Effects of hardening and freezing stress on membrane lipids and CO2 fixation of Ceratodon purpureus protonemata

Membrane lipids and steady-state CO₂ fixation rates were studied in moss protonemata in order to evaluate separately the effects of growth temperature, freezing stress and the achievement of frost hardiness. Protonemata of Ceratodon purpureus (Hedw.) Brid, were grown at 20 and 4°C and parts of both materials were then hardened. The low growth temperature increased the content and unsaturation level of membrane lipids significantly. This did not, however, cause a noticeable increase in the frost hardiness. Nor was the achievement of frost hardiness in this material accompanied by further changes in the amount or unsaturtion level of any membrane lipid class. Cytoplasmic membranes were abundant in both unhardened and hardened materials grown at 4°C, which agreed with the high phospholipid content of these protonemata. The only significant difference in membrane lipids between unhardened and hardened materials was a 50% lower level of trans 16:1 fatty acid in the phosphatidylglycerol fraction of hardened protonemata.
In hardened protonemata monogalactosyldiacylglycerol (MGDG) was the membrane lipid most liable to decrease during the freeze-thaw cycle. The loss of MGDG was accompanied by partial inhibition of CO₂ fixation. Provided the content of phospholipids remained unchanged (freeze-thaw cycle with – 10°C in hardened protonemata), this inhibition was mostly reversible. If loss of the phospholipids also had occurred during the freeze-thaw cycle, as was the case in unhardened material at or below -10°C, CO₂ fixation was severely and nearly irreversibly inhibited after thawing.     

Hydroxyproline O‐arabinosyltransferase mutants oppositely alter tip growth in Arabidopsis thaliana and Physcomitrella patens

Hydroxyproline O‐arabinosyltransferases (HPATs) are members of a small, deeply conserved family of plant‐specific glycosyltransferases that add arabinose sugars to diverse proteins including cell wall‐associated extensins and small signaling peptides. Recent genetic studies in flowering plants suggest that different HPAT homologs have been co‐opted to function in diverse species‐specific developmental contexts. However, nothing is known about the roles of HPATs in basal plants. We show that complete loss of HPAT function in Arabidopsis thaliana and the moss Physcomitrella patens results in a shared defect in gametophytic tip cell growth. Arabidopsis hpat1/2/3 triple knockout mutants suffer from a strong male sterility defect as a consequence of pollen tubes that fail to fully elongate following pollination. Knocking out the two HPAT genes of Physcomitrella results in larger multicellular filamentous networks due to increased elongation of protonemal tip cells. Physcomitrella hpat mutants lack cell‐wall associated hydroxyproline arabinosides and can be rescued with exogenous cellulose, while global expression profiling shows that cell wall‐associated genes are severely misexpressed, implicating a defect in cell wall formation during tip growth. Our findings point to a major role for HPATs in influencing cell elongation during tip growth in plants.     

Promotion of Pylaisiella selwynii growth and gametophore formation by octopine and cytokinin

Growth, branching and gametophore formation by protonema developing from spores of the moss Pylaisiella selwynii were promoted by octopine, an unusual amino acid found in crown gall tumors. Cytokinin (0.01 μ M ) in combination with octopine (0.1 μ M to 1 m M ) increased the number of gametophores formed and decreased the time required for their development. The combined effect of these two compounds was similar to that obtained with virulent agrobacteria. At higher cytokinin concentrations, protonemal buds formed abnormal masses of cells rather than normal gametophores. Common amino acids and auxins, alone or in combination with cytokinin, had relatively little effect on the development of the moss.     

Branch formation induced by microbeam irradiation ofAdiantum protonemata

Branches were induced in centrifugedAdiantum protonemal cells by partial irradiation with polarized red light. Nuclear behavior and microtubule pattern change during branch formation were investigated. A branch formed at any part where a red microbeam was focused along a long apical cell. The nucleus moved towards the irradiated area and remained there until a branch developed. The pattern of microtubules changed from parallel to oblique at the irradiated area and then a transverse arrangement of microtubules appeared on both sides of the area. It appeared as if the nucleus was suspended between two microtubule rings. This nuclear behavior and the changes in microtubule pattern were different from those observed during branch formation under whole cell irradiation. From the results of this work we suggest that there is an importance for precise control of experimental conditions.     

