Studies on the spread of Gaeumannomyces graminis var. tritici in wheat.

The effect of cultivations on the spread of Gaeumannomyces graminis var. tritici from a line source of inoculum, consisting of naturally infected stubble and roots, was recorded in the field over 2 yr. With the aid of cultivations, spread in the first wheat crop occurred frequently to a distance of 0·9 m and occasionally to a distance of 2·5 m. However, the following wheat crop was uniformly infected, probably as a result of a rapid build-up of background inoculum in the first wheat, so that any spread by cultivations was masked.     

Disease in wheat genotypes naturally infected with Gaeumannomyces graminis var. tritici

Wheat genotypes consisting of seven homozygous lines and 40 segregate families were studied at two sites naturally infested with the take-all pathogen, Gaeumannomyces graminis var. tritici. The numbers of seminal and coronal roots infected with G. graminis and other root pathogens were recorded. The genotypes differed in infection with G. graminis, with little evidence of genotype × environment interactions. The incidence of G. graminis and Rhizoctonia solani was negatively associated, but the association did not greatly influence differences between wheats in infection with G. graminis. The distribution of R. solani was negatively associated with the severity of take-all at only one site. Of symptoms of infection with G. graminis, wheat genotypes differed most in incidence of deadheads, but differences were not consistent over environments, and were associated with earliness of maturity. Wheats differed more in expression of disease than in infection with G. graminis. The course of disease was deduced from associations between the incidence of pathogens and components of plant growth and yield. G. graminis was the dominant pathogen at both sites, and caused a yield loss of 0–15% at one site, and an average 62% loss at the other. More components of yield were affected where disease was most severe.     

小麦全蚀病菌致病性遗传规律研究*

利用弱致病性白化突变菌株与强致病菌株进行有性重组,研究了小麦全蚀病菌(禾顶囊壳小麦变种,Gaeumannomyces gramimis var. tritici)致病性的遗传规律。结果表明,以接种株地上部干重为致病性指标时,该菌对小麦的致病性为数量遗传性状,估计控制致病性的基因数为5 ~ 7个。弱致病菌株与强致病菌株杂交F1代致病性由弱至强呈连续分布,子代致病性的平均水平接近种亲代致病性的平均值。致病性的遗传力为54.36% ~ 71.00%,平均为62.32%。致病性表型易受环境因素的影响。菌落颜色与致病性无明显相关。     

Occurrence of Phialophora radicicola var. graminicola and Gaeumannomyces graminis var. tritici on roots of wheat in field crops

Assessments of Phialophora radicicola var. graminicola (PRG) and Gaeumannomyces graminis var. tritici (GGT) were made by culturing and by direct microscopic examination of pieces of seminal roots from 16 winter wheat crops grown in different cropping sequences and with different phosphate manuring. PRG occurred on all wheat crops, but was abundant only on wheat after grass, where it seemed to delay the onset of damaging take-all by 1 yr. Delayed occurrence of take-all by phosphate fertiliser was not related to differences in populations of PRG. Wheat grown in ‘take-all decline’ soils had only small amounts of PRG, indicating that the development and the decline of take-all epidemics may be influenced by different biological control mechanisms; breaking sequences of wheat crops by 1 yr grass leys might harness the advantages of both mechanisms.     

Purification and Characterization of a Secreted Laccase of Gaeumannomyces graminis var. tritici

