Epigenetic memory transmission through mitosis and meiosis in plants

Gene activities can be regulated by epigenetic modifications of nucleotides and chromatin that are stably propagated through somatic cell divisions and, in some cases, across generations. The mechanisms that control epigenetic marks have recently been uncovered using model organisms, such as the flowering plant Arabidopsis thaliana. In Arabidopsis, perturbation of epigenetic gene activity often results in heritable developmental phenotypes. Stable, but potentially reversible, changes in epigenetic status can also be sources for phenotypic variations in natural plant populations.     

Phylogenetic relationships among Lactuca (Asteraceae) species and related genera based on ITS-1 DNA sequences

Internal transcribed spacer (ITS-1) sequences from 97 accessions representing 23 species of Lactuca and related genera were determined and used to evaluate species relationships of Lactuca sensu lato (s.l.). The ITS-1 phylogenies, calculated using PAUP and PHYLIP, correspond better to the classification of Feráková than to other classifications evaluated, although the inclusion of sect. Lactuca subsect. Cyanicae is not supported. Therefore, exclusion of subsect. Cyanicae from Lactuca sensu Feráková is proposed. The amended genus contains the entire gene pool (sensu Harlan and De Wet) of cultivated lettuce (Lactuca sativa). The position of the species in the amended classification corresponds to their position in the lettuce gene pool. In the ITS-1 phylogenies, a clade with L. sativa, L. serriola, L. dregeana, L. altaica, and L. aculeata represents the primary gene pool. L. virosa and L. saligna, branching off closest to this clade, encompass the secondary gene pool. L. virosa is possibly of hybrid origin. The primary and secondary gene pool species are classified in sect. Lactuca subsect. Lactuca. The species L. quercina, L. viminea, L. sibirica, and L. tatarica, branching off next, represent the tertiary gene pool. They are classified in Lactuca sect. Lactucopsis, sect. Phaenixopus, and sect. Mulgedium, respectively. L. perennis and L. tenerrima, classified in sect. Lactuca subsect. Cyanicae, form clades with species from related genera and are not part of the lettuce gene pool.     

Effects of podophyllin and 8-hydroxyquinoline on meiosis

The induced meiotic abnormalities as a result of sprayingVicia faba plants with aqueous saturated solutions of podophyllin and 8-hydroxyquinoline were studied. The 2 drugs induced the same types of abnormalities including lagging chromosomes, stickiness, and bridges. The main difference between the 2 agents was the induction of polyploid P.M.Cs. by 8-hydroxyquinoline.     

The development of nuclear vacuoles during meiosis in plants

Vacuoles formed by the invagination of the inner membrane of the nuclear envelope have been observed during meiotic prophase in a wide range of plants. In the angiosperm Lycopersicon their formation was found to coincide with the completion of synaptonemal complex formation, and this timing is analogous to that observed during this stage in the silkworm Bombyx. The implications of this activity in relation to the process of chromosome movement are discussed. In the gymnosperm Pinus, the heterosporous fern Marsilea and homosporous ferns Pteridium and Dryopteris the formation of nuclear vacuoles begins much earlier, coinciding with the condensation of chromatin during leptotene. They enlarge and become more elaborate as meiosis proceeds, and may eventually become detached from the nuclear envelope. It is therefore thought unlikely that theyfulfil functions connected with chromosome movement in the manner proposed for the silkworm and the tomato. During diplotene/diakinesis they contain electron-opaque granules and fibrils, and the possible origin and significance of this material is discussed.     

The cytogenetics of homologous chromosome pairing in meiosis in plants

Three activities hallmark meiotic cell division: homologous chromosome pairing, synapsis, and recombination. Recombination and synapsis are well-studied but homologous pairing still holds many black boxes. In the past several years, many studies in plants have yielded insights into the mechanisms of chromosome pairing interactions. Research in several plant species showed the importance of telomere clustering on the nuclear envelope (telomere bouquet formation) in facilitating alignment of homologous chromosomes. Homologous pairing was also shown to be tied to the early stages of recombination by mutant analyses in Arabidopsis and maize. In contrast, little is known about the mechanisms that guide homolog interaction after their rough alignment by the bouquet and before the close-range recombination-dependent homology search. The relatively large and complex genomes of plants may require additional mechanisms, not needed in small genome eukaryotes, to distinguish between local homology of duplicated genes or transposable elements and global chromosomal homology. Plants provide an excellent large genome model for the study of homologous pairing and dissection of this process.     

Transactivation of Ds elements in plants of lettuce (Lactuca sativa)

The maize transposable element, Activator (Ac), is being used to develop a transposon mutagenesis system in lettuce, Lactuca sativa. In this paper, we describe somatic and germinal transactivation of Ds by chimeric transposase genes in whole plants. Constructs containing either the Ds element or the Ac transposase open reading frame (ORF) were introduced into lettue. The Ds element was located between either the 35S or the Nos promoter and a chimeric spectinomycin resistance gene (which included a transit peptide), preventing expression of spectinomycin resistance. The genomic coding region of the Ac transposase was expressed from the 35S promoter. Crosses were made between 104 independent R₁ plants containing Ds and three independent R₁ plants expressing transposase. The excision of Ds in F₁ progenies was monitored using a phenotypic assay on spectinomycin-containing medium. Green sectors in one-third of the F₁ families indicated transactivation of Ds by the transposase at different developmental stages and at different frequencies in lettuce plants. Excision was confirmed using PCR and by Southern analysis. The lack of green sectors in the majority of F₁ families suggests that the majority of T-DNA insertion sites are not conducive to excision. In subsequent experiments, the F₁ plants containing both Ds and the transposase were grown to maturity and the F₂ seeds screened on medium containing spectinomycin. Somatic excision was again observed in several F₂ progeny; however, evidence for germinal excision was observed in only one F₂ family.     

