Root exudation and microflora populations in rhizosphere of crop genotypes differing in tolerance to micronutrient deficiency

Crop genotypes differ in their tolerance to micronutrient-deficient soils, but the underlying mechanisms are poorly understood. This paper reviews information on mechanisms of tolerance to Zn and Mn deficiency, concentrating on plant-induced changes in chemistry and biology of rhizosphere that alter availability of Zn and Mn.When grown under conditions of Zn deficiency, wheat genotypes more tolerant of Zn deficiency released greater amounts of phytosiderophore, 2-deoxymugineic acid, than the sensitive genotypes. In addition, Zn deficiency increased numbers of fluorescent pseudomonads in rhizosphere of all wheat genotypes tested, but the effect was particularly obvious for genotypes tolerant of Zn deficiency.Rhizosphere of wheat genotypes contained an increased proportion of Mn reducers under Mn-deficiency compared to Mn-sufficiency conditions. When grown in soils of low Mn availability, some wheat genotypes tolerant of Mn deficiency (like cv. Aroona) had a greater ratio of Mn-reducers to Mn-oxidisers in the rhizosphere compared to the sensitive genotypes. In contrast, microflora in the rhizosphere of other wheat genotypes tolerant of Mn deficiency (like C8MM) did not show the same response as Aroona. It therefore appears that different mechanisms may underlie the expression of tolerance to Mn deficiency in wheat genotypes.It is concluded that wheat genotypes tolerant of Zn or Mn deficiency have a capacity to alter chemical and biological properties of the rhizosphere, thus increasing availability of critical micronutrients.     

Manganese nutrition and accumulation of phenolics and lignin as related to differential resistance of wheat genotypes to the take-all fungus

Differential resistance of four Triticum aestivum L. genotypes to isolates of take-all fungus (Gaeuman-nomyces graminis var. ritici Walker) was tested in a complete factorial experiment set up in a growth chamber using Mn-deficient Wangary sand amended with four rates of Mn. Mn-efficient cultivars produced more dry matter at low supply of Mn. Fertilization with Mn significantly increased its accumulation in roots and shoots. The most sensitive measure of take-all infection was the total length of root stellar lesions; these lesions were reduced by Mn fertilization and were shorter in Mn-efficient genotypes. The resistance-enhancing effect of Mn was the most obvious in the Mn-inefficient genotype (Bayonet) and the least obvious in the Mn-efficient one (C8MM). Phenolics biosynthesis in roots was clicited by fungal infection, especially in the case of the highly virulent isolate. The weakly virulent isolate increased phenolics concentration in roots much more if no Mn was added, indicating that the resistance-enhancing effect of Mn may not be directly exerted through the effects on phenolics biosynthesis. Lignin concentration in roots decreased due to Mn fertilization, while no effect of take-all infection was noted. It appears that biosynthesis of phenolics and lignin in wheat roots has a low Mn requirement which can be satisfied at environmental Mn concentrations below those necessary for optimum plant growth. ei]Section editor: A C Borstlap ei]Section editor: H Lambers     

Comparison of responses to Mn deficiency between the UK wheat genotypes Maris Butler, Paragon and the Australian wheat genotype C8MM

Wheat grown in Mn-deficient soil has been widely observed to produce much reduced yields. Breeding for Mn-efficient wheat genotypes adapted to Mn-deficient soils would represent a long-term solution for wheat agronomy, To characterize the physiological basis of Mn efficiency in wheat genotypes would facilitate the breeding programs for producing Mn-efficient wheat. Using a solution culture and a soil culture system in the present study, a Mn-efficient UK wheat genotype Maris Butler and a Mn-inefficient UK wheat genotype Paragon have been compared with a Mn-efficient Australian wheat genotype C8MM in the responses to Mn deficiency In order to characterize the Mn efficiency in these wheat genotypes.Results showed that in solution culture, Marls Butler grown under Mn deficiency had 77% relative dry matter yield of control plants that were grown under Mn sufficiency, whereas C8MM and Paragon had 60% and 58% relative dry matter yield of their respective controls. Results from the soil culture demonstrated that relative dry matter yield remained high for Marie Butler and C8MM (53% and 56%, respectively), whereas the value for Paragon dropped to 33%. In terms of dry matter yield and photosynthetic efficiency, Mads Butler demonstrated Mn efficiency in both solution culture and soil culture, whereas C8MM showed Mn efficiency only In soil culture. Results also demonstrated that under Mn-depleted supply in soil, plants of C8MM had a significantly higher ability in Mn uptake, whereas plants of Marls Butler showed a higher internal Mn usa efficiency in comparison with plants of Paragon. Results from the present study indicate that the ability of C8MM to accumulate higher amounts of Mn is the basis of the improved Mn efficiency of this genotype in comparison with Paragon, and in Marls Butler there is a higher internal use of Mn expressed as an improved photosynthetic efficiency in conferring its Mn efficiency. It is suggested that more than one mechanism has arisen in wheat to confer tolerance to Mn deficiency.     

