Silicon distribution and accumulation in shoot tissue of the tropical forage grass Brachiaria brizantha

Silicon (Si) accumulation in organs and cells is one of the most prominent characteristics of plants of the family Poaceae. Many species from this family are used as forage plants for animal feeding. The present study investigates in Brachiaria brizantha (Hochst. ex A. Rich.) Stapf. cv. Marandu: (1) the dry matter production and Si content in shoot due to soil Si fertilizations; (2) the Si distribution among shoot parts; and (3) the silica deposition and localization in leaves. Plants of B. brizantha cv. Marandu were grown under contrasting Si supplies in soil and nutrient solution. Silica deposition and distribution in grass leaf blades were observed using light microscope and scanning electron microscope equipped with an energy dispersive X-ray spectrometer (SEM-EDXS). Silicon concentration in the B. brizantha shoot increased according to the Si supply. Silicon in grass leaves decreased following the order: mature leaf blades > recently expanded leaf blades > non-expanded leaf blades. Silicon accumulates mainly on the upper (adaxial) epidermis of the grass leaf blades and, especially, on the bulliform cells. The Si distribution on adaxial leaf blade surface is non uniform and reflects a silica deposition exclusively on the cell wall of bulliform cells.     

Genetic Diversity and Population Structure of the Brachiaria brizantha Germplasm

Brachiaria brizantha (Hochst. ex A. Rich.) Stapf. (syn. Urochloa brizantha (Hochst. ex A. Rich.) R.D. Webster) is a species used primarily as forage in tropical America and Southeast Asia. B. brizantha has been extensively researched since the 1980s with the initiation of the Tropical Forages Breeding Program conducted by the Brazilian Agricultural Research Corporation (Empresa Brasileira de Pesquisa Agropecuária; EMBRAPA), holding one of the largest germplasm collections in the world. This work has identified 15 new microsatellite markers for this species, which have been used in addition to five previously reported markers, to estimate the genetic similarities among 172 accessions and six cultivars of this species. Similarity index values ranged from 0.40 to 1.00. Two duplications were found in the germplasm. A Bayesian analysis performed using the STRUCTURE 2.3.3 program revealed the presence of three clusters with different allelic pools. This analysis is valuable for the performance of crosses to explore heterosis; however, the mode of reproduction of the accessions and ploidy barriers must be observed for effective exploration. A grouping analysis using the neighbor-joining method was consistent with the STRUCTURE analysis, and a combination approach suggested that this germplasm collection does not exhibit considerable genetic variability despite the presence of three distinct allelic pools. The lack of correlation between the genetic and geographic distances is also discussed.     

Direct evidence of pseudogamy in apomictic Brachiaria brizantha (Poaceae)

Brachiaria brizantha is a forage grass that has several apomictic accessions. B. brizantha cv. Marandu is a natural tetraploid aposporous apomict widely cultivated in Brazil. Pseudogamy was detected in this species by observation that seed set is suppressed in plants that have had the stigmas excised from the flowers. The egg cell develops parthenogenetically in the apomictic plants, meaning that fertilisation is necessary for the formation of the endosperm. A thorough knowledge of all the events of seed formation in natural apomictic plants is essential for a complete understanding of this mode of reproduction. In this paper, we show direct evidence of pseudogamy in B. brizantha through the cytological analysis of polar nucleus fertilisation and the determination of triploid level of the endosperm tissue. The development of the male gametophyte gives rise to a reduced tri-celled pollen, the viability of which varies throughout the year, reaching 88% in the peak of the flowering period. Discharge of the male gamete takes place around 10 h after pollination and monospermy is the predominant system observed. Precocious embryony was also observed in these plants; embryos arise from egg cells. Endosperm development followed the free nucleus model and was associated with the presence of an embryo. Cellularisation and reserve uptake occurred 2 days after pollination (DAP) and mature endosperm was observed 8 DAP. The triploid level of the endosperm in the apomictic accession confirmed the 2:1 maternal:paternal ratio of genome contribution in the tissue.     

