Fine structure of protonemal apical cells of the mossPhyscomitrium turbinatum

Summary Elongating caulonemal apical cells of the mossPhyscomitrium turbinatum were cultivatedin vitro and observed during successive stages of cell elongation and division. Actively-growing cells which had completed approximately half of their growth in length were examined by electron microscopy. The distribution of many organelles changes progressively from the cell tip to the distal edge of the large basal vacuole, establishing an apical-basal gradient in organization. Whereas the vacuoles become progressively more extensive in more mature parts of the cell, the dictyosomes, chloroplasts and smooth endoplasmic reticulum are more numerous in younger regions. Some mitochondria in the younger regions of the cell contain localized areas of membrane invagination. Attempts were made to clarify the origin and growth of vacuoles, which become increasingly prominent as the apical cell elongates.Morphological evidence suggests that vacuoles arise in close association with endoplasmic reticulum and dictyosomes as a result of ER dilation and/or cytoplasmic sequestration. The number of vacuolar profiles is highest at the cell tip, decreasing progressively toward the base of the cell, Conversely, the mean area of vacuolar profiles increases progressively toward more basal regions of the cell. These features, along with the increasing number of closely grouped vacuolar profiles along an apical-basal gradient are compatible with the concept of vacuolar growth by coalescence, culminating in their union with the basal vacuole.     

Unequal distribution of nuclear pores in oryzalin-induced mini-nuclei in protonema cells of the mossFunaria hygrometrica: influence of the nucleolus

Summary Mini-nuclei, formed in tip cells ofFunaria caulonemata after oryzalin treatment, have unequally distributed nuclear pores. The region of the nuclear envelope near the nucleolus, in a distance of up to 3 m, is devoid of pores. In other areas pores occur with a high frequency.     

Blue light-regulation of cell division in Adiantum protonemata: an approach with pulse stimulation

Abstract Cell division of the single-celled Adiantum protonemata produced by red-light (RL) incubation of germinated spores is induced by transfer to darkness and is stimulated by blue light (BL). It is known that the cellular process leading to this cell division includes one cell cycle and the BL response results from shortening of the Gl phase. The authors studied this BL regulation of cell cycle by giving a pulse of BL after RL termination and measuring changes in the proportion of divided cells. To minimize phytochrome responses arising from BL irradiation, the plants were kept in continuous far-red light instead of total darkness after the RL incubation. The response to a pulse (10–100 s) approached saturation with increasing rluences in a manner that reciprocity is valid. The sensitivity to BL, investigated by measuring the response to a saturating pulse, showed an increase in the first several hours after RL termination, followed by a sustained sensitivity for 20 h. Time courses of the pulse-induced responses showed a lag of about 12 h, which was considerably shorter than in the non-stimulated control; the lag was approximately independent of the strength of BL stimulation or the timing of BL application after RL termination, and the major difference occurred in the slope. It is concluded that the sensitivity to BL is retained during the time span in which the dark-dependent Gl phase progresses, and that the BL response is initiated independently of the reactions involved in the dark-dependent Gl phase. A minimal reaction model of Gl phase is suggested to unify the results.     

Blue light-regulation of cell division in Adiantum protonemata: kinetic properties of the photosystem

Abstract Blue-light-induced cell division in single-celled Adiantum protonemata was studied by using two or three pulses (pulse duration: 30 s) separated by various periods and by using relatively long irradiation (e.g. 30 min). The results showed: (1) that the response is saturated by a single pulse, (2) that after the application of a saturating pulse, the protonemata gradually become responsive to another pulse, showing time-dependent saturation to the second pulse, and (3) that although reciprocity holds in the pulse-induced response, it becomes invalid as the exposure duration extends in the range of minutes. These results were analysed in view of a reaction model in which a molecular component is considered to exist in two forms A and B. The response to a single pulse is considered to result from a light-dependent conversion of the component (A→B), and the restoration response measured by two pulses, from its dark reversion (B→A). The analyses yielded (1) the value of the constant which relates the fluence rate to the rate constant of the light-induced reaction, and (2) the rate constant of the dark reaction. The model was extended to formulate the responses to long irradiations as a function of the integrated concentration of B over time. The responses predicted by the formula by using the parameter values estimated from the pulse responses were able to explain the responses measured for long irradiation.     

Does the silver moss Bryum argenteum exhibit sex-specific patterns in vegetative growth rate,asexual fitness or prezygotic reproductive investment?

Background and Aims

Expected life history trade-offs associated with sex differences in reproductive investment are often undetected in seed plants, with the difficulty arising from logistical issues of conducting controlled experiments. By controlling genotype, age and resource status of individuals, a bryophyte was assessed for sex-specific and location-specific patterns of vegetative, asexual and sexual growth/reproduction across a regional scale.

