Confirmation of <Emphasis Type="Italic">Peronospora agrimoniae</Emphasis> as a distinct species

Leaves with typical symptoms of downy mildew were found on common agrimony in the Czech Republic in 2014 and 2015 and at several locations in Germany from 2010 to 2014. The causal agent of downy mildew of agrimony was often reported as Peronospora agrimoniae, but sometimes also as P. sparsa. Morphological characteristics of the pathogens found in both countries are in the range of previous works for P. agrimoniae, but also other downy mildews parasitic on Rosaceae, rendering their discrimination based on published observations difficult. For molecular identification sequencing of several loci (nrITS rDNA, cox1 and cox2) was performed. Phylogenetic analyses based on nrITS rDNA clearly separated P. agrimoniae from other Peronospora species infecting Rosaceae. Thus, considering P. agrimoniae as separate species seems justified. Two German specimens were identical to two Czech samples in both nrITS rDNA and cox1 mtDNA sequences, but differed in a single nucleotide substitution in cox2 region. To our knowledge, this is the first verified record of P. agrimoniae on common agrimony in the Czech Republic.     

The exceptionality of stress response in Magnaporthe oryzae: a set of “salt stress-induced” genes unique to the rice blast fungus

Journal of Plant Diseases and Protection - The ability of pathogens to signal perception and adaptation to environmental changes is an important prerequisite for successful colonization of the host...     

Effect of nitrogen species supply and mycorrhizal colonization on organosulfur and phenolic compounds in onions

The aim of the present study was to test whether variations in the root environment affect the content of health-related organosulfur compounds, total phenolic compounds, and flavonol glycoside concentrations in onions. For this purpose, greenhouse-grown onions ( Allium cepa L.) were either inoculated with a commercial arbuscular mycorrhizal inoculum or a sterile inoculum and were provided with two NH(4)(+):NO(3)(-) ratios as a nitrogen source. Onion growth, arbuscular mycorrhizal colonization rate, sugars, and nutrient element concentrations were also quantified. The plant antioxidant activity and quercetin monoglucoside and organosulfur compound concentrations increased with dominant nitrate supply. Furthermore, mycorrhizal colonization increased the antioxidant activity and also concentrations of the major quercetin glucosides. The present study provides clear evidence that antioxidant activity, quercetin glycosides, and organosulfur compounds can be increased in sufficiently supplied onion plants by dominant nitrate supply or application of arbuscular mycorrhizal fungi. This was probably due to increased precursor production and induced defense mechanisms.     

Phosphorus uptake by cowpea plants from sparingly available or soluble sources as affected by nitrogen form and arbuscular‐mycorrhiza‐fungal inoculation

In most plant species, nutrient uptake is facilitated upon root association with symbiotic arbuscular mycorrhizal (AM) fungi. The aim of the present experiment was to test how the form in which nitrogen (N) is supplied to the growth medium affects substrate pH, AM development, and contribution of the symbiosis to phosphorus (P) uptake from sparingly available or soluble resources. Cowpea (Vigna unguiculata L. Walp) plants inoculated or noninoculated with AM fungi (Glomus sp.) were grown in pots with a sand substrate supplied with nutrient solution. The nutrient solution was prepared either with a high or a low concentration of soluble P, and NO ‐N : NH ‐N ratios of 9:1 or 5:5. The substrate supplied with low‐P nutrient solution was either or not additionally amended with ground rock phosphate. Despite a high level of root colonization, AM fungi used in the present study did not appear to increase plant availability of rock phosphate. It cannot be excluded that the ability of AM root systems to acquire P from sparingly available resources differs depending on the plant and fungal genotypes or environmental conditions. The absence from the growth substrate of P‐solubilizing microorganisms able to associate with AM mycelia might also have been a reason for this observation in our study. Increased supply of NH relative to NO improved plant P availability from rock phosphate, but also had a negative effect on the extent of AM‐fungal root colonization, irrespective of the plant P‐nutritional status. Whether increasing levels of NH can also negatively affect the functioning of the AM symbiosis in terms of plant element uptake, pathogen protection or soil‐structure stabilization deserves further investigation.     

Influence of nitrogen forms and mycorrhizal colonization on growth and composition of Chinese bunching onion

In recent years, interest has grown in cultivating Allium species with enhanced health benefits and/or distinct flavor. Concentrations of phytochemicals determining these desired characteristics may be influenced by nitrogen forms (ammonium or nitrate) and arbuscular mycorrhizal (AM) fungi. We examined these relations with the test plant bunching onion (Allium fistulosum L.). Three different ammonium‐to‐nitrate (NH : NO ) ratios were supplied in combination with or without inoculation with an AM fungus (Glomus mosseae). The plants were evaluated for dry weight, leaf number, and content of nutrients (N, NO , P, S), sugars (glucose, fructose, and sucrose), and organosulfur compounds (measured as pyruvic acid). The experiment was carried out under controlled conditions in a greenhouse. Plants were grown on perlite amended twice a day with nutrient solution. In nonmycorrhizal plants, the application of nutrient solution with predominant NO or NH₄NO₃ as N source supported adequate growth of Allium fistulosum while predominant NH supply resulted in decreased growth and occurrence of wilting symptoms. Mycorrhizal inoculation significantly increased dry weight and leaf number of predominantly NH ‐fed or NH₄NO₃‐fed plants. While shoot P concentration increased with higher NH supply, shoot N concentration increased in predominantly NH ‐fed plants only. Nitrogen form and AM colonization had little effect on shoot S or sugar concentrations. The total content in organosulfur compounds was significantly affected by both, N form and AM colonization. The optimal growth condition for a high formation of organosulfur compounds in this experiment was a nutrient solution with predominant NO supply, but when supported by AM fungi, Allium fistulosum produced similar amounts of pyruvic acid in NH₄NO₃‐fed plants.     