Physical mapping of rDNA genes corroborates allopolyploid origin in apomictic Brachiaria brizantha

Brachiaria (Trin.) Griseb belongs to the family Poaceae, and within the genus, apomixis or sexuality is present in different accessions of the same species. The majority of Brachiaria species are polyploid and apomictic, making strategies for crop improvement by breeding very intricate. In spite of the high frequency of apomictic polyploids, the relationship of polyploidy and hybridization with apomixis in Brachiaria is still unclear. Further analysis requires detailed knowledge regarding the genomic composition of the polyploids. The present work introduces the use of fluorescent in situ hybridization (FISH) into cytogenetic analysis of Brachiaria. Physical mapping of heterologous rDNA sequences, associated with conventional karyotyping of the B. brizantha diploid sexual (BRA 002747) and the tetraploid apomictic (BRA000591) accessions, provided evidence of the latter being of allopolyploid origin. Based on our results and on previous knowledge on apomixis in B. brizantha, we suggest that the origin of apomixis was probably a consequence of hybridization.     

Effect of the tropical grass Brachiaria brizantha (Hochst. ex A. Rich.) Stapf on microbial population and activity in petroleum-contaminated soil

The effect of the tropical pasture grass Brachiaria brizantha on numbers of bacteria, fungi and degraders of alkanes, aromatics, cycloalkanes and crude oil in petroleum hydrocarbon contaminated and uncontaminated savannah soil was evaluated. Substrate induced soil respiration and soil pH were compared between planted and unplanted soil. B. brizantha had a mostly increasing effect on microbial numbers. As an exception, growth of bacteria was not or negatively affected. Microbial respiration and pH were always lower in planted than in unplanted soil. Low pH may result from enhanced oil degradation in planted soil leading to an accumulation of organic acids. A comparable stimulation of crude oil degraders and fungi in planted soil points to the importance of fungi. Since they tolerate lower pH values than bacteria, they are considered to play a central role in oil degradation. Given that the enhancement of crude oil degradation under the influence of B. brizantha could not clearly be correlated to microbial numbers and activity, other factors like oxygen availability, plant enzymes and synergistic degradation by microbial consortia have to be considered.     

An endophyte of the tropical forage grass Brachiaria brizantha: isolating, identifying, and characterizing the fungus, and determining its antimycotic properties

Brachiaria, predominantly an African genus, contains species, such as B. brizantha, an apomictic C4 grass, that are commercially important forage grasses in tropical America, where they now cover about 55 million hectares. From B. brizantha accession CIAT 6780, we isolated an endophytic fungus that may be economically significant. The fungus was identified as Acremonium implicatum (J. Gilman & E.V. Abott). 18S rDNA and ITS rDNA sequences were used to characterize isolates of the endophyte, and showed that they belonged to the Acremonium genus, being close to A. strictum and A. kiliense. Using the random amplified polymorphic DNA (RAPD) technique, involving arbitrary primers of 10 bases, we showed that the isolates were highly similar to each other. Antiserum produced from a monoconidial culture of A. implicatum isolated from B. brizantha 6780, differentiated the isolates consistently in line with the DNA data. When we compared endophyte-free with endophyte-infected B. brizantha CIAT 6780 plants, both artificially inoculated with the pathogenic Drechslera fungus, we found that the endophyte-infected plants had fewer and smaller lesions than did the endophyte-free plants. Sporulation of Drechslera sp. on artificially inoculated leaf sheath tissues was also much less on tissue infected with the endophyte.     

Influence of fertilizer levels on phytoremediation of crude oil-contaminated soils with the tropical pasture grass Brachiaria brizantha (hochst. ex a. rich.) stapf

Determination of fertilizer levels in phytoremediation of petroleum hydrocarbons is a complex issue, since nutrient demands of the plant and of degrading microorganisms in the rhizosphere have to be considered In the present work, three fertilizer levels were tested in a greenhouse experiment with the aim of optimizing growth of the tropical pasture grass Brachiaria brizantha and enhance microbial degradation of heavy crude oil in soil Fertilizer was applied twice in a concentration of 200, 300, and 400 mg each of N, P, and K per kg soil before and after the first sampling (14 wk). The medium fertilizer concentration resulted in best root growth and highest absolute oil dissipation (18.4%) after 22 wk The highest concentration produced best shoot growth and highest relative oil dissipation after 14 wk (10.5% less than unplanted control). In general, degradation of total oil and grease was higher in planted than in unplanted soil, but differences diminished toward the end of the experiment. Next to fertiizer quantity, its composition is an important factor to be further studied, including the form of available nitrogen (N-NO3- vs. N-NH4+). Field trials are considered indispensable for further phytoremediation studies, since greenhouse experiments produce particular water and nutrient conditions.     

