Microcystis and Anabaena (Dolichospermum) are among the most toxic cyanobacterial genera and often succeed each other during harmful algal blooms. The role allelopathy plays in the succession of these genera is not fully understood. The allelopathic interactions of six strains of Microcystis and Anabaena under different nutrient conditions in co-culture and in culture-filtrate experiments were investigated. Microcystis strains significantly reduced the growth of Anabaena strains in mixed cultures with direct cell-to-cell contact and high nutrient levels. Cell-free filtrate from Microcystis cultures proved equally potent in suppressing the growth of nutrient replete Anabaena cultures while also significantly reducing anatoxin-a production. Allelopathic interactions between Microcystis and Anabaena were, however, partly dependent on ambient nutrient levels. Anabaena dominated under low N conditions and Microcystis dominated under nutrient replete and low P during which allelochemicals caused the complete suppression of nitrogen fixation by Anabaena and stimulated glutathione S-transferase activity. The microcystin content of Microcystis was lowered with decreasing N and the presence of Anabaena decreased it further under low P and high nutrient conditions. Collectively, these results indicate that strong allelopathic interactions between Microcystis and Anabaena are closely intertwined with the availability of nutrients and that allelopathy may contribute to the succession, nitrogen availability, and toxicity of cyanobacterial blooms.
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Succession and toxicity of 'Microcystis' and 'Anabaena' ('Dolichospermum') blooms are controlled by nutrient-dependent allelopathic interactions
Effects of increased zooplankton biomass on phytoplankton and cyanotoxins: A tropical mesocosm study
Zooplankton are important biocontrol agents for algal blooms in temperate lakes, while their potential in tropical and subtropical environments is not well understood. The aim of the present study was to evaluate the influence of increased zooplankton biomass on phytoplankton community and cyanotoxins (microcystins and saxitoxin) content of a tropical reservoir (Ipojuca reservoir, Brazil) using in situ mesocosms. Mesocosms consisted of 50 L transparent polyethylene bags suspended in the reservoir for twelve days. Phytoplankton populations were exposed to treatments having 1 (control), 2, 3 and 4 times the biomass of zooplankton found in the reservoir at the beginning of the experiment. Filamentous cyanobacteria such as Planktothrix agardhii and Cylindrospermopsis raciborskii were not negatively influenced by increasing zooplankton biomass. In contrast, the treatments with 3 and 4 times zooplankton biomass negatively affected the Cyanobacteria Aphanocapsa sp., Chroococcus sp., Dolichospermum sp., Merismopedia tenuissima, Microcystis aeruginosa and Pseudanabaena sp.; the diatom Cyclotella meneghiniana; and the cryptophyte Cryptomonas sp. Total microcystin concentration both increased and decreased at different times depending on zooplankton treatment, while saxitoxin level was not significantly different between the treatments and control. The results of the present study suggest that zooplankton biomass can be manipulated to control the excessive proliferation of non-filamentous bloom forming cyanobacteria (e.g. M. aeruginosa) and their associated cyanotoxins.
In vitro development of sugarcane seedlings using ethephon or gibberellin
The use of plant growth regulators is directly related to the success of in vitro propagation, which is an advantageous alternative to obtain seedlings on a commercial scale. This study aimed to evaluate the in vitro development of ‘IAC 95-5000’ sugarcane seedlings after the addition of different doses of ethephon (0, 25, 50, 100 and 200 mg L-1) or gibberellic acid (0, 2.5, 5.0, 7.5 and 10.0 mg L-1) to the culture medium. Ethephon increased the number of tillers (up to 231.70%), reduced height of the main tiller (44.66 to 60.47%), and did not affect the shoot´s fresh and dry mass. On the other hand, gibberellin decreased the number of tillers and negatively changed biomass partitioning. It is concluded that the use of ethephon is a potential strategy to enhance in vitro production of ‘IAC 95-5000’ sugarcane seedlings, since it increased the number of usable shoots in subsequent subcultures, and its effects on height reduction can be reversible. However, the use of the tested doses of gibberellic acid is not recommended, because it impaired seedling development of this sugarcane variety.
