Plant regeneration via somatic embryogenesis in the seeded diploid banana Musa ornata Roxb

Somatic embryos of a seeded diploid ornamental banana (Musa ornata Roxb.) were obtained from zygotic embryos cultured on semi-solid Murashige and Skoog (MS) (1962) medium with the auxin 2,4-D (0.5, 1, 2 mg/l) and 5% CW. Removal of 2,4-D and transferral to Schenk and Hildebrandt (SH) (1972) salts with CW followed by basal MS led to embryo germination and growth. Plantlet production was obtained using filter paper bridges in liquid half-strength SH medium with 1% sucrose. The remarkable phenotypic fidelity of somatic embryos to that of zygotic embryos and the presence of a haustorium-like outgrowth on the somatic embryos is described.

Coordinate accumulation of homeologous transcripts of seven cotton Lea gene families during embryogenesis and germination

One of two related patterns of total transcript accumulation are seen during embryogenesis for 18 cotton Lea (Late embryogenesis-abundant) gene families in the allotetraploid cotton Gossypium hirsutum L. cv Coker 201. Coordinate accumulation in each class is complex, suggesting that Lea mRNA abundance is regulated by several events. Each of the Lea gene families probably contains two active homeologous genes (alloalleles), one in each of cotton's two subgenomes. It is of interest whether both transcripts of a Lea family are regulated the same or whether the complexity of total transcript accumulation is due to different regulation of the two transcripts. The two polypeptides encoded by the homeologous transcripts of 7 Lea families can be distinguished on two-dimensional gels. The majority of Lea transcripts in total RNAs is shown to be functional in vitro throughout development; thus in vitro translation should faithfully measure their relative abundance. The ratio of the two transcripts of each was followed during embryo maturation when Lea transcript concentration increases an average of 70-fold and during the first 12 hr of germination when Lea transcripts decline in concentration an average of 50-fold. For 6 of the Lea families, the relative level of the two transcripts is invariant throughout this period. The two constituent transcripts of the seventh, Lea9, change 5-fold in relative concentration during late maturation and 3-fold during germination. Both transcripts still follow the same temporal pattern of accumulation; only their rates of change are somewhat different. These Lea families are a random sample of the 18 described. If multiple events affect the mRNA abundance of each Lea family, such events then each affect their individual transcripts in a similar way.

Accumulation kinetics of cotton late embryogenesis-abundant mRNAs and storage protein mRNAs: coordinate regulation during embryogenesis and the role of abscisic acid

The accumulation of total RNA transcripts of 18 late embryo-abundant (Lea) gene families, each encoding two closely related Lea mRNAs, was measured in cotyledon total RNA during embryogenesis and germination of Gossypium hirsutum L. by RNA dot hybridization. Transcript abundance of the three storage protein families was also followed. The Lea mRNAs belong to only two related groups of commonly regulated mRNAs. The transcript level of each of the 6 members of Class I has two transient maxima during early maturation and a maximum level at 3 days prior to desiccation. The transcript level of each of the 12 members of Class II increases abruptly in late maturation with a maximum concentration at 3 days before desiccation (Class IIA) or at desiccation (Class IIB). Several patterns of early accumulation also exist within Class II, some overlapping with those of storage protein or Lea Class I mRNAs. The concentrations of Lea mRNAs increase at least 10- to 1700-fold during embryogenesis and decline 15- to 220-fold during the first day of germination. Earlier studies indicated that most Lea mRNAs, but no storage protein mRNAs, are induced in excised embryos exposed to abscisic acid (ABA). Free (+)ABA was measured during embryogenesis using a monoclonal antibody-based ELISA. The ABA concentration shows maxima correlating with the maxima in early Class I Lea mRNA abundance. However, ABA declines during the accumulation of Class I and Class II Lea mRNAs in late embryogenesis. Consequently, ABA could be an endogenous regulator of Class I Lea mRNAs during early maturation but is not the primary regulator of Lea mRNAs during late maturation. In contrast, cotyledon water potential exhibits a decrease that correlates with the late induction of Lea mRNAs. These findings are consistent with the hypothesis that low water potential is the endogenous regulator of some ABA-inducible mRNAs. The cessation of vascular flow to the embryo may also be involved in the disappearance of storage protein mRNAs and the late induction of Lea mRNAs.

Ionic currents traversing cell clusters from carrot suspension cultures reveal perpetuation of morphogenetic potential as distinct from induction of embryogenesis

Patterns of ionic currents accompanying shape and surface changes during growth of cell clusters from carrot suspension cultures were examined using a vibrating probe. Electrical polarity was established in clusters undergoing apparently disorganized proliferation in the presence of 2,4-dichlorophenoxyacetic acid (2,4-D). This electrical polarity is similar to that found in organized somatic embryos which form upon removal of 2,4-D, and implies that the proliferating clusters are actually suppressed embryos. This implication is further supported by observations using scanning electron microscopy. We conclude that the potential to undergo embryogenesis is present even before the 2,4-D is removed, but cannot be realized in the presence of the auxin.

