Błaszk. V. Blanke, C. Renker & F. Buscot
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SPORES occur singly in the soil and usually are closely adherent to roots, as well as frequently form within roots; origin blastically at the tip of extraradical hyphae of mycorrhizal roots. Spores light yellow (4A4) to yellow ochre (5C7); globose to subglobose; (23-)50(-70) µm diam; sometimes ovoid; 20-55 x 45-100 µm; with a single subtending hypha.
SUBCELLULAR STRUCTURE OF SPORES of one wall with three layers (swl1-3).
Layer 1 rigid, smooth, hyaline, ca. 0.5 µm thick, continuous with a one-layered subtending hypha of the most juvenile spores, then darkening to light yellow (4A4) and thickening to (1.2-)1.8(-2.7) µm.
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In PVLG
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Layer 2 rigid, smooth, hyaline, (0.2-)0.8(-1.2) µm thick, closely adherent to layer 1.
In mature spores, layer 1 sometimes more or less deteriorates with age, whereas layer 2 always remains intact.
Layer 3 smooth, light yellow (4A4) to yellow ochre (5C7), (0.7-)1.6(-2.2) µm thick.
In crushed spores, layers 1 and 2 usually are adherent or sometimes separate from each other. However, the two layers easily separate from layer 3.
None of the spore wall layers reacts in Melzer’s reagent.
The wall of youngest spores consists of layer 1 only. Then, layer 2 origins, and layer 3 begins to form after a full differentiation of layer 2.
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In PVLG+Melzer's reagent |
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Wall of subtending hypha light yellow (4A4) to yellow ochre (5C7); (0.5-)0.9(-1.5) µm thick at the spore base; composed of three layers (shwl1-3) continuous with spore wall layers 1-3; subtending hyphal wall layers 1 and 2 extend up to 70 µm below the spore base.
Pore occluded by a septum, 1.2-2.7 µm wide, continuous with the innermost lamina of spore wall layer 3.
GERMINATION. Not observed.
MYCORRHIZAE. Glomus xanthium was associated in the field with vesicular-arbuscular mycorrhizae of Xanthium cf. spinosum L. in Greece, Cenothera drummondi Hook in Israel, and Ammophila arenaria (L.) Link in Spain and Turkey (Błaszkowski et al. 2004).
In one-species cultures with Zea mays L. as the plant host, Gl. xanthium formed mycorrhizae with arbuscules, vesicles, as well as intra- and extraradical hyphae. Arbuscules were numerous and generally evenly distributed along the root fragments examined. Vesicles occurred numerously and were globose to subglobose, (20-)27(-40) µm diam, or ellipsoid, 17.5-40.0 x 22.5-120 µm. Intraradical hyphae varied in thickness from (0.9-)5.9(-12.7) µm, grew parallel to the root axis, and sometimes formed Y- or H-shaped branches and coils, 12.5-27.5 x 17.5-55.0 µm. Extraradical hyphae were abundant, frequently associated with spores, and measured (3.2-)4.7(-7.4) µm wide. In 0.1% trypan blue, arbuscules stained violet white (16A2) to deep violet (16D8), vesicles violet white (16A2) to deep violet (16E8), intraradical hyphae violet white (16A2) to deep violet (16E8), coils violet white (16A2) to reddish violet (16B6), and extraradical hyphae deep violet (16D8-E8).
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In roots of Z. mays |
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PHYLOGENETIC POSITION. Sequence data and phylogenetic analyses (Błaszkowski et al. 2004) placed Glomus xanthium in Glomus Group A sensu Schüßler et al. (2001). Sequences of Gl. xanthium fell into a separate cluster (Fig. 1) or even formed a separate lineage (Fig. 2) within Glomus Group A, distinct from all well known species of this group (e. g. Gl. mosseae (Nicol. & Gerd.) Gerd. & Trappe, Gl. coronatum Giovann., Gl. caledonium (Nicol. & Gerd.) Trappe & Gerd., Gl. geosporum (Nicol. & Gerd) C. Walker). However, Gl. xanthium clustered close to a preliminarily named Glomus sp. ‘Bad Sachsa’ (with no further correlation to morphological features) from a gypsum slope of the southern Harz mountains (Germany; Renker et al. 2003, Börstler et al. unpubl. data), displaying identities between 90 and 94% (Fig. 3).
DISTRIBUTION. Spores of Gl. xanthium were for the first time isolated from a trap culture established with a soil sample collected under X. cf. spinosum colonizing maritime sand dunes adjacent to Veriko in northern Greece (22o35’E, 40o08’N; (Błaszkowski et al. 2004). This fungus was not found in the field-sampled soil. The fungi occurring in the field soil from which Gl. xanthium inoculum originated included two unrecognized Glomus spp. and Scutellospora persica (Koske & C. Walker) C. Walker & F.E. Sanders. The arbuscular mycorrhizal fungal species associated with Gl. xanthium in trap cultures were Gl. clarum Nicol. & N.C. Schenck, Gl. gibbosum Blaszk., and an undescribed Glomus sp.