Secretory structures in Aldama species (Heliantheae–Asteraceae): morphology, histochemistry and composition of essential oils
Aldama representatives are particularly noteworthy for producing essential oils, and the chemical composition identification of these oils can help in the search for possible bioactive compounds, and provide useful information for taxonomic studies. A correct description of the occurrence and positioning of the secretory structures related to the production of these metabolites in the plant body could provide a basis for future studies on biological and pharmacological activity, indicating which organ merits further investigation. The four Aldama species investigated here were chosen due to their aromatic and resiniferous potential and because they are morphologically very similar. They were investigated phytochemically and their secretory structures were identified. Four types of secretory structures were found for the species herein: hydathodes, glandular trichomes, canals and cavities. Except for the hydathodes, all of them are responsible for the essential oil production. The highest yields of essential oils were obtained the underground organs. Seventy-nine compounds were identified for the four Aldama species, whereas 13 compounds occurred in the essential oils from all species and some of them were unique to a given species. The chemical composition of the four species, described herein for the first time, can be used for identification purposes and, also reveals several compounds of proven biological activity, confirming the potential usefulness of Aldama species.
Underground organs of Brazilian Asteraceae: testing the CLO-PLA database traits
Not all plant traits from all regions have been standardized or databased. Some ecosystems, such as tropical grasslands, are under-represented in such databases owing to the difficulty in assessing bud banks and evaluating clonal growth. This study aimed to (i) determine whether Brazilian morphological traits of belowground organs can be translated into categories used in the CLO-PLA database and (ii) assess the applicability of clonal and bud bank traits standardized in the CLO-PLA database for Brazilian Aldama species, which have specialized belowground organs and are able to resprout. In all, 165 species, including herbs, subshrubs and shrubs, of 37 genera from different Brazilian ecosystems, were evaluated. Not all the traditional Brazilian morphological categories could be translated into CLO-PLA traits, resulting in a lower number of categories and loss of information regarding plant morphology. Furthermore, clonal and bud bank traits could be only partially evaluated for Aldama, since some traits showed seasonal variation. The CLO-PLA classification focused on the organs in relation to the soil surface, the connection between mother and daughter shoots, and the origin of buds from which daughter shoots sprout. In the Brazilian classification, by contrast, anatomical features or early ontogeny of the organ are very important. Nevertheless, our results might form the basis for future comparative studies across ecosystems and biomes, for which common trait standardization is necessary. However, further research is needed to assess the functional morphology of clonal and bud bank traits in tropical regions.
Sugarcane smut: shedding light on the development of the whip-shaped sorus
Background and Aims Sugarcane smut is caused by the fungus Sporisorium scitamineum (Ustilaginales/Ustilaginomycotina/Basidiomycota), which is responsible for losses in sugarcane production worldwide. Infected plants show a profound metabolic modification resulting in the development of a whip-shaped structure (sorus) composed of a mixture of plant tissues and fungal hyphae. Within this structure, ustilospores develop and disseminate the disease. Despite the importance of this disease, a detailed histopathological analysis of the plant–pathogen interaction is lacking.
Plant–insect–pathogen interactions: a naturally complex ménage à trois
Under environmental conditions, plants are constantly exposed to a wide range of biotic interactions, which include insects, and pathogens. Usually scientists are tempted to study each association individually, which reduces the complexity of the interaction. This restricted view of the problem does not consider that plants are the ballroom in which a multitude of organisms are constantly interacting with each other affecting not only plant responses but also how one organism responds to the other. Plants attacked by insects and pathogens display profound physiological, morphological and chemical changes or adaptations that result in organism attraction or avoidance, depending on the species involved. Therefore, many researchers worldwide have decided to study this phenomenon in a more holistic view, integrating genetics, ecology and physiology to depict these complex interactions. In this review, we will discuss how plant infection by pathogens may affect insect behavior and vice-versa and how plants cope with these multitude of biotic stresses.
