Parasitic plants, including the root holoparasites Orobanche spp., cause devastating damage to crops worldwide. Arabidopsis thaliana is widely used as an amenable model plant system to study host–pathogen interactions. Understanding the molecular basis involved in host–parasite interactions will provide practical tools for the detection of genes responsible for incompatibility and resistance responses. In preliminary petri dish experiments, A. thaliana induced seed germination of O. aegyptiaca, O. minor, and O. ramosa at the rate of 87, 72, and 67% of maximum seed germination, respectively. Arabidopsis thaliana induction of O. crenata and O. cumana seeds was negligible (less than 2% of maximum germination). In additional polyethylene bag studies, A. thaliana was parasitized by O. aegyptiaca, O. ramosa, and O. minor resulting in 12, 5, and 2 parasites per host plant, respectively. The results facilitate the use of A. thaliana in host-parasitic plant interaction research. Nomenclature: Orobanche aegyptiaca Pers. ORAAE, Egyptian broomrape; Orobanche crenata Forsk. ORACR, crenate broomrape; Orobanche cumana = Orobanche cernua Loefl. ORACE, nodding broomrape; Orobanche minor Sm. ORAMI, small broomrape; Orobanche ramosa L. ORARA, branched broomrape; Arabidopsis thaliana (L.) Heynh. ARBTH ‘Columbia’, mouse-ear cress.

Parasitization by Orobanche is a complex process, one that is mediated by host-derived chemical signals that control parasite seed germination and haustorium initiation and one that ultimately results in the union of two plant species. Experiments were conducted to characterize Orobanche parasitization of the model plant Arabidopsis thaliana and to begin to explore the role of host flavonoid metabolism in the interaction. Arabidopsis thaliana stimulated seed germination and allowed tubercle development of O. aegyptiaca and O. ramosa but did not significantly stimulate seeds of O. crenata, O. minor, or O. cernua. However, if Orobanche seeds were artificially stimulated, O. crenata and O. minor successfully established tubercles on A. thaliana. When compared to the recognized crop hosts, Daucus carota and Nicotiana tabacum, A. thaliana stimulated less O. aegyptiaca germination but allowed for formation of equivalent numbers of tubercles. These findings indicate that A. thaliana is not a large-scale producer of germination stimulant but is highly susceptible to the parasite once Orobanche seeds have germinated. Experiments comparing wild-type A. thaliana plants to mutant lines deficient in flavonoid biosynthesis revealed no differences in the ability to stimulate germination or to allow tubercle formation, indicating that host flavonoid production is not essential for Orobanche parasitization. The results of this work support the use of A. thaliana as a valuable host in understanding Orobanche parasitization.Nomenclature: Arabidopsis thaliana L. Heynh., ARBTH, mouseearcress; Daucus carota L. ‘Danver half long’, DAUCS, carrot; Nicotiana tabacum L. ‘Coker 319’, NIOTA, tobacco; Orobanche aegyptiaca Pers., ORAAE, Egyptian broomrape; Orobanche cernua Loefl., ORACE, nodding broomrape; Orobanche crenata Forsk., ORACR, crenate broomrape; Orobanche minor Smith, ORAMI, small broomrape; Orobanche ramosa L., ORARA, branched broomrape.

The extensive natural variation of Arabidopsis thaliana ecotypes is being increasingly exploited as a source of variants of genes which control (agronomically) important traits. We have subjected 19 different Arabidopsis thaliana ecotypes to an analysis using the amplified fragment length polymorphism (AFLP) technique in order to estimate their genetic diversity. The genetic diversity was estimated applying the method of Nei and Li (1979) and a modified version of it and using 471 informative polymorphisms. The data obtained revealed that within this small set of ecotypes a group of three ecotypes and a further single ecotype exhibit considerable genetic diversity in comparison to the others. These ecotypes clustered at positions significantly separated from the bulk of the ecotypes in the generated similarity plots. The analysis demonstrated the usefulness of the AFLP method for determinating intraspecies genetic diversity as exemplified with Arabidopsis thaliana ecotypes. Results are discussed and compared with data obtained with other methods.

