G. P. Rédei

Single locus heterosis

SummaryExperimental evidence is reported in favor of superdominance in over-all vigor inArabidopsis thaliana in connection with two x-ray induced mutants. The tests adopted (outcrossing to unrelated tester, repeated recombination for 15 generations) failed to reveal any additional genetic variation. The genetic and physiolgical mechanism of superdominance is discussed in light of recent information on gene structure and function.

Extra-chromosomal mutability determined by a nuclear gene locus in arabidopsis

Abstract Two recessive mutations isolated after ethyl methanesulfonate treatment produce variegation when homozygous with excellent penetrance and variable expressivity. The variegation involves different degrees of pigmentation and morphology of the leaves as well as fertility. The altered phenotypes observed in the mutants display non-mendelian inheritance though the primary cause of the alteration is the mutation at a chromosomal locus ( chm ) in linkage group 3. The two mutants seem allelic but non-identical. The results of the genetic analysis are consistent with the interpretation that a nuclear mutator locus is inducing several different type of hereditary alterations in the plastome. The plastome mutations reveal involvement of the plastome in the control of its own morphology as well as in cellular and organ differentiation. The system indicates that though the plastome is endowed with a considerable autonomy in some functions its mutability is subject also to nuclear control. The informational content of the 20–80 plastids per cell is apparently identical. The occurrence of cells with different types of plastids reveals that “epistasis” is not of major importance among the plastids within a single cell. The independent expression and inheritance of color variegation from morphological alterations indicate a separate localization (absence of linkage) of the pertaining genetic elements. The predominantly joint appearance of other phenes point to the existence of pleiotropy or linkage within the plastids.

Estimation of mutation rate in autogamous diploids

The principle and practice of a procedure for the estimation of mutation frequency in diploid multicellular tissues is outlined and discussed. The formula of calculation is as follows: R=MS×GECN×2×D R = mutation rate M = number of M2 families segregating for recessive mutants S = number of surviving plants GECN = genetically effective cell number of the mutagen-treated tissues (germline) 2 = correction factor for diploidy D = dose of the mutagen Full-size table Table options View in workspace Download as CSV

Thiamine mutants of the crucifer, Arabidopsis

In higher plants the only obligate organoauxotrophic mutants available are concerned with the synthesis of thiamine, vitamin B1. In Arabidopsis 65 mutants, representing four loci controlling four different steps in the thiamine pathway, have been obtained and identified. Heteroalleles were distinguished within two loci by qualitative and quantitative responses to low and high temperatures. The availability of auxotrophs in higher plants makes it possible to employ highly selective genetic techniques in well-differentiated diploid organisms.

Mutation Induction and Detection in Arabidopsis

Mutation is an infrequent event, and the stability of the various gene loci is quite different. Additional complexity appears when mutability is studied in different taxonomic groups displaying a variety of genome organization. Higher plants permit the study of mutation in three types of genomes: nuclear, plastidic and mitochondrial. In Arabidopsis the size of the nuclear genome is about 1. 3 × 1012 dalton whereas the chloroplast and the mitochondrial genomes are estimated to be 4 to 5 orders of magnitude smaller. The nuclear genome of Arabidopsis is much less redundant than that of other higher plants, thus mutation can be detected at an estimated 19,688 loci during embryonic development and the total number of loci with visible mutations appears to be 27,875.

Induction of Auxotrophic Mutations in Plants

Auxotrophic mutants in microorganisms have contributed most importantly to the rapid development of genetics and biochemistry in the last three decades. Their usefulness is not limited to elucidation of the genetic control of biochemical pathways but they are indispensable for the study of protein synthesis, regulation, various mechanisms of information transfer, the nature of mutation etc.