Tryptamine and related compounds in plants

The occurrence of the tryptamines and related compounds in fungi and higher plants is listed on a taxonomic basis. Several of these amines have considerable physiological activity in higher animals.     

Phenethylamine and related compounds in plants

A comprehensive and up-to-date listing is provided of the distribution of phenethylamines in the Plant Kingdom. Such a listing is of importance because of their considerable physiological activity in higher animals. Their distribution in plants is also of some taxonomic interest.     

Effects of several meiotic mutations on female meiosis in maize

A modified enzyme digestion technique of ovary isolation followed by staining and squash preparation has allowed us to observe female meiosis in normal maize meiotically dividing megaspore mother cells (MMCs). The first meiotic division in megasporogenesis of maize is not distinguishable from that in mi-crosporogenesis. The second female meiotic division is characterized as follows: (1) the two products of the first meiotic division do not simultaneously enter into the second meiotic division; as a rule, the chalazal-most cell enters division earlier than the micropylar one, (2) often the second of the two products does not proceed with meiosis, but degenerates, and (3) only a single haploid meiotic product of the tetrad remains alive, and this cell proceeds with three rounds of mitoses without any intervening cell wall formation to produce the eight-nucleate embryo sac. This technique has allowed us to study the effects of five meiotic mutations (aml, aml-pral, afdl, dsy *-9101, and dvl) on female meiosis in maize. The effects of the two alleles of the aml gene (aml and aml-pral) and of the afdl and dsy *-9101mutations are the same in both male and female meiosis. The aml allele prevents the entrance of MMCs into meiosis and meiosis is replaced by mitosis; the aml-pral permits MMCs to enter into meiosis, but their progress is stopped at early prophase I stages. The afdl gene is responsible for substitution of the first meiotic (reductional) division by an equational division including the segregation of sister chromatid centromeres at anaphase I. The dsy * -9101 gene exhibits abnormal chromosome pairing; paired homologous chromosomes are visible at pachytene, but only univalents are observed at diakinesis and metaphase I stages. These mutation specific patterns of abnormal meiosis are responsible for the bisexual sterility of these meiotic mutants. The abnormal divergent shape of the spindle apparatus and the resulting abnormal segregation of homologous chromosomes observed in micro-sporogenesis in plants homozygous for the dv1 mutation have not been found in meiosis of megasporogenesis. Only male sterility is induced by the dv1 gene in the homozygous condition. © 1993 Wiley-Liss, Inc.     

Effects of kaurane diterpene derivatives on germination and growth of Lactuca sativa seedlings

Kaurenoic and grandiflorenic acid, isolated from Wedelia paludosa (Asteraceae), some derivatives from these acids (alcohols, esters, amides, lactones, oximes) and other naturally occurring kaurane diterpenes were tested for their action on the growth of radical and shoot of Lactuca sativa. Gibberellic acid, GA3, a commercially available phytohormone, belonging to the same class of diterpenes, was also tested. Some of the tested substances showed a remarkable activity either in the inhibition or in stimulation of L. sativa growth. The activity, in some cases, was even higher than that of GA3.     

二氢血根碱对莴苣幼苗根生长及其细胞分裂的影响

以莴苣幼苗为受体,用培养皿法检测从细果角茴香中分离得到的二氢血根碱对莴苣幼苗根生长和根毛发育的影响,并采用根尖细胞有丝分裂检测和单细胞凝胶电泳法对其可能的作用机制进行了初步研究.结果显示:较低浓度(25、50μmol/L)二氢血根碱能显著促进莴苣根的生长,较高浓度(200、300μmol/L)二氢血根碱显著抑制根的生长;二氢血根碱(10、20、30、40、50μmol/L)对莴苣幼苗根毛发育有极显著的抑制作用,且两者均表现了浓度依赖性.较低浓度(25、50μmol/L)二氢血根碱使根尖细胞有丝分裂指数显著增加,而对根尖细胞DNA没有显著影响;较高浓度(200、300μmol/L)二氢血根碱使根尖细胞有丝分裂指数显著下降,同时根尖细胞DNA受到显著性损伤.研究发现,低浓度的二氢血根碱对莴苣幼苗根生长的促进作用主要是由于根尖细胞有丝分裂活力增加所致;而高浓度二氢血根碱对莴苣幼苗根的抑制作用极可能是由于根尖细胞DNA受到损伤,使得细胞分裂活力降低,分裂期细胞数目减少,从而导致根生长受到抑制.     

An assessment of genetic variability in somacloned lettuce plants (Lactuca sativa) and their offspring

Plants were regenerated from callus derived from cotyledons and first true-leaves of the lettuce cultivars Salad Bowl, Lobjoits Cos and Pennlake. Sexual progeny of these regenerants were assessed under glasshouse and field conditions for variation including reaction to lettuce mosaic virus (LMV) and downy mildew (Bremia lactucae). All three cultivars exhibited somaclonal variation. Mutations detected at the seedling stage included reduced vigour, albinism and changes in chlorophyll content, with most being recessive. Variation for leaf shape and vigour was detected in mature plants. One line exhibited increased yield and chlorophyll content together with early flowering. Enhanced and reduced susceptibility to both LMV and B. lactucae were observed. Reduced susceptibility to B. lactucae was indicated by an extended latent period following inoculation in two lines. Reduced susceptibility to LMV in glasshouse trials could not be confirmed in the field although one such line exhibited an improved yield and a second line segregated 1:1 in glasshouse tests for plants which were obviously infected and those without symptoms. All variable lines were diploid.