Unusual patterns of somatic embryogenesis in the domesticated carrot: developmental effects of exogenous auxins and auxin transport inhibitors

The effects of various exogenous auxins and polar auxin transport inhibitors on somatic embryogenesis in carrot cultures were investigated. Indole-3-acetic acid and 2,4-dichlorophenoxyacetic acid do not disrupt the sequence or the polarity of individual stages in embryo development, but tend to cause developing embryos to revert to undifferentiated callus, with increasing frequency in later embryo stages. The transport inhibitors, N-(1-naphthyl)phthalamic acid and 2,3,5-triiodobenzoic acid, block morphological transitions to the subsequent stage; for example, they cause the formation of enlarged globular and oblong embryos. Heart embryos in these treatments usually develop additional lateral growth axes. These results shed light on the role of auxin and its polar transport in somatic embryogenesis.

Concepts of target cells in plant differentiation

The regeneration of whole plants from fragments of mature organs is compelling evidence that plant cells retain genetic totipotency throughout differentiation. Within the intact plant, however, strict regulatory controls operate to maintain a co-ordinated pattern of growth. Not every cell differentiates along the same developmental pathway. Cell performance is determined by mechanisms that permit subtle discriminations in recognition of and response to an array of environmental and hormonal cues. Much effort is currently directed to understanding these control systems in plants. Certain positionally differentiated cells and tissues have been characterized by their specific signal recognition and their precise responses in gene expression. In a few, the competence to respond to particular signals has been distinguished by the presence of cell-specific protein markers. This article discusses some recent studies that help towards an understanding of the target nature of cells in plant development.

Plant seed embryogenesis as a tool for molecular biology

Aspects of the embryogenesis of higher plants that are of interest to molecular and cell biologists are reviewed. What is known about the changing population of developmentally regulated mRNA is summarized, and the properties of the gene products that most distinguish embryogenesis, the nutritional storage proteins, are collated.

Partial purification and properties of a transamidinase from Lathyrus sativus seedlings. Involvement in homoarginine metabolism and amine interconversions

A transamidinase was purified 463-fold from Lathyrus sativus seedlings by affinity chromatography on homoarginine--Sepharose. The enzyme exhibited a wide substrate specificity, and catalysed the reversible transfer of the amidino groups from donors such as arginine, homoarginine and canavanine to acceptors such as lysine, putrescine, agmatine, cadaverine and hydroxylamine. The enzyme could not be detected in the seeds, and attained the highest specific activity in the embryo axis on day 10 after seed germination. Its thiol nature was established by strong inhibition by several thiol blockers and thiol compounds in the presence of ferricyanide. In the absence of an exogenous acceptor, it exhibited weak hydrolytic activity towards arginine. It had apparent mol.wt. 210000, and exhibited Michaelis--Menten kinetics with Km 3.0 mM for arginine. Ornithine competitively inhibited the enzyme, with Ki 1.0 mM in the arginine--hydroxylamine amidino-transfer reaction. Conversion experiments with labelled compounds suggest that the enzyme is involved in homoarginine catabolism during the development of plant embryo to give rise to important amino acids and amine metabolites. Presumptive evidence is also provided for its involvement in the biosynthesis of the guanidino amino acid during seed development. The natural occurrence of arcain in L. sativus and mediation of its synthesis in vitro from agmatine by the transamidinase are demonstrated.

[Different behaviour of paternal and maternal genomes during embryogenesis in the fern, Marsilea (author's transl)]

The DNA of gametes of the water fern, Marsilea vestita, has been radioactively labelled during gemetogenesis. The distribution of the originally present paternal and maternal genomes could thus be followed during the development of the embryo. It has been found that the maternal chromatin is uniformly distributed among the daughter cells of the several successive cell generations. However, the paternal chromatin divides equally only during the first three embryonic divisions. From this stage on, the partition varies according to the different areas of the embryo. Only the organogenic cells retain a high degree of radioactivity. This pattern of inheritance concerns chiefly the apical cells of shoots and roots. This peculiar behaviour of the paternal genome can be explained in different ways. At present, a non-random distribution of paternal chromatids, i. e. a selective segregation of old and new strands of DNA to opposite poles of embryonic mitoses, appears most likely.

[Accelerated production of permanent preparations of whole plant embryo sacs and pollen complexes]

Original methods are described for producing constant preparations of plant integral embryo sacs and pollen complexes isolated by means of tissue maceration with Helix pomatia gastric juice enzymes. The Feulgen stained preparations are put into the Canada balsam. According to this procedure there is no need of paraffin treatment and work with a microtome; it makes use of other methods which highly accelerate the production of constant preparations. In connection with putting the object in Canada balsam the method of treatment and the succession of operations are changed as compared to the routine procedure of producing total preparations of embryo sacks.