Aneuploidy detection with a short-term hexaploid wheat assay

A new type of assay for the identification of agents causing aneuploidy is described. This assay takes advantage of allohexaploid wheat in which monosomic and nullisomic cell lineages can be genetically detected. The wheat strain used (Neatbys virescens) was homozygous for a pair of recessive alleles (v1) which in homozygous condition interfere with normal pigmentation of the leaves at low temperature whereas at higher temperature nearly normal green color formation is permitted. In a single dose this allele cannot suppress the formation of green color even at low temperature, i.e., it is hemizygous ineffective. This locus is in the short arm of chromosome 3B near the centromere. As a consequence of non-disjunction of this chromosome twin sectors may be detected, in which the monosomic cell lineages appear green whereas the trisomic sectors display with color on a cream-colored background at low temperature. This genetic system can also be used for the detection of deletions or duplications involving the short arm of chromosome 3B, and to some extent the A- and the D-genome homeologues. We have determined the pattern of differentiation of the shoot apex and on that basis we can separate the independent genetic events from reappearance of the sectors of common origin in the successive leaves. Such an understanding of development of the leaf sectors permits a quantitative estimation of the genetic response of the plants to mutagenic factors. We have found that X-rays, gamma-rays, p-fluorophenylalanine, 3-aminotriazole, caffeine, vinblastin sulfate, benzo[a]pyrene and auramine significantly increased aneuploidy, and diethylstilbestrol, sulfacetamide, safrole and dichlorvos caused some increase of sectoring. Cytological data on root tips of irradiated seeds support the interpretation of the mechanism of sector formation in the leaves. The test is simple, fast, inexpensive, and it does not require elaborate facilities or highly trained technicians. The trials were well reproducible during a period of 3 years in 2 laboratories. Therefore we consider the new assay a useful complement to other tests of chemicals or physical agents that may cause non-disjunction and other chromosomal aberrations in human populations.

An anomaly of the genetic regulation of the de novo pyrimidine pathway in the plant Arabidopsis

Humans afflicted by hereditary orotic aciduria are characterized by insufficiencies in the de novo pyrimidine pathway. Mutants at a nuclear gene locus in Arabidopsis, in contrast, exhibit increased activities of orotidylic acid (O5P) pyrophosphorylase (2.4.2.10) and O5P-decarboxylase (4.1.1.23). In the plants, as well as in human cells, the symptoms of the genetic disorder can be partly cured by feeding the pyrimidine analogue 6-azauracil. In normal human cells, the supply of the antimetabolite 6-azauridine leads to augmented levels of these enzymes, and in the cell cultures of patients suffering from orotic aciduria type I nearly normal levels of the enzymes are observed. In the tissues of Arabidopsis, on 6-azauracil administration, the level of O5P-pyrophosphorylase decreases while that of O5P-decarboxylase is elevated. The genetic alteration may involve a regulatory function in both humans and plants.

Sensitivity, Specificity and Accuracy of the Arabidopsis Assay in the Identification of Carcinogens

In 61 laboratories using a variety of prokaryotic and eukaryotic short-term assays a maximum of 42 chemicals were tested. The purpose of the study was to identify the best battery of assays for screening carcinogens. None of the assay systems correctly identified all the proven carcinogens or “noncarcinogens”. The individual assays were characterized by sensitivity (% of correct identification of carcinogens), specificity (% of correct identification of “noncarcinogens”), predictivity (% of carcinogens correctly identified among all the compounds tested) and hypersensitivity (% of compounds classified as carcinogens among all the compounds tested.

Mutagenicity of ascorbic acid for the plant Arabidopsis

Abstract Mutagenicity of ascorbic acid and ascorbate has been demonstrated in Arabidopsis. The grequency of mutation at a very large number of loci consistently exceeded the spontaneous level when mature seeds were treated with 0.05–2.00% solution of this vitamin although the induced rate of mutation was apparently not much affected by the concentrations within this range. In six experiments the mutational events were scored by the phenotype of immature embryos. In another series of experiments, seedling mutations were assessed in M 2 generations involving over 330 000 genomes treated and approximately equal size concurrent control. Although the induced rate of mutation was low (0.00024 over all loci) in the latter series, it was highly significantly above the spontaneous level (0.00010).