Role of reactive oxygen and nitrogen species in the vascular responses to inflammation

Inflammation is a complex and potentially life-threatening condition that involves the participation of a variety of chemical mediators, signaling pathways, and cell types. The microcirculation, which is critical for the initiation and perpetuation of an inflammatory response, exhibits several characteristic functional and structural changes in response to inflammation. These include vasomotor dysfunction (impaired vessel dilation and constriction), the adhesion and transendothelial migration of leukocytes, endothelial barrier dysfunction (increased vascular permeability), blood vessel proliferation (angiogenesis), and enhanced thrombus formation. These diverse responses of the microvasculature largely reflect the endothelial cell dysfunction that accompanies inflammation and the central role of these cells in modulating processes as varied as blood flow regulation, angiogenesis, and thrombogenesis. The importance of endothelial cells in inflammation-induced vascular dysfunction is also predicated on the ability of these cells to produce and respond to reactive oxygen and nitrogen species. Inflammation seems to upset the balance between nitric oxide and superoxide within (and surrounding) endothelial cells, which is necessary for normal vessel function. This review is focused on defining the molecular targets in the vessel wall that interact with reactive oxygen species and nitric oxide to produce the characteristic functional and structural changes that occur in response to inflammation. This analysis of the literature is consistent with the view that reactive oxygen and nitrogen species contribute significantly to the diverse vascular responses in inflammation and supports efforts that are directed at targeting these highly reactive species to maintain normal vascular health in pathological conditions that are associated with acute or chronic inflammation.     

Production and nitrogen responses of the African dwarf shrub Indigofera spinosa to defoliation and water limitation

Summary The dwarf shrub Indigofera spinosa Forsk. (Papilionacea), a native forage species of arid Northwest Kenya, was propogated from seed, grown in a controlled environment, and subjected to three treatments of defoliation and watering frequencies in a factorial experimental design. Biomass production and nitrogen accumulation in tissue components were measured to determine defoliation responses in a water-limited environment. We hypothesized that plants would maintain biomass and nitrogen flows despite removal of aboveground meristems and tissues by defoliation. Principal experimental results included a slight reduction (11%; P=0.08) of total biomass production by clipping ca. 1/3 or 2/3 of new leaves and stems and all apical meristems every month. Total aboveground production was not affected by clipping, while final root biomass was reduced 17% by the 2/3 clipping. The least water stressed plants were affected most negatively by defoliation, and the unclipped plants responded more negatively to greater water limitation. Plants achieved partial biomass compensation through alterations in shoot activity and continued allocation of photosynthate to roots. A smaller fraction of leaf production was directed to litter in clipped plants although clipping only removed the youngest tissues, suggesting that clipping increased leaf longevity. In turn, each leaf probably contributed a greater total quantity of photosynthate. Photosynthetic rates were also likely to have been increased by clipping water-stressed plants. In contrast to biomass, plants overcompensated for nitrogen lost to defoliation. Total nitrogen uptake by individual plants was stimulated by defoliation, as there was more total nitrogen in leaves and stems. Increased nitrogen uptake was achieved by clipping stimulation of total uptake per unit of root rather than of total root mass.     

Pollen source and resource limitation to fruit production in the rare species Eremosparton songoricum (Fabaceae)

Eremosparton songoricum (Litv.) Vass. is a rare, central Asian desert species which shows lower fruit set and seed set (<16%) than most hermaphroditic species. We hypothesized that fruit production was limited by pollen and resources. To evaluate potential fruit abortion due to pollen limitation, supplemental hand‐pollination was undertaken, the mating system was investigated and the foraging behavior of pollinators was recorded. To investigate possibile resource limitation, flowers and young pods were artificially removed and fertilization were manipulated. The results showed that under natural pollination, the number of pollen deposited on the stigma greatly exceed the number of ovules per ovary. Mating system experiments showed that the species is self‐compatible, but depended on pollinators to set seeds. Supplemental outcross pollination increased fruit set significantly. The most frequent effective pollinator Megachile terminate Morawitz, was observed pollinating many flowers of the same individual plant (74.5±1.3%). These results suggested that fruit production is affected by insufficient outcross pollen rather than by pollen quantity. Removal of 2/3 of the flowers and young pods led to significantly higher fruit set, as did addition of fertilizers (N–P–K: 0.025–0.05–0.013 g, N–P–K: 0.05–0.1–0.025 g) showing that reducing resource acceptors and increasing inorganic resources both helps to improve fruit set. We therefore conclude that reproductive success of E. songoricum is limited by both outcross pollen and available nutrients.     

