Two genes from Arabidopsis thaliana related to the auxin-inducible Aux28 and Aux22 genes of soybean have been isolated. These genes belong to a small multi-gene family and are similar to the soybean Aux gene family in the sequence of the predicted proteins, intron/exon locations, and auxin-enhanced expression of their transcripts. Application of auxin to 8-day old Arabidopsis plants, 4-day old etiolated seedlings, and suspension culture cells all resulted in enhanced Aux transcript levels. Comparison of the promoter sequences from the soybean and Arabidopsis genes yielded no significant sequence conservation; however, three regions of near sequence identity are present between the two Arabidopsis Aux genes.

Visual examination of roots from 12,000 mutagenized Arabidopsis seedlings has led to the identification of more than 40 mutants impaired in root hair morphogenesis. Mutants from four phenotypic classes have been characterized in detail, and genetic tests show that these result from single nuclear recessive mutations in four different genes designated RHD1, RHD2, RHD3, and RHD4. The phenotypic analysis of the mutants and homozygous double mutants has led to a proposed model for root hair development and the stages at which the genes are normally required. The RHD1 gene product appears to be necessary for proper initiation of root hairs, whereas the RHD2, RHD3, and RHD4 gene products are required for normal hair elongation. These results demonstrate that root hair development in Arabidopsis is amenable to genetic dissection and should prove to be a useful model system to study the molecular mechanisms governing cell differentiation in plants.

A simple method for analysis of meiosis in anthers of Arabidopsis thaliana is described. Anthers showing a high frequency of microsporocytes undergoing meiotic divisions were obtained from buds of 18—20-d-old plants at the rosette stage of growth just prior to bolting. All stages of meiosis were readily observed.

The identification of a cDNA (MR19) corresponding to a maize alpha-tubulin and homologous genomic clones (MG19/6 and MG19/14) is described. The cDNA has been isolated by differential screening of a cDNA maize root library. We have found two alpha-tubulin genes in a tandem arrangement in the genomic clones, separated by approximately 1.5 kbp. One of the genes (gene I) contains an identical nucleotide sequence which corresponds to the cDNA clone. The two deduced proteins from DNA sequences are very similar (only two conservative replacements in 451 amino acids) and they share a high homology as compared with the published alpha-tubulin sequences from other systems and in particular with the Arabidopsis thaliana and Chlamydomonas reinhardtii sequences reported. The structure of both genes is also very similar; it includes two introns, of 1.7 kbp and 0.8 kbp respectively, in each gene and only one intron placed at a homologous position in relation to Arabidopsis thaliana genes. By using specific 3' probes it appears that both genes are preferentially expressed in the radicular system of the plant. The alpha-tubulin gene family of Zea mays seems to be represented by at least 3 or 4 members.

The Ta1 elements are a low copy number, copia-like retrotransposable element family of Arabidopsis thaliana. Six Ta1 insertions comprise all of the Ta1 element copies found in three geographically diverse A. thaliana races. These six elements occupy three distinct target sites: Ta1-1 is located on chromosome 5 and is common to all three races (Col-0, Kas-1 and La-0). Ta1-2 is present in two races on chromosome 4 (Kas-1 and La-0), and Ta1-3, also located on chromosome 4, is present only in one race (La-0). The six Ta1 insertions share >96% nucleotide identity, yet are likely to be incapable of further transposition due to deletions or nucleotide changes that alter either the coding capacity of the elements or conserved protein domains required for retrotransposition. Nucleotide sequence comparisons of these elements and the distribution of Ta1 among 12 additional A. thaliana geographical races suggest that Ta1-1 predated the global dispersal of A. thaliana. As the species spread throughout the world, two additional transposition events occurred which gave rise first to Ta1-2 and finally to Ta1-3.

A population of Arabidopsis thaliana growing locally in a suburb of Zurich called Weiningen was observed to be infected with downy mildew. Plants were collected and the progress of infection was investigated in artificial inoculations in the laboratory. The plants proved to be highly susceptible, and pronounced intercellular mycelial growth, haustoria formation, conidiophore production, and sporulation of the causal organism Peronospora parasitica were all observed. The formation of oogonia, antheridia, and oospores also occurred. In contrast, Arabidopsis strain RLD was resistant to infection and none of the above structures was formed. The fungus was localized very soon after penetration of RLD leaf cells, which responded with a typical hypersensitive reaction. The differential interaction of an isolate of P. parasitica with two strains of Arabidopsis opens up the possibility of cloning resistance determinants from a host that is very amenable to genetic and molecular analysis.

We have isolated and an analyzed a pre-ferredoxin gene from Arabidopsis thaliana. This gene encodes a 148 amino acid precursor protein including a chloroplast transit peptide of 52 residues. Southern analysis shows the presence of a single copy of this ferredoxin (Fd) gene in the A. thaliana genome. Its expression is tissue-specific and positively affected by light. Response times, both to dark and light conditions, are remarkably rapid. A chimeric gene consisting of a 1.2 kb Fd promoter fragment fused to the beta-glucuronidase reporter gene was transferred to tobacco. This fusion gene is expressed in a tissue-specific way; it shows high levels of expression in green leaves, as compared to root tissue.