Subsequently, this fungus was revealed in 15 trap cultures with rhizosphere soils of other dune sites of the Mediterranean Sea. They were collected from under C. drummondi growing near Tel-Aviv (32º4’N, 34º46’E), Israel, in 1997 (one sample) and 2000 (7 samples), from among roots of A. arenaria growing near Cape Salinas (36o19’N, 3o2’E), Majorca, Spain, in 2001 (2 samples), from under A. arenaria growing near Karabucak-Tuzla (36o43’N, 34o59’E), Turkey, in 2001 (4 samples), and under A. arenaria growing near Calambrone (43o35’N, 10o18’E), Italy, in 2002 (one sample). No study of the composition of arbuscular mycorrhizal fungal species in the field-collected soils was undertaken. The arbuscular mycorrhizal fungi co-occurring with Gl. xanthium in the trap cultures with Israeli soils were Archaeospora trappei (R.N. Ames & Linderman) J.B. Morton & D. Redecker, Gl. constrictum Trappe, Gl. claroideum N.C. Schenck & G.S. Sm., Gl. coronatum, Pacispora scintillans (S.L. Rose & Trappe) Sieverd. & Oehl, and Scutellospora pellucida (Nicol. & N.C. Schenck) C. Walker & F.E. Sanders. The Majorca’s cultures contained Ar. trappei, Gl. constrictum, Gl. corymbiforme Blaszk., S. calospora (Nicol. & Gerd.) C. Walker & F.E. Sanders, those from Turkey Gl. constrictum, Gl. coronatum, Gl. fasciculatum (Thaxt.) Gerd. & Trappe emend. C. Walker & Koske, Gl. aurantium Blaszk. et al., and S. calospora, and those from Italy Acaulospora bireticulata F.M. Rothwell & Trappe, Gl. aurantium, Gl. microcarpum Tul. & C. Tul., and S. persica.
NOTES. Two properties mainly distinguish Gl. xanthium from other Glomus species. First, spores of the new fungal species tend to form within or tightly adherent to roots. Second, the spores are relatively small, with the outermost layer usually thicker than the innermost layer of the 3-layered spore wall.
The pattern of spore wall differentiation in Gl. xanthium is similar to that of Glomus species investigated to date (Błaszkowski 1997; Błaszkowski and Tadych 1997; Morton 1996; Stürmer and Morton 1997), with discrete layers formed successively.
When observed under a dissecting microscope, spores of Gl. xanthium most resemble small-spored isolates of Gl. aggregatum N.C. Schenck & Sm. emend. Koske and Gl. intraradices N.C. Schenck & G.S. Sm. These species produce yellow-coloured spores that frequently occur in both aggregates tightly associated with roots and inside them (Schenck and Smith 1982; Stürmer and Morton 1997).
Using a light microscope, examination of spores crushed in a mixture of PVLG and Melzer’s reagent readily separates the three fungi. The spore wall of Gl. xanthium is composed of two, usually adherent rigid, semi-permanent and permanent layers, respectively, readily separating from a laminate innermost layer when crushed. The spore wall of Gl. aggregatum and Gl. intraradices also consists of three layers, of which two outer ones usually detach from the innermost laminate layer in crushed spores (Schenck and Smith 1982; Stürmer and Morton 1997). However, the two outer spore wall layers of the latter fungi are short-lived and usually are completely sloughed in mature spores (Stürmer and Morton 1997). Additionally, the outermost wall layer of both Gl. aggregatum and Gl. intraradices stains red to purple in Melzer’s reagent (Stürmer and Morton 1997), whereas that of Gl. xanthium ramains non-reactive in this reagent. Finally, the unique character of Gl. aggregatum is the production of spores inside their parent spores by internal proliferation (Koske 1985).
Although morphology placed Gl. xanthium close to Gl. intraradices, molecular data did not confirm this estimation. Unfortunately, there is lack of sequence data for Gl. aggregatum. Based on available molecular data, Gl. xanthium can be considered a member of Glomus Group A sensu Schüßler at al. (2001). While all the well known species of this group are distinct from Gl. xanthium, Glomus sp. ‘Bad Sachsa’-sequences were found to be the closest relatives in the phylogentic analyses. Firstly detected by Landwehr et al. (2002) at a gypsum slope in the southern Harz mountains (Germany), similar sequence types were found in further studies within Germany (Renker et al. 2003, Börstler et al., unpubl. data). Quite recently, Wubet et al. (2003) detected a Glomus sp. in Ethiopia colonizing roots of Prunus africana ITS sequences of this fungus fall into the same sequence cluster like Gl. xanthium and the original Glomus sp. ‘Bad Sachsa’ sequence.
Glomus xanthium is probably adapted to warm soils of southern hemisphere. They have not been found in any of ca. 3000 soil samples collected in different dune and non-dune soils of northern Europe (Błaszkowski 2003). Koske (1987) found temperature to be the main abiotic factor influencing the structure of arbuscular fungi of the barrier dunes extending from New Jersey to Virginia. According to Pirozynski (1968), temperature is the major factor determining the distribution and occurrence of fungi in general.
REFERENCES
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Landwehr M., Hildebrandt U., Wilde P., Nawrath K., Tóth T., Biró B., Bothe H. 2002. The arbuscular mycorrhizal fungus Glomus geosporum in European saline, sodic and gypsum soils. Mycorrhiza 12, 199-211.
Koske R. E. 1985. Glomus aggregatum emended: a distinct taxon in the Glomus fasciculatum complex. Mycologia 77, 619-630.
Koske R. E. 1987. Distribution of VA mycorrhizal fungi along a latitudinal temperature gradient. Mycologia 79, 55-68.
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Schüßler A., Schwarzott D., Walker C. 2001. A new fungal phylum, the Glomeromycota: phylogeny and evolution. Mycol. Res. 105, 1413-1421.
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Wubet T., Weiß M., Kottke I., Teketay D., Oberwinkler F. 2003. Molecular diversity of arbuscular mycorrhizal fungi in Prunus africana, an endangered medicinal tree species in dry Afromontane forests of Ethiopia. New Phytol. 161, 517-528.