The Protonema of Chara fragilis Desv.: Regenerative Formation,Photomorphogenesis, and Gravitropism

When exposed to constant white light for four weeks, isolated nodes of Chara fragilis Desv. regenerated side branches, rhizoids, and multicellular protonemata, the latter being similar to those germinated from oospores. When kept in darkness the nodes developed protonemata exclusively. These were single-celled, colourless, and tip-growing and, with the light microscope, they looked like rhizoids. Upon exposure to blue light, but not to red or far-red, the growth rates of the protonemata rapidly declined, the cell apices swelled, and the nucleus migrated acropetally. Within 24 h the cells went through the first of a series of divisions resulting in the formation of multicellular protonemata. When returned to darkness after a blue light pulse of 5 h the cell divisions proceeded normally, but the protonemata showed etiolated growth. While growth of the internode was drastically promoted, the development of the multicellular apex and the lateral initial were suppressed. Both uni- and multicellular etiolating protonemata showed negative gravitropism but were phototropically insensitive. It is argued that the single-celled protonema is an organ specialized for the penetration of mud covering the nodes or oospores of Chara and thus serves to search for light, comparable to etiolated hypocotyls and stems in seedlings of higher plants.     

Change in the rate of organelle movement during progression of the cell cycle inAdiantum protonemata

Summary The rate of organelle movement during progression of the cell cycle in single-celled protonemata of the fernAdiantum capillus-veneris is determined microscopically with a time-lapse video system. Under red light organelle movement is very slow (1.8 m/min) in early G₁ in the apical 100-m region. The rate of organelle movement becomes higher in proportion to distance from the nuclear region, reaching a plateau in the neighborhood of 300 m from the tip. Organelle movement during the progression of G₁ and S phases in the dark does not show a significant difference from that in early G₁ under red light. In M phase, however, organelle movement in the nuclear region slows down a few minutes after nucleolar disappearance and then stops until the beginning of cell plate formation. Organelle movement in the basal region of the protonema slows down, but does not stop, shortly after movement in the nuclear region has ceased. This indicates that a message is sent from the nuclear region to the basal region.     

Generational differences in response to desiccation stress in the desert moss Tortula inermis

BACKGROUND AND AIMS: Active growth in post-embryonic sporophytes of desert mosses is restricted to the cooler, wetter months. However, most desert mosses have perennial gametophytes. It is hypothesized that these life history patterns are due, in part, to a reduced desiccation tolerance for sporophytes relative to gametophytes. METHODS: Gametophytes with attached post-embryonic sporophytes of Tortula inermis (early seta elongation phenophase) were exposed to two levels of desiccation stress, one rapid-dry cycle and two rapid-dry cycles, then moistened and allowed to recover, resume development, and/or regenerate for 35 d in a growth chamber. KEY RESULTS: Gametophytes tolerated the desiccation treatments well, with 93 % survival through regenerated shoot buds and/or protonemata. At the high stress treatment, a significantly higher frequency of burned leaves and browned shoots occurred. Sporophytes were far more sensitive to desiccation stress, with only 23 % surviving after the low desiccation stress treatment, and 3 % surviving after the high desiccation stress treatment. While the timing of protonemal production and sporophytic phenophases was relatively unaffected by desiccation stress, shoots exposed to one rapid-dry cycle produced shoots more rapidly than shoots exposed to two rapid-dry cycles. CONCLUSIONS: It is concluded that sporophytes of Tortula inermis are more sensitive to rapid drying than are maternal gametophytes, and that sporophyte abortion in response to desiccation results from either reduced desiccation tolerance of sporophytes relative to gametophytes, or from a termination of the sporophyte on the part of the gametophyte in response to stress.     

垂蒴真藓原丝体发育特征的研究

在室内培养垂蒴真藓(Bryum uliginosum)孢子的基础上,对其孢子萌发、原丝体发育及配子体发生的全过程进行了跟踪观察。结果表明：该种孢子萌发后直接产生绿丝体。而轴丝体和假根仅在绿丝体上产生;配子体原始细胞产生于绿丝体基部细胞或轴丝体上。由此可以看出：垂蒴真藓属于真藓型孢子萌发型(Bryum-type)。     

N-(2-氯-4-吡啶基)-N'-苯基脲(CPPU)对无疣墙藓原丝体发育与芽体发生的影响

0.2、0.4、1.0及1.5mg·L-1 4个浓度的N-(2-氯-4-吡啶基)-N'-苯基脲(CPPU)对无疣墙藓的孢子萌发没有显著影响,但抑制绿丝体伸长和侧枝生成,对芽体的分化有明显的促进,且芽体发生数随浓度升高而增加.