Comparison and characterization of maize stripe and maize line viruses

Two morphologically similar viruses isolated from maize in East Africa induced two distinct symptom types in maize. One, designated maize stripe virus (MSV), showed broad yellow stripes or a yellowing of the entire leaf, acute bending of the shoot apex and severe stunting. The second, maize line virus (MLV), induced continuous, narrow yellow lines along the leaf veins and did not cause apical bending or stunting. MSV and MLV were both transmitted by Peregrinus maidis (Delphacidae), but not by Cicadulina mbila (Jassidae) or by sap inoculation. Both viruses were purified by extracting systemically infected leaves in 0–5 M sodium citrate buffer and clarifying with 7 ml n-butanol/100 ml extract, followed by differential, and finally sucrose density gradient, centrifugation. Partially purified preparations of both viruses contained isometric viruslike particles of two sizes: MSV particles were 35 and 40 nm in diameter with sedimentation coefficients (so₂o, w) ^I09 ^anI0o respectively; MLV particles were 28 and 34 nm in diameter. Antisera prepared against MSV and MLV had dilution end points of 1/128 and 1/64 respectively in agar-gel diffusion tests. In tests with low-titre antisera, MSV did not react with MLV antiserum and MLV did not react with MSV antisreum; in the presence of antiserum containing antibodies to both MSV and MLV, the two viruses formed precipitin bands which crossed in the pattern of non-identity. Maize streak virus and maize mottle virus showed no serological relationship with MSV or MLV. On the basis of particle size and serology MSV and MLV are shown to be two distinct and possibly unrelated viruses. MSV and MLV apparently are dissimilar from any characterized viruses of the Gramineae.     

Description of maize

Peter Kalm (1716–1779), a student of Linnaeus and later Professor of Economy at the University of Åbo, Sweden,³ was sent to North America in 1748 by the Swedish Academy of Science to obtain seed and other planting stock of New World herbs, shrubs and trees for possible use in Sweden. Kalm was a close, objective observer of things botanical and cultural and had a distinct sense of the utilitarian. HisTravels into North America; containing its Natural History, and a circumstantial Account of its Plantations and Agriculture in general, with the Civil, Ecclesiastical and Commercial State of the Country, the Manners of the Inhabitants, and several curious and Important Remarks on the various Subjects were, as the English title of the 1770 translation of John Reinhold Forster suggests, wide ranging in interests and observations. Kalm’s own title,En Resa til Norra America, in its conciseness better reflects the student of Linnaeus. In the Travels there are a number of references to maize and maize growing. From 1749 to 1778 Kalm contributed seventeen articles on North American subjects to the Kongliga Svenska Vetenskap-Academiens Handlingar. “Beskrifning om Mays,” published in two parts, in 1751 and 1752, has not previously been available in English translation. Kalm’s keen observations on maize culture and utilization in the late colonial period should prove of interest to students of maize and of the history of North American agriculture.     

Paramutation in maize

Paramutation is a heritable change in gene expression induced by allele interactions. This review summarizes key experiments on three maize loci, which undergo paramutation. Similarities and differences between the phenomenology at the three loci are described. In spite of many differences with respect to the stability of the reduced expression states at each locus or whether paramutation correlates with DNA methylation and repeated sequences within the loci, recent experiments are consistent with a common mechanism underlying paramutation at all three loci. Most strikingly, trans-acting mutants have been isolated that prevent paramutation at all three loci and lead to the activation of silenced Mutator transposable elements. Models consistent with the hypothesis that paramutation involves heritable changes in chromatin structure are presented. Several potential roles for paramutation are discussed. These include localizing recombination to low-copy sequences within the genome, establishing and maintaining chromatin domain boundaries, and providing a mechanism for plants to transmit an environmentally influenced expression state to progeny.     