Methods

Twelve genotypes (six male, six female) of the dioecious bryophyte Bryum argenteum were subcultured to remove environmental effects, regenerated asexually to replicate each genotype 16 times, and grown over a period of 92 d. Plants were assessed for growth rates, asexual and sexual reproductive traits, and allocation to above- and below-ground regenerative biomass.

Key Results

The degree of sexual versus asexual reproductive investment appears to be under genetic control, with three distinct ecotypes found in this study. Protonemal growth rate was positively correlated with asexual reproduction and sexual reproduction, whereas asexual reproduction was negatively correlated (appeared to trade-off) with vegetative growth (shoot production). No sex-specific trade-offs were detected. Female sex-expressing shoots were longer than males, but the sexes did not differ in growth traits, asexual traits, sexual induction times, or above- and below-ground biomass. Males, however, had much higher rates of inflorescence production than females, which translated into a significantly higher (24x) prezygotic investment for males relative to females.

Conclusions

Evidence for three distinct ecotypes is presented for a bryophyte based on regeneration traits. Prior to zygote production, the sexes of this bryophyte did not differ in vegetative growth traits but significantly differed in reproductive investment, with the latter differences potentially implicated in the strongly biased female sex ratio. The disparity between males and females for prezygotic reproductive investment is the highest known for bryophytes.     

Correlation of electrical current influx with nuclear position and division inFunaria caulonema tip cells

Summary Ionic currents around caulonema tip cells of the filamentous protonema of the mossFunaria hygrometrica were examined using a nonintrusive vibrating microelectrode to map electrical current before and during mitosis. Tip cells in interphase generate inward electrical currents that are maximal at the nuclear region. These currents remain concentrated over the nucleus as it migrates forward maintaining a constant distance from the growing tip. Just prior to mitosis this inward current increases twofold. During mitosis and cytokinesis current at the nuclear zone increases to four times the resting level and fluctuates, falling to zero after cell plate fusion with parental walls. The locus of outward current could not be dectected. These results suggest that plasma membrane ion currents may regulate both nuclear positioning and subsequent temporal and spatial control of cell division.     

Development and differentiation in plants

An overview of plant development has been presented. In lower plants like mosses, auxin and cytokinin regulate the creation and the stability of the differentiated state of various cell types. The differentiated state is plastic and all cell types dedifferentiate to ground state, the chloronema. Even in higher plants, embryonic cells become only roughly committed during shoot meristem formation. Their terminal destiny becomes specified during the post-germination phase when the rough outline gets refined. The lack of a firm determined state, clonally heritable through mitosis, indicates that the development in plants is unlikely to be specified by a rigid programme.     

Storage of the phytochrome-mediated phototropic stimulus of moss protonemal tip cells

A phytochrome-regulated phototropic response of the moss Ceratodon purpureus was investigated. Chlorotetracycline (CTC) was used to visualize membrane-associated calcium gradients in the tip cell of moss caulonemal filaments. A tip-to-base Ca²⁺ gradient was observed. The ionophore monensin rapidly inhibited the growth of the tip cell and abolished the CTC fluorescence. Six hours after transferring to inhibitor-free medium, protonemal growth resumed and reached the normal growth rate within 12 h. The growth was accompanied by a reappearance of the CTC-fluorescence gradient. Unilateral irradiation given during the monensin treatment or after the treatment during the period when growth inhibition persisted led, with the re-initiation of growth, to a typical positive phototropic bending in complete darkness. Far-red light applied just before the growth response started, or during growth inhibition, abolished the phototropic response. The phytochrome-mediated signal was qualitatively (position) and quantitatively (degree of bending) memorized. Signal perception and response could be separated temporally. This result indicates that at least under some circumstances, e.g. under the influence of monensin, the phytochrome-mediated signal can be stored for several hours in darkness. Calcium seems to be essential for the processing of polar growth only. A specific function (second messenger) in phytochrome-dependent signal transduction could not be confirmed.Abbreviation CTC chlorotetracycline     

Epiphytic Bacteria Associated with the Bryophyte Funaria hygrometrica: Effects of Methylobacterium Strains on Protonema Development

Abstract: Bacteria represent a substantial fraction of the microorganisms that inhabit leaf surfaces. We collected samples of the moss Funaria hygrometrica (L.) in the field and analysed the epiphytes on the gametophyte by the agar impression method and scanning electron/fluorescence microscopy. On the phylloid surface numerous bacteria were detected, notably in the grooves between adjacent lamina cells. Methanol-ammonium salts agar surfaces impressed with isolated phylloids of green gametophytes resulted in the growth of methylotrophic colonies. Two Methylobacterium strains ( M. mesophilicum and M. sp., isolated from the Funaria phylloids) were found to simulate the well-known effect of cytokinin application on bud formation in Funaria protonemata. In addition, Methylobacterium inoculation promoted the growth of protonemal filaments. The significance of this novel Methylobacterium -land plant interaction is discussed.