Time-course of Biosynthesis of Phenolics and Lignin in Roots of Wheat Genotypes Differing in Manganese Efficiency and Resistance to Take-all Fungus

Inhibition of lignin biosynthesis in Triticum aestivum L. rootsby Mn deficiency has been suggested as the mechanism of reducedresistance of Mn-deficient wheat roots to infection by the take-allfungus (Gaeumannomyces graminis var. tritici). This study evaluatedphenolics and lignin accumulation in roots of wheat genotypesdiffering in Mn efficiency (measured as growth and yield inMn-deficient soils) and take-all resistance. Seedlings of theMn-inefficient, take-all sensitive genotype Bayonet and theMn-efficient, more take-all resistant genotype C8MM were grownin nutrient solution without added Mn for 18 d and then transferredto a Mn-deficient sandy soil fertilized with Mn at 0 or 30 mgkg^-1. Both genotypes had Mn-deficient roots and shoots at thetime of transfer to the soil. Roots of both genotypes were inoculatedwith the take-all fungus 0, 1, 3 and 7 d after transfer. Twenty-fourhours after inoculation, take-all fungus penetrated the rootstele of take-all sensitive Bayonet but not of more resistantC8MM wheat. Rates of phenolics and lignin accumulation in rootsdeclined steadily during growth in soil for up to 8 d, werehigher in mature, fully differentiated parts of the root systemcompared to distal, younger root tissue, and were higher inBayonet than in C8MM. Manganese fertilization did not significantlyinfluence rates of phenolics and lignin accumulation but reduceddepth of radial penetration by hyphae in both genotypes. Therate of phenolics accumulation was positively (r = 0·91to 0·96) correlated with the rate of lignin accumulation.Mn-efficient C8MM had a higher rate of lignin accumulation perunit of phenolics than Mn-inefficient Bayonet over a wide rangeof phenolics synthesis rates. From this we suggest that C8MMhas a more efficient mechanism for conversion of phenolics tolignin, the trait which appears related to higher take-all resistanceof this genotype.Copyright 1994, 1999 Academic Press Gaeumannomyces graminis var. tritici, lignin, manganese, phenolics, resistance, roots, Triticum aestivum     

Root exudation and Fe uptake and transport in wheat genotypes differing in tolerance to Zn deficiency

Tolerance to Zn deficiency in wheat germplasm may be inversely related to uptake and transport of Fe to shoots. The present study examined eight bread (Triticum aestivum) and two durum (T. turgidum L. conv. durum) wheat genotypes for their capacity to take up and transport Fe when grown under either Fe or Zn deficiency. Bread wheat genotypes Aroona, Excalibur and Stilleto showed tolerance to Zn and Fe deficiency, while durum wheat genotypes are clearly less tolerant to either deficiency. Roots of bread wheats tolerant to Zn deficiency exuded more phytosiderophores than sensitive bread and durum genotypes. Greater amounts of phytosideophores were exuded by roots grown under Fe than Zn deficiency. A relatively poor relationship existed between phytosiderophore exudation or the Fe uptake rate and relative shoot growth under Fe deficiency. At advanced stages of Zn deficiency, genotypes tolerant to Zn deficiency (Aroona and Stilleto) had a greater rate of Fe uptake than other genotypes. Zinc deficiency depressed the rate of Fe transport to shoots in all genotypes in early stages, while advanced Zn deficiency had the opposite effect. Compared with Zn-sufficient plants, 17-day-old Zn-deficient plants of genotypes tolerant to Zn deficiency had a lower rate of Fe transport to shoots, while genotypes sensitive to Zn deficiency (Durati, Yallaroi) had the Fe transport rate increased by Zn deficiency. A proportion of total amount of Fe taken up that was transported to shoots increased with duration of either Fe or Zn deficiency. It is concluded that greater tolerance to Zn deficiency among wheat genotypes is associated with the increased exudation of phytosiderophores, an increased Fe uptake rate and decreased transport of Fe to shoots. This revised version was published online in June 2006 with corrections to the Cover Date.     

