Plant communities and environmental factors in the Guayana Highlands: monitoring for conservation under future climate change

The Guayana Highlands (GH) constitute a highly diverse, but relatively poorly studied Neotropical biome, comprised of ～50 flat-topped mountain summits (called tepuis). Previous studies based on warming forecasts for the region suggested that an upward displacement of environmental conditions of 500–700 m could occur by 2100, potentially resulting in the extinction of c. 50% of its endemic flora due to total habitat loss. To assess the ecological responses of the species to climate change, and select the appropriate conservation measures, long-term monitoring of the GH plant communities will be necessary. In this study, the baseline state for future comparisons was established for the best explored tepui in terms of its flora, Roraima-tepui (2810 m), through a floristic characterization of its different vegetation types. We also identified the environmental gradients underlying the major plant communities, and assessed the effects of human activities on the chemistry of soils and water at three field camps. Our results yielded five main community types: three meadows, one shrubland, and one forest, with their corresponding diagnostic species. The herbaceous communities were mainly influenced by the presence of flat sandy soils, with varying flooding capacity. Shrublands and forests were characterized by irregular organic soils with very low pH. Finally, pH values below 3 were measured on an organic soil of a field camp, although further studies will be necessary to attribute this deviation to human activities.     

Rescue of cytochrome P450 oxidoreductase (Por) mouse mutants reveals functions in vasculogenesis, brain and limb patterning linked to retinoic acid homeostasis

Cytochrome P450 oxidoreductase (POR) acts as an electron donor for all cytochrome P450 enzymes. Knockout mouse Por(-/-) mutants, which are early embryonic (E9.5) lethal, have been found to have overall elevated retinoic acid (RA) levels, leading to the idea that POR early developmental function is mainly linked to the activity of the CYP26 RA-metabolizing enzymes (Otto et al., Mol. Cell. Biol. 23, 6103-6116). By crossing Por mutants with a RA-reporter lacZ transgene, we show that Por(-/-) embryos exhibit both elevated and ectopic RA signaling activity e.g. in cephalic and caudal tissues. Two strategies were used to functionally demonstrate that decreasing retinoid levels can reverse Por(-/-) phenotypic defects, (i) by culturing Por(-/-) embryos in defined serum-free medium, and (ii) by generating compound mutants defective in RA synthesis due to haploinsufficiency of the retinaldehyde dehydrogenase 2 (Raldh2) gene. Both approaches clearly improved the Por(-/-) early phenotype, the latter allowing mutants to be recovered up until E13.5. Abnormal brain patterning, with posteriorization of hindbrain cell fates and defective mid- and forebrain development and vascular defects were rescued in E9.5 Por(-/-) embryos. E13.5 Por(-/-); Raldh2(+/-) embryos exhibited abdominal/caudal and limb defects that strikingly phenocopy those of Cyp26a1(-/-) and Cyp26b1(-/-) mutants, respectively. Por(-/-); Raldh2(+/-) limb buds were truncated and proximalized and the anterior-posterior patterning system was not established. Thus, POR function is indispensable for the proper regulation of RA levels and tissue distribution not only during early embryonic development but also in later morphogenesis and molecular patterning of the brain, abdominal/caudal region and limbs.     

Capsular arabinomannans from Mycobacterium avium with morphotype-specific structural differences but identical biological activity

The capsules of two colony morphotypes of Mycobacterium avium strain 2151 were investigated, i.e. the virulent smooth-transparent (SmT1) and the nonvirulent smooth-opaque (SmO) types. From both morphotypes we separated a nonacylated arabinomannan (AM) from an acylated polysaccharide fraction by affinity chromatography, of which the AMs were structurally characterized. The AMs from the virulent morphotype, in contrast to that from the nonvirulent form, possessed a larger mannan chain and a shorter arabinan chain. Incubation of murine bone marrow-derived macrophages and human dendritic cells showed that the acylated polysaccharide fractions were potent inducers of tumor necrosis factor-alpha, interleukin-12, and interleukin-10 compared with nonacylated AMs, which led to only a marginal cytokine release. Further in vitro experiments showed that both the acylated polysaccharide fractions and the nonacylated AMs were able to induce in vitro anti-tumor cytotoxicity of human peripheral blood mononuclear cells. Thus, morphotype-specific structural differences in the capsular AMs of M. avium do not correlate with biological activity; however, their acylation is a prerequisite for effective stimulation of murine macrophages and human dendritic cells.     

