Epiplastome Variation of the Number of Chloroplasts in Stomata Guard Cells of Sugar Beet (Beta vulgaris L.)

In stomata guard cells of sugar beet, variation in the number of chloroplasts was studied in successive generations: (1) hybrid generation; (2) generation yielded by uniparental apozygotic seed reproduction; (3) generation obtained after seed treatment with a colchicine solution; (4) generation obtained after seed treatment with 5-azacytidine. As compared to hybrid generation, uniparental seed reproduction increases the average number of chloroplasts in stomata guard cells (from 13.5 to 15.0) and decreases distribution variance of this trait by a factor of 3 to 4. Colchicine increases both average number of chloroplasts in stomata guard cells (from 13.5 to 18.2) and distribution variance (about twice). 5-Azacytindine reduces the number of chloroplasts in cells (from 15.0 to 12.9) but enhances distribution variance (about 1.5 times). Variation in the number of chromosomes in stomata cells is related to myxoploidy in meristem tissue, on the one hand, and to the rate of cell division, on the other. Uniparental seed reproduction is suggested to enhance the number of organelles per cell due to high myxoploidy in cell populations. Colchicine blocks spindle division and sharply increases the level of myxoploidy in cell populations and the number of organelles per cell. 5-Azacytidine hypomethylates chromosome DNA, increases the rate of cell divisions, and reduces the number of organelles per cell. The described changes in the number of chloroplasts are inherited in cell lineage (cell hereditary memory) and successive sporophyte generations.     

Structural and Transcriptional Variation of Mitochondrial DNA in Pollen-Sterile Agamospermous Sugar Beet (<Emphasis Type="Italic">Beta vulgaris</Emphasis> L.) Progeny

The cytoplasm status according to mitochondrial sequence tags was determined in agamospermous sugar beet progenies characterized by unstable manifestation of cytoplasmic male sterility. The detected variations in the ratios of homologous sequences related to the N and S mtDNA types did not correlate with pollen phenotypes of the plants. Polymerase chain reaction allowed semiquantitative evaluation of these variations and their detection. A cDNA corresponding to a hitherto unknown minor RNA and containing a consensus sequence of dicotyledonous plant promoters was detected by mtRNA display. Probable sources of CMS variability in agamospermous sugar beet progenies are discussed.     

Variability of Chloroplast Number in Populations of Stomata Guard Cells in Sugar Beet Beta vulgaris L.

Diploid sugar beet plants demonstrate a broad variability of the number of chloroplasts in stomata guard cells, which is related to mixoploidy of cell populations in leaf apical meristems (epigenomic variability). In addition to random organelle segregation between daughter cells, this variability is affected by factors disrupting the mitotic cycle: (1) plant treatment with a mitotic poison, such as colchicine; (2) duration of the life cycle of a plant; the variability in second-year plants is greater than in first-year ones; (3) the mode of plant reproduction; the variability in inbred plants is greater than in the initial population. Treatment of germinating seeds with a diluted colchicine solution increases the number of organelles in cells in the myxoploid generation (generation C₀) and the variance of the distributions in the first vegetation year. The variability in the organelle number in stomata cells correlates with that in maternal meristem cells. It is concluded that the epidermal cell monolayer (including stomata guard cells) keeps record of the epigenomic and epiplastome variability in meristem cells. The variability of the number of chloroplasts in stomata guard cells is approximated by binomials with negative powers.     

Effect of the epimutagen 5-azacytidine on the structure of floral-stalk metameres in sugar beet Beta vulgaris L.

This study is dedicated to the effect of the epimutagen 5-azacytidine on the structure of floral-stalk metameres in sugar beet Beta vulgaris L. Simple phytomeres consist of separate flowers (unianthous plants; UA phenotype), while complex metameres occur in synanthous plants (SA phenotype). Treatment of the synanthous line msSOAN-5 dramatically reduced the number of flowers on phytomeres as early as in generation zero (A₀Az₀). For the first time, plants with simple phytomeres were found in this line. The proportion of plants with the UA phenotype in generation A₀Az₀ was 7.7%. In generation A₁Az₁, the tendency for reduction of the number of flowers on phytomeres persisted. The proportion of plants with simple phytomeres reached 77%; i.e., the frequency of UA phenotypes in the progeny (A₁Az₁) increased tenfold in comparison with the parents A₀Az₀. The high frequency of morphogenetic changes in the flower stem structure under the influence of the epimutagen suggests that the variability of the UA-SA character in beet populations is of epigenetic rather than mutational nature.     

Analysis of serial sequences of seeds in sugar beet plants by apozygotic reproduction

Dichotomic serial sequences of fruits of two types (with and without seeds) on shoots were analyzed in sugar beet upon apozygotic reproduction. Serial sequences were analyzed using serial statistical tests. The distribution of fruits of the two types was nonrandom in most cases. The variation of serial sequences is considered in terms of the epigenetic variation of discrete characters in plants.     