Genotypic Differences in the Production and Partitioning of Carbohydrates Between Roots and Shoots of Wheat Grown under Zinc or Manganese Deficiency

The partitioning of soluble carbohydrates and starch betweenroots and shoots was investigated in wheat genotypes differingin Zn or Mn efficiency. The plants were grown for 11 d in achelate-buffered nutrient solution with sufficient or deficientZn and Mn supply. The Zn-efficient cultivar Warigal had a greatershoot fresh weight under sufficient Zn compared with the Zn-inefficientcultivar Durati. When supplied with sufficient Zn, Warigal hada greater concentration and content of soluble carbohydratesin roots and shoots in comparison with Durati. Under deficientZn supply, Durati had a greater concentration and content ofstarch in roots and shoots compared with Warigal. In an experimentwith varying supply of Mn, the Mn-efficient genotype C8MM hada greater shoot fresh weight than the Mn-inefficient cultivarBayonet under sufficient or deficient Mn supply. The concentrationof soluble carbohydrates in roots and shoots was decreased bydeficient Mn supply in C8MM but not in Bayonet. Starch accumulatedin the roots of Bayonet under deficient Mn supply. The resultssuggest that synthesis of carbohydrates is decreased under Zndeficiency, while they are preferentially partitioned to theroots to increase growth and thus the surface area availablefor Zn uptake. In the case of Mn deficiency, carbohydrate productionwas limited, but partitioning between roots and shoots was notaltered.Copyright 1997 Annals of Botany Company Carbohydrate; deficiency; manganese; assimilate partitioning; starch; Triticum aestivum; zinc     

Control of Mn status and infection rate by genotype of both host and pathogen in the wheat take-all interaction

Take-all is a world-wide root-rotting disease of cereals. The causal organism of take-all of wheat is the soil-borne fungus Gaeumannomyces graminis var tritici (Ggt). No resistance to take-all, worthy of inclusion in a plant breeding programme, has been discovered in wheat but the severity of take-all is increased in host plants whose tissues are deficient for manganese (Mn). Take-all of wheat will be decreased by all techniques which lift Mn concentrations in shoots and roots of Mn-deficient hosts to adequate levels. Wheat seedlings were grown in a Mn-deficient calcareous sand in small pots and inoculated with four field isolates of Ggt. Infection by three virulent isolates was increased under conditions which were Mn deficient for the wheat host but infection by a weakly virulent isolate, already low, was further decreased. Only the three virulent isolates caused visible oxidation of Mn in vitro. The sensitivity of Ggt isolates to manganous ions in vitro did not explain the extent of infection they caused on wheat hosts. In a similar experiment four Australian wheat genotypes were grown in the same Mn-deficient calcareous sand and inoculated with one virulent isolate of Ggt. Two genotypes were inefficient at taking up manganese and were very susceptible to take-all, one was very efficient at taking up manganese and was resistant to take-all, and the fourth genotype was intermediate for both characters. All genotypes were equally resistant under Mn-adequate conditions.     

Phytosiderophore release in bread and durum wheat genotypes differing in zinc efficiency

The effect of the zinc (Zn) nutritional status on the rate of phytosiderophore release was studied in nutrient solution over 20 days in four bread wheat (Triticum aestivum cvs. Kiraç-66, Gerek-79, Aroona and Kirkpinar) and four durum wheat (Triticum durum cvs. BDMM-19, Kunduru-1149, Kiziltan-91 and Durati) genotypes differing in Zn efficiency.Visual Zn deficiency symptoms, such as whitish-brown necrosis on leaves and reduction in plant height appeared first and more severe in Zn-inefficient durum wheat genotypes Kiziltan-91, Durati and Kunduru-1149. Compared to the bread wheat genotypes, all durum wheat genotypes were more sensitive to Zn deficiency. BDMM-19 was the least affected durum wheat genotype. Among the bread wheat genotypes, Kirkpinar was the most sensitive genotype. In all genotypes well supplied with Zn, the rate of phytosiderophore release was very low and did not exceed 1 mol 32 plants^-1 3h^-1, or 0.5 mol g^-1 root dry wt 3h^-1. However, under Zn deficiency, with the onset of visual Zn deficiency symptoms, the release of phytosiderophores was enhanced in bread wheat genotypes up to 7.5 mol 32 plants^-1 3h^-1, or 9 mol g^-1 root dry wt 3h^-1, particularly in Zn-efficient Kiraç-66, Gerek-79 and Aroona. In contrast to bread wheat genotypes, phytosiderophore release in Zn-deficient durum wheat genotypes remained at a very low rate. Among the durum wheat genotypes BDMM-19 had highest rate of phytosiderophore release. HPLC analysis of root exudates showed that 2-deoxymugineic acid (DMA) is the dominating phytosiderophore released from roots of Zn-efficient genotypes. In root extracts concentration of DMA was also much higher in Zn-efficient than in inefficient genotypes. The results demonstrate that enhanced synthesis and release of phytosiderophores at deficient Zn supply is involved in Zn efficiency in wheat genotypes. It is suggested that the expression of Zn efficiency mechanism is causally related to phytosiderophore-mediated enhanced mobilization of Zn from sparingly soluble Zn pools and from adsorption sites, both in the rhizosphere and plants.     