Axillary shoot apical meristems initiate post-embryonically in the axils of leaves. Their developmental fate is a main determinant of the final plant body plan. In Arabidopsis, usually a single axillary meristem initiates in the leaf axil even though there is developmental potential for formation of multiple branches. While the wild-type plants rarely form multiple branches in the leaf axil, tfl1-2 plants regularly develop two or more branches in the axils of the rosette leaves. Axillary meristem formation in Arabidopsis occurs in two waves: an acropetal wave forms during plant vegetative development, and a basipetal wave forms during plant reproductive development. We report here the morphological and anatomical changes, and the STM expression pattern associated with the formation of axillary and accessory meristems during Arabidopsis vegetative development.

A project investigating the biology of protein kinases and phosphatases in Arabidopsis thaliana and other plants.

Histidine (His)-to-Aspartate (Asp) phosphorelay signal transduction systems are generally made up of a “sensor histidine (His)-kinase”, a “response regulator”, and a “histidine-containing phosphotransmitter (HPt)”. In the higher plant, Arabidopsis thaliana, results from recent intensive studies suggested that the His-to-Asp phosphorelay mechanism is at least partly responsible for propagation of environmental stimuli, such as phytohormones (e.g. ethylene and cytokinin). Here we compiled the members of the HPt family of phosphotransmitters in Arabidopsis thaliana (AHP- series, Arabidopsis HPt phosphotransmitters), based on both database and experimental analyses, in order to provide a comprehensive basis at the molecular level for understanding the function of the AHP phosphotransmitters that are implicated in the His-to-Asp phosphorelay of higher plants.

A Brassica napus skp1-like gene promoter drives GUS expression in Arabidopsis thaliana male and female gametophytes. 16. International Congress on Sexual Plant Reproduction

Common structural and amino acid motifs among cloned plant disease-resistance genes (R genes), have made it possible to identify putative disease-resistance sequences based on DNA sequence identity. Mapping of such R-gene homologues will identify candidate disease-resistance loci to expedite map-based cloning strategies in complex crop genomes. Arabidopsis thaliana expressed sequence tags (ESTs) with homology to cloned plant R genes (R-ESTs), were mapped in both A. thaliana and Brassica napus to identify candidate R-gene loci and investigate intergenomic collinearity. Brassica R-gene homologous sequences were also mapped in B. napus. In total, 103 R-EST loci and 36 Brassica R-gene homologous loci were positioned on the N-fo-61-9 B. napus genetic map, and 48 R-EST loci positioned on the Columbia x Landsberg A. thaliana map. The mapped loci identified collinear regions between Arabidopsis and Brassica which had been observed in previous comparative mapping studies; the detection of syntenic genomic regions indicated that there was no apparent rapid divergence of the identified genomic regions housing the R-EST loci.

A Brassica napus skp1-like gene promoter drives GUS expression in Arabidopsis thaliana male and female gametophytes. 16. International Congress on Sexual Plant Reproduction

Post-translational modification of histones, in particular acetylation, is an important mechanism in the regulation of eukaryotic gene expression. Histone deacetylases are enzymes that remove acetyl groups from the core histones and play a key role in the repression of transcription. HD2 is a maize histone deacetylase, which shows no sequence homology to the histone deacetylases identified from other eukaryotes. We have identified two putative HD2-like histone deacetylase cDNA clones, AtHD2A and AtHD2B, from Arabidopsis thaliana by screening the expressed sequence tag database. AtHD2A and AtHD2B encode putative proteins of 246 and 305 amino acids, and share 44% and 46% amino acid identity to the maize HD2, respectively. Northern blot analysis indicated that AtHD2A was highly expressed in flowers and young siliques of Arabidopsis plants, whereas AtHD2B was widely expressed in stems, leaves, flowers and young siliques. AtHD2A repressed transcription when directed to a promoter containing GAL4-binding sites as a GAL4 fusion protein. Deletion of the extended acidic domain or the domain containing predicted catalytic residues of AtHD2A resulted in the loss of gene repression activity, revealing the importance of both domains to AtHD2A function. Arabidopsis plants were transformed with a gene construct comprising an AtHD2A cDNA in the antisense orientation driven by a strong constitutive promoter, -394tCUP. Silencing of AtHD2A expression resulted in aborted seed development in transgenic Arabidopsis plants, suggesting that the AtHD2A gene product was important in the reproductive development of Arabidopsis thaliana.