Similarity of maize and sorghum genomes as revealed by maize RFLP probes

Summary Densely saturated genetic maps of neutral genetic markers are a prerequisite either for plant breeding programs to improve quantitative traits in crops or for evolutionary studies. cDNA and genomic clones from maize were utilized to initiate the construction of a RFLP linkage map in Sorghum bicolor. To this purpose, an F₂ population was produced from starting parental lines IS 18729 (USA) and IS 24756 (Nigeria) that were differentiated with regard to many morphological and agronomical traits. A total of 159 maize clones were hybridized to the genomic DNA of the two parents in order to detect polymorphism: 154 probes hybridized to sorghum and 58 out of these were polymorphic. In almost all of the cases hybridization patterns were similar between maize and sorghum. The analysis of the segregation of 35 polymorphic clones in an F₂ population of 149 individuals yielded five linkage groups. The three principal ones recall regions of maize chromosomes 1, 3 and 5: in general, colinearity was maintained. A possible inversion, involving a long region of maize chromosome 3, was detected. Simulations were also performed to empirically obtain a value for the lowest number of individuals of the F₂ population needed to obtain the same linkage data.Prof. E. Ottaviano, to whom this paper is dedicated, suddenly died on June 7^th, 1991     

The allotetraploidization of maize

Summary Four barley telotrisomics (Triplo 3S, 5S, 6S, and 7S) were studied. No major qualitative differences in morphology between the telotrisomics and their diploid sibs were found. The pollen and seed fertility of these telotrisomics was comparable to their diploid sibs. The meiotic study showed that the average frequency of 6_II + 1_III at diakinesis and metaphase I was 84.2% and 71.7%, respectively. The normal chromosome separation ranged from 77.2% to 89.4% at anaphase I through telophase II. The transmission rate of the extra telocentric chromosomes averaged 28.4% upon selfing and 28.7% through the female. All four telotrisomics showed various degrees of pollen transmission, the average being 3.6%. Ditelotetrasomic plants (2n = 14 + 2 homologous telocentrics) were obtained in the progenies of selfed monotelotrisomic plants of all four types. These ditelotetrasomic plants were viable and showed various degrees of seed fertility.Contribution from the Department of Agronomy. Supported by USDA-CSU Cooperative Research Project No. 58-82HW-6-8 and CSU Hatch Project.     

Cleavage polyembryony in maize

Summary Two types of cleavage polyembryony are described in the inbred line VIR 17 of maize. Suspensorial embryony was observed to occur spontaneously. Typical cleavage of the zygotic proembryo occurred spontaneously, but could also be induced by treating the developing caryopses with 2,4-Dichlorophenoxyacetic acid (2,4-D) on the second day after pollination. 2,4-D was active as a decorelative factor also evoking the expression of totipotency in individual proembryonal cells.     

The allotetraploidization of maize

Summary Allotetraploidization is the creation of artificial allotetraploids. Allotetraploidization of maize can be accomplished by concentrating differential pairing affinity (DPA) factors into lines by a recurrent selection breeding system. Selection will be based on changes in genetic ratios which are the result of changes in the relative frequencies of various pairing configurations caused by DPA. Part 1 of this series gave extensive data on gene segregation in trisomic and tetraploid heterozygotes. Some of these tetraploids behaved like segmental allotetraploids. Part 2 presented a model for gene segregation in segmental allotetraploids. This paper presents an analogous model for gene segregation in trisomic heterozygotes. The pairing configurations of trisomes are analyzed by considering pairing in single arms which then are combined to obtain pairing configurations for whole chromosomes. The chromosome disjunction patterns of the various pairing configurations are hypothesized and expected genetic ratios are given that result from different levels of DPA expressed in several hypothetical trisomes. The model analyzes the effect of random pairing in one arm and non-random pairing in the other arms. Also, the effect of crossing over is taken into account. Because crossing over rates are affected by the environment, part of the variability in the data (Part 1) is explained. In addition, an hypothesis is advanced to explain the frequent enhancement of pairing affinity following x-irradiation.Contribution from Agricultural Research/Science and Education Administration, US Department of Agriculture, University of Missouri, Columbia, Missouri, Missouri Agric. Exp. Sta. Jounal Series No. 8 670     

