Brassinosteroid-independent functions of the BRI1-associated kinase BAK1/SERK3

Eukaryotes have evolved programmed cell death (PCD) mechanisms that play important roles in both, development and immunity.^1–3 We demonstrated a requirement for the Arabidopsis thaliana leucine-rich repeat receptor-like kinase (LRR-RLK), BAK1/SERK3 (BRI1-Associated receptor Kinase 1/Somatic Embryogenesis Receptor Kinase 3) in regulating the containment of microbial infection-induced necrosis. BAK1-deficient plants showed constitutive expression of defense-related genes and developed spreading cell death upon infection by necrotizing pathogens that result in enhanced susceptibility to necrotrophic pathogens. This reaction was not inducible by exposition of bak1 mutants to general stresses but appeared to be solely inducible by necrotizing pathogen infection. BAK1 is known to interact with the brassinosteroid receptor, BRI1, and thereby facilitates plant growth and development in a brassinolide (BL)-dependent manner.^4,5 Surprisingly, the cell death-related phenotype in bak1 mutants is brassinolide-independent. In this addendum we want to present recent new data on BAK1 and discuss its role as a general regulator in plant processes being as diverse as brassinosteroid signaling in development, perception of pathogen associated molecular patterns (PAMPs), and cell-death control in innate immunity.Key words: LRR-RLK, cell-death control, immunity, brassinosteroids, BAK1, SERK3, BRI1, FLS2     

Cell envelope stress response in Gram-positive bacteria

The bacterial cell envelope is the first and major line of defence against threats from the environment. It is an essential and yet vulnerable structure that gives the cell its shape and counteracts the high internal osmotic pressure. It also provides an important sensory interface and molecular sieve, mediating both information flow and the controlled transport of solutes. The cell envelope is also the target for numerous antibiotics. Therefore, the monitoring and maintenance of cell envelope integrity in the presence of envelope perturbating agents and conditions is crucial for survival. The underlying signal transduction is mediated by two regulatory principles, two-component systems and extracytoplasmic function sigma factors, in both the Firmicutes (low-GC) and Actinobacteria (high-GC) branches of Gram-positive bacteria. This study presents a comprehensive overview of cell envelope stress-sensing regulatory systems. This knowledge will then be applied for in-depth comparative genomics analyses to emphasize the distribution and conservation of cell envelope stress-sensing systems. Finally, the cell envelope stress response will be placed in the context of the overall cellular physiology, demonstrating that its regulatory systems are linked not only to other stress responses but also to the overall homeostasis and lifestyle of Gram-positive bacteria.     

Bacitracin sensing in Bacillus subtilis

The extracellular presence of antibiotics is a common threat in microbial life. Their sensitive detection and subsequent induction of appropriate resistance mechanisms is therefore a prerequisite for survival. The bacitracin stress response network of Bacillus subtilis consists of four signal-transducing systems, the two-component systems (TCS) BceRS, YvcPQ and LiaRS, and the extracytoplasmic function (ECF) σ factor σ^M. Here, we investigated the mechanism of bacitracin perception and the response hierarchy within this network. The BceRS–BceAB TCS/ABC transporter module is the most sensitive and efficient bacitracin resistance determinant. The ABC transporter BceAB not only acts as a bacitracin detoxification pump, but is also crucial for bacitracin sensing, indicative of a novel mechanism of stimulus perception, conserved in Firmicutes bacteria. The Bce system seems to respond to bacitracin directly (drug sensing), whereas the LiaRS TCS and σ^M respond only at higher concentrations and indirectly to bacitracin action (damage sensing). The YvcPQ–YvcRS system is subject to cross-activation via the paralogous Bce system, and is therefore only indirectly induced by bacitracin. The bacitracin stress response network is optimized to respond to antibiotic gradients in a way that maximizes the gain and minimizes the costs of this stress response.     

6-MSAS-like polyketide synthase genes occur in lichenized ascomycetes

Lichenized and non-lichenized filamentous ascomycetes produce a great variety of polyketide secondary metabolites. Some polyketide synthase (PKS) genes from non-lichenized fungi have been characterized, but the function of PKS genes from lichenized species remains unknown. Phylogenetic analysis of keto synthase (KS) domains allows prediction of the presence or absence of particular domains in the PKS gene. In the current study we screened genomic DNA from lichenized fungi for the presence of non-reducing and 6-methylsalicylic acid synthase (6-MSAS)-type PKS genes. We developed new degenerate primers in the acyl transferase (AT) region to amplify a PKS fragment spanning most of the KS region, the entire linker between KS and AT, and half of the AT region. Phylogenetic analysis shows that lichenized taxa possess PKS genes of the 6-MSAS-type. The extended alignment confirms overall phylogenetic relationships between fungal non-reducing, 6-MSAS-type and bacterial type I PKS genes.     