We purified a secreted fungal laccase from filtrates of Gaeumannomyces graminis var. tritici cultures induced with copper and xylidine. The active protein had an apparent molecular mass of 190 kDa and yielded subunits with molecular masses of 60 kDa when denatured and deglycosylated. This laccase had a pI of 5.6 and an optimal pH of 4.5 with 2,6-dimethoxyphenol as its substrate. Like other, previously purified laccases, this one contained several copper atoms in each subunit, as determined by inductively coupled plasma spectroscopy. The active enzyme catalyzed the oxidation of 2,6-dimethoxyphenol (K_m = 2.6 × 10⁻⁵ ± 7 × 10⁻⁶ M), catechol (K_m = 2.5 × 10⁻⁴ ± 1 × 10⁻⁵ M), pyrogallol (K_m = 3.1 × 10⁻⁴ ± 4 × 10⁻⁵ M), and guaiacol (K_m = 5.1 × 10⁻⁴ ± 2 × 10⁻⁵ M). In addition, the laccase catalyzed the polymerization of 1,8-dihydroxynaphthalene, a natural fungal melanin precursor, into a high-molecular-weight melanin and catalyzed the oxidation, or decolorization, of the dye poly B-411, a lignin-like polymer. These findings indicate that this laccase may be involved in melanin polymerization in this phytopathogen’s hyphae and/or in lignin depolymerization in its infected plant host.     

Purification and characterization of a secreted laccase of Gaeumannomyces graminis var. tritici.

We purified a secreted fungal laccase from filtrates of Gaeumannomyces graminis var. tritici cultures induced with copper and xylidine. The active protein had an apparent molecular mass of 190 kDa and yielded subunits with molecular masses of 60 kDa when denatured and deglycosylated. This laccase had a pI of 5.6 and an optimal pH of 4.5 with 2,6-dimethoxyphenol as its substrate. Like other, previously purified laccases, this one contained several copper atoms in each subunit, as determined by inductively coupled plasma spectroscopy. The active enzyme catalyzed the oxidation of 2, 6-dimethoxyphenol (Km = 2.6 x 10(-5) +/- 7 x 10(-6) M), catechol (Km = 2.5 x 10(-4) +/- 1 x 10(-5) M), pyrogallol (Km = 3.1 x 10(-4) +/- 4 x 10(-5) M), and guaiacol (Km = 5.1 x 10(-4) +/- 2 x 10(-5) M). In addition, the laccase catalyzed the polymerization of 1, 8-dihydroxynaphthalene, a natural fungal melanin precursor, into a high-molecular-weight melanin and catalyzed the oxidation, or decolorization, of the dye poly B-411, a lignin-like polymer. These findings indicate that this laccase may be involved in melanin polymerization in this phytopathogen's hyphae and/or in lignin depolymerization in its infected plant host.     

Incidence of Gaeumannomyces graminis var. tritici and K deficiency increase rhizospheric denitrification

Wheat inoculated with the root pathogen Gaeumannomyces graminis var. tritici (Ggt) was grown in quartz silt at two levels of potassium nutrition. While in plants well supplied with K the incidence of Ggt did not affect plant growth, it reduced shoot and root weight of K deficient plants. Denitrification, measured by the acetylene inhibition technique and expressed as N₂O/mg root weight, was increased either by low K nutrition or by Ggt infection. Highest denitrification in the rhizosphere of plants was found with a combination of both, K deficiency and Ggt attack.     

Evidence for resistance in wheat cultivars grown in sand culture to the take-all pathogen, Gaeumannomyces graminis var. tritici

A laboratory method to inoculate seedlings uniformly with Gaeumannomyces graminis var. tritici is described. Resistance is defined via the rate of hyphal entry into the vascular tissue of host seedlings, and is measured by direct observation and by early stelar lesion development in seminal roots. The two scores for resistance are compared and evaluated, for infection with an isolate of low virulence. Evidence was obtained for resistance in the roots of wheat seedlings to G. graminis.     

小麦全蚀病菌原生质体制备的条件及再生菌株的致病性

分别利用崩溃酶、溶壁酶、纤维素酶和蜗牛酶酶解小麦全蚀病菌,进行原生质体的制备试验。结果显示,4种酶均能消化该菌细胞壁,获得一定数量的原生质体;产生原生质体效率最高的是溶壁酶,该酶在浓度为16 mg/mL时产生的原生质体数量最多,最佳的酶解时间为2~3 h,最适作用温度为28℃。制备的原生质体可以再生并与原始出发菌株具有相同的致病能力。     

Thickening and browning of cortical cell walls in seminal roots of wheat seedlings infected with Gaeumannomyces graminis var. tritici

This paper examines a little studied reaction of wheat roots to invasion by Gaeumannomyces graminis var. tritici, namely the thickening and browning of cortical cell walls. Examination was confined to distal segments of young seminal roots grown in sand. Browning and thickening of walls of cells were associated with the lignification of tissue and appeared to be a response of living or recently live cells to invasion by weakly virulent isolates. Wheat genotypes differed in their ability to thicken and lignify cell walls, and the difference between two wheat cultivars appeared to be under simple genetic control. There was some evidence that cortical browning temporarily retarded radial invasion by hyphae of G. graminis into young seminal roots of wheats.     