Dry matter production and distribution of zinc in bread and durum wheat genotypes differing in zinc efficiency

Six bread wheat (Triticum aestivum cvs. Kiraç-66, Gerek-79, Aroona, ES 91-12, ES-14 and Kirkpinar) and four durum wheat (Triticum durum cvs. BDMM-19, Kunduru-1149, Kiziltan-91 and Durati) genotypes were grown under controlled environmental conditions in nutrient solution for 20 days to study the effect of varied supply of Zn (0 to 1 µM) on Zn deficiency symptoms in shoots, root and shoot dry matter production, and distribution of Zn in roots and shoots.Visual Zn deficiency symptoms, such as whitish-brown lesions on leaves, appeared rapidly and severly in durum wheats, particularly in Kiziltan-91 and Durati. Among the durum wheats, BDMM-19 was less affected by Zn deficiency, and among the bread wheats Kiraç-66, ES 91-12, Aroona and Gerek-79 were less affected than ES-14 and Kirkpinar.Under Zn deficiency, shoot dry matter production was decreased in all genotypes, but more distinctly in durum wheat genotypes. Despite severe decreases in shoot growth, root growth of all genotypes was either not affected or even increased by Zn deficiency. Correspondingly, shoot/root dry weight ratios were lower in Zn-deficient than in Zn-sufficient plants, especially in durum wheat genotypes.The distinct differences among the genotypes in sensitivity to Zn deficiency were closely related with the Zn content (Zn accumulation) per shoot but not with the Zn concentration in the shoot dry matter. On average, genotypes with lesser deficiency symptoms contained about 42% more Zn per shoot than genotypes with severe deficiency symptoms. In contrast to shoots, the Zn content in roots did not differ between genotypes. Shoot/root ratios of total Zn content were therefore greater for genotypes with lesser deficiency symptoms than for genotypes with severe deficiency symptoms (i.e. all durum wheat genotypes).The results suggest that the enhanced capacity of genotypes for Zn uptake and translocation from roots to shoot meristems under deficient Zn supply might be the most important factor contributing to Zn efficiency in wheat genotypes. The results also demonstrate that under severe Zn deficiency, Zn concentration in the shoot dry matter is not a suitable parameter for distinguishing wheat genotypes in their sensitivity to Zn deficiency.     

Physiological responses of wheat genotypes grown in chelator-buffered nutrient solutions with increasing concentrations of excess HEDTA

The chelator-buffered nutrient solutions containing excess chelator have been used frequently in the micronutrient research, but potential toxicity of the excess chelator has not been ascertained. The present study was conducted to test effects of four concentrations of excess HEDTA [ N-(2-hydroxyethyl)ethylenedinitrilotriacetic acid] and two levels of total Zn on growth, root exudation, and nutrient uptake and transport by Triticum aestivum L. (cv. Aroona) and Triticum turgidum L. conv. durum (Desf.) MacKey (cv. Durati) genotypes differing in tolerance to Zn deficiency. Excess HEDTA at 50 μM reduced root and shoot growth and caused visual toxicity symptoms (necrotic lesions) on leaves; these effects were generally absent at lower concentrations of excess HEDTA. Root exudation of phytosiderophores increased with increasing concentrations of excess HEDTA at deficient and sufficient Zn levels, and was higher in Zn-deficiency-tolerant Aroona than in Zn-deficiency-sensitive Durati wheat. Shoot and root Zn concentrations showed a saturable response to increasing Zn²⁺ activities in solution. Excess HEDTA at 50 μM caused an increase in shoot concentrations of Fe and a decrease in concentrations of Mn and Cu. An average rate of Zn uptake increased with an increase in Zn²⁺ ionic activity in solution, with Zn-deficiency-tolerant Aroona having a higher rate of Zn uptake than Zn-deficiency-sensitive Durati in the deficiency range of Zn²⁺ activities. Average uptake rates of Mn and Cu decreased with an increase in concentration of excess HEDTA. Similar observations were noted for transport of Mn and Cu to shoots, while Zn transport to shoots was proportional to Zn²⁺ activities in solution. It was concluded that excess HEDTA at 50 μM adversely affects wheat growth and physiology, while excess of 25 μM or less does not cause measurable toxicity. This revised version was published online in June 2006 with corrections to the Cover Date.     