When family matters: an analysis of Thelotremataceae (Lichenized Ascomycota: Ostropales) as bioindicators of ecological continuity in tropical forests

We analysed patterns of habitat and microhabitat preferences of 19 families (comprising 135 genera and 950 species) of crustose, corticolous lichens in Costa Rica (Arthoniaceae, Arthopyreniaceae, Coenogoniaceae, Graphidaceae, Lecanoraceae, Letrouitiaceae, Monoblastiaceae, Pertusariaceae, Physciaceae, Pilocarpaceae, Porinaceae, Pyrenulaceae, Ramalinaceae, Roccellaceae, Strigulaceae, Teloschistaceae, Thelenellaceae, Thelotremataceae, Trypetheliaceae), in order to test whether Thelotremataceae are suitable predictors of undisturbed tropical rain forest and can be used as bioindicators of ecological continuity. The dataset consisted of 12,215 specimen samples and six environmental parameters recorded for each sample (altitude, degree of seasonality, vegetation type, disturbance level, substrate nature, light exposure), which were analysed by a multivariate approach using principal component analysis (PCA). The analysis showed that three of the 19 families, Letrouitiaceae, Porinaceae, and Thelotremataceae, showed significant preferences for undisturbed primary to old growth secondary forest, fully shaded to semi-exposed microhabitats, and the bark of mature tree trunks, parameters assumed to be correlated with ecological continuity of closed rain forest habitats. Thelotremataceae had broader altitudinal range than Letrouitiaceae and Porinaceae and significantly higher genus and species diversity (16 genera, 130 species) compared to Porinaceae (4 genera, 40 species) and Letrouitiaceae (1 genus, 5 species). Our results support the hypothesis that Thelotremataceae perform best as predictors of undisturbed dry and lowland to montane rain forest and are the most suitable lichen bioindicators of ecological continuity in these ecosystems. In contrast, Lecanoraceae, Pertusariaceae, Physciaceae, and Teloschistaceae, were found to be predictors of disturbed and pioneer (micro-)habitats. We also found that, among a variety of parameters tested, the Index of Ecological Continuity (IEC), adapted to the use of Thelotremataceae in tropical forests, performs best in terms of predicting disturbance levels and site history. A semi-taxonomic approach identifying morphotypes rather than genera or species yielded the same results, making this method suitable for a broader spectrum of potential users.     

Sex-specific responses to vocal convergence and divergence of contact calls in orange-fronted conures (Aratinga canicularis)

We investigated the signal function of vocal imitation of contact calls in orange-fronted conures (OFCs; Aratinga canicularis) in Costa Rica. OFCs live in dynamic social systems with frequent flock fusions and fissions. Exchanges of contact calls precede these flock changes. During call exchanges, the similarity between the contact calls of different individuals may either increase (converge) or decrease (diverge). We conducted a playback experiment on wild-caught captive birds in which we simulated convergent, divergent and no-change interaction series with male and female contact calls. OFCs responded differently to convergent and divergent series of contact calls, but only when we considered the sex of the test birds. Males called most in response to convergent series, whereas females demonstrated high calling rates in response to both convergent and divergent interactions. Both sexes responded most to contact calls from the opposite sex, but overall females produced more calls and had shorter latencies to calling than males. These results demonstrate that OFCs can discriminate between male and female contact calls and that subtle changes in contact call structure during interactions have signal function. The stronger overall response to convergent series suggests that convergence of contact calls is an affiliative signal.     

Organic carbon dynamics in mangrove ecosystems: A review

Our current knowledge on production, composition, transport, pathways and transformations of organic carbon in tropical mangrove environments is reviewed and discussed. Organic carbon entering mangrove foodwebs is either produced autochthonously or imported by tides and/or rivers. Mangrove litter and benthic microalgae are usually the most important autochthonous carbon sources. Depending on local conditions, phytoplankton and seagrass detritus imported with tides may represent a significant supplementary carbon input. Litter handling by the fauna not only affects microbial carbon transformations, but also the amount of organic carbon available for export. Most mangrove detritus that enters the sediment is degraded by microorganisms. Aerobic respiration and anaerobic sulfate reduction are usually considered the most important microbial respiration processes, but recent evidence suggests that iron respiration may be important in mangrove sediments as well. Organic carbon that escapes microbial degradation is stored in sediments and in some mangrove ecosystems, organic-rich sediments may extend to several meters depth. Many mangrove forests also lose a significant fraction of their net primary production to coastal waters. Large differences occur between mangrove forests with respect to litter production and export. Mangrove-derived DOC is also released into the water column and can add to the total organic carbon export. Numerous compounds have been characterized from mangrove tissues, including carbohydrates, amino acids, lignin-derived phenols, tannins, fatty acids, triterpenoids and n-alkanes. Many of these may, together with stable isotopes, exhibit a strong source signature and are potentially useful tracers of mangrove-derived organic matter. Our knowledge on mangrove carbon dynamics has improved considerably in recent years, but there are still significant gaps and shortcomings. These are emphasized and relevant research directions are suggested.