‐DNA integration mechanism itself. Rice will require approximately the same number of T‐DNA insertions for saturation mutagenesis as will Arabidopsis. A database of the T‐DNA insertion sites in rice is publicly available at our web site (http://www.genomics.zju.edu.cn/ricetdna).