J. A. Van Assche

Heavy metal tolerance in some ectomycorrhizal fungi

The metal tolerance of isolates of some common ectomycorrhizal fungi of Pin us and Betula spp., collected on heavily metal-polluted sites was compared with that of strains of the same species from non-polluted areas. The strains isolated from carpophores on unpolluted soils were greatly inhibited by the metals, whereas most of the strains derived from polluted soils were strongly tolerant to heavy metals: some strains were still able to grow very well at concentrations of 1000 Vwg g-1 zinc. The mechanisms for zinc (Zn) and copper (Cu) tolerance are completely different, since zinc tolerance does not result in copper tolerance and vice versa. We suppose that ectomycorrhizal fungi are submitted to a selection for metal tolerance in polluted soils. The possible role of tolerant strains in excluding metals from the host plant is discussed. Key-words: Metal tolerance, ectomycorrhizal fungi, zinc, copper, Pinus, Betula, Suillus, Amanita, Paxillus

The role of temperature on the dormancy cycle of seeds of Rumex obtusifolius L.

Seeds of Rumex obtusifolius L. were buried at different depths in bare soil, under a grass turf and under shrubs. The temperature was measured daily at seed level. During 3 years, monthly samples were exhumed and the germination was tested at different temperature regimes. The dormancy levels follow a seasonal pattern. Additional germination experiments were set up in order to explain the underlying principles of a season-sensing mechanism of buried seeds. In this respect, the importance of a sudden shift to higher temperatures for induction of germination is demonstrated. Key-words: Dormancy, Rumex, germination, temperature, seeds

Trehalase Activity in Dormant and Activated Spores of Phycomyces blakesleeanus

SummaryHeat treatment of Phycomyces sporangiospores, which breaks dormancy, causes a very rapid 10- to 15fold increase in trehalase activity; soon after the heat shock the enzyme activity decays. This phenomenon can be repeated several times by repeating the heat shocks. Prolonging the heat treatment over the minimum required time delays the decay of enzyme activity. Cycloheximide does not prevent the rise in enzyme activity. It is suggested that heat treatment converts temporarily an inactive form of trehalase into an active one. Optimal enzyme activity is obtained at pH 7.5 and the enzyme requires metal ions for maximal activity. The possible role of trehalase in the spore-activation process is discussed.

Cell wall carbohydrates in Phycomyces blakesleeanus burgeff

The carbohydrate composition of the cell walls from spores, mycelium and sporangiophores of Phycomyces blakesleeanus was analyzed. Spore wall polysaccharides contained over 50% glucose, about 20% uronic acids, 10% mannose and 10% amino-sugars. During the growth of the hyphae amino-sugars became the main carbohydrate (45%); uronic acids contributed some 25%, glucose and fucose 10% and galactose nearly 6%. Sporangiophores contained almost 90% aminosugars and some 6% uronic acids. Traces of rhamnose were found in all wall preparations. A similar picture emerged from studies on the incorporation of [U-14C]-glucose into wall materials.Furthermore we looked for a GDP-fucose synthesizing system and found an increasing activity during early germination. This rise in activity was inhibited by cycloheximide but not by 5-fluorouracil.

Effect of 5-fluorouracil and cycloheximide on the early development of Phycomyces blakesleeanus spores and the activity of N-acetylglucosamine synthesizing enzymes

The development of germinating Phycomyces spores was not inhibited by 5-fluorouracil (1 mM) until the emergence of the germination tube. Fluorouracil was incorporated into RNA as efficiently as uracil; it did not inhibit the synthesis of proteins and the increase in respiratory activity during early develpment. Cycloheximide inhibited development as well as the increase in respiration and protein synthesis. This suggested that protein synthesis or some other cycloheximide dependent process, but no mRNA synthesis, was needed for the first developmental stages. The activity of two enzymes involved in the synthesis of N-acetylglucosamine increased markedly during germination. This increase was inhibited by both 5-fluorouracil and cycloheximide; this suggested that those enzymes were synthesized on mRNA formed during germination.

