Susan D. Waaland

CHLOROPLAST STRUCTURE AND PIGMENT COMPOSITION IN THE RED ALGA GRIFFITHSIA PACIFICA: REGULATION BY LIGHT INTENSITY1

The ratio of accessory phycobiliproteins to chlorophyll a is controlled by light intensity in the marine red alga Griffithsia pacifica. The greatest changes in pigment ratios are observed below 300 ft‐c; above 300 ft‐c the response approaches saturation. Ultrastructural examination of chloroplasts of plants grown at different intensities reveals that the number of phycobilisomes per unit of photosynthetic thylakoid changes in direct proportion to the pigment ratios and in inverse proportion to the light intensity.

Analysis of Cell Elongation in Red Algae by Fluorescent Labelling

SummaryThe mechanism of cell elongation in five red algae, Griffithsia pacifica Kylin, G. tenuis C. Agardh, G. globulifera Harvey, Antithamnion kylinii Gardner, and Callithamnion sp. was studied using Calcofluor White ST as a vital, fluorescent cell-wall stain. In each alga elongation of intercalary shoot cells occurs primarily by the localized addition of new cell-wall material rather than by extension of pre-existing cell wall. Cell extension is localized in narrow bands in the lateral walls of a cell; there may be one or two bands per cell and these may be located at the top or bottom of the lateral wall. The number and location of bands of elongation are constant within a species but vary from species to species. Cell walls of elongating intercalary cells of each of these algae are essentially isotropic, indicating a net random orientation of cell-wall microfibrils.

Morphogenesis in the Red Alga, Griffithsia pacifica: Regeneration from Single Cells

SummaryThe regeneration of plants of the red alga Griffithsia pacifica from single, isolated cells is described. Regeneration can start from any cell and is triggered by the removal of an abutting cell. An isolated, single shoot cell forms a shoot and a rhizoidal cell within one day. The shoot then adds new cells by apical division at the rate of 1–2 cells/day; branches are formed at predictable but not fixed locations by budding of subapical cells. Each shoot cell enlarges for 6–8 days. The resulting plant consists of uniseriate, pseudodichotomously-branched shoot filaments with multicellular rhizoidal filaments at their base. The predictability and rapidity of this development combined with the large size of these cells (1.0×0.2 mm) facilitate developmental studies on this organism.

Development in the red alga, Griffithsia pacifica: Control by internal and external factors

SummaryDevelopment in the red alga Griffithsia pacifica is affected by both external and internal factors. Under 16:8 photoperiods, both cell division and cell elongation show a diurnal rhythm. The rhythm of division persists for at least 7 cycles in continuous light, and can be reset; this indicates that the timing of cell division is controlled by an endogenous rhythm. Both cell division and elongation require light, but the rate of division of apical cells and the rate of cell elongation are both relatively insensitive to either light intensity or photoperiod. In contrast. division in nodal cells, which leads to branch formation, is strongly promoted by high light intensity or long photoperiods. By manipulating the conditions of illumination, one can obtain Griffithsia plants varying from unbranched to highly branched.

Cell repair through cell fusion in the red alga, Griffithsia pacifica

SummaryWhen an intercalary shoot cell of the red algaGriffithsia pacifica is killed, the cell may be replaced through the wound-healing process of cell repair. During cell repair the cells on either side of the dead cell cut off new cells towards the dead cell. The superjacent cell produces a rhizoid; the subjacent cell produces an atypical shoot cell. The two new cells grow towards each other through the lumen of the dead cell. When they meet, they fuse; the resulting cell expands laterally to fill the cavity of the dead cell and is transformed into a typical intercalary shoot cell, morphologically and physiologically indistinguishable from the killed cell it replaces. The entire cell repair process takes 24–30 hours. Three aspects of cell repair suggest that intercellular communication occurs across the dead cell; these are a precocious division of the cell below the dead cell, a reversible change in the morphology and growth of the shoot cell which participates in repair, and a definite attraction between the two cells which fuse. Thus during cell repair we find evidence not only for cellular redifferentiation through cell fusion, but also for extracellular substances which change pathways of morphogenesis.

Evidence for a species-specific cell fusion hormone in red algae

SummaryIn plants of the red algal genus,Griffithsia, dead intercalary cells are often replaced by the process of cell repair. During this process two cells are produced: one a rhizoid, induces the formation of, stimulates the elongation of and attracts the second, a repair shoot cell. The two cells fuse to form a single shoot cell. The interactions between the rhizoid and the repair shoot appear to be mediated through a diffusible hormone,rhodomorphin. It is possible to induce cell repair, including cell fusion, between two different plants by juxtaposing a rhizoid and a freshly decapitated filament (in vitro cell repair). Rhizoids of three different species ofGriffithsia (G. pacifica, G. tenuis, andG. globulifera) were tested in thein vitro cell repair system to see if rhizoids of one species could induce repair shoot formation and cell fusion in filaments of the other two species. Cell fusion was only induced between plants of the same species; there was no interspecific cell fusion. This suggests that the cell fusion hormone of each species is specific for that species and cannot induce cell repair in other species.When three strains of the speciesG. pacifica were tested in thein vitro repair system, rhizoids of each strain induced repair shoots in decapitated filaments of all three strains and successful fusions occurred in each case. Thus hybrid cells were produced which contained nuclei of different genetic content. However there was evidence of cytoplasmic incompatibility in hybrid cells of some of the crosses.