Interactions Between Nitrogen and Manganese Nutrition of Barley Genotypes Differing in Manganese Efficiency

Ammonium-fed plants may acidify the rhizosphere and thus increaseavailability of Mn in calcareous alkaline soils. The importanceof N nutrition in the differential expression of tolerance toMn deficiency among cereal genotypes is not yet clear. Two factorialexperiments testing effects of the NH₄-N/NO₃-N ratio and Mnfertilization on growth of barley genotypes differing in toleranceto Mn deficiency were conducted in two calcareous alkaline soilsin pots in a controlled environment. In the soil containing80% CaCO₃at pH 8.5, better root and shoot growth and highershoot Mn concentrations were achieved with nitrate supply, especiallyat lower rates of Mn fertilization. The Mn-efficient genotypeWeeah (tolerant of Mn deficiency) achieved better root and shootgrowth than Mn-inefficient Galleon barley (sensitive to Mn deficiency)regardless of experimental treatment. Fertilization with Mndid not influence total N concentration in barley roots andshoots. In the soil containing 5% CaCO₃at pH 7.8, ammonium-fedplants had better root and shoot growth and, at shoot Mn concentrationsabove the critical level, Mn-inefficient Galleon performed betterthan Mn-efficient Weeah barley. It appears that differentialexpression of Mn efficiency among barley genotypes is not associatedwith differences in Mn availability expected to be producedby differential rhizosphere acidification as a response to differentforms of N supply. There is an apparent preference of locallyselected barley genotypes for nitrate nutrition when grown onthe highly calcareous alkaline soils of southern Australia. Ammonium; calcareous soil; Hordeum vulgare ; manganese; nitrate; nitrogen form; nutrient efficiency; rhizosphere     

Manganese reduction in the rhizosphere of mycorrhizal and nonmycorrhizal maize

The influence of rhizosphere microorganisms and vesicular-arbuscular (VA) mycorrhiza on manganese (Mn) uptake in maize (Zea mays L. cv. Tau) plants was studied in pot experiments under controlled environmental conditions. The plants were grown for 7 weeks in sterilized calcareous soil in pots having separate compartments for growth of roots and of VA mycorrhizal fungal hyphae. The soil was left either uninoculated (control) or prior to planting was inoculated with rhizosphere microorganisms only (MO-VA) or with rhizosphere microorganisms together with a VA mycorrhizal fungus [Glomus mosseae (Nicol and Gerd.) Gerdemann and Trappe] (MO+VA). Mycorrhiza treatment did not affect shoot dry weight, but root dry weight was slightly inhibited in the MO+VA and MO-VA treatments compared with the uninoculated control. Concentrations of Mn in shoots decreased in the order MO-VA > MO+VA > control. In the rhizosphere soil, the total microbial population was higher in mycorrhizal (MO+VA) than nonmycorrhizal (MO-VA) treatments, but the proportion of Mn-reducing microbial populations was fivefold higher in the nonmycorrhizal treatment, suggesting substantial qualitative changes in rhizosphere microbial populations upon root infection with the mycorrhizal fungi. The most important microbial group taking part in the reduction of Mn was fluorescent Pseudomonas. Mycorrhizal treatment decreased not only the number of Mn reducers but also the release of Mn-solubilizing root exudates, which were collected by percolation from maize plants cultivated in plastic tubes filled with gravel quartz sand. Compared with mycorrhizal plants, the root exudates of nonmycorrhizal plants had two fold higher capacity for reduction of Mn. Therefore, changes in both rhizosphere microbial population and root exudation are probably responsible for the lower acquisition of Mn in mycorrhizal plants.     

Quorum-sensing system influences root colonization and biological control ability in Pseudomonas fluorescens 2P24

Pseudomonas fluorescens 2P24 is a biocontrol agent isolated from a wheat take-all decline soil in China. This strain produces several antifungal compounds, such as 2,4-diacetylphloroglucinol (2,4-DAPG), hydrogen cyanide and siderophore(s). Our recent work revealed that strain 2P24 employs a quorum-sensing system to regulate its biocontrol activity. In this study, we identified a quorum-sensing system consisting of PcoR and PcoI of the LuxR–LuxI family from strain 2P24. Deletion of pcoI from 2P24 abolishes the production of the quorum-sensing signals, but does not detectably affect the production of antifungal metabolites. However, the mutant is significantly defective in biofilm formation, colonization on wheat rhizosphere and biocontrol ability against wheat take-all, whilst complementation of pcoI restores the biocontrol activity to the wild-type level. Our data indicate that quorum sensing is involved in regulation of biocontrol activity in P. fluorescens 2P24.     

