Elimination of hop latent viroid upon developmental activation of pollen nucleases

Hop latent viroid (HLVd) is not transmissible through hop generative tissues and seeds. Here we describe the process of HLVd elimination during development of hop pollen. HLVd propagates in uninucleate hop pollen, but is eliminated at stages following first pollen mitosis during pollen vacuolization and maturation. Only traces of HLVd were detected by RT-PCR in mature pollen after anthesis and no viroid was detectable in in vitro germinating pollen, suggesting complete degradation of circular and linear HLVd forms. The majority of the degraded HLVd RNA in immature pollen included discrete products in the range of 230–100 nucleotides and therefore did not correspond to siRNAs. HLVd eradication from pollen correlated with developmental expression of a pollen nuclease and specific RNAses. Activity of the pollen nuclease HBN1 was maximal during the vacuolization step and decreased in mature pollen. Total RNAse activity increased continuously up to the final steps of pollen maturation. HBN1 mRNA, which is abundant at the uninucleate microspore stage, encodes a protein of 300 amino acids (34.1 kDa, isoeletric point 5.1). Sequence comparisons revealed that HBN1 is a homolog of S1-like bifunctional plant endonucleases. The developmentally activated HBN1 and pollen ribonucleases could participate in the mechanism of HLVd recognition and degradation.

Pollen Development in Orchids. 5. A Generative Cell Domain Involved in Spatial Control of the Hemispherical Cell Plate

The unequal first pollen mitosis in moth orchids (Phalaenopsis) is followed by an unusual form of cytokinesis that isolates a small lens-shaped generative cell from a large vegetative cell. No preprophase band of microtubules predicts the division plane and the new cell plate grows completely around the generative cell rather than fusing with the parental wall. Development of the phragmoplast cytoskeleton consisting of fusiform bundles of microtubules and F-actin occurs in three major stages: (1) the initial asymmetrical phragmoplast conforming to the shape of the interzonal region, which tapers from the broad mass of chromosomes at the generative pole to the rounded mass at the vegetative pole; (2) the symmetrical plate-like phragmoplast; and (3) the hemispherical phragmoplast, which curves around the generative nucleus. Microtubules of the generative half of the hemispherical phragmoplast are nuclearbased, while those on the vegetative side terminate in endoplasmic reticulum. The path of the phragmoplast appears to outline a cytoplasmic domain denned by a radial system of microtubules emanating from the generative nucleus.

Differential effects of specific chromosomal deficiencies on the development of the maize pollen grain

A study was made of the differential effects of specific chromosomal deficiencies on the development of the maize pollen grain. Twenty-six B–A translocations involving 17 of the 20 chromosome arms were used to produce hypoploid plants in which one half of the microspores had a predictable chromosomal deletion. Breakpoints of the translocations were proximally located in most cases, although some were more distal. Deficient and normal male gametophytes from these hypoploids were studied cytologically to characterize developmental changes. Generally, loss of part of a chromosome arm caused abnormal microspore development, a slowing of the normal mitotic or developmental processes in the male gametophyte, or a termination of development. Slowing of development was observed as early as the quartet stage in deficient microspores from TB-1Sb and TB-9Sd hypoploids, while in others the developmental delay occurred later, mostly during the first pollen mitosis. The abnormal or blocked development associated with other deficiencies began as early as the quartet stage in deficient microspores of TB-1La hypoploids and as late as the first mitotic telophase in those of TB-6Lb, TB-6Lc, TB-9La, and TB-9Lc. The developmental modifications induced by the diverse deficiencies are dependent on the particular segment lost, demonstrating that components of normal microspore development identified in this study are controlled by genes located in specific parts of the genome.Key words: pollen, gametophyte, deletion, B–A translocations.

Maize microsporogenesis

The use of maize (Zea mays L.) pollen for basic scientific research has been well documented, but the progression of clear cytological features of maize microsporogenesis has not been fully documented. This study was undertaken to identify cytologically the different developmental stages of maize pollen and to correlate them with morphological features of the developing maize tassel. Morphological changes in the length of the tassel, floret, and anther were recorded and correlated with six cytologically defined stages of microsporogenesis: premeiosis, meiosis, uninucleate stage, first pollen mitosis, second pollen mitosis, and mature pollen.Key words: cytogenetics, gametophyte, maize, microsporogenesis, pollen.