Isolation of a cell-fusion hormone from Griffithsia pacifica kylin, a red alga.

Filaments of Griffithsia pacifica replace dead cells by the process of cell repair. When an intercalary cell is killed, but its cell wall remains intact holding the two halves of the plant together, the cell above it produces a repair rhizoid cell; the cell below it produces a specialized, rhizoid-like repair shoot cell. The repair rhizoid and shoot grow towards each other, meet, and fuse to form a single shoot cell. Evidence from observations of cell repair in vivo has indicated that the repair rhizoid produces a hormone or hormones which induce the production of the repair shoot, maintain the rhizoid-like morphology and growth of the repair shoot, and attract it to the repair rhizoid for fusion. This hormone has been named rhodomorphin. Using an artificial cell-fusion system we show that repair rhizoids and normal rhizoids, but no shoot cell, can induce decapitated filaments to form repair shoot cells. Decapitated filaments form repair shoot cells only when they are exposed to the hormone within 4–6 h after decapitation; after this time they lose their sensitivity to the hormone. A method has been developed for isolating, and assaying for, the cell-fusion hormone. Rhodomorphin retains its activity for several days at room temperature and for at least two years at-16° C.

An investigation of the role of transcellular ion currents in morphogenesis ofGriffithsia pacifica Kylin

SummaryTranscellular ion currents are thought to play a role in the induction and maintenance of localized growth in plant cells. In the marine red algaGriffithsia pacifica, two types of cells elongate by localized tip growth, rhizoidal and repair shoot cells. The pattern of growth and morphogenesis in these cells can be altered by environmental and hormonal parameters. We examined the role of localized currents in four developmental processes inG. pacifica: 1. normal elongation of rhizoids, 2. the phototropic response of rhizoids, 3. the re-initiation of growth in dark-starved rhizoids, and 4. morphogenesis of repair shoot cells in the presence and absence of rhodomorphin, an endogenous hormone which regulates growth of these cells.We have found that there is a localized region of inflowing current at the growing tips of both rhizoids and repair shoot cells. The current density at these apices, measured approx. 20 μm from the cell surface, fluctuates in the range of 0.6 to 8 μA cm−2 with occasional periods of either very large current (> 20 μA cm−2) or no measurable current; however, the current density is not correlated with the rate of elongation. In addition, currents of similar magnitudes are found at the tips of non-growing cells. Rhizoids which have lost their cytoplasmic polarity and have stopped elongating, following prolonged periods in total darkness, can reestablish a polar distribution of organelles and restart localized growth in the absence of any measurable current at their tips. Thus, it appears that inG. pacifica localized transcellular currents are neither sufficient or necessary for the maintenance or reinitation of sites of localized growth and organelle accumulation.

Parasexually produced hybrids between female and male plants of Griffithsia tenuis C. Agardh, a red alga.

Somatic cell fusion between vegetative cells of a male and a female isolate of Griffithsia tenuis, a marine red alga, has been obtained. Hybrid cells have been isolated and they have regenerated new plants. Almost all these hybrid plants made reproductive structures. In nearly half these cases the first 3–10 cells of the hybrid filament produced reproductive structures chracteristic of the tetrasporic (diploid) phase rather than the sexual (haploid) phase of the life cycle of this alga. However as these filaments continued to grow, cells further along the filament began to produce sexual, either female or male, reproductive structures. The observations suggest that the production of tetrasporangial branches does not require the fusion of male and female nucleic; rather, male and female nucleic remaining separate, act in concert to produce these structures, and in subsequent cell divisions the nuclei of one sex may be left behind allowing the nuclei of the remaining sex to direct the production of sexual branches.

Cytoplasmic incompatibility following somatic cell fusion in Griffithsia pacifica Kylin, a red alga

SummarySomatic cell fusion between two isolates ofG. pacifica is followed by a cytoplasmic incompatibility reaction (CIR) in the cytoplasm donated by only one of the isolates. This CIR is characterized by the aggregation, fusion and lysis of chloroplasts of the sensitive strain; the chloroplasts of the other strain are unaffected. In addition, the nuclei of both strains retain a normal distribution during the fusion and lysis events. Cell elongation and nuclear division stop in CIR-affected cells. The CIR begins in the hybrid cell and then appears sequentially in adjacent cells of the sensitive strain; this transfer occurs only between living cells which share a crosswall. There is a lag between hybrid cell formation and the initiation of the CIR. This lag is more than 3 times as long at 17 ‡C than at 24 ‡C; over this range, the rate of movement of the CIR along a filament is temperature-insensitive. Thus it appears that a temperature dependent process, perhaps the synthesis of CIR-inducing agents, is required for the initiation of the CIR; subsequent movement of such agents appears to occur by diffusion.