Zinc deficiency-induced phytosiderophore release by the Triticaceae is not consistently expressed in solution culture

The effects of zinc (Zn) and iron (Fe) deficiencies on phytosiderophore (PS) exudation by three barley (Hordeum vulgare L.) cultivars differing in Zn efficiency were assessed using chelator-buffered nutrient solutions. A similar study was carried out with four wheat (Triticum aestivum L. and T. durum Desf.) cultivars, including the Zn-efficient Aroona and Zn-inefficient Durati. Despite severe Zn deficiency, none of the barley or wheat cultivars studied exhibited significantly elevated PS release rates, although there was significantly enhanced PS exudation under Fe deficiency. Aroona and Durati wheats were grown in a further study of the effects of phosphate (P) supply on PS release, using 100 μM KH₂PO₄ as high P, or solid hydroxyapatite as a supply of low-release P. Phytosiderophore exudation was not increased due to P treatment under control or Zn-deficient conditions, but was increased by Fe deficiency. Accumulation of P in shoots of Zn- and Fe-deficient plants was seen in both P treatments, somewhat more so under the KH₂PO₄ treatment. Zinc-efficient wheats and grasses have been previously shown to exude more PS under Zn deficiency than Zn-inefficient genotypes. We did not observe Zn-deficiency-induced PS release and were unable to replicate the results of previous researchers. The tendency for Zn deficiency to induce PS release seems to be method dependent, and we suggest that all reported instances may be explained by an induced physiological deficiency of Fe. Received: 25 October 1999 / Accepted: 3 December 1999     

Role of 2,4-diacetylphloroglucinol-producing fluorescent Pseudomonas spp. in the defense of plant roots

Plants have evolved strategies of stimulating and supporting specific groups of antagonistic microorganisms in the rhizosphere as a defense against diseases caused by soilborne plant pathogens owing to a lack of genetic resistance to some of the most common and widespread soilborne pathogens. Some of the best examples of natural microbial defense of plant roots occur in disease suppressive soils. Soil suppressiveness against many different diseases has been described. Take-all is an important root disease of wheat, and soils become suppressive to take-all when wheat or barley is grown continuously in a field following a disease outbreak; this phenomenon is known as take-all decline (TAD). In Washington State, USA and The Netherlands, TAD results from the enrichment during monoculture of populations of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing Pseudomonas fluorescens to a density of 10 (5) CFU/g of root, the threshold required to suppress the take-all pathogen, Gaeumannomyces graminis var. tritici. 2,4-DAPG-producing P. fluorescens also are enriched by monoculture of other crops such as pea and flax, and evidence is accumulating that 2,4-DAPG producers contribute to the defense of plant roots in many different agroecosystems. At this time, 22 distinct genotypes of 2,4-DAPG producers (designated A - T, PfY and PfZ) have been defined by whole-cell repetitive sequence-based (rep)-PCR analysis, restriction fragment length polymorphism (RFLP) analysis of PHLD, and phylogenetic analysis of PHLD, but the number of genotypes is expected to increase. The genotype of an isolate is predictive of its rhizosphere competence on wheat and pea. Multiple genotypes often occur in a single soil and the crop species grown modulates the outcome of the competition among these genotypes in the rhizosphere. 2,4-DAPG producers are highly effective biocontrol agents against a variety of plant diseases and ideally suited for serving as vectors for expressing other biocontrol traits in the rhizosphere.     

Identity and frequency of occurrence ofTrichoderma spp. in roots of wheat and rye-grass in Western Australia and their effect on root rot caused byGaeumannomyces graminis var.tritici

