Development of the resting sporangia ofSynchytrium endobioticum,the causal agent of potato wart disease

Summary An ultrastructural study of the development of the resting sporangium ofSynchytrium endobioticum (Schilb.) Perc. infecting potato cells is presented. The resting sporangium is found to have a single large, centrally placed nucleus with a prominent nucleolus through its entirein situ development. The cytoplasmic organization of the resting sporangium is further characterized by numerous membrane-bound lipid bodies and osmiophilic bodies. The latter have a characteristic sieve-like appearance, probably because certain storage components have been extracted during preparation for electron microscopy. Because of the similar location and appearance of these osmiophilic bodies it is suggested that they are identical to what has earlier (based on light microscopy) been described as chromatin granules; and the ultrastructural studies presented here show that nucleolar discharge which was described from light microscopic observations as leading to chromatin granules in the cytoplasm, and finally forming the nuclei of the zoospores (bally 1912,curtis 1921,percival 1910) simply does not occur.The appearance of dense fibrillar-like structures on the sporangial surface at an early stage of resting sporangium development ultrastructurally distinguishes the resting sporangium from the zoosporangium. The development of the layered portion of the thick sporangial wall is shown to be due to the fusion of vacuoles containing pre-made wall fibrils with the cell membrane. It is suggested that the inner compact wall layer which is essentially substructureless is formed by the membrane itself.The characteristic wings of the matureS. endobioticum resting sporangium originate from the potato host cell wall. Remnants of host cell organelles in the outermost layer of the resting sporangium wall show that degradation of the host cell cytoplasm contributes to wall formation of the parasite.     

Germination and parasitation of the resting sporangia ofSynchytrium endobioticum

Summary The cytoplasmic organization of the long-lived, thick walled resting stage of the sporangium ofSynchytrium endobioticum (Schilb.) Perc. is described. The cytoplasm of the resting sporangium contains a large number of closely packed lipid bodies and irregular electron dense bodies, which are interspaced with fine channels of cytoplasm. These ultrastructural observations are discussed in relation to the hypothesis ofBally (1912) andCurtis (1921) that zoospore primordia are already present during the resting stage. It is shown that the zoospore primordium is actually a lipid body and an osmiophilic body and the strands postulated to connect the individual zoospore primordia are actually the fine channels of cytoplasm.A new inner wall layer is laid down prior to the start of the germination. It is this wall layer which will protrude to form the vesicle in which sporogenesis takes place. The germination process observed, protrusion of a vesicle through a crack in the sporangial wall, the migration of the sporangial content into the vesicle, and the formation of a single, membrane-bound sporangium within this vesicle, is in full agreement with the recent light microscopic studies ofSharma andCammack (1976). These observations support the transfer ofS. endobioticum from the subgenusMesochytrium to the subgenusMicrosynchytrium (bothsensu Karling 1964).A major objective of the study, to obtain ultrastructural evidence for the location of the meiotic divisions in the life cycle, was not fulfilled.Three different fungi were observed to parasitize the resting sporangium ofS. endobioticum. These infections are discussed in relation to other mycoparasites of plant pathogenic fungi. The possibility of using a mycoparasite for the biological control of potato wart disease is considered to be without practical relevance.     

Meiosis in the aquatic fungusCatenaria allomycis

Summary Catenaria allomycis Couch (Blastocladiales) is an endobiotic fungal parasite primarily of species of the genusAllomyces. The life cycle ofC. allomycis contains both sexual and asexual phases. Synaptonemal complexes have been found in young developing resistant sporangia (RS) suggesting that meiosis occurs within the thick walled RS prior to syngamy. Ultrastructural evidence suggests that meiosis proceeds through pachytene in the developing RS and is arrested in diplotene of prophase I until the sporangia are induced to germinate at which time the meiotic process is completed. Quantitative nuclear counts in developing RS support the ultrastructural observations. Meiotic nuclei are characterized by polar fenestrae in the nuclear envelope and intranuclear plaque-like microtubule organizing centers (MTOC).Portion of a Ph.D. dissertation submitted by the senior author to the Graduate School, University of Georgia.     

