Belowground microbes mitigate plant-plant competition.

Dimorphandra wilsonii, a Cerrado endemic Fabaceae tree, is threatened by land-use changes. The few remaining individuals occur in areas dominated by alien grasses like Urochloa decumbens. We tested the impact of nitrogen (N) availability and symbionts' presence on mitigating the effects of competition from U. decumbens. Dimorphandra wilsonii seedlings were 50-week pot-cultivated under limiting (3mM) or non-limiting (10mM) N, with or without U. decumbens, and inoculated or not with a N-fixer (Bradyrhizobium sp.) and an arbuscular mycorrhizal fungus (AMF - Glomus etunicatum), both forming symbioses in the field. Since D. wilsonii seedlings grew more and 'lost' fewer nutrients under the symbionts' presence, symbionts mitigated plant-plant competition. Under limiting N, inoculated D. wilsonii seedlings grew more (despite no nodulation), but N fixation was only suggested when inoculated D. wilsonii seedlings competed with U. decumbens. D. wilsonii13C, and substrate's carbon and respiration suggest that only the microbes performing key functions received plant carbon. Under non-limiting N, inoculated D. wilsonii seedlings became enriched in 13C, substrate accumulated carbon and microbial respiration increased, suggesting a more generalist microbial community. Data suggest inoculating D. wilsonii seeds/seedlings with AMF and N-fixers as a conservation measure. However, long-term field-studies need to confirm these conclusions.

Integrative effects of zinc and temperature on germination in Dimorphandra wilsonii rizz.: Implications of climate changes.

The integrative effects of zinc (Zn; 0 mg L-1 , 75 mg L-1 , 150 mg L-1 , and 200 mg L-1 ) and temperature (25 °C, 30 °C, and 35 °C) on seed germination of the threatened Brazilian species Dimorphandra wilsonii were evaluated. Zinc effects on seed germination were only observed at 30 °C and 35 °C. By stimulating respiration rates, rising temperatures accentuate hydrogen peroxide (H2 O2 ) formation in germinating seeds in the presence of Zn. Seed Zn tolerance was related to the activation of enzymatic antioxidants, and ascorbate peroxidase (APX) activity had a central role in H2 O2 scavenging under the highest temperatures tested. Increased APX activity allowed successful germination, whereas decreasing APX activity was accompanied by decreasing germination rates in Zn-treated seeds at 35 °C. Within a scenario of future climate change, it will be extremely important to avoid increasing Zn concentrations in natural habitats that would threaten conservation efforts directed toward this endangered plant species. Environ Toxicol Chem 2017;36:2036-2042. © 2017 SETAC.

Effects of glyphosate acid and the glyphosate-commercial formulation (Roundup) on Dimorphandra wilsonii seed germination: Interference of seed respiratory metabolism.

Glyphosate-formulations are widely used in the Brazilian Cerrado (neotropical savanna) with little or no control, threatening population of the endangered species Dimorphandra wilsonii. We investigated the toxicity of different concentrations (0, 5, 25 and 50 mg l-1) of glyphosate acid and one of its formulations (Roundup®) on seed germination in D. wilsonii. Glyphosate acid and Roundup drastically decreased seed germination by decreasing seed respiration rates. The activation of antioxidant enzymes, ascorbate peroxidase and catalase assure no hydrogen peroxide accumulation in exposed seeds. Glyphosate acid and the Roundup-formulation negatively affected the activities of enzymes associated with the mitochondrial electron transport chain (ETC), with Complex III as its precise target. The toxicity of Roundup-formulation was greater than that of glyphosate acid due to its greater effects on respiration. The herbicide glyphosate must impair D. wilsonii seed germination by disrupting the mitochondrial ETC, resulting in decreased energy (ATP) production. Our results therefore indicate the importance of avoiding (or closely regulating) the use of glyphosate-based herbicides in natural Cerrado habitats of D. wilsonni as they are toxic to seed germination and therefore threaten conservation efforts. It will likewise be important to investigate the effects of glyphosate on the seeds of other species and to investigate the impacts of these pesticides elsewhere in the world.

Nodulation in Dimorphandra wilsonii Rizz. (Caesalpinioideae), a threatened species native to the Brazilian Cerrado.

The threatened caesalpinioid legume Dimorphandra wilsonii, which is native to the Cerrado biome in Brazil, was examined for its nodulation and N(2)-fixing ability, and was compared with another, less-threatened species, D. jorgei. Nodulation and potential N(2) fixation was shown on seedlings that had been inoculated singly with five bradyrhizobial isolates from mature D. wilsonii nodules. The infection of D. wilsonii by two of these strains (Dw10.1, Dw12.5) was followed in detail using light and transmission electron microscopy, and was compared with that of D. jorgei by Bradyrhizobium strain SEMIA6099. The roots of D. wilsonii were infected via small transient root hairs at 42 d after inoculation (dai), and nodules were sufficiently mature at 63 dai to express nitrogenase protein. Similar infection and nodule developmental processes were observed in D. jorgei. The bacteroids in mature Dimorphandra nodules were enclosed in plant cell wall material containing a homogalacturonan (pectic) epitope that was recognized by the monoclonal antibody JIM5. Analysis of sequences of their rrs (16S rRNA) genes and their ITS regions showed that the five D. wilsonii strains, although related to SEMIA6099, may constitute five undescribed species of genus Bradyrhizobium, whilst their nodD and nifH gene sequences showed that they formed clearly separated branches from other rhizobial strains. This is the first study to describe in full the N(2)-fixing symbiotic interaction between defined rhizobial strains and legumes in the sub-family Caesalpinioideae. This information will hopefully assist in the conservation of the threatened species D. wilsonii.