Light and Electron Microscopy Studies on the Infection of a Wild‐type and Metalaxyl‐resistant Isolate of Phytophthora sojae in Soybean Hypocotyls

Resistance of soybean cultivars, depending on single dominant genes to Phytophthora sojae, may easily be overcome by emerging new virulent races. Light microscopy (LM) and electron microscopy (EM) were used to study the infection process of the wild‐type isolate Ps411 and metalaxyl‐resistant mutant Ps411‐M of P. sojae in hypocotyls of soybean seedlings grown from untreated and metalaxyl‐treated seeds. The isolate Ps411‐M of P. sojae exhibited a high degree of resistance to metalaxyl compared to Ps411. The pathogenic fitness of Ps411‐M in hypocotyls of soybean seedlings was lower compared to Ps411. LM observations showed distinct differences in the infection process of both isolates in hypocotyls of treated soybean seedlings. EM studies revealed differences in the prepenetration stage between Ps411 and Ps411‐M on hypocotyls grown from seeds treated with 0.02% metalaxyl until the whole seed surface coated. The number of infection sites was markedly reduced and few hyphae continued to spread. Numerous ultrastructural alterations in hyphae were observed in treated hypocotyls infected with Ps411, including pronounced thickening of hyphal cell walls and encasement of haustorium‐like bodies; electron‐dense material was deposited in host cell walls in contact with hyphal cells. Neither the prepenetration process nor penetration or spread of hyphae in the hypocotyls of the resistant isolate was affected in treated compared to non‐treated tissue. While in treated hypocotyls infected with the wild‐type isolate, host defence reactions were induced, no such reactions were detected in treated hypocotyls infected with the resistant isolate. Hypocotyls from metalaxyl‐treated seeds infected with the wild‐type isolate resembled an incompatible interaction, whereas during infection with the metalaxyl‐resistant mutant, the compatible interaction was not changed.     

Studies on groundnut hypocotyl exudates and the behaviour ofRhizoctonia solani in influencing the disease

Summary The behaviour ofRhizoctonia solani on the surface of groundnut seedling hypocotyls was studied at different stages of growth. Characteristic variation in the growth and production on infection cushions was observed. No infection cushions were observed as the seedling reaches the age of three weeks. Hypocotyl exudates were collected at various stages of growth and analysed for total phenols, carbohydrates and ninhydrin positive compounds. The exudates were also analysed for individual amino acids, sugars and organic acids. Gradual decline in the total quantities of various compounds was observed with age. Quantitative and qualitative change in the individual compounds was also evident and some of the compounds were not at all detected as age increases. The possible correlation between the exudation and behaviour of the fungus on host surface is discussed.     

Influence of Plant Growth Regulators on Growth and Virulence of Rhizoctonia solani (Kühn) Pathogenic to Groundnut Seedlings (Arachis hypogaea L. var. TMV 2)

Growth of R. solani was markedly influenced by GA₃, kinetin but not by lAA. The mycelium used for inoculation of groundnut hypocotyls was obtained from cultures grown on solid media supplemented with various concentrations of GA₃, kinetin and IAA. The virulence was evaluated by measuring the lesion size 48 h after inoculation. Gibberellic acid followed by kinetin caused enhanced lesion size at 0.5 μg/ml in the medium. A correlation was observed between the infection cushions produced and the virulence of the pathogen.     

Effectiveness of Different Frankia Cell Types as Inocula for the Actinorhizal Plant Casuarina

The soil bacterium Frankia of the Actinomycetales, capable of forming N₂-fixing symbiotic root nodules on a diverse array of actinorhizal plants, has several morphological forms when grown in pure culture. Fresh hydrated preparations of whole cells, hyphae, and spores were all infective on seedlings of Casuarina at different dilutions. Desiccated hyphae showed no infection capacity, while desiccated spores remained infective, although at a reduced level. On the basis of most-probable-number statistics, spore suspensions were 3 orders of magnitude more infective than hyphae.     

Studies on turfgrass snow mold caused byTyphula ishikariensis. II. Microscopical observation of infected bentgrass leaves

Light and transmission electron microscopy revealed thatTyphula ishikariensis penetrated into bentgrass leaves either through cuticles or stomata either by single hyphae or infection cushions formed on host surfaces. Time course study on infected leaves showed that penetration through stomatal subsidiary cells and their adjacent cells seemed to occur earlier than that through epidermal cells located farther from stomata. More than 30% of epidermal cells were infected by 10 days after inoculation. When hyphae penetrated through an intact cuticle of epidermal cells, they seemed to dissolve host cell walls enzymatically at penetration sites. Physical pressure also seemed to be involved in penetration.     