Nutrient and water uptake by roots of forest trees

Although per growing season nutrient uptake of adequately growing forest trees is less than the nutrient uptake of annual crop species, nutrient uptake per unit root length in trees is considerable. Because of high heterogeneity of soil conditions and root growth in forest soils, modelling of uptake processes is even more difficult for forest than for crop stands. Detailed studies show that white lips of growing tree roots have a high nutrient uptake capacity. However, most root tips are usually colonised by mycorrhizal fungi. These fungi can participate substantially in tree nutrient uptake, in particular in the utilisation of organically-bound phosphorus and nitrogen in soils. Mycorrhizal hyphae, root tips, and older root zones can all absorb water, but their actual contribution is difficult to assess. In this review, experimental results from our laboratory and literature data are used to describe the potential activity of tree roots and mycorrhizas in nutrient and water uptake. Methodology for in situ measurements must be developed to quantify at different forest sites the actual contribution of mycorrhizas and different root parts.     

Phylogenetic investigations in the genus <Emphasis Type="Italic">Pseudoperonospora</Emphasis> reveal overlooked species and cryptic diversity in the <Emphasis Type="Italic">P. cubensis</Emphasis> species cluster

Pseudoperonospora cubensis is one of the most devastating diseases of cucurbitaceous crops. The pathogen has a worldwide distribution and occurs in all major cucurbit growing areas. It had been noticed for the first time at the end of the 19th century, but it became a globally severe disease as recently as 1984 in Europe and 2004 in North America. Despite its economic importance, species concepts in Pseudoperonospora are debated. Here, we report that the genus Pseudoperonospora contains cryptic species distinct from the currently accepted ones. Pseudoperonospora on Celtis is split into two phylogenetic lineages and Pseudoperonospora humuli is confirmed as a species distinct from the Cucurbitaceae-infecting lineages. A cryptic species occupying a basal position within the Pseudoperonospora cubensis complex is revealed to be present on Humulus japonicus, thus providing evidence that the host jump that gave rise to Pseudoperonospora cubensis likely occurred from hops. Notably, Cucurbitaceae infecting pathogens are present in two cryptic sister species or subspecies. Clade 1 contains primarily specimens from North America and likely represents Pseudoperonospora cubensis s.str.. Pre-epidemic isolates in clade 2 originate from Japan and Korea, suggesting this cryptic species or subspecies is indigenous to East Asia. Recent samples of this lineage from epidemics in Europe and the United States cluster together with clade 2. It thus seems possible that this lineage is associated with the recent severe epidemics of cucurbit downy mildew and is now naturalised in North America and Europe.     

Molecular phylogenetic analysis of <Emphasis Type="Italic">Peronosclerospora</Emphasis> (Oomycetes) reveals cryptic species and genetically distinct species parasitic to maize

Downy mildews are amongst the most widespread and economically important pathogens of cultivated grasses in the tropics and subtropics. Despite their importance, molecular methods, particularly DNA sequence analysis, have rarely been applied to either species identification or to the determination of phylogenetic relationships between species. Here we report the presence of several cryptic species in the genus Peronosclerospora. Further we confirm that maize can be parasitised by several species of Peronosclerospora, including P. eriochloae, which has not been reported previously as a pathogen of maize. The presence of 14 distinct phylogenetic lineages, including three that are parasitic to maize, highlights the current fragmentary knowledge on the diversity and classification of species within Peronosclerospora. Species identification in Peronosclerospora has been traditionally based on the host genus and a set of variable morphological characteristics, which has meant that the identification of species is often unreliable. This situation is primed for the application of molecular techniques for the identification of species. One of the lineages parasitic to maize in Australia has not yet been formally described and its distribution is not known. Future investigation including a broad sampling of downy mildews from maize and other cultivated and native grasses on a world-wide basis is a prerequisite to a re-evaluation of quarantine regulations aimed at restricting or limiting their spread.     

N₂reduction and hydrogenation to ammonia by a molecular iron-potassium complex

The most common catalyst in the Haber-Bosch process for the hydrogenation of dinitrogen (N(2)) to ammonia (NH(3)) is an iron surface promoted with potassium cations (K(+)), but soluble iron complexes have neither reduced the N-N bond of N(2) to nitride (N(3-)) nor produced large amounts of NH(3) from N(2). We report a molecular iron complex that reacts with N(2) and a potassium reductant to give a complex with two nitrides, which are bound to iron and potassium cations. The product has a Fe(3)N(2) core, implying that three iron atoms cooperate to break the N-N triple bond through a six-electron reduction. The nitride complex reacts with acid and with H(2) to give substantial yields of N(2)-derived ammonia. These reactions, although not yet catalytic, give structural and spectroscopic insight into N(2) cleavage and N-H bond-forming reactions of iron.