Katsunori Isobe

Community structure of arbuscular mycorrhizal fungi in a primary successional volcanic desert on the southeast slope of Mount Fuji

Community structure of arbuscular mycorrhizal fungi (AMF), evaluated as spore samples and mycorrhizal roots of four herbaceous plant species, was investigated at different altitudes in a primary successional volcanic desert on Mount Fuji using molecular methods (fragment and sequence analysis of the large ribosomal subunit RNA gene). In total, 17 different AMF clades were identified, and most were members of the Glomaceae, Acaulosporaceae, and Gigasporaceae. The AMF community structures detected by spore sampling were inconsistent with those from plant roots. Of all AMF clades, six (35.3%) were detected only on the basis of spores, six (35.3%) only in roots, and five corresponded to both spores and roots (29.4%). Although an Acaulospora species was the most dominant among spores (67.1%), it accounted for only 6.8% in root samples. A species analysis of AMF communities at different altitudes demonstrated that AMF species diversity increased as altitude decreased and that the species enrichment at lower altitudes resulted from the addition of new species rather than species replacement. The inconsistencies in the species composition of spore communities with those in roots and the change in species diversity with altitude are discussed.

Arbuscular mycorrhizal colonization of the dominant plant species in primary successional volcanic deserts on the Southeast slope of Mount Fuji.

Arbuscular mycorrhizal (AM) colonization was observed on four plant species in primary successional volcanic deserts on the Southeast slope of Mount Fuji. The AM colonization of the dominant species, Polygonum cuspidatum, contradicts the conclusion that Polygonaceae are often regarded as being non-mycorrhizal species. The secondary dominant species, Polygonum weyrichii var. alpinum, formed no mycorrhizas. The roots of Cirsium purpuratum, Clematis stans and Campanula punctata ssp. hondoensis, showed a higher percentage of AM colonization than P. cuspidatum. AM colonization and spore density in the rhizosphere soil of P. cuspidatum significantly decreased as elevation increased. AM colonization in roots of Cirsium purpuratum and Clematis stans also tended to decrease with increased altitudes. Cirsium purpuratum and Campanula punctata ssp. hondoensis formed single structural types of Arum- and Paris-type, respectively, whereas P. cuspidatum and Clematis stans formed both Arum- and Paris-type morphologies.

Impact of a 5-year winter cover crop rotational system on the molecular diversity of arbuscular mycorrhizal fungi colonizing roots of subsequent soybean

The impact of winter cover crops, specifically wheat (Triticum aestivum L.), red clover (Trifolium pratense L.), and rapeseed (Brassica napus L.) or winter fallow, on community composition of arbuscular mycorrhizal fungi (AMF) in subsequent soybean roots was investigated in a 5-year field trial on andosolic soils in Japan. Soybean roots were sampled at full-flowering and analyzed for AMF communities using a partial LSU rDNA region. Phylogenetic analysis detected 22 AMF phylotypes, including eight Glomus, three Gigaspora, two Scutellospora, three Acaulospora, two Rhizophagus, and one of Funneliformis, Diversispora, Paraglomus, and an unknown glomeromycete in the roots. The 5-year rotation of different winter cover crops or winter fallow did not impact the molecular diversity of AMF communities colonizing the roots of subsequent soybean. In all of the rotations, Glomus and Gigaspora phylotypes were common to soybean roots over the 5-year period. Redundancy analysis (RDA) demonstrated that AMF communities in the roots of subsequent soybean were not significantly different among winter cover crop rotations or fallow. However, AMF communities in soybean roots were clearly influenced by rotation year suggesting that climate or other environmental factors were more important than winter cover cropping system management.

Distribution of Arbuscular Mycorrhizal Fungi in Upland Field Soil of Japan: 2. Spore Density of Arbuscular Mycorrhizal Fungi and Infection Ratio in Soybean and Maize Fields