The Development and Behaviour of Mycelial Strands in Merulius lacrymans (Wulf.) Fr.: II. Hyphal Behaviour during Strand Formation

The system of hyphal branching by Merulius lacrymans was observedin mycelium which had grown from a wood food-base on to glassslides during incubation in sterile moist chambers. A hierarchyof branches and sub-branches arose from the region of clampconnexions, or nodes, of relatively wide main hyphae. Therewas evidence that the sequence of branches occurring at nodesin basipetal succession represented the time sequence of branchdevelopment at any one node. Later-formed branches at any nodewere smaller than earlier branches, but such earlier branchesusually became smaller towards the tip as growth continued.Mycelial strands were built up by growth and branching of thigmo-tropicallysensitive ‘tendril’ hyphae in association with thewide main hyphae. Tendril hyphae were characteristically narrow,thin-walled hyphae arising both as later-formed branches fromthe nodes of the main hyphae and as the narrowed tips of earlierbranches. Although this branching behaviour could be seen amongstaerial hyphae growing over agar media, hyphae growing in contactwith or within the agar behaved differently and did not formstrands.     

Endogenous Development of Adventitious Root Primordia in Lettuce Hypocotyls

Rudimentary adventitious root primordia were observed in optically-sectionedFeulgen-stained hypocotyls of 8-d-old lettuce seedlings germinatedin the dark and in a low light intensity environment. It issuggested that formation of these primordia may represent anaspect of normal development in the seedling. Adventitious, root primordia, hypocotyls, lettuce, Lactuca sativa     

Infection of Onion by the White Rot Pathogen, Sclerotium cepivorum

Infection of onion tissue by Sclerotium cepivorum occurred from germ tubes penetrating between adjacent epidermal cell walls or directly, via penetration pegs produced from slightly swollen hyphal tips or from beneath dome shaped infection cushions. After passing through the cuticle, the infection peg enlarged to form an infection hypha within the primary cell wall. Extensive degradation of the epidermal cell wall occurred, often at a distance of 2–3 cells from the advancing hyphae. As infection advanced, hyphae spread rapidly from the epidermis to the cortex growing between and within dead/dying host cells. Extensive host cell death resulted in localized collapse of the tissue around infection points. Complete colonization of the internal tissues of the root and stem base occurred within 5–7 days of inoculation.     

The Effects of Griseofulvin on Certain Phytopathogenic Fungi

In vitro and in vivo investigations have been carried out intothe reaction of actively growing hyphae of Botrytis spp. onencountering an environment containing griseofulvin. In vitro, the reactions noted were slowing of the growth rate,stimulation to branching, abnormal appearance of hyphal protoplasm,and loss of rigidity of the hyphal membrane in the region ofthe tip. The antibiotic is not translocated within the mycelium. In vivo, Botrytis tulipae was incapable of penetrating cellmembranes in the stem of tulips watered with griseofulvin. Theantibiotic was shown to delay or to prohibit the penetrationof onion epithelium by B. allii, depending on concentration.The characteristic curling and stunting of hyphae caused bygriseofulvin have both been observed to occur in plant cells.     

Ulodendron: Lindley and Hutton and its Cuticle

The Carboniferous fossil stem genus Ulodendron has been knownsince 1831 when it was first described by Lindley and Hutton.Since that time ideas about its generic value and what shouldbe included in it have varied considerably. Specimens referableto U. majus L. & H. have been examined and cuticle pre-parationsmade from them. Ulodendron is shown to have persistent leavesattached to leaf cushions. Stomata are present on both leavesand leaf cushions. The presence of ligule pits is also demonstratedproving the genus to be ligulate.     

The RHG Gene Is Involved in Root and Hypocotyl Gravitropism in Arabidopsis thaliana

In higher plants, shoots show negative gravitropism and rootsshow positive gravitropism. To elucidate the molecular mechanismsof root and hypocotyl gravitropism, we segregated the secondmutation from the original phyB-1 mutant line which impairedboth root and hypocotyl gravitropism and characterized thisnovel mutation named rhg (for root and hyzypocotyl gravitropism).The rhg is a single recessive nuclear mutation and it is mappedon the lower part of the chromosome 1. Analyses on the gravitropicresponses of the rhg mutant indicate that root and hypocotylgravitropism are severely impaired but inflorescence stem gravitropismis not affected by the rhg mutation. In the rhg mutant seedlings,amyloplasts (statoliths for gravity-perception) were presentin the presumptive statocytes of roots and hypocotyls. Phototropismby roots and hypocotyls was not impaired in the rhg mutant.These results suggest that the RHG gene product probably actson the gravity-perception and/or the gravity-signal transductionin root and hypocotyl gravitropism. This is the first reportabout the genetic locus specifically involved in both root andhypocotyl gravitropism but not inflorescence stem gravitropism,supporting our hypothesis that the mechanisms of gravitropismare genetically different between hypocotyls and inflorescencestems. (Received March 11, 1997; Accepted April 17, 1997)     

Effects of hypergravity on growth and cell wall properties of cress hypocotyls

Elongation growth of etiolated hypocotyls of cress (Lepidiumsativum L.) was suppressed when they were exposed to basipetalhypergravity at 35 g and above. Acceleration at 135 g causeda decrease in the mechanical extensibility and an increase inthe minimum stress-relaxation time of the cell wall. Such changesin the mechanical properties of the cell wall were prominentin the lower regions of hypocotyls. The amounts of cell wallpolysaccharides per unit length of hypocotyls increased underthe hypergravity condition and, in particular, the increasein the amount of cellulose in the lower regions was conspicuous.Hypergravity did not influence the neutral sugar compositionof either the pectin or the hemicellulose fraction. The amountof lignin was also increased by hypergravity treatment, althoughthe level was low. The data suggest that hypergravity modifiesthe metabolism of cell wall components and thus makes the cellwall thick and rigid, thereby inhibiting elongation growth ofcress hypocotyls. These changes may contribute to the plants'ability to sustain their structures against hypergravity. Key words: Cell wall extensibility, cellulose, hypergravity, Lepidium sativum L., lignin     

Growth responses of sunflower hypocotyl disks inoculated with Escherichia coli and Agrobacterium tumefaciens I. Morphological observations

Disks of sunflower hypocotyls 1 mm thick grown in light anddarkness on agar containing mineral salts and sucrose to whichIAA was added in varying concentrations, were inoculated withE. coli, A. tumefaciens or sterile synthetic medium. Light-growndisks inoculated with E. coli proliferated from the lower surfaceand formed numerous long roots but dark-grown disks were usuallyinhibited in comparison with uninfected disks. Inoculation withA. tumefaciens induced proliferation mainly from the upper surfaceand a few short roots were formed. Uninfected disks grown with0.01 ppm IAA proliferated in a manner similar to that of E.coli-infected light-grown disks on simple medium but in thedark similar treatments produced quite different morphologicaleffects. The form of proliferation induced by E. coli underall conditions of growth could not be equated with that inducedby A. tumefaciens or by different concentrations of IAA. (Received December 5, 1967; )     

Internal Changes in Hyphae of Rhizopus sexualis (Smith) Callen and Mucor hiemalis Wehm. associated with Zygospore Formation

The distribution of nucleic acids, nuclei, mitochondria, andreserve foods in vegetative hyphae, zygophores, and developingzygospores of Rhizopus sexualis and Mucor hiemalis were examinedby differential staining. The extreme tips and growing zones of vegetative hyphae containeda high concentration of RNA and numerous mitochondria. Nucleiwere not present at the extreme tip but were numerous just behindit. In older parts of the hyphae the concentration of RNA waslow and both nuclei and mitochondria were fewer than in thezone of elongation. Glycogen and lipids were present in all parts of the livinghyphae except the extreme tips and were more highly concentratedin the older parts of the hyphae. Young zygophores showed a much lower RNA/DNA ratio than thatfound in the vegetative hyphal tips. Transfer of colonies from20? C to temperatures of less than 10? C, which is known toprevent zygospore initiation, caused some but not all recognizablezygophores of R. sexualis, but not those of M. hiemalis, torevert to the RNA/DNA ratio characteristic of vegetative hyphae.Some zygophores of Rhizopus and most of those of Mucor developedinto sporangiophores at low temperature, retaining the relativelylow RNA/DNA ratio throughout development. It is suggested thata reduction in the RNA/DNA ratio is an early step in the changefrom the vegetative state to the reproductive one. At firstthis step is reversible, but soon becomes irreversible by anadditional step, the nature of which is unknown. For some timeafter this the reproductive hyphae are capable of either producingasexual sporangia or of conjugating to produce zygospores. Onceconjugation has taken place development either ceases or continuesuntil the spore is fully mature, but it cannot under any circumstancesthen be reversed. The development and maturation of the zygospore involves a greatincrease in number of both nuclei and mitochondria and in theconcentration of glycogen and lipids.     

