Role of ABA in Maturation of Rapeseed Embryos

Development of Brassica napus L. cv Tower embryos of different ages cultured in vitro with and without abscisic acid (ABA) was compared with normal development in situ to investigate the role of ABA in embryo maturation. Endogenous ABA levels were measured by radioimmunoassay, and sensitivity to ABA was assayed in terms of its ability to suppress precocious germination and stimulate accumulation of storage protein and storage protein mRNA. During development in situ, the levels of endogenous ABA and 12S storage protein mRNA both reach their peaks just before the embryos begin to desiccate. The ABA levels during this phase of development also correlate with the time required in culture before germination is evident. Following these peaks, increasing concentrations of exogenous ABA are required to both suppress germination and continue storage protein accumulation in vitro. Thus, both endogenous ABA and ABA sensitivity decline during maturation. The concentrations of exogenous ABA required to suppress germination at these later stages result in abnormally high levels of endogenous ABA and appear to be toxic. These results are consistent with the hypothesis that in maturing rapeseeds, low water content rather than ABA prevents germination during the later stages of development.     

GABAergic inhibition shapes temporal and spatial response properties of pyramidal cells in the electrosensory lateral line lobe of gymnotiform fish

1. The amplitude-coding pyramidal neurons of the first-order nucleus in weakly electric gymnotiform fish (Eigenmannia), the electrosensory lateral line lobe (ELL), exhibit 2 major physiological transformations of primary afferent input. Pyramidal cells rapidly adapt to a step change in amplitude, and they have a center/surround receptive-field organization. This study examined the physiological role of GABAergic inhibition on pyramidal cells. GABAergic synapses onto the somata of pyramidal cells primarily originate from granule-cell interneurons along with descending input. 2. Pyramidal cells fall into two physiologically distinct categories: E units, which are excited by a rise in stimulus amplitude, and I units, which are inhibited by a rise in stimulus amplitude. Microiontophoretic application of bicuculline methiodide onto both types of pyramidal cells increased the time constant of adaptation, defined as the time required for the neuron's response to decay to 37% of its maximum value, by 70-90%. The peak firing rate of E units to a step increase in stimulus amplitude increased by 49%, while the firing rate of I units did not change significantly. 3. Bicuculline application demonstrated that GABAergic inhibition may contribute to the strict segregation of E and I response properties. In the presence of bicuculline, many E units (normally excited only by stimulus amplitude increases) became excited by both increases and decreases; many I units (normally excited only by amplitude decreases) also became excited to increases. 4. The size of the excitatory receptive-field of E units was not affected by bicuculline, although response magnitude increased. The inhibitory surround increased in spatial extent by 175% with bicuculline administration. Neither the size of the I unit receptive-field center nor the response magnitude changed in the presence of bicuculline. The antagonistic surround of I units, however, increased by 49%. 5. The anatomy of the ELL is well understood (see Carr and Maler 1986). The physiological results obtained in this study, along with the results of Bastian (1986a, b), further our understanding of the functional role of the ELL circuitry. Our results suggest that spatial and temporal response properties of pyramidal cells are regulated by different but interacting inhibitory interneurons, some of which use GABA as a neurotransmitter. The activity of these interneurons is in turn controlled by descending feedback systems.     

Two closely linked tomato HKT coding genes are positional candidates for the major tomato QTL involved in Na+/K+ homeostasis

The location of major quantitative trait loci (QTL) contributing to stem and leaf [Na⁺] and [K⁺] was previously reported in chromosome 7 using two connected populations of recombinant inbred lines (RILs) of tomato. HKT1;1 and HKT1;2, two tomato Na⁺‐selective class I‐HKT transporters, were found to be closely linked, where the maximum logarithm of odds (LOD) score for these QTLs located. When a chromosome 7 linkage map based on 278 single‐nucleotide polymorphisms (SNPs) was used, the maximum LOD score position was only 35 kb from HKT1;1 and HKT1;2. Their expression patterns and phenotypic effects were further investigated in two near‐isogenic lines (NILs): 157‐14 (double homozygote for the cheesmaniae alleles) and 157‐17 (double homozygote for the lycopersicum alleles). The expression pattern for the HKT1;1 and HKT1;2 alleles was complex, possibly because of differences in their promoter sequences. High salinity had very little effect on root dry and fresh weight and consequently on the plant dry weight of NIL 157‐14 in comparison with 157‐17. A significant difference between NILs was also found for [K⁺] and the [Na⁺]/[K⁺] ratio in leaf and stem but not for [Na⁺] arising a disagreement with the corresponding RIL population. Their association with leaf [Na⁺] and salt tolerance in tomato is also discussed.     

