NADPH-generating system: Influence on microsomal mono-oxygenase stability during incubation for the liver-microsomal assay with rat and mouse S9 fractions

Activity levels of 7-ethoxycoumarin O-deethylase (ED), aminopyrine N-demethylase (APD), p-nitroanisoleO-demethylase (p-NAD) and glucose-6-phosphate dehydrogenase (G-6-PDH) were determined in incubation mixtures for the liver-microsomal assay (LMA) at time 0 and after 1 and 2 h incubation under conditions for mutagenic assay. The experiments were performed with S9 liver fractions from mice (induced with Na-phenobarbital and β-naphthoflavone) and rats (induced with Aroclor 1254) with and without G-6-PDH in the incubation mixtures.

In the absence of G-6-PDH the activities were significantly lower at time 0 in the mouse. The pattern of stability, however, was similar for the activities, with an increase of stability after 1 and 2 h of pre-incubation (an exception for p-NAD).

Only ED activity showed a similar behaviour in the rat. No differences were present for APD and p-NAD activities at time 0 in the rat, but the enzyme stabilities were significantly decreased after 2 h of incubation (about 15% and 10% for APD and p-NAD respectively) in the absence of G-6-PDH.

At time 0, the amounts of G-6-PDH differed between mouse and rat fractions; however, during the incubations for LMA they decreased by about 57% and 53% for the two species, respectively. In addition to the above biochemical results, the presence of exogenous G-6-PDH in the incubations for the mutagenic assay, significantly increased the mitotic gene conversion and mitotic crossing-over of dimethylnitrosamine (DMN) and AR2MNFN (a nitroimidazo[2,1-b]thiazole) in the D₇ strain of Saccharomyces cerevisiae.     

Cellular functions of NSF: not just SNAPs and SNAREs

N-ethylmaleimide sensitive factor (NSF) is an ATPases associated with various cellular activities protein (AAA), broadly required for intracellular membrane fusion. NSF functions as a SNAP receptor (SNARE) chaperone which binds, through soluble NSF attachment proteins (SNAPs), to SNARE complexes and utilizes the energy of ATP hydrolysis to disassemble them thus facilitating SNARE recycling. While this is a major function of NSF, it does seem to interact with other proteins, such as the AMPA receptor subunit, GluR2, and beta2-AR and is thought to affect their trafficking patterns. New data suggest that NSF may be regulated by transient post-translational modifications such as phosphorylation and nitrosylation. These new aspects of NSF function as well as its role in SNARE complex dynamics will be discussed.     

Are correlations among foliar traits in ferns consistent with those in the seed plants?

Broad-based studies of gymnosperms and angiosperms reveal consistent and functionally significant correlations among foliar traits such as leaf mass per area (LMA), maximum photosynthetic rate (A(area)), foliar nitrogen (N(area)), foliar chlorophyll (Chl) and leaf longevity. To assess the generality of these relationships, we studied 20 fern species growing in the understorey of a temperate deciduous forest. We found that foliar N(area) increases with LMA, and that foliar N(area) and A(area) are positively correlated with one another, as are foliar N(area) and Chl. The ferns in general have very low LMA compared with most seed plants; A(area), N(area) and Chl are below median values for seed plants but are not extreme. Species with overwintering fronds have significantly higher LMA than species with fronds that senesce at the end of the growing season, as well as a significantly higher C : N ratio in frond tissue and relatively high foliar N on an areal basis. Correlations among foliar traits associated with gas exchange in these forest understorey ferns are in accordance with patterns reported for seed plants, suggesting a high degree of functional constraint on the interrelationships among key elements in foliar design.     

Genome size evolution in relation to leaf strategy and metabolic rates revisited

BACKGROUND AND AIMS: It has been proposed that having too much DNA may carry physiological consequences for plants. The strong correlation between DNA content, cell size and cell division rate could lead to predictable morphological variation in plants, including a negative relationship with leaf mass per unit area (LMA). In addition, the possible increased demand for resources in species with high DNA content may have downstream effects on maximal metabolic efficiency, including decreased metabolic rates. METHODS: Tests were made for genome size-dependent variation in LMA and metabolic rates (mass-based photosynthetic rate and dark respiration rate) using our own measurements and data from a plant functional trait database (Glopnet). These associations were tested using two metrics of genome size: bulk DNA amount (2C DNA) and monoploid genome size (1Cx DNA). The data were analysed using an evolutionary framework that included a regression analysis and independent contrasts using a phylogenetic tree with estimates of molecular diversification times. A contribution index for the LMA data set was also calculated to determine which divergences have the greatest influence on the relationship between genome size and LMA. KEY RESULTS AND CONCLUSIONS: A significant negative association was found between bulk DNA amount and LMA in angiosperms. This was primarily a result of influential divergences that may represent early shifts in growth form. However, divergences in bulk DNA amount were positively associated with divergences in LMA, suggesting that the relationship may be indirect and mediated through other traits directly related to genome size. There was a significant negative association between genome size and metabolic rates that was driven by a basal divergence between angiosperms and gymnosperms; no significant independent contrast results were found. Therefore, it is concluded that genome size-dependent constraints acting on metabolic efficiency may not exist within seed plants.     

The relationship between nuclear DNA content and leaf strategy in seed plants

BACKGROUND AND AIMS: Species' 2C-values (mass of DNA in G(1) phase 2n nuclei) vary by at least four orders of magnitude among seed plants. The 2C-value has been shown to be co-ordinated with a number of other species traits, and with environmental variables. A prediction that species 2C-values are negatively related to leaf life span (LL) and leaf mass per area (LMA) is tested. These leaf traits are components of a major dimension of ecological variation among plant species. METHODS: Flow cytometry was used to measure the 2C-values for 41 Australian seed plant species, 40 of which were new to the literature. Where possible, LL and LMA data from the global literature were combined with 2C-values from our data set and online C-value databases. KEY RESULTS: Across all species, weak positive relationships were found between 2C-values and both LL and LMA; however, these did not reflect the relationships within either angiosperms or gymnosperms. Across 59 angiosperm species, there were weak negative relationships between 2C-values and both LL (r2 = 0.13, P = 0.005) and LMA (r2 = 0.15, P = 0.002). These relationships were the result of shifts to longer LL and greater LMA in woody compared with herbaceous growth forms, with no relationships present within growth forms. It was not possible to explain a positive relationship between 2C-values and LMA (r2 = 0.30, P = 0.024) across 17 gymnosperm species. The 2C-value was not related to LL or LMA either across species within orders (except for LMA among Pinales), or as radiation divergences in a model phylogeny. CONCLUSIONS: Gymnosperms appear to vary along a spectrum different from angiosperms. Among angiosperms, weak negative cross-species relationships were associated with growth form differences, and traced to a few divergences deep in the model phylogeny. These results suggest that among angiosperms, nuclear DNA content and leaf strategy are unrelated.     

Latitudinal Variation in Leaf and Tree Traits of the Mangrove Avicennia germinans (Avicenniaceae) in the Central Region of the Gulf of Mexico

The variation in leaf mass per area, leaf nutrients (% carbon, nitrogen and phosphorus), and the allometric relation between tree height and diameter of the black mangrove, Avicennia germinans, were explored in nine mangrove forests in similar environments along a 5° latitudinal gradient in the central region of the Gulf of Mexico, as indicated by a southward increase in temperature and precipitation. There was no correlation between leaf nitrogen or phosphorus content and latitude. Leaf mass per area and leaf carbon content were positively correlated with latitude and negatively correlated with temperature and annual rainfall, whereas asