R. Larry Peterson

Root hairs: specialized tubular cells extending root surfaces

Root hairs are tubular extensions of epidermal cells that have their origin either in any protoderm cell or in specialized protoderm cells called trichoblasts. These latter cells are the result of an asymmetric cytokinesis determined by the positioning of a pre-prophase band of microtubules. The smaller sibling cell is the trichoblast and specializes physiologically and structurally prior to root hair outgrowth. Several genes are involved in the initiation and outgrowth of root hairs. Elongation of root hairs is by tip growth, and, correlated with this, cytoplasmic organelles and cytoskeletal elements show a polarized distribution; the apical dome consists of numerous vesicles, many associated with cell wall synthesis. The relationship between cellulose microfibril deposition and the pattern of cortical microtubules has received considerable attention, as has the role of the cytoskeleton and calcium in controlling cytoplasmic streaming. Root hairs extend the absorbing surface of the root and therefore have been studied in terms both of physiological characteristics of the plasma membrane and uptake of water and of various ions in the soil solution. Many plant species develop soil sheaths (rhizosheaths) which protect the root surface from desiccation and harbour various microorganisms; root hairs are intimately involved in these sheaths. Various growth regulators have been studied in terms of their effect on the structure and function of root hairs. Root hairs play a significant role in the interaction between plants and nitrogen-fixing microorganisms (e.g.,Rhizobium, Frankia) and symbiotic mycorrhizal fungi.

Associations between microfungal endophytes and roots: do structural features indicate function?

Roots encounter a plethora of microorganisms in the soil environment that are either deleterious, neutral, or beneficial to plant growth. Root endophytic fungi are ubiquitous. These include dark septate endophytes whose role in plant growth and the maintenance of plant communities is largely unknown. The objectives of this review were to assess the structural features of the interactions between dark septate endophytic fungi and the roots of both angiopsperms and conifers, and to suggest avenues for further research. Several light microscopy studies of endophyte–root interactions have revealed a variety of structural features, depending on host species and plant growth conditions. In some cases, when fungal hyphae enter roots they cause cell breakdown, whereas in other situations there is little noticeable effect. In some tree species, associations with these endophytes may mimic ectomycorrhizas or ectendomycorrhizas. The few ultrastructural studies indicate that intracellular hyphae lack a host-derived p...

Ectendomycorrhizal associations - characteristics and functions

Abstract. Mycorrhizal symbioses are widespread mutualistic associations of many plant hosts found in many habitats. One type of putative mycorrhizal association, ectendomycorrhiza, is confined to Pinus and Larix spp. and is common in conifer nurseries and in disturbed habitats. This association is characterized by the unique combination of a fungal mantle, Hartig net, and intracellular hyphae, the latter forming soon after Hartig net development. Many reports of the occurrence of ectendomycorrhizas from field-collected specimens are likely erroneous and instead may represent senescent ectomycorrhizas. The fungus species involved in the formation of ectendomycorrhizas were initially called E-strain fungi and their identification was based on characteristics of hyphae and chlamydospores. With the discovery of teleomorphs for some of these fungi, they were found to be ascomycetes. More recently, molecular methods have been used to clarify their systematics and phylogeny and it is apparent that most of the isolates belong to two species, Wilcoxina mikolae and Wilcoxina rehmii. Two species of dematiaceous fungi and a member of the Pezizales, Sphaerosporella brunnea, also have been reported to form ectendomycorrhizas. These fungi can form ectendomycorrhizas with their hosts over a broad pH range and may utilize many substrates as a carbon source. Ectendomycorrhizas may be important in the revegetation of disturbed sites and in the establishment of conifer seedlings in post-fire situations.

The co-occurrence of ectomycorrhizal, arbuscular mycorrhizal, and dark septate fungi in seedlings of four members of the Pinaceae

Although roots of species in the Pinaceae are usually colonized by ectomycorrhizal (EM) fungi, there are increasing reports of the presence of arbuscular mycorrhizal (AM) and dark septate endophytic (DSE) fungi in these species. The objective of this study was to determine the colonization patterns in seedlings of three Pinus (pine) species (Pinus banksiana, Pinus strobus, Pinus contorta) and Picea glauca x Picea engelmannii (hybrid spruce) grown in soil collected from a disturbed forest site. Seedlings of all three pine species and hybrid spruce became colonized by EM, AM, and DSE fungi. The dominant EM morphotype belonged to the E-strain category; limited colonization by a Tuber sp. was found on roots of Pinus strobus and an unknown morphotype (cf. Suillus–Rhizopogon group) with thick, cottony white mycelium was present on short roots of all species. The three fungal categories tended to occupy different niches in a single root system. No correlation was found between the percent root colonized by EM and percent colonization by either AM or DSE, although there was a positive correlation between percent root length colonized by AM and DSE. Hyphae and vesicles were the only AM intracellular structures found in roots of all species; arbuscules were not observed in any roots.

Structural features of a Lophodermium endophyte during the cryptic life-cycle phase in the foliage of Pinus strobus

Needles of Pinus strobus (white pine) were cleared and stained to survey the occurrence and location of Lophodermium sp., a fungal endophyte. Cytoplasmically dense endophytic hyphae with a pronounced lobed morphology and containing lipid bodies were localized intercellularly between the epidermis and hypodermis. These fungal infections did not appear quiescent, but rather exhibited signs of continual slow growth. A few associated host cells exhibited a hypersensitive response. Material embedded in resin and examined by light microscopy and transmission electron microscopy confirmed the location of hyphae between epidermal and hypodermal cells, and the presence of lipid bodies within the hyphae. In senescing needles, aggressive colonization of needle tissues occurred. Thus, for Lophodermium in white pine, endophytic infection is active rather than quiescent, and displays an alternate hyphal strategy to that seen in the reproductive phase.

The Use of Plant Mutants to Study Regulation of Colonization by AM Fungi

Colonization of roots by arbuscular mycorrhizal (AM) fungi and the formation of structures that characterize the AM symbiotic association, i.e. appressoria, extraradical and intraradical hyphae, arbuscules and vesicles, involve a complex series of events. Plant mutants that show a block at some stage in the colonization process are useful in determining the factors involved in the interaction between hyphae and root cells at each step in that process. Most of the mutants identified to date are legumes that also show some impairment in nodulation. There is a tight correlation between the degree of impairment in nodulation and AM formation. For example, failure to initiate nodules (nod-) in several legume species is associated with the failure of roots to be colonized by AM fungi (myc-). Mycorrhiza formation in these mutants is blocked at the stage of appressorium formation and involves chemical and structural changes in the root epidermis. Colonization of other mutants by mycorrhizal fungi is blocked at other stages of mycorrhizal development. Although the molecular basis for the various steps in nodulation has been worked out in detail, there is less information for the AM symbiosis. It has been suggested, based on some experimental evidence, that these symbioses may share some common genes. A mutant of a non-legume species, tomato, has been isolated recently that shows various degrees of reduction in AM formation depending on the fungus species used as inoculum. Mutants of additional non-legume species are needed for the study of the developmental regulation of this symbiosis since the majority of AM fungus host plants are non-legumes.

Genetic relationships of endophytic Lophodermium nitens isolates from needles of Pinus strobus

The foliage of Pinus strobus (eastern white pine), as that of all other conifers examined, is occupied by endophytic fungi, the most frequent of which is Lophodermium nitens. The number and extent of endophytic infections and the genetic relationship of individual isolates within living needles as well as their relationship to isolates from forest floor needles is unknown. To examine these and related questions, forest floor isolates and foliar endophytes from needle segments were obtained for ribosomal DNA sequencing and randomly amplified polymorphic DNA (RAPD) analysis. Molecular and morphological data were compared and infection frequency determined as a function of position along the needle. Ribosomal DNA sequences of foliar and ascospore isolates showed high levels of genetic similarity (> 97% identity) for the internal transcribed spacer region. RAPD profiles were able to distinguish ascospore siblings from non-siblings, and also revealed that many needle isolates belonged to the same genotype as adjacent neighbour isolates, as would be expected from mycelial spread within the needle. Morphotype evaluation and RAPD profiles showed similar patterns: identical morphotypes grouped together and showed little or no genetic difference under RAPD analysis. Both morphological and molecular data indicated that the majority of infections were contained within 1 mm needle segments but could extend to about 4 mm in length. Infection frequency increased along the length of the needle from the proximal (shoot) end to the distal tip, with markedly higher rates in the distal quarter. Thus, endophytic infections of L. nitens in white pine needles consist of many localized, discrete infections, originating from ascospores and differentially distributed along the length of the needle. In the course of this work, it was found that GenBank accession no. AF203470 under the name Meloderma desmaszieresii appeared not to be that species but L. nitens on the basis of the ITS sequences.

Cytoskeleton in mycorrhizal symbiosis

An understanding of the role played by the cytoskeleton in formation and function of mycorrhizas has been hampered by the technical difficulty of working with mycorrhizal material. Recently, however, improved labelling techniques suitable for both plant and fungal symbionts in combination with either epifluorescence microscopy or laser scanning confocal microscopy have resulted in new information. As well, molecular methods have made it possible to monitor changes of cytoskeletal elements during mycorrhiza development. Currently we know that the cytoskeletal systems of both plant and fungal partners undergo changes during both ecto- and endomycorrhizal symbiosis. However, little information is available concerning the regulatory factors or the cause and effect relationship of cytoskeletal changes and cellular events. In this article, research involving the cytoskeleton of mycorrhizas is reviewed in detail, whereas basic information of the cytoskeleton of plant and fungal cells is only briefly discussed as background. A brief comparison is also made between the information on mycorrhizas with that of biotrophic pathogenic fungi and the Rhizobium–legume symbiosis.

The interface between the arbuscular mycorrhizal fungus Glomus intraradices and root cells of Panax quinquefolius: a Paris-type mycorrhizal association

Two major types of arbucular mycorrhizal associations, the Arum-type and the Paris-type, have been identified based on morphological features. Although the Paris-type is the most common, it is the Arum-type that has been most intensively studied in terms of structure/function because of its prevalence in agronomically important plant species. In this study, the interface between the host cell cytoplasm and intracellular hyphae (extensive hyphal coils and arbusculate coils), which typify the Paris-type mycorrhiza, was studied. Using immunofluorescence techniques combined with laser scanning confocal microscopy, dramatic changes in the cytoskeleton in colonized cells were observed. Changes in the positioning of both host cell microtubules and actin filaments occurred in colonized plant cells. Both microtubules and actin filaments were associated with the hyphal coils and the arbusculate coils. An interfacial matrix, of host origin, was demonstrated between hyphal coils and arbusculate coils using various affinity techniques. It formed an apoplastic compartment consisting of cellulose and pectins between the fungus and host cell cytoplasm. There was less labelling adjacent to the fine branches of arbusculate coils compared to the hyphal coils. These observations show some similarities to those seen with Arum-type mycorrhizas.

Soil microbial communities from an elevational cline differ in their effect on conifer seedling growth

Sub-alpine environments consist of altitudinal gradients associated with dramatic changes in plant growth and community composition, but the role of soil feedbacks and microbe interactions is largely unknown. Here, we examine the influence of the overall soil microbial community, with a focus on ectomycorrhizal and dark septate endophytic root colonizing fungi, from low, mid, and high elevations on the growth of Pinus contorta and Picea glauca × engelmannii. The influence of the soil microbial community was tested on seedlings from the same three elevations in order to determine ‘home’ versus ‘away’ effects on conspecifics of differing elevations. The low elevation soil was the most fertile and harbored a soil microbial community with an overall negative effect on seedling growth. In contrast, the high elevation soil was the least fertile and had a microbial community that enhanced seedling growth. However, only the soil microbial community in the highest elevation soil resulted in a stronger influence on the native P. contorta seedlings than seedlings originating from lower elevations. Despite the overall influence of the soil microbial community, ectomycorrhizal colonization was significantly correlated with P. glauca × engelmannii growth rates, but colonization by dark septate endophytes showed no relationship with seedling growth. The results provide evidence that plant—soil microbial community relationships are dependent on soil environment. Moreover, our results provide further support for the importance of soil microbes in facilitating seedling growth toward the edge of their elevational range.