Trichoderma hamatum, T. harzianum andT. koningii were isolated from wheat and rye-grass roots from a field in Western Australia. Frequency of occurrence ofTrichoderma spp. was higher on roots subjected to washing only, for both wheat and rye-grass than the roots which were surface-sterilized with 0.6% or 1.25% NaOCl.Trichoderma spp. were recovered at a higher frequency on PDA amended with lactic acid (pH 4.5) than on PDA alone (pH 5.6) or PDA with streptomycin. In general,Trichoderma spp. were isolated at a higher frequency from roots of wheat than that of rye-grass.T. hamatum occurred at a higher frequency in rye-grass roots than in wheat, whereasT. harzianum was more common in roots of wheat than in rye-grass, especially in seedling and milky ripe stages.T. koningii was recovered at a higher frequency from roots at seedling stage of rye-grass than wheat, the reverse being true at tillering stage.T. koningii was not recovered from roots of either host in any sampling when they were surface sterilized with 1.25% NaOCl.The take-all fungus was isolated from wheat and rye-grass roots more frequently at tillering and stem extension stages than others. It was severely pathogenic to both hosts in sterilized and non-sterilized soil.Addition of lactic acid, HCl or streptomycin to PDA did not affect the growth of theTrichoderma spp. tested, but the growth was slower on Martin's medium than on other media. In generalT. harzianum andT. koningii grow faster thanT. hamatum. The growth of the three species were not different at 20 and 25°C, but at 15°c growing of all species was significantly reduced.Incorporation of lactic acid into PDA prevented the bacterial growth in all treatments. Streptomycin too reduced but to a lesser degree than lactic acid. Surface sterilization with NaOCl decreased the recovery of both bacteria and fungi. T. hamatum andT. koningii reduced the mortality of wheat and rye-grass plants inoculated with the take-all fungus in sterilized and non-sterilized soil, whereT. harzianum did not protect wheat or rye-grass from infection by the take-all fungus.     

Grain zinc, iron and protein concentrations and zinc-efficiency in wild emmer wheat under contrasting irrigation regimes

Micronutrient malnutrition, and particularly deficiency in zinc (Zn) and iron (Fe), afflicts over three billion people worldwide, and nearly half of the world’s cereal-growing area is affected by soil Zn deficiency. Wild emmer wheat [Triticum turgidum ssp. dicoccoides (Körn.) Thell.], the progenitor of domesticated durum wheat and bread wheat, offers a valuable source of economically important genetic diversity including grain mineral concentrations. Twenty two wild emmer wheat accessions, representing a wide range of drought resistance capacity, as well as two durum wheat cultivars were examined under two contrasting irrigation regimes (well-watered control and water-limited), for grain yield, total biomass production and grain Zn, Fe and protein concentrations. The wild emmer accessions exhibited high genetic diversity for yield and grain Zn, Fe and protein concentrations under both irrigation regimes, with a considerable potential for improvement of the cultivated wheat. Grain Zn, Fe and protein concentrations were positively correlated with one another. Although irrigation regime significantly affected ranking of genotypes, a few wild emmer accessions were identified for their advantage over durum wheat, having consistently higher grain Zn (e.g., 125 mg kg^?1), Fe (85 mg kg^?1) and protein (250 g kg^?1) concentrations and high yield capacity. Plants grown from seeds originated from both irrigation regimes were also examined for Zn efficiency (Zn deficiency tolerance) on a Zn-deficient calcareous soil. Zinc efficiency, expressed as the ratio of shoot dry matter production under Zn deficiency to Zn fertilization, showed large genetic variation among the genotypes tested. The source of seeds from maternal plants grown under both irrigation regimes had very little effect on Zn efficiency. Several wild emmer accessions revealed combination of high Zn efficiency and drought stress resistance. The results indicate high genetic potential of wild emmer wheat to improve grain Zn, Fe and protein concentrations, Zn deficiency tolerance and drought resistance in cultivated wheat.     

Biological control of take-all in wheat by endophytic Bacillus subtilis E1R-j and potential mode of action

The bacterial strain E1R-j, isolated as an endophyte from wheat roots, exhibited high antifungal activity to Gaeumannomyces graminis var. tritici (Ggt). Strain E1R-j was identified as Bacillus subtilis based on morphological, physiological and biochemical methods as well as on 16S rDNA analysis. This strain inhibited mycelium growth in vitro of numerous plant pathogenic fungi, especially of Ggt, Coniothyrium diplodiella, Phomopsis sp. and Sclerotinia sclerotiorum. In greenhouse experiments, soil drenches with cell densities of 10⁶, 10⁹ and 10¹² CFU ml⁻¹ E1R-j reduced significantly take-all disease, caused by Ggt, in wheat seedling by 62.6%, 68.6% and 70.7%, respectively, compared to the inoculated control, 4 weeks after sowing. Growth parameters such as lengths and fresh weights of roots and shoots of Ggt-inoculated control plants were significantly lower compared to Ggt-inoculated and E1R-j treated plants. Field experiments in the season 2006/2007, heights of wheat plants in the Ggt inoculated plots were significantly reduced compared to the non inoculated treatments. Yield parameters such as kernels per head and thousand kernel weight (TKW) in inoculated control plants were lower compared to the other treatments. In the experimental year 2007/2008, independent treatments with the bacterial strain E1R-j and the fungicide Triadimefon reduced take-all disease in wheat roots by 55.3% and 61.9%, compared to the inoculated control plants. In this season plant height in inoculated control was significantly lower and also the yield parameters seeds per head and especially TKW were drastically reduced compared to the other treatments. E1R-j treatment alleviated the detrimental effects of take-all on grain yield parameters to a similar extent as Triadimefon application. SEM studies revealed that in the presence of E1R-j, hyphae of Ggt showed leakage, appeared ruptured, swollen and shriveled. Following root drench, strain E1R-j was able to colonize endophytically roots and leaves of wheat seedlings. While the population of the bacterial strain in wheat roots steadily increased from the second to the fourth leaf stage, in the leaf tissue the population of the strain rapidly declined. TEM studies also showed that cells of E1R-j were present in roots of wheat seedlings and effectively retarded infection and colonization of Ggt in root tissue; suppression of Ggt by E1R-j was accompanied by disintegration of hyphal cytoplasm. In addition, in the presence of E1R-j cells in Ggt-infected root tissue morphological defense reactions were triggered such as formation of wall appositions and papillae. The results presented indicate that the endophytic strain E1R-j of B. subtilis meets demands required for biocontrol of take-all.     