Nodulation of actinorhizal plants byFrankia strains capable of both root hair infection and intercellular penetration

Summary A morphological analysis of the initiation and development of root nodules ofElaeagnus angustifolia andMyrica cerifera inoculated with pure-culturedFrankia strains DDB 011610 or DDB 020110 was undertaken. From ultrastructural observations it was determined that both of theseFrankia strains can infectElaeagnus by an intercellular penetration mechanism andMyrica by the root hair infection mechanism. This indicates that both of these strains have the ability to infect host plant roots by either of two mechanisms. The reverse, thatElaeagnus orMyrica could be infected by both mechanisms, was not observed. The infection and nodule development processes of these two plants in combination with these strains were similar to observations made in previous studies (Miller andBaker 1985,Torrey andCallaham 1979). However, one exception was identified in the development of the prenodule ofMyrica when infected with strain 011610, in that endophytic hyphae developed vesicles within the cells of the prenodule. This event has not been described before for any of the actinorhizal genera and may be an indication of less than optimal compatibility between the host plant and the symbiont.Contribution no. 876 of the Battelle-Kettering Laboratory.     

The activity of powdery-mildew haustoria after feeding the host cells with different sugars,as measured with a potentiometric cyanine dye

The biotrophic parasite Erysiphe graminis f. sp. hordei produces haustoria within the cells of its host Hordeum vulgare. To determine the physiological activity of these haustoria, the electric potential across the membranes in the mitochondria of the haustorium was studied. The membrane potential was estimated with the fluorescent potentiometric cyanine dye 3,3-dibutyloxacarbocyanine iodide. The addition of depolarizing agents (carbonylcyanide m-chlorophenylhydrazone, 2,4-dinitrophenol or KCN) to infected cells resulted in an increase of fluorescence after the addition of low concentrations or a decrease of fluorescence after the addition of higher concentrations. When the infected host cell was fed with increasing concentrations of d-glucose (25, 50, 75 mM), corresponding decreases of fluorescence were measured immediately in the mitochondria of the fungal haustoria. Sucrose induced a similar reduction of fluorescence about 20 min late. d-Galactose and d-fructose induced a somewhat smaller reduction of fluorescence, l-glucose and d-glucitol had no effect. The results indicate that haustoria take up glucose from the host cells immediately. Sucrose, d-galactose and d-fructose seem to require time to be metabolized before their products reach the fungal haustorium or mitochondria.Abbreviations CCCP carbonylcyanide m-chlorophenylhydrazone - DiOC₄(3) 3,3-dibutyloxacarbocyanine iodide - DNP 2,4-dinitrophenol     

The fine structure and cytology of the association between the parasitic red algaHolmsella australis and its red algal hostGracilaria furcellata

Summary Holmsella australis Noble andKraft ms. is a colourless red algal parasite, forming whitish pustules on its photosynthetic red algal host,Gracilaria furcellata Harvey. In the infected region, host cortical tissue continues to grow and enclose the expanding pustule. Filaments of both host and parasite grow apically, the cells being connected by primary pit connections (PCs). Secondary PCs form between cells of the same species, and in addition,H. australis initiates the formation of secondary PCs with cells ofG. furcellata. All three types of secondary PC are morphologically distinct. In hostparasite PCs the surface adjoining the host cell is similar in structure to a host-host PC, while that adjoining the parasite cell has the structure of a parasite-parasite PC. The plasma membrane is continuous between the cells of the unrelated host and parasite. In addition, a cap membrane is typically produced only on the host surface, though occasionally the parasite side is enclosed by a cap membrane as well. Cap membranes are absent from parasite-parasite PCs (making them intracellular), while host-host PCs are typically extracellular, both cells producing cap membranes. The presence or absence of a cap membrane in certain positions appears to vary, and suggests that cells may be able to regulate its presence. Since transport of nutrients would be expected to occur from host to parasite cells, and between parasite cells, the morphological evidence presented here suggests the PCs may be the pathway.     

Growth and reproduction in culture of ten Phytophthora fragariae races

Cultural characteristics were employed to develop a basis for characterizing each of the ten recognized races ofPhytophthora fragariae Hickman in the United States. Investigations were made into the relationships of temperature to growth and colony morphology, the effects of various media and ß-sitosterol on growth and oospore production, and relative zoosporangium production capabilities of the ten races. On the basis of this information, the ten races were divided into two groups according to their colony morphologies. Further characterization was possible on the basis of differential oospore and zoosporangium production in different media, and, to a lesser extent, with contrasting increments in mycelial mass and linear extension.     

A wet-sieving and flotation technique for the detection of resting sporangia of Synchytrium endobioticum in soil

A wet-sieving technique is described for the extraction of resting sporangia of Synchytrium endobioticum from soil. Fractions between 71 and 25 μm. were held on sieves of aperture size 40 and 25 μm. Resting sporangia in these fractions were then separated from soil particles of similar size by chloroform flotation and centrifugation. The viability of the sporangia was assessed microscopically by viewing them by transmitted light.     