The pattern of protein and nucleic acid synthesis in germinating spores of Phycomyces blakesleeanus

SummarySynthesis of proteins, RNA and DNA is measured by incorporation of labelled precursors at different times during germination of Phycomyces spores.RNA and protein synthesis increases immediately after activation. DNa synthesis begins at a later stage (± 8 h) of germination when germ tubes are already present. Nuclear division occurs earlier in germination (±4–5 h) and is accompanied by a decrease in RNA synthesis. It can be concluded that at least most of the dormant spores are in the G2 phase of the cell cycle.Analysis of ribosomal RNA after pulse-chase labelling shows only three labelled compounds: a precursor molecule (2.25×106 daltons) and the two mature ribosomal RNA compounds (1.4×106 and 0.7×106 daltons). This suggests that the two rRNAs are formed directly from the precursor molecule. Cycloheximide totally blocks the transformation of the ribosomal precursor molecule into mature rRNA.

Temperature conditions control embryo growth and seed germination of Corydalis solida (L.) Clairv., a temperate forest spring geophyte

Spring is often the most suitable period for seedling establishment of temperate woodland species. Different physiological mechanisms resulting in spring emergence have evolved in seeds of such plants. The aim of this study was to determine the requirements for breaking dormancy and for seed germination of the European perennial spring geophyte Corydalis solida (Fumariaceae). Ripe seeds of C. solida contain an underdeveloped embryo, consisting of no more than a clump of cells. As a consequence, the embryo has to differentiate and grow to a critical length before germination can occur. In nature, seeds are dispersed in spring, while growth of the embryo starts in the autumn and continues in winter. Germination starts in late winter, immediately after embryo growth is completed, resulting in seedling emergence in the following spring. Experiments in controlled conditions showed that temperature is the main factor controlling dormancy and germination. Incubation at autumn temperatures (15/6 degrees C; 20/10 degrees C) for at least 8 weeks is required to initiate embryo growth, while a transfer to 5 degrees C is needed for completion of embryo growth and germination. Growth of the embryo of C. solida occurs at different temperatures over an extended period, a feature typical of temperate forest herbs. Our results indicate that the dormancy mechanism in seeds of C. solida is very similar to mechanisms in other Corydalis species studied thus far, suggesting that stasis in the dormancy trait has occurred.

A combined physical and physiological dormancy controls seasonal seedling emergence of Geranium robertianum

Temperate forest herbs with seeds exhibiting both a physical and a physiological dormancy mechanism are rare, and knowledge on the factors regulating germination of these species is fragmentary. The biennial Geranium robertianum L. grows mainly in temperate woodlands, but can also be found in exposed habitats. Seedlings of G. robertianum are known to emerge from spring until autumn, but little is known about the environmental factors regulating germination. In this study, phenology of seedling emergence and of physical dormancy loss was examined for seeds buried at shaded or sunny exposed locations. The role of temperature in regulating dormancy and germination was analysed by incubating seeds in temperature sequences simulating temperatures that seeds experience in nature. The results indicate that most seeds of G. robertianum buried in sunny conditions germinate immediately after physical dormancy loss in summer. Seeds buried in shaded conditions also lose physical dormancy mainly during summer, but remain physiologically dormant and do not germinate until late winter or early spring. Besides physical dormancy, seeds of G. robertianum also initially have a high level of physiological dormancy, which is reduced during dry storage. Physiological dormancy is reduced through chilling in winter, thus enabling the seeds to germinate at low temperatures. We conclude that a complex combination of physical and physiological dormancy ensures that G. robertianum seeds germinate in summer at exposed sites and in early spring at shaded sites.

EFFECT OF N-ALCOHOLS AND HIGH PRESSURE ON THE HEAT ACTIVATION OF THE SPORES OF PHYCOMYCES BLAKESLEEANUS

Publisher Summary This chapter discusses the effect of N-alcohols and high pressure on the heat activation of the spores of Phycomyces blakesleeanus. The dormancy and germination of fungal spores has been studied intensively as it is considered to be a model system for the study of cell development. Dormant fungal spores do not germinate when they are placed in a medium containing all the nutrients necessary for growth and complete development. The dormancy of the spores can be broken by a heat treatment. The dormancy of the spores of Phycomyces blakesleeanus can be broken completely by a heat treatment of 3 min at 50°C. When pressure is applied during the activation of the spores, two kinds of effects are seen: (1) at low pressures, the activation temperature is raised by about 2–4 °C per 1000 atm and (2) at higher pressures, however, the effect is reversed and a lowering of the activation temperature by about 5°C per 1000 atm occurs in the pressure range of 1000–2000 atm. The dual effect of pressure on the activation of the spores clearly indicates a protein involvement—low pressures counteract, while high pressures enhance the thermal denaturation of proteins.