The germ track and stem cells in higher plants

A. Weisman’s conception of the germ track is considered within a historical context and the fundamental distinctions in the methods of forming the germ track cells in animals and plants are underscored. In animals differentiation of germ track cells is realized once at the start of embryogenesis, whereas in higher plants this process is achieved repeatedly throughout the entire life of the plant. Fundamental differences in morpho-and embryogenesis in animals and plants as well as differences in the properties of somatic and germ cells provide plants with special paths of hereditary variability and evolution which are not present in animals.     

Epigenetic variation of the choriflowered-symflowered character in the symflowered sugar beet line SOAN-14

The effect of 5-azacytidine on the variation of the choriflowered (CF)-symflowered (SF) character in sugar beet was studied in several generations obtained via seed reproduction. The epimutagen (5-azacytidine) significantly reduced the number of flowers in glomerate inflorescences in the year of seed treatment and in the next generation (Az₁), leading to the appearance of plants with single flowers in bract axils of a flower stalk. The CF character resulting from epimutagene treatment of sugar beet seeds (plants with genotype M ^Z M ^Z ) was inherited as a monohybrid character in both zygotic and apozygotic seed progenies. The proportion of the CF and SF phenotypes in the progenies was affected by the chromatid number in the chromosomes (mixoploidy of the cell populations). Alleles of the Mm locus were found to affect the variation in phytomere organization of flower stalks.     

Fruit set variation associated with apozygotic reproduction in sugar beet <Emphasis Type="Italic">Beta vulgaris</Emphasis> L.

Fruit set upon apozygotic reproduction was studied for four years in the msSOAN-5 pollen-sterile inbred sugar beet line. The progenies obtained from pollen-sterile plants by apozygotic reproduction had both fruits with normal seeds and parthenocarpic fruits without seeds, which was not an occasional event. Growth conditions proved to strongly affect the fruit set and seed quality. For instance, water deficiency during early plant development increased the proportion of parthenocarpic seeds. Water deficiency combined with a lower temperature during flowering additionally caused a substantial decrease in the total number of fruits. Under the same growth conditions, related accessions did not differ in seed productivity, but varied in the proportion of normal (with seeds) and parthenocarpic (without seeds) fruits.     

Expression of CMS in Zygotic and Apozygotic Progenies of Sugar Beet Beta vulgaris L.

Zygotic and apozygotic progenies of sugar beet exhibit high phenotypic variation with respect to cytoplasmic male sterility (CMS). There are progenies with completely sterile, semisterile, semifertile, and fertile pollen. The proportions of semifertile and fertile plants in zygotic and apozygotic progenies varied from zero to 28% and from zero to 17.8%, respectively. Comparison of the phenotypic distributions in zygotic and apozygotic progenies did not reveal significant differences in the CMS expression, although the latter is determined by the maternal S-plasmotype and both maternal and paternal (pollinator) genotypes in zygotic progenies and only by the maternal S-plasmotype and genotype in apozygotic progenies. It has been hypothesized that the instability of the CMS expression in apozygotic progenies is determined by epigenetic variation in the activities of the genes that control the maintenance of the pollen-grain sterility. Inactivated dominant alleles R f ⁰ ₁ and R f ⁰ ₂ in homozygous state may function as sterility maintenance genes, whereas activation of these alleles during ontogeny results in a partial or complete restoration of pollen-grain fertility. It was demonstrated that pollen fertility of mother plants withS cytoplasm did not affect the CMS expression in two sib progenies. Conversely, in two other progenies, the proportion of fertile plants was significantly higher in the sib progenies of mother plants with fertile pollen and S cytoplasm (inheritance of epigenetic variation).     

Expression of Alleles of the Mdh1Locus in Apozygotic Progenies of Sugar Beet Beta vulgarisL.

Expression of Mdh1alleles has been studied in 60 apozygotic (agamospermic) sugar beet progenies. Seed progenies were obtained by uniparental (pollenless) mode of seed reproduction: selfing of pollen-sterile plants isolated with paper bags. The apozygotic seed progenies demonstrate a disomic gamete autosegregation, i.e., the ratio between genotypes in the progenies correspond to the gamete segregation in a duplex heterozygote of an autotetraploid. It was shown that the ratio between theMdh1phenotypes in apozygotic progenies is strongly affected by spontaneous inactivation of one of the alleles. In most progenies, the excess of FF phenotypes and the deficit of SS phenotypes were observed. In our opinion, such deviations in genotype and phenotype frequencies result from conversion of the active Mdh1-Sinto the inactive Mdh1-S ⁰allele (epigenetic gene inactivation). The spontaneous inactivation of one allele results in extremely variable frequencies of heterozygous Mdh1-F/Mdh1-Sgenotypes and phenotypes in the apozygotic seed progenies. The empirical distribution of the frequencies of heterozygous genotypes in the apozygotic seed progenies is given by a negative binomial distribution describing the expected time of occurrence of random events.