Microfluorometric analysis of the nuclear DNA contents of the somatic tissues of Arabidopsis thaliana has revealed extensive endoreduplication, resulting in tissues that comprise mixtures of polyploid cells. Endoreduplication was found in all tissues except those of the inflorescences and was developmentally regulated according to the age of the tissues and their position within the plant.

Analysis of 11 genomic clones containing the S-adenosylmethionine synthetase I gene (sam1) of Arabidopsis thaliana revealed the presence of a 431-base-pair (bp) insertion in the 3' end of sam1 in one of these clones. The inserted sequence, called Tat1, shows structural features of a transposon. It is flanked by a 5-bp duplication of the target site DNA and has 13-bp inverted repeats at its termini. Two highly homologous elements situated in a different genomic context were isolated from a genomic library. Genomic Southern analysis indicates that there are at least four copies of Tat1 present in the A. thaliana ecotype Columbia genome. Different hybridization patterns are observed with DNAs derived from different ecotypes of Arabidopsis thaliana, indicating that the element has moved since the divergence of these ecotypes. In two populations of A. thaliana, linear extrachromosomal Tat1-homologous DNA has been observed. The presented data are consistent with the hypothesis that Tat1 is an active transposable element.

Three populations of Arabidopsis thaliana (L.) Heynh. were inoculated with three isolates of Plasmodiophora brassicae Woron. Inoculation of young Arabidopsis plants caused clubbing of roots and, in the late flowering population CrGC 9-4, infection of shoots. In this population, the number of inoculated plants reaching the flowering stage was reduced, and the majority of plants died prematurely. Symptoms of shoot infections were compressed rosettes with thickened and stunted leaves containing resting spores of P. brassicae. The results showed clearly that A. thaliana is susceptible to P. brassicae.

Mit der Etablierung eines Resistenztests fuer das System Arabidopsis thaliana/Plasmodiophora brassicae und dem Auffinden unterschiedlicher Krankheitsreaktionen von Arabidopsis-Linien gegenueber einem P. brassicae Isolat erfolgten erste Schritte auf dem Weg zur Charakterisierung der Kohlhernieresistenz in Arabidopsis. Die zur Zeit laufenden Kreuzungsexperimente werden Aufschluss geben ueber die Vererbung der Resistenz und ueber die Anzahl der beteiligten Gene. Somit ist eine Grundlage geschaffen fuer eine Isolierung von Resistenzgenen in diesem Modellorganismus.

Phototropic curvature has been measured for etiolated Arabidopsis thaliana seedlings with and without a preirradiation. A bilateral preirradiation with 450-nm light at a fluence greater than about 0.1 micromole per square meter causes a rapid desensitization to a subsequent 450-nanometer unilateral irradiation at 0.5 micromole per square meter. Following a refractory period, the capacity to respond phototropically recovers to the predesensitization level, and the response is then enhanced. The length of the refractory period is between 10 and 20 minutes. Both the time needed for recovery and the extent of enhancement increase with increasing fluence of the bilateral preirradiation. Based on the relative spectral sensitivities of desensitization and enhancement, these responses can be separated. Desensitization is induced by blue light but not by red light. Enhancement, however, is induced by both blue and red light. Thus, enhancement can be induced without desensitization but not vice versa. Both desensitization and enhancement affect only the magnitude of the response and do not affect the fluence threshold.

We describe the structure of an Arabidopsis thaliana genomic clone containing two classes of repetitive DNA elements derived from the centromere region of chromosome 1. One class is comprised of tandem arrays of a highly reiterated repeat containing degenerate telomere sequence motifs. Adjacent to these telomere-similar repeats were found a dispersed repetitive element reiterated approximately five times in the A. thaliana genome. The nucleotide sequence of the dispersed repeat is unusual, being extremely AT-rich and composed of numberous, overlapping repeat motifs.

Xanthomonas campestris pv. campestris caused black rot disease when inoculated via the hydathodes and caused a spreading chlorosis when infiltrated into the leaves of Arabidopsis thaliana race Pr0. In contrast, leaves of Arabidopsis race Columbia were resistant to X. c. campestris and were slightly chlorotic or symptomless when inoculated by either method of inoculation. By genetic analysis, we found that resistance to X. c. campestris was determined by a dominant allele of a single, nuclear gene.