Detoxification of Benzoxazolinone Allelochemicals from Wheat by Gaeumannomyces graminis var. tritici, G. graminis var. graminis, G. graminis var. avenae, and Fusarium culmorum

The ability of phytopathogenic fungi to overcome the chemical defense barriers of their host plants is of great importance for fungal pathogenicity. We studied the role of cyclic hydroxamic acids and their related benzoxazolinones in plant interactions with pathogenic fungi. We identified species-dependent differences in the abilities of Gaeumannomyces graminis var. tritici, Gaeumannomyces graminis var. graminis, Gaeumannomyces graminis var. avenae, and Fusarium culmorum to detoxify these allelochemicals of gramineous plants. The G. graminis var. graminis isolate degraded benzoxazolin-2(3H)-one (BOA) and 6-methoxy-benzoxazolin-2(3H)-one (MBOA) more efficiently than did G. graminis var. tritici and G. graminis var. avenae. F. culmorum degraded BOA but not MBOA. N-(2-Hydroxyphenyl)-malonamic acid and N-(2-hydroxy-4-methoxyphenyl)-malonamic acid were the primary G. graminis var. graminis and G. graminis var. tritici metabolites of BOA and MBOA, respectively, as well as of the related cyclic hydroxamic acids. 2-Amino-3H-phenoxazin-3-one was identified as an additional G. graminis var. tritici metabolite of BOA. No metabolite accumulation was detected for G. graminis var. avenae and F. culmorum by high-pressure liquid chromatography. The mycelial growth of the pathogenic fungi was inhibited more by BOA and MBOA than by their related fungal metabolites. The tolerance of Gaeumannomyces spp. for benzoxazolinone compounds is correlated with their detoxification ability. The ability of Gaeumannomyces isolates to cause root rot symptoms in wheat (cultivars Rektor and Astron) parallels their potential to degrade wheat allelochemicals to nontoxic compounds.     

Linear relationship between Gaeumannomyces graminis var. tritici (Ggt) genotypic frequencies and disease severity on wheat roots in the field

In order to investigate potential links existing between Gaeumannomyces graminis var. tritici (Ggt) population structure and disease development during polyetic take-all epidemics in sequences of Ggt host cereals, seven epidemics in fields with different cropping histories were monitored during the seasons 2001/2002 (two fields), 2002/2003 (two fields) and 2003/2004 (three fields). Take-all incidence and severity were measured at stem elongation and Ggt populations were characterized. The 73 isolates collected in the two fields in 2001/2002 were distributed into two multilocus genotypes, G1 and G2 according to amplified fragment length polymorphism analysis. A monolocus molecular marker amplified by F-12 random amplification polymorphism DNA primer sizing between 1.9 and 2.0 kb that gave strictly the same distinction between the two multilocus genotypes was further applied to measure G1/G2 frequencies among Ggt populations in all fields (266 isolates). The ratios of G1 to G2 differed between fields with different cropping histories. A linear relationship between G2 frequency among Ggt populations and disease severity at stem elongation was measured during the three cropping seasons. When take-all decline was observed, G2 frequencies were low in first wheat crops, highest in short-term sequences and intermediate in longer sequences of consecutive crops of Ggt host cereals. This pattern could be the result of population selection by environmental conditions, in particular by microbial antagonism during the parasitic phase of the fungus. In order to better understand take-all epidemic dynamics, the distinction between these two genotypes could be a basis to develop models that link approaches of quantitative epidemiology and advances in population genetics of Ggt.