An account of some physiological studies in two species of phytophthora

Summary The paper deals with some physiological studies of two typical strains ofPhytophthora palmivora Butl. andP. parasitica Dast. var.macrospora Ashby causing severe fruit rots ofAchras sapota L. andAnona squamosa L. respectively. The effect of temperature on the germination of sporangia had a marked effect. Maximum percentage of indirect germination (by formation of zoospores) occurred at 20° C, whereas direct germination of sporangia (by formation of germ tube) was at 30° C, which further continued upto 37° C. Maximum indirect germination at 20° C shows the favourable cool temperature for the epidemic of the diseases (Fruit rots) in nature. The best substrate (medium) for germination of sporangia was found to be tap water. Next to this was the host decoction or extract. Glucose solution also accelerated sporangial germination. The effect of two dyes viz., Malachite green and crystal violet was also studied in relation to growth and sporulation ofPhytophthora isolates. Their addition to the medium in various fractional dilutions had a profound influence in the rate of growth and sporulation. An interesting observation noted was that growth of the isolates was inversely porportional to the various concentrations of the dyes, under study. An attempt was also made to study the influence of various vitamins. In all, six vitamins were included in the study. Out of these, thiamine and riboflavin were found to be the best sources promoting good growth and sporulation of both the species ofPhytophthora under study.Forms a part of Senior author's M. Sc. (Agric.) Thesis, University of Poona, Poona (India).Respectively, Ex-Junior Research Fellow, I.C.A.R., New Delhi; Professor of Plant Pathology and Principa, College of Agriculture, Junagad (Gujarat); Plant Pathologist, Wheat Rust Research Station, Mahabaleshwar, Poona, India.     

Floral anatomy and systematic position ofVivianiaceae

The floral anatomy of four species ofViviania has been studied. In the basic floral plan and essential floral anatomical featuresViviana closely resembles theGeraniaceae. Evidence from vegetative and floral anatomy, ultrastructural studies on phloem as well as phytochemistry supports geranialean affinity ofViviania; the placement within thePittosporales sensuHutchinson being unnatural.     

Endophyte differentiation inCasuarina actinorhizae

Summary This is an ultrastructural study of development of infected cells in nitrogen fixing root nodules ofCasuarina spp. While several aspects of development are similar to those found in many other actinorhizae, unusual aspects of development of the host cell and differentiation of the endophyte inCasuarina are correlated with unusual changes in the wall of the infected cell. Instead of vesicles the endophyte forms atypical hyphae in mature infected cells. These unusual hyphal forms are termed intracellular hyphae. Intracellular hyphae are nonseptate hyphae which originate and terminate within the same host cell, and have a varying diameter and a multidirectional growth and branching pattern. A laminate surface layer previously undescribed on hyphae ofFrankia is a feature common to mostCasuarina endophytic hyphae and is probably similar chemically to the laminae comprising the multilamellate envelope of endophytic vesicles in other actinorhizae.This paper is Florida Agricultural Experiment Station Journal Series No. 7350.     

The structure and formation of host-parasite pit connections between the red algal alloparasiteHarveyella mirabilis and its red algal hostOdonthalia floccosa

Summary Harveyella mirabilis is a colourless red algal alloparasite which grows on and within its photosynthetic hostOdonthalia floccosa. Cells ofHarveyella establish secondary pit connections (PCs) with other parasite cells and with cells of the host. Small, uninucleate conjunctor cells are produced by parasite cells and remain connected to them by PCs. Conjunctor cells may fuse with either an adjacent host or parasite cell, with the parasite-conjunctor cell PC becoming either a host-parasite or parasite-parasite secondary PC. Occasionally the conjunctor cell does not fuse with an adjacent cell (either host or parasite) and degenerates. The secondary pit plug which forms between a parasite cell and its conjunctor cell always develops with two structurally distinct surfaces characteristic of a host-parasite pit plug. Only if the conjunctor cell fuses with another parasite cell will the structure of the pit plug be altered to that of a parasite-parasite pit plug. Fungal hyphae also invade the region of infection, andHarveyella cells respond by producing nonfunctional conjunctor cells that grow towards adjacent hyphae. Evidence suggests that secondary PCs may be induced to form mechanically, by the physical presence of another cell, rather than in direct response to a message received from an adjacent cell. The mechanism of secondary PC formation described here is similar to that reported for the closely related alloparasiteHolmsella and may be common to a number of red algal parasitic associations. Helen Margaret Quirk, B. Sc. (Hons), M. Sc. (1953–1982), student, research assistant and friend, died after a long illness on October 24, 1982.     