Toxicity of Cd to signal grass (Brachiaria decumbens Stapf.) and Rhodes grass (Chloris gayana Kunth.)

Given that Cd accumulates within plant tissues to levels that are toxic to animals, it is necessary to understand the role of plants in highly Cd-contaminated systems and their subsequent impact on the health of animals. A solution culture experiment was conducted to elucidate the effects of increasing Cd²⁺ activity ({Cd²⁺}) on growth of Rhodes grass (Chloris gayana Kunth.) and signal grass (Brachiaria decumbens Stapf.). The shoot and root fresh mass of both Rhodes grass and signal grass was reduced by 50% at ca. 0.5 µM {Cd²⁺}. Elevated {Cd²⁺} resulted in a significant decrease in the tissue Mn concentration for both the shoots and roots, and caused a chlorosis of the veins in the shoots. Root hair growth was prolific even at high {Cd²⁺}, thus root hair growth appeared to be less sensitive to elevated Cd than was root growth per se. The critical shoot tissue concentrations (50% reduction in growth), 230 µg g^?1 for Rhodes grass and 80 µg g^?1 for signal grass, exceeded the maximum level of Cd tolerated in the diet of animals (ca. 5 µg g^?1). When assessing the risk associated with the revegetation of Cd-contaminated sites with Rhodes grass or signal grass, careful consideration must be given, therefore, to the transfer of toxic concentrations of Cd to grazing animals and through the wider food chain.     

Decoupling of nitrogen and phosphorus in dominant grass species in response to long-term nitrogen addition in an Alpine Grassland in Central Asia

Plant Ecology - Increased nitrogen (N) availability induced by fertilizer use, rapid urbanization, and livestock cultivation has important effects on the biogeochemical cycles of plant N and...     

Dynamic responses of Pseudomonas fluorescens DF57 to nitrogen or carbon source addition

Cultures of Pseudomonas fluorescens DF57 were grown on different carbon and nitrogen sources. Glucose, succinate and acetate were used as carbon source and pulsed to an aerobic steady-state cultivation of P. fluorescens DF57 at D = 0.1 h(-1) with citrate as limiting carbon source. Glucose was utilised with the fastest uptake rate (19.4 C mmol l(-1) h(-1)) compared to succinate (8.8 C mmol l(-1) h(-1)) and acetate (4.3 C mmol l(-1) h(-1)). Acetate triggered an inhibition of cellular metabolism, which resulted in 2-h long growth arrest after its addition to the steady-state culture. The influence of the nitrogen source was investigated in an aerobic cultivation on a mixture of ammonium and nitrate as limiting nitrogen sources and citrate as non-limiting carbon source. When ammonia and nitrate were pulsed to the steady-state culture, they were mainly assimilated into biomass with a maximum uptake rate of 111 and 33 mg N l(-1) h(-1), respectively. Nitrate uptake was never complete as the residual concentration in the chemostat cultivation was 30 mg N l(-1) nitrate. A pulse of nitrite in the cultivation broth resulted in an inhibition of the growth but not of the primary metabolism, as nitrite was taken up at 38 mg N l(-1) h(-1), citrate was consumed and cofactors were produced continuously. In all experiments, oxygen was used as electron acceptor.     

Contrasting impacts of grass species on nitrogen cycling in a grazed Sudanian savanna

We investigated the impact of perennial and annuals grass species on nitrogen cycling in a Sudanian savanna of Burkina Faso. We also analysed how the local context in terms of grazing and soil properties modifies these impacts. We selected four plots differing both by the intensity of grazing by cattle and soil depth, and used soil and grass biomass ¹⁵N as integrative indicators of N cycle. If perennials are able to foster a more efficient nitrogen cycling there should be lower ¹⁵N abundances in their biomass and soil. If soil depth and cattle pressure significantly modify nitrogen fluxes, soil depth and cattle pressure should influence ¹⁵N signatures. Our results suggest that perennial grasses are more conservative for nitrogen (inhibition of nitrification, less leaching via a perennial root system, slower cycling). The increase in leaf δ¹⁵N with N concentration is steeper in Loudetia togoensis than in the three other grasses. No significant difference was found between the ¹⁵N signatures of the four plots. Our results on ¹⁵N signatures and the fact that perennial grasses are much more abundant in the plots that are less grazed and have deeper soils, confirm that the switch from perennial to annual grasses is linked to a degradation in soil fertility and pasture quality. This suggests that ¹⁵N signatures can be used as indicators of fertility.