Abstract In this study, soil samples were collected from upland fields where maize and soybeans had been cultivated and the density of AM (arbuscular mycorrhizal) fungal spores and the percentage of soybean roots infected with AM fungi (infection ratio) were assessed to determine the factors of the soil chemical properties affecting the mycorrhizal infection. The roots and rhizosphere soil were sampled from 9 soybean fields and 8 maize fields in the summer of 2004. The soil samples were examined for chemical properties (pH, electric conductivity, total phosphate, available phosphate, and phosphate absorption coefficient) and the density of AM fungal spores. Soybean roots were stained with trypan blue to determine the infection ratio. There was a significant difference in soil pH and available phosphorus content with the sampling site. The phosphorus absorption coefficient markedly varied with the sampling site and there was a significant difference in the phosphorus absorption coefficient with the site. The spore density in the soybean and maize fields markedly differed with the sampling site and there was a significant difference spore density with the sampling site. The density of AM fungal spores in the soybean field was negatively correlated with the available phosphorus content, and showed a positive correlation with the phosphate adsorption. This means that an increase in the available soil phosphorus due to the application of phosphate fertilizers will lower the density of AM fungal spores in the soil and that the density of AM fungi spores is generally higher in soils with a higher phosphate absorption coefficient. It is considered that this tendency is marked in the soil with a low phosphate adsorption coefficient. The infection ratio was positively correlated with spore density, and negatively with the available phosphorus content. To increase mycorrhizal infection of soybeans, we need to decrease the amount of available soil phosphorus and simultaneously to increase the density of AM fungal spores. Excessive application of phosphorus fertilizers should be avoided.

Inoculation with Arbuscular Mycorrhizal Fungi or Crop Rotation with Mycorrhizal Plants Improves the Growth of Maize in Limed Acid Sulfate Soil

Abstract Arbuscular mycorrhizal fungi (AMF) improve the uptake of immobile mineral nutrients such as phosphate, thereby improving plant growth. In acid sulfate soil (ASS), AMF spore density is generally low which impacts root colonization and phosphate uptake. Thus, inoculation may help increase AMF colonization of crops grown in ASS. AMF spore density decreases after cultivation of a non-host crop or bare fallow. In addition, preceding crops affect the growth and yield of subsequent crops. The production of AMF inocula requires AMF-compatible plants. The objective of the present study is to elucidate the effect of preceding crops on the persistence of inoculated AMF and growth of succeeding maize under an ASS condition with lime application. Spore density of AMF after cultivation of preceding crops (soybean or job’s tears) was maintained in comparison to fallow leading to higher AMF colonization of maize and improved plant growth. Thus, maintenance of AMF spore density, either through selection of preceding crops or application of AMF inoculum, may be a viable strategy to improve maize growth in limed ASS of Thailand.

Distribution of Arbuscular Mycorrhizal Fungi in Upland Field Soil of Japan

Abstract To Quantify The Effects of Arbuscular Mycorrhizal Fungi (Am Fungi) On The Growth of Upland Field Crops Cultivated in Japan, We Analyzed Soil Samples From 124 Sites in 18 Japanese Prefectures For Available P Content, Ph and Am Fungal Spore Density. The Am Fungal Spore Density in The 124 Soil Samples Was 1.7 Per G Dw On The Average, and Lower Than 1.0 Per G Dry Soil (Dw) in About Half of The Soil Samples. The Maximum Spore Density Was 20.6 Spores Per G Dw. The Density of Am Fungal Spore Did Not Vary Significantly With The Sampling Site and The Kind of Cultivated Crop in The Sampling Field. The Ph of The Soil With A High Spore Density Ranged From 6 To 8, and in The Soil Samples With A Ph Lower Than 6 and Higher Than 8, TheSpore Density Was Lower Than 5 Spores Per G Dw. Thus, in The Acid Or Alkaline Soil, The Sporogenesis of Am Fungi Is Suppressed. Because Available P Content Was Consistently Low in The Soil Samples With A High Spore Density, P Content Was Considered To Correlate With The Am Fungal Spore Density. Therefore, Crop Cultivation With Limited P Fertilizer Application and Reduced Available P Content May Be Important To increase Am Fungal Spore Density in Upland Field Soil.

Molecular diversity and spore density of indigenous arbuscular mycorrhizal fungi in acid sulfate soil in Thailand

Acid sulfate soil (ASS) has an extremely low pH (3.0) and a high capacity to fix phosphate; symptoms of phosphate deficiency are commonly observed in many crop plants. Arbuscular mycorrhizal (AM) fungi form mutualistic relationships with plant roots, and improve uptake of phosphate from soil. However, there is little information on the actual situation of AM fungi in ASS in Thailand. The purpose of the present study is to determine the indigenous AM fungal density and species in ASS in Thailand. AM fungal spores were retrieved and identified by molecular approaches from ASS field at the central plain of Thailand. This study showed that AM fungal spore density in ASS was 0.232 spores per g dry soil. Among the plant species growing in the natural ASS, there was no AM fungal colonization in the roots of four plant species, i.e. Digitaria sp., Fimbristylis sp., Mimosa pudica L., and Sesbania sp.; however, AM colonization was found in Wedelia roots. Using phylogenetic analysis, four operational taxonomic units (OTUs), i.e., one Glomus, one Entrophospora, one Paraglomus and one unknown species were identified from the AM fungal spores. Five OTUs, i.e., two Glomus, one Acaulospora, one Entrophospora and one unknown Glomeromycota were indentified from Wedelia roots. To our knowledge, this is the first report of the actual situation of AM fungi in ASS in Thailand determined by using molecular approaches.