Chemistry and micromorphology of compound epicuticular wax crystalloids (Strelitzia type)

Certain non-liliiflorous taxa within the monocotyledons (e.g.,Strelitzia, Heliconia, Typha) are characterized by compound epicuticular wax rodlets (Strelitzia type). Similar rodlets are also encountered on the surface of the dicotyledonous plantBenincasa hispida (Thunb.)Cogn. Chemical analysis of the surface wax from both sources showed that the rodlets are chemically distinct. The rodlets of the monocotyledons consist exclusively of aliphatic wax lipids, mainly wax esters. In contrast, the rodlets ofBenincasa are cheifly composed of triterpenol acetates and triterpenols. Formation of rodlets is therefore interpreted as ultrastructural convergency. It is concluded that taxonomical studies on wax crystalloids can be misleading when interpreted in terms of micromorphology of crystalloids only.     

Monocentric fungi on species of rhizosolenia from saline habitats

Summary Eight monocentric fungi are reported in two provisionally identified species ofRhizosolenia from sea-water. Only two species are assigned specific names. These areRhizophydium planktonicum Canter andR. fragilariae Canter. A fungus with depressed-globose sporangia is included as a form ofR. fragilariae. Two unidentified species ofChytridium are reported. A fungus tentatively assigned toZygorhizidium and a form provisionally assigned toPetersenia are also reported and described insofar as their characteristics are known.     

The influence of phosphorus limitation of the diatomAsterionella Formosa on the zoospore production of its fungal parasiteRhizophydium Planktonicum

The influence of phosphorus limitedAsterionella on the zoospore production of its fungal parasiteRhizophydium planktonicum was measured, using laboratory cultures of host and parasite. At saturated phosphorus concentrations the host reached a specific growth rate of 0.95.d^–1. Growing on these host cells, the mean parasite zoospore production was 26 spores per sporangium, and the mean development time of a sporangium was 45 hours. Growing on phosphorus limited hosts, the zoospore production decreased to less then 9 spores per sporangium, and the development time decreased to 40 hours. On phosphorus limited hosts, zoospores were produced at a slower rate. The algal growth rate was reduced to a greater extent than the fungal growth rate. Therefore, it could be concluded that phosphorus limitation ofAsterionella will facilitate the development of an epidemic of its parasiteRhizophidium, at least at high diatom densities, when possible differences in infectability of the algae play a minor role.     

An ultrastructural comparison betweenSpermatozopsis andDunaliella (Chlorophyceae)

The ultrastructure of the type species of the genusDunaliella, D. salina, has been reinvestigated in an attempt to clarify the relationships betweenDunaliella andSpermatozopsis. Dunaliella salina differs in the following ultrastructural characters fromSpermatozopsis (as exemplified byS. similis Preisig etMelkonian): presence of a distinctive surface coat covering the plasmalemma; presence of a prominent pyrenoid (with pairs of thylakoids partially entering the pyrenoid matrix); dictyosomes parabasal; endoplasmic reticulum closely underlying the plasmalemma around most of the cell; contractile vacuoles absent; cell form ovoid to elongated and not spirally twisted; mitochondrial profiles near the flagellar apparatus. Differences in the ultrastructure of the flagellar apparatus: basal body angle more or less fixed; distal connecting fibre cross-striated; system II fibre (rhizoplast) present, associated with mitochondrial profile; system I fibre underlying two-stranded microtubular root; mating structure present. These ultrastructural differences justify distinction between the two taxa at generic level. The problematical status of freshwater species ofDunaliella is briefly discussed.     

Synaptonemal complex formation and meiosis in the resting sporangium ofBlastocladiella emersonii

Summary Blastocladiella emersonii Cantino etHyatt (Phycomycetes, Blastocladiales) has aBrachyallomyces type of life cycle (sensu Emerson 1941) or aBlastocladiella type of life cycle (sensu Karling 1973), with a regular formation of zoosporangia and resting sporangia and no sexual stages.Synaptonemal complex formation (a characteristic stage in meiotic prophase—pachytene) occurs during development of the resting sporangium inB. emersonii.During resting sporangium germination, two meiotic nuclear divisions give rise to nuclei which have approximately one-fourth of the nuclear volume of the diplotene nucleus. The life cycle ofBlastocladiella emersonii resembles that ofCatenaria anguillulae. The time of diploidization (n2 n) has not been determined.     

Serial reconstruction of the mitochondrial reticulum in the antarctic flagellate,Pyramimonas gelidicola (Prasinophyceae,Chlorophyta)

Summary A serial reconstruction ofPyramimonas gelidicola McFadden, Moestrup andWetherbee has revealed a large reticulated mitochondrion branching throughout the cell. The possibility of single mitochondria in other members of thePrasinophyceae and the uniformity of the morphology of this organelle within the class is discussed.