Ant versus bird exclusion effects on the arthropod assemblage of an organic citrus grove

1. Predation‐exclusion experiments have highlighted that top‐down control is pervasive in terrestrial communities, but most of these experiments are simplistic in that they only excluded a single group of predators and the effect of removal was evaluated on a few species from the community. The main goal of our study was to experimentally establish the relative effects of ants and birds on the same arthropod assemblage of canopy trees. 2. We conducted 1‐year long manipulative experiments in an organic citrus grove intended to quantify the independent effects of bird and ant predators on the abundance of arthropods. Birds were excluded with plastic nets whereas ants were excluded with sticky barriers on the trunks. The sticky barrier also excluded other ground dwelling insects, like the European earwig Forficula auricularia L. 3. Both the exclusion of ants and birds affected the arthropod community of the citrus canopies, but the exclusion of ants was far more important than the exclusion of birds. Indeed, almost all groups of arthropods had higher abundance in ant‐excluded than in control trees, whereas only dermapterans were more abundant in bird‐excluded than in control trees. A more detailed analysis conducted on spiders also showed that the effect of ant exclusion was limited to a few families rather than being widespread over the entire diverse spectrum of spiders. 4. Our results suggest that the relative importance of vertebrate and invertebrate predators in regulating arthropod populations largely depends on the nature of the predator–prey system.     

14-3-3beta binds to and negatively regulates the tuberous sclerosis complex 2 (TSC2) tumor suppressor gene product,tuberin

TSC2, or tuberin, is the product of the tuberous sclerosis tumor suppressor gene TSC2 and acts downstream of the phosphatidylinositol 3-kinase-Akt signaling pathway to negatively regulate cellular growth. One mechanism underlying its function is to assemble into a heterodimer with the TSC1 gene product TSC1, or hamartin, resulting in a reduction in phosphorylation, and hence activation, of the ribosomal subunit S6 kinase (S6K). We identified a novel interaction between TSC2 and 14-3-3beta. We found that 14-3-3beta does not interfere with TSC1-TSC2 binding and can form a ternary complex with these two proteins. Association between 14-3-3beta and TSC2 requires phosphorylation of TSC2 at a unique residue that is not a known Akt phosphorylation site. The overexpression of 14-3-3beta compromises the ability of the TSC1-TSC2 complex to reduce S6K phosphorylation. The antagonistic activity of 14-3-3beta toward TSC is dependent on the 14-3-3beta-TSC2 interaction, since a mutant of TSC2 that is not recognized by 14-3-3beta is refractory to 14-3-3beta. We suggest that 14-3-3 proteins interact with the TSC1-TSC2 complex and negatively regulate the function of the TSC proteins.     

On temporal partitioning of a community of ectomycorrhizal fungi

Several mechanisms may contribute to the high species richness often reported in ectomycorrhizal (ECM) fungal communities, including spatial and temporal partitioning. Here, we focus on temporal partitioning. Using molecular methods, we determined the frequencies of occurrence of ECM fungal species detected as hyphae and ECM roots in the forest floor of a Pinus resinosa plantation during a 13-month period. We then used a novel statistical procedure to place the most frequently occurring ECM fungal species into groups distinguished by their patterns of relative frequency over time. Three groups with contrasting temporal patterns were distinguishable for fungal species detected as hyphae. Two groups were distinguishable for species detected as ECM roots. Our results support the hypothesis that temporal partitioning occurs among the species of ECM fungi in this community, but we did not address its causes, which may have involved interactions among species' physiological tolerances, temporal environmental variability, temporal patterns of root production, and variation in fungal genet lifespan. These interactions should be the subjects of future research.