Ultrastructural analysis of cell component distribution in the apical cell of Ceratodon protonemata

Quantitative analysis of organelle distribution and dynamics in Physcomitrella patens protonemal cells

BACKGROUND

In the last decade, the moss Physcomitrella patens has emerged as a powerful plant model system, amenable for genetic manipulations not possible in any other plant. This moss is particularly well suited for plant polarized cell growth studies, as in its protonemal phase, expansion is restricted to the tip of its cells. Based on pollen tube and root hair studies, it is well known that tip growth requires active secretion and high polarization of the cellular components. However, such information is still missing in Physcomitrella patens. To gain insight into the mechanisms underlying the participation of organelle organization in tip growth, it is essential to determine the distribution and the dynamics of the organelles in moss cells.

RESULTS

We used fluorescent protein fusions to visualize and track Golgi dictyosomes, mitochondria, and peroxisomes in live protonemal cells. We also visualized and tracked chloroplasts based on chlorophyll auto-fluorescence. We showed that in protonemata all four organelles are distributed in a gradient from the tip of the apical cell to the base of the sub-apical cell. For example, the density of Golgi dictyosomes is 4.7 and 3.4 times higher at the tip than at the base in caulonemata and chloronemata respectively. While Golgi stacks are concentrated at the extreme tip of the caulonemata, chloroplasts and peroxisomes are totally excluded. Interestingly, caulonemata, which grow faster than chloronemata, also contain significantly more Golgi dictyosomes and fewer chloroplasts than chloronemata. Moreover, the motility analysis revealed that organelles in protonemata move with low persistency and average instantaneous speeds ranging from 29 to 75 nm/s, which are at least three orders of magnitude slower than those of pollen tube or root hair organelles.

CONCLUSIONS

To our knowledge, this study reports the first quantitative analysis of organelles in Physcomitrella patens and will make possible comparisons of the distribution and dynamics of organelles from different tip growing plant cells, thus enhancing our understanding of the mechanisms of plant polarized cell growth.

UV-A induces two calcium waves in Physcomitrella patens

Our understanding of the role of Ca2+ in blue/UV-A photoreceptor signaling in a single cell is limited. Insight into calcium signaling has now been attained in Physcomitrella patens and its cryptochrome and phototropin knock-outs. Physcomitrella patens caulonemal filaments grow in the dark by apical extension and their apical cells are highly polarized. Fura-2-dextran ratio images of the apical cell from wild type (WT), Ppcry1a/1b and PpphotA2/B1/B2 were obtained immediately following UV-A exposure (30 microW cm(-2) at 340 nm for 1,000 ms plus 30 microW cm(-2) at 380 nm for 1,000 ms) [abbreviated as 1,000 ms (340/380 nm)] and demonstrated two intracellular waves: a Ca2+ wave from the growing apical tip through the apical cap, and a wave from the junction of the neighboring cell through the vacuolar, nuclear and plastid regions. In WT, the UV-A-induced tip wave increase had a magnitude of 454.0 +/- 40 nM, traveled at a rate of 3.4 +/- 0.7 microm s(-1) and was complete within 26.6 +/- 2.3 s, while the basal vacuolar wave had a magnitude of 596.8 +/- 110 nM, a rate of 8.4 +/- 0.8 microm s(-1) and duration of 25.3 +/- 4.9 s. Subsequent Ca2+ spikes of similar magnitude followed these waves. The amplitude of the Ca2+ waves in the apical cap and basal vacuolar regions of Ppcry1a/1b were higher than those in the WT, while the duration of those in PpphotA2/B1/B2 was longer. Subsequent Ca2+ spikes occurred in WT and Ppcry1a/1b but not in PpphotA2/B1/B2. When Mn2+ was added to the culture medium, the [Ca2+](cyt) increase was delayed, did not move as a wave and lasted longer. The results indicate that plants respond to blue light and UV-A radiation by generating a wave of changes in the [Ca2+](cyt). The characteristics of these Ca2+ waves were dependent upon cryptochrome and phototropin. Blue/UV-A signaling in P. patens appears to differ from that in Arabidopsis.

The density of apical cells of dark-grown protonemata of the moss Ceratodon purpureus

Determinations of plant or algal cell density (cell mass divided by volume) have rarely accounted for the extracellular matrix or shrinkage during isolation. Three techniques were used to indirectly estimate the density of intact apical cells from protonemata of the moss Ceratodon purpureus. First, the volume fraction of each cell component was determined by stereology, and published values for component density were used to extrapolate to the entire cell. Second, protonemal tips were immersed in bovine serum albumin solutions of different densities, and then the equilibrium density was corrected for the mass of the cell wall. Third, apical cell protoplasts were centrifuged in low-osmolarity gradients, and values were corrected for shrinkage during protoplast isolation. Values from centrifugation (1.004 to 1.015 g/cm3) were considerably lower than from other methods (1.046 to 1.085 g/cm3). This work appears to provide the first corrected estimates of the density of any plant cell. It also documents a method for the isolation of protoplasts specifically from apical cells of protonemal filaments.

Curvature induced by amyloplast magnetophoresis in protonemata of the moss Ceratodon purpureus

After gravistimulation of Ceratodon purpureus (Hedw.) Brid. protonemata in the dark, amyloplast sedimentation was followed by upward curvature in the wild-type (WT) and downward curvature in the wwr mutant (wrong way response). We used ponderomotive forces induced by high-gradient magnetic fields (HGMF) to simulate the effect of gravity and displace the presumptive statoliths. The field was applied by placing protonemata either between two permanent magnets at the edge of the gap, close to the edge of a magnetized ferromagnetic wedge, or close to a small (<1 mm) permanent magnet. Continuous application of an HGMF in all three configurations resulted in plastid displacement and induced curvature in tip cells of WT and wwr protonemata. WT cells curved toward the HGMF, and wwr cells curved away from the HGMF, comparable to gravitropism. Plastids isolated from protonemal cultures had densities ranging from 1.24 to 1.38 g cm-3. Plastid density was similar for both genotypes, but the mutant contained larger plastids than the WT. The size difference might explain the stronger response of the wwr protonemata to the HGMF. Our data support the plastid-based theory of gravitropic sensing and suggest that HGMF-induced ponderomotive forces can substitute for gravity.