Plant competition and disease in genetically diverse wheat populations

Summary The direct and indirect effects of plant genetic diversity on epidemics and the influence of disease on plant competition were investigated using the wheat (Triticum aestivum)/stripe rust (Puccinia striiformis) system. Replacement series consisting of a susceptible and a resistant wheat genotype or two wheat genotypes susceptible to different races of stripe rust were grown in the presence and absence of the pathogen. Stripe rust severity, number of seed heads, seed yield, and seed weight were determined separately for each wheat genotype in the mixtures and the pure stands. The frequency of susceptible genotypes in a mixture explained up to 67% of the variation in disease severity. However, competitive interactions among plant genotypes sometimes appeared to alter susceptibility and obscured the relationship. In pure stands of single genotypes, disease severity explained between 52 and 58% of the variation in seed yield. In mixtures, coefficients of determination were only 10 and 31%, suggesting a strong influence of plant-plant interactions on seed yield. These results suggest that host-parasite coevolutionary models need to account for the strong effect that specific plant genotype combinations may have on disease severity and plant reproduction.Paper No. 9818 of the journal series of the Oregon Agricultural Experiment Station     

Wheat Cultivar-Specific Selection of 2,4-Diacetylphloroglucinol-Producing Fluorescent <Emphasis Type="Italic">Pseudomonas</Emphasis> Species from Resident Soil Populations

An emerging body of evidence indicates a role for plant genotype as a determinant of the species and genetic composition of the saprophytic microbial community resident to the rhizosphere. In this study, experiments were conducted to determine the capacity of five different wheat cultivars to enhance resident populations and support introduced strains of 2,4-diacetylphloroglucinol (2,4-DAPG)-producing fluorescent pseudomonads, a group of bacteria known to provide biological control of several soilborne diseases. When soils were cropped with three successive 28-day growth cycles of wheat, the 2,4-DAPG-producing strains were consistently recovered from the rhizosphere of the cultivar Lewjain, and commonly were present at populations higher than those recovered from other wheat cultivars. Based on restriction fragment length polymorphism and sequence analyses of phlD, a key gene involved in 2,4-DAPG production, two previously undefined phlD+ genotypes, referred to as genotypes PfZ and PfY, were discovered. Wheat cultivar Lewjain was the primary source of genotype PfY while cultivar Penawawa yielded the majority of genotype PfZ. Based on 16S rDNA sequence analysis, both new phlD genotypes were classified as P. fluorescens. Comparison of the rhizosphere competence of 2,4-DAPG-producing P. fluorescens Q2-87 (genotype B) and P. fluorescens LR3-A28 (genotype PfY) showed that both strains persisted at similar populations in the rhizosphere of all cultivars tested over a 30 day period when introduced as a seed inoculant. However, when strain LR3-A28 was applied as a soil inoculant, this strain was recovered at higher populations from the rhizosphere of wheat cultivar Lewjain than from the rhizospheres of two other cultivars. No cultivar effects were shown for strain Q2-87. Collectively, these results add further to evidence indicating a degree of specificity in interactions between plant cultivars and specific members of the saprophytic microbial community. Furthermore, as 2,4-DAPG-producing fluorescent Pseudomonas spp. have a central role in the spontaneous reduction in severity of take-all disease of wheat in response to continuous wheat monoculture, we postulate that the use of specific cultivars, such as Lewjain, which possess a superior capacity to enhance resident soil populations of these bacteria may have potential to reduce the length of the monoculture period required to induce natural suppressiveness of soils toward this disease.