Cytological aspects of isolated microspore culture of Brassica napus

Preculture cytological events within anthers of two genotypes of Brassica napus were examined to detect differences between microspores that have embryogenic potential and those that do not. Microspores of five anthers per bud were cultured, while the sixth anther was fixed for cytological observation. A series of buds of increasing maturity were individually sampled from specific racemes. Correlations of bud, anther, microspore, and nuclear size were drawn to establish physical parameters for each cytological stage. Cytological events and cytophotometrically monitored DNA content were noted for spores of each anther size class. Best embryogenic responses were among populations of microspores in the late uninucleate stage, immediately prior to first pollen mitosis. Typical, rod-shaped, cotyledonous embryos could be generated within 30 days from microspores at this stage at a rate up to 1300/anther. Slightly younger or older microspores had a drastically reduced embryogenic performance. Postculture observations (within 6 h after culture) indicated that the embryogenic spores appeared spherically swollen, distinctly vacuolate, and with a clear cytoplasm.

Extrême précocité et conditions thermiques du développement apical et floral chez Claytonia caroliniana var. caroliniana

The apical and floral development of Claytonia caroliniana var. caroliniana has been studied concurrently with soil temperature, in a sugar maple forest of the Stoneham mountain, Québec. Apical cellular activity begins early in May, while the flowering stems of the year are present. At the beginning of July, external apical development becomes visible. In the first days of August, 9 months before flowering, the foliar and floral structures of the next year are already present in the soil. Meiosis takes place at the beginning of October and first pollen mitosis follows shortly after, in the middle of the same month. From that time, well developed individuals, without chlorophyll, are present just under the litter. They can occasionally turn green and reach the upper surface of the litter in November or December, where they will spend wintertime under the snow, at a temperature oscillating between 0 and −4 °C. This behaviour is quite close to the survival strategy of hemicryptophytes. The active epigeous growth period begins in the middle of April, with the melting of snow. Second pollen mitosis and flowering take place at this time, rapidly followed by seed setting, dissemination, and destruction of the aerial portion of the plant. Cytoecological investigations to study possible influence of environmental factors on chromosomal anomalies in primordia should thus be conducted during the year preceding the flowering of Claytonia.

Relative rate of development of aneuploid and euploid microspores in a tertiary trisomic of rye, Secale cereale L.

Meiotic configurations were studied in pollen mother cells of a tertiary trisomic of rye. Chains of five and chains of three, in alternate orientation, were the most frequent configurations. Assuming loss of univalents in anaphase I or single chromatids in anaphase II, a total of 58.1% of the viable gametes resulting after meiosis were expected to contain the normal haploid complement, whereas 41.9% were expected to have the translocated chromosome in addition. The percentages of uninucleate and binucleate microspores in anthers containing dividing microspores provided a time scale for the development of euploid and aneuploid spores during first pollen mitosis. Microspores containing the extra translocated chromosome tended to divide at a later stage than euploid microspores. The slower development was also illustrated by the course of the mitotic index of both types. It was found that 58.1% of all microspores passing through pollen mitosis contained seven chromosomes and 41.9% contained eight chromosomes, which means that up to the end of first pollen mitosis aneuploid spores were not lost significantly more than euploid spores. It is likely that the delay in development already starts immediately after meiosis.Key words: rye, tertiary trisomic, euploid microspores, aneuploid microspores, rate of development.

Biology of Australian seagrasses: Pollen development and submarine pollination in Amphibolis antarctica and Thalassodendron ciliatum (Cymodoceaceae)

Development of the filiform pollen of the sea nymph Arnphibolis antarctica (Labill.) Sonder & Aschers. ex Aschers. has been characterized by high resolution light and electron microscopic methods. First pollen mitosis occurs at the end of the young spore period immediately preceding the vacuolate period, in contrast to many terrestrial pollens. Mature pollen is trinucleate, and is spirally coiled within the anther. The mature pollen wall shows a positive reaction for acid phosphatase like the intine of terrestrial pollens but is devoid of the outer exine layer, as judged by light and electron microscopic evidence. Development and arrangement of Thalassodendron ciliatum (Forssk.) Den Hartog pollen are similar. The adaptation of the pollen of aquatic plants for submarine pollination is reviewed in the light of evidence from 18th and 19th century work.