Anatomical Changes in Prosopis cineraria (L.) Druce Seedlings Growing at Different Levels of NaCl Salinity

Seedlings of Prosopis cineraria were grown in artificial substrateswith additions of 0, 50, 100, 200, 400 and 600 mM NaCl. At anNaCl concentration of 600 mM the germination was greatly hindered.At 400 mM the radicles scarcely emerged. An anatomical studyof seedlings grown in the presence of 0–200 mM NaCl showedthat salinity induced structural changes in the roots, hypocotyls,epicotyls and leaflets. The secondary xylem differentiated earlierin treated plants than in the controls especially in the rootand epicotyl as salinity rose. This could be an adaptive strategyto facilitate water transport. The number of water storage cellsin the epidermal and cortical layers of the hypocotyls increasedwith salt concentration. Leaflets of seedlings grown in increasingsalinity tended to show a less dorsiventral and more isolateralorganization, and a greater thickness of leaf mesophyll. A preliminarycomparison with other species of the genus Prosopis is brieflydiscussed. Prosopis cineraria (L.) Druce, seedling, salinity, anatomical changes     

Biology and life cycle ofAtelocauda koae,an unusual demicyclic rust

Atelocauda koae, a rust of the native HawaiianAcacia koa, is considered as a demicyclic species, having spermogonial, aecial, and telial states, but is unusual in production of aeciospores simultaneously with teliospores rather than consecutively. Host inoculation with spores of each state separately confirmed that the life cycle was perpetuated by the telial state, but the aeciospores, while capable of germination and stomal penetration, did not produce detectable infection. This rust therefore behaves as a microcyclic species, and appears to be in evolutionary transition toward this reduced state. Teliospores produced vestigial, permanently attached basidiosopores which germinated to produce infective hyphae. The hyphae entered the host either through stomata or penetrated the epidermis directly, with the latter method being more common. Unusual nuclear associated with teliospore germination, in which meiosis occurs in more than one diploid nucleus was observed, in confirmation of an earlier study.     

Starch Grain Casts and Moulds in Eocene (Tertiary) Fossil Mangrove Hypocotyls

Further examples of the recently reported fossil starch graincasts and moulds in the fossil mangrove Ceriops cantiensis Chandlerfrom the London Clay (Eocene) of the Isle-of-Sheppey, Kent,England have now been discovered. In addition, similar structureshave been found in the fossil hypocotyls of Palaeobruguieraelongata Chandler from Herne Bay, Kent and also in four recentlycollected hypocotyls of a new species of fossil mangrove fromSheppey. Starch grains from herbarium material of the extantmangrove genera, Bruguiera and Ceriops, have been examined forcomparison with the fossil forms. starch grain casts, fossil, mangrove, hypocotyls, palaeobotany, Eocene, framboids, Rhizophoraceae, Bruguiera sexangula, Ceriops cantiensis, Palaeobruguiera elongata     

Stress relaxation properties of the cell wall of growing intact plants

The cell wall of dark-grown Avena coleoptiles and the epidermisof light-grown mungbean hypocotyls was subjected to stress-relaxationanalysis and the following results were obtained. 1. Actively growing apical regions of the organs, either coleoptilesor hypocotyls, had certain threshold values of minimum stress-relaxationtime, T_O, 0.04 sec for coleoptile cell wall and 0.03 sec forthe epidermal cell wall of hypocotyls. The cell wall of thebasal region of the organs, which were mature and not growing,had a higher value of To. 2. When the apical regions of the organs, either coleoptilesor hypocotyls, ceased to grow, their cell walls showed T_O valuesabove these thresholds. 3. The relaxation rate, b, was small in the cell wall of activelygrowing regions of the organs, compared with that of non-growingregions. 4. The maximum relaxation time, T_m, was variable and no significantrelationship with growth capacity was found. 5. The extensibility, mm/gr, was large not only in activelygrowing regions of the organs but also in fully grown regions,suggesting that the value represents complex properties of thecell wall including the history of cell wall extension. From these results, we concluded that biochemical modificationsoccur in the cell wall matrix of actively growing organs ofeither monocots or dicots, and these are the bases of the capacityof the cell wall to extend and are represented chiefly by T_oand possibly by b. (Received August 12, 1974; )