BAK1 is involved in AtRALF1-induced inhibition of root cell expansion
The rapid alkalinization factor (RALF) peptide negatively regulates cell expansion, and an antagonistic relationship has been demonstrated between AtRALF1, a root-specific RALF isoform in Arabidopsis, and brassinosteroids (BRs). An evaluation of the response of BR signaling mutants to AtRALF1 revealed that BRI1-associated receptor kinase1 (bak1) mutants are insensitive to AtRALF1 root growth inhibition activity. BAK1 was essential for the induction of AtRALF1-responsive genes but showed no effect on the mobilization of Ca2+ and alkalinization responses. Homozygous plants accumulating AtRALF1 and lacking the BAK1 gene did not exhibit the characteristic semi-dwarf phenotype of AtRALF1-overexpressors. Biochemical evidence indicates that AtRALF1 and BAK1 physically interact with a Kd of 4.6 μM and acridinium-labeled AtRALF1 was used to demonstrate that part of the specific binding of AtRALF1 to intact seedlings and to a microsomal fraction derived from the roots of Arabidopsis plants is BAK1-dependent. Moreover, AtRALF1 induces an increase in BAK1 phosphorylation, suggesting that the binding of AtRALF1 to BAK1 is functional. These findings show that BAK1 contains an additional AtRALF1 binding site, indicating that this protein may be part of a AtRALF1-containing complex as a co-receptor, and it is required for the negative regulation of cell expansion.
Arabidopsis pollen tube integrity and sperm release are regulated by RALF-mediated signaling
In flowering plants, fertilization requires complex cell-to-cell communication events between the pollen tube and the female reproductive tissues, which are controlled by extracellular signaling molecules interacting with receptors at the pollen tube surface. We found that two such receptors in Arabidopsis, BUPS1 and BUPS2, and their peptide ligands, RALF4 and RALF19, are pollen tube–expressed and are required to maintain pollen tube integrity. BUPS1 and BUPS2 interact with receptors ANXUR1 and ANXUR2 via their ectodomains, and both sets of receptors bind RALF4 and RALF19. These receptor-ligand interactions are in competition with the female-derived ligand RALF34, which induces pollen tube bursting at nanomolar concentrations. We propose that RALF34 replaces RALF4 and RALF19 at the interface of pollen tube–female gametophyte contact, thereby deregulating BUPS-ANXUR signaling and in turn leading to pollen tube rupture and sperm release.
Arabidopsis thaliana rapid alkalinization factor 1-mediated root growth inhibition is dependent on calmodulin-like protein 38
Arabidopsis thaliana rapid alkalinization factor 1 (AtRALF1) is a small secreted peptide hormone that inhibits root growth by repressing cell expansion. Although it is known that AtRALF1 binds the plasma membrane receptor FERONIA and conveys its signals via phosphorylation, the AtRALF1 signaling pathway is largely unknown. Here, using a yeast two hybrid system to search for AtRALF1-interacting proteins in Arabidopsis, we identified calmodulin-like protein 38 (CML38) as an AtRALF1-interacting partner. We also found that CML38 and AtRALF1 are both secreted proteins that physically interact in a Ca2+- and pH-dependent manner. CML38- knockout mutants generated via T-DNA insertion were insensitive to AtRALF1, and simultaneous treatment with both AtRALF1 and CML38 proteins restored sensitivity in these mutants. Hybrid plants lacking CML38 and having high accumulation of the AtRALF1 peptide did not exhibit the characteristic short-root phenotype caused by AtRALF1 overexpression. Although CML38 was essential for AtRALF1-mediated root inhibition, it appeared not to have an effect on the AtRALF1-induced alkalinization response. Moreover, acridinium-labeling of AtRALF1 indicated that the binding of AtRALF1 to intact roots is CML38 dependent. In summary, we describe a new component of the AtRALF1 response pathway. The new component is a calmodulin-like protein that binds AtRALF1, is essential for root growth inhibition, and has no role in AtRALF1 alkalinization.