Flavonoids in the extract and exudate of the roots of leguminous crops

The colonization level of arbuscular mycorrhizal fungi varies with the crop species (I so be et al., 1998), owing to differences in root structure and in the composition of root exudates (Baon et al., 1994; Mandelbaum et al., 2000). Root exudates affect the spore germination, hyphal growth and colonization of arbuscular mycorrhizal fungi (Gianinazzi-Pearson et al., 1989; Vierheilig et al., 1990). Root exudates contain many types of compound, such as amino acids, reducing sugars, organic acids, flavonoids and plant hormones. Moreover, many reseachers have reported that flavonoids stimulate the spore germination, hyphal growth and colonization of arbuscular mycorrhizal fungi (Baptista et al., 1994; Becard et al., 1992; Siqueira et al., 1991; Tsai et al., 1991; Vierheilig et al., 1998). However, Becard et al. (1992) reported that the effect of the flavonoid on the hyphal growth of arbuscular mycorrhizal fungi varies with the kind of flavonoid. To date, however, there have been few reports on the flavonoid composition in root extracts and exudates. In this study we examined the flavonoid content in the root extracts of various crops and root exudates of kidney bean.

Effect of Winter Crop Species on Arbuscular Mycorrhizal Fungal Colonization and Subsequent Soybean Yields

Abstract We evaluated how the cultivation of arbuscular mycorrhizal fungi (AMF) host (wheat) and non-host (rapeseed) crops affects the subsequent soybean crop by assessing AMF spore density and AMF colonization, phosphorus (P) uptake by soybean and yields of soybean over a 4-year period. Every year wheat or rapeseed was cultivated from autumn to spring and soybean from spring to autumn. From the first to fourth year, AMF spore density was higher in the plot after the cultivation of wheat (wheat plot) than in the rapeseed plot. From the second to fourth year, the AMF colonization ratio was higher in the wheat plot than in the rapeseed plot. In the first year, there was no difference in the AMF colonization ratio, growth, and P uptake by soybean plants between the rapeseed plot and wheat plot. However, from the second to fourth year, AMF colonization ratio, plant growth, and P uptake by soybean in the wheat plot were higher than those in the rapeseed plot. The soybean yields in both plots gradually decreased from the first to fourth year, but, in the second and the fourth year, soybean yields were higher in the wheat plot than in the rapeseed plot. Soybean yield was significantly correlated with the AMF colonization ratio, but not with AMF spore density. Therefore, we concluded that AMF colonization is not determined by AMF spore density alone, and other factors influence the AMF colonization in subsequent soybean plants. It is important to increase the AMF colonization ratio to increase soybean yield.

Characteristics of newly isolated ammonia-oxidizing bacteria from acid sulfate soil and the rhizoplane of leucaena grown in that soil

Abstract We isolated four new ammonia-oxidizing bacterial strains from leucaena (Leucaena leucocephala) grown in fields of an acid sulfate soil (ASS) in Thailand; 17SS from the surface soil and 17RS from the rhizoplane of a non-limed plot; 9SS from the surface soil and 9RS from the rhizoplane in a limed plot. The cells of all strains had the typical lobate shape of the genus Nitrosospira (“Nitrosolobus”). The percentage similarity of the 16S rRNA genes of these strains to that of Nitrosospira (“Nitrosolobus”) multiformis ATCC25196T (ATCC25196T) was 99.52% (strains 17SS, 17RS and 9SS) and 99.66% (strain 9RS). These newly isolated bacteria were all identified as Nitrosospira sp. Each strain, especially 17SS isolated from ASS, had higher urease activity than that of ATCC25196T. The o-acetylserine(thiol)lyase activity of 17SS was the highest of all the isolates. At pH 6.0, every isolate and ATCC25196T were able to utilize urea as the sole nitrogen source, in particular, strain 17SS grew best. The isolates from ASS showed higher urea utility than the isolates from the rhizoplane. Strain 17SS tolerated copper at levels up to 6.3 mmol L−1, but ATCC25196 T was inhibited at that concentration.