Anther cultures of maize

The presence of synthetic auxins (2,4-D at a concentration as low as of 0.5 to 5.0 mg I^-1, NAA at least at 5 mg I^-1) in the cultivation medium was essential for the induction of callogenesis in anther cultures ofZea mays L. The application of IAA was ineffective. Kinetin induced bursting, darkening and a rapid anther necrosis, but at an appropriate concentration ratio with 2,4-D it stimulated pollen maturation at the same time.     

Sequencing the maize genome

Sequencing of complex genomes can be accomplished by enriching shotgun libraries for genes. In maize, gene-enrichment by copy-number normalization (high C(0)t) and methylation filtration (MF) have been used to generate up to two-fold coverage of the gene-space with less than 1 million sequencing reads. Simulations using sequenced bacterial artificial chromosome (BAC) clones predict that 5x coverage of gene-rich regions, accompanied by less than 1x coverage of subclones from BAC contigs, will generate high-quality mapped sequence that meets the needs of geneticists while accommodating unusually high levels of structural polymorphism. By sequencing several inbred strains, we propose a strategy for capturing this polymorphism to investigate hybrid vigor or heterosis.     

The allotetraploidization of maize

Summary Allotetraploidization is the creation of artifical allotetraploids from a normally diploid species. The possible value of allotetraploid maize has been discussed in Section I of this series. Allotetraploidization of maize can be achieved by restructuring a maize genome so that its chromosomes will not pair with those of the standard maize genome. This restructuring can be done by concentrating differential pairing affinity (DPA) factors into a single line by a recurrent selection type of breeding program. Because the divergence of the maize genome is a gradual process, it is necessary to devise a model for chromosome pairing and gene segregation in segmental allotetraploids. This has been done by considering pairing in each arm separately and then combining paired arms to form pairing configurations for whole chromosomes. The chromosome disjunction patterns are hypothesized and genetic ratios in relation to different levels of DPA are suggested.Contribution from the Science and Education Administration, U.S. Department of Agriculture, and the Agronomy Department, University of Missouri, Columbia, Missouri, Agricultural Experiment Station Journal Series No. 8090     

Cross sterility in maize

Molecular Genetics and Genomics -     

The maize pathogenesis-related PRms protein localizes to plasmodesmata in maize radicles.

Pathogenesis-related (PR) proteins are plant proteins induced in response to infection by pathogens. In this study, an antibody raised against the maize PRms protein was used to localize the protein in fungal-infected maize radicles. The PRms protein was found to be localized at the contact areas between parenchyma cells of the differentiating protoxylem elements. By using immunoelectron microscopy, we found that these immunoreactive regions correspond to plasmodesmal regions. This was also true for the parenchyma cells filling the central pith of the vascular cylinder, although PRms mRNA accumulation was not detected in these cells. These findings suggest that for one cell type, the parenchyma cells of the central pith, the protein is imported rather than synthesized. The localization of the PRms protein indicates the possible existence of mechanisms for sorting of plant proteins to plasmodesmata and suggests that this protein may have a specialized function in the plant defense response. These findings are discussed with respect to the structure and function of plasmodesmata in cell-to-cell communication processes in higher plants.     

Cell death in maize

Cell death occurs in plants as a part of normal development and as a response to toxins, pathogens and other environmental stimuli or insults. When cell death occurs as an orderly disassembly of the cell under the control of a genetically determined program, the process is referred to as programmed cell death (PCD). The PCD mechanisms of plants show many striking similarities to, but also intriguing differences from, those of animals. The extensive genetic, developmental and physiological characterizations of maize have made it an excellent system for the study of cell death. We describe the recent advances in the study of cell death in maize in light of what is known in plants and animals.