Physiological response of tomatoes at drought,heat and their combination followed by recovery

In nature, crops encounter a combination of abiotic stresses that severely limit yield. Our aim was to dynamically expose the changes of tomatoes' physiological parameters to drought, heat and their combination and thereby clarify the relationship between the responses to single and combined stress. We studied the effect of single and combined drought and heat stresses on the shoot and root of two tomato cultivars (Sufen No.14 as CV1; Jinlingmeiyu as CV2). After being exposed to combined stress for 6 days, the dry weight of shoot and root significantly decreased. The F_q′/F_m′ (quantum yield of photosystem II) was significantly lower in CV1 upon drought and combined stress and in CV2 subjected to combined stress (between days 4 and 6) compared to control. The relative water content during combined stress was significantly lower than control from day 4 to recovery day 2. On days 3 and 6, the water loss rate significantly increased under heat stress and decreased at drought and combined stress, respectively. The combined stress caused severe damages on photosystem II and chloroplast ultrastructure. The root activity after stress recovered even though drought significantly increased the activity from day 2 to day 6. Combined stress result in complex responses during tomato growth. The CV1 was more heat tolerant than CV2, but there was no varietal difference at drought and combined stress. This study contributes to the understanding of the underlying physiological response mechanism of plant to combined stress and crop improvement by providing valuable information for abiotic stress‐tolerant tomato breeding.     

Near-critical phenomena in intracellular metabolite pools

The supply and consumption of metabolites in living cells are catalyzed by enzymes. Here we consider two of the simplest schemes where one substrate is eliminated through Michaelis-Menten kinetics, and where two types of substrates are joined together by an enzyme. It is demonstrated how steady-state substrate concentrations can change ultrasensitively in response to changes in their supply rates and how this is coupled to slow relaxation back to steady state after a perturbation. In the one-substrate system, such near-critical behavior occurs when the supply rate approaches the maximal elimination rate, and in the two-substrate system it occurs when the rates of substrate supply are almost balanced. As systems that operate near criticality tend to display large random fluctuations, we also carried out a stochastic analysis using analytical approximations of master equations and compared the results with molecular-level Monte Carlo simulations. It was found that the significance of random fluctuations was directly coupled to the steady-state sensitivity and that the two substrates can fluctuate greatly because they are anticorrelated in such a way that the product formation rate displays only small variation. Basic relations are highlighted and biological implications are discussed.     

Sinorhizobium meliloti acpXL mutant lacks the C28 hydroxylated fatty acid moiety of lipid A and does not express a slow migrating form of lipopolysaccharide

Lipid A is the hydrophobic anchor of lipopolysaccharide (LPS) in the outer membrane of Gram-negative bacteria. Lipid A of all Rhizobiaceae is acylated with a long fatty acid chain, 27-hydroxyoctacosanoic acid. Biosynthesis of this long acyl substitution requires a special acyl carrier protein, AcpXL, which serves as a donor of C28 (omega-1)-hydroxylated fatty acid for acylation of rhizobial lipid A (Brozek, K.A., Carlson, R.W., and Raetz, C. R. (1996) J. Biol. Chem. 271, 32126-32136). To determine the biological function of the C28 acylation of lipid A, we constructed an acpXL mutant of Sinorhizobium meliloti strain 1021. Gas-liquid chromatography and mass spectrometry analysis of the fatty acid composition showed that the acpXL mutation indeed blocked C28 acylation of lipid A. SDS-PAGE analysis of acpXL mutant LPS revealed only a fast migrating band, rough LPS, whereas the parental strain 1021 manifested both rough and smooth LPS. Regardless of this, the LPS of parental and mutant strains had a similar sugar composition and exposed the same antigenic epitopes, implying that different electrophoretic profiles might account for different aggregation properties of LPS molecules with and without a long acyl chain. The acpXL mutant of strain 1021 displayed sensitivity to deoxycholate, delayed nodulation of Medicago sativa, and a reduced competitive ability. However, nodules elicited by this mutant on roots of M. sativa and Medicago truncatula had a normal morphology and fixed nitrogen. Thus, the C28 fatty acid moiety of lipid A is not crucial, but it is beneficial for establishing an effective symbiosis with host plants. acpXL lies upstream from a cluster of five genes, including msbB (lpxXL), which might be also involved in biosynthesis and transfer of the C28 fatty acid to the lipid A precursor.