Pollen ultrastructure in anther cultures of Datura innoxia. II. The generative-cell wall

In young pollen grains of Datura innoxia, a wall of the usual hemispherical type separates the 2 gametophytic cells initially and, in the electron microscope, appears as an electron-translucent matrix which is contiguous with the intine. Before detachment of the generative cell from the intine, the matrix decreases in thickness and in places is dispersed altogether leaving the plasmalemmae on either side of it in close apposition. A particularly prominent zone, triangular in profile, is left where the wall joins with the intine. After detachment of the cell, remnants of the matrix can be seen distributed irregularly around the cell and it is supposed that these are partly derived from material in the triangular zone as the cell is drawn away from the intine. The wall residues persist throughout the maturation phase of the pollen and are considered to be either callose resulting from incomplete digestion of the initial wall, or some other polysaccharide material which is unevenly laid down along the wall and concentrated at the junction with the intine. In pollen induced into embryogenesis by anther culture, wall material is also distributed irregularly around the detached cell in a series of discrete zones, but these are more extensive than in vivo, closer together and in many instances highly dilated. The wall profiles thus have a beaded appearance, the ‘beads’ being connected together by short links of the 2 apposed plasmalemmae. The contents of the swollen zones have a similar electron density to that of the matrix in vivo but also show traces of a fibrillar component. It is postulated that this unusual swelling is a prelude to dispersal of the wall by disruption of the plasmalemmal links and to the establishment of cytoplasmic continuity between the 2 cells. The significance of such binucleate pollen grains in the formation of non-haploid embryos is discussed.

Pollen ultrastructure in anther cultures of Datura innoxia. III. Incomplete microspore division

During the microspore division in Datura innoxia, the mitotic spindle is oriented in planes both perpendicular (PE) and oblique (OB) to the spore wall against which the nucleus is situated. However, irrespective of polarity, the usual type of hemispherical wall is laid down at cytokinesis and isolates the generative cell from the rest of the pollen grain (type A). In PE spores the vegetative nucleus initially occupies a central position in the pollen grain, whereas in OB spores the vegetative nucleus lies at the periphery of the grain close to the generative cell. In anther cultures initiated just before the microspore division is due to take place, no marked change can be observed in either orientation or symmetry of the mitotic spindle when the spores divide. In some, however, cytokinesis is disrupted and deposition of the hemispherical wall arrested. In the absence of a complete wall, differentiation of the generative cell cannot take place and binucleate pollen grains are formed having 2 vegetative-type nuclei (type B). The 2 nuclei in the B pollens are always situated against the pollen-grain wall, suggesting that the disruption phenomenon is related to the OB spores. The incomplete wall always makes contact with the intine on the intine-side of the spindle. Wall material may be represented merely as short stubs projecting out from the intine into the cytoplasm, in which event the 2 nuclei lie close to each other and are separated by only a narrow zone of cytoplasm. In other grains the wall is partially developed between the nuclei and terminates at varying distances from the tonoplast; in these, the nuclei are separated by a wider zone of cytoplasm. The significance of these binucleate grains in pollen embryogenesis is discussed.

Pollen ultrastructure in anther cultures of Datura innoxia. I. Division of the presumptive vegetative cell

Ultrastructural features of embryogenic pollen in Datura innoxia are described, just prior to, during, and after completion of the first division of the presumptive vegetative cell. In anther cultures initiated towards the end of the microspore phase and incubated at 28 degrees C in darkness, the spores divide within 24 h and show features consistent with those of dividing spores in vivo. Cytokinesis is also normal in most of the spores and the gametophytic cell-plate curves round the presumptive generative nucleus in the usual highly ordered way. Further differentiation of the 2 gametophytic cells does not take place and the pollen either switches to embryogenesis or degenerates. After 48–72 h, the remaining viable pollen shows the vegetative cell in division. The cell, which has a large vacuole and thin layer of parietal cytoplasm carried over from the microspore, divides consistently in a plane parallel to the microspore division. The dividing wall follows a less-ordered course than the gametophytic wall and usually traverses the vacuole, small portions of which are incorporated into the daughter cell adjacent to the generative cell. The only structural changes in the vegetative cell associated with the change in programme appear to be an increase in electron density of both plastids and mitochondria and deposition of an electron-dense material (possibly lipid) on the tonoplast. The generative cell is attached to the intine when the vegetative cell divides. Ribosomal density increases in the generative cell and exceeds that in the vegetative cell. A thin electron-dense layer also appears in the generative-cell wall. It is concluded that embryogenesis commences as soon as the 2 gametophytic cells are laid down. Gene activity associated with postmitotic synthesis of RNA and protein in the vegetative cell is switched off. The data are discussed in relation to the first division of the embryogenic vegetative cells in Nicotiana tabacum.

Orderly arrangement of ribosomes in the embryogenic callus tissue of Corylus avellana L

Cultured callus tissue of hazel (Corylus avellana L.), which has the potency of somatic embryogenesis, was used for the study of cell ultrastructure in the course of callus growth and embryoid formation. The meristematic cells of this tissue exhibit a specific organization of rough endoplasmic reticulum (RER), stacked into extensive parallel sheets. The membranes of the aggregated RER are associated with orderly arrays of bound ribosomes. The high regularity of the alignment of the attached ribosomes seems to be influenced by the distance between the two neighbouring membranes in the RER aggregate. The RER aggregates with orderly attached ribosomes are more frequently found in callus cells and in early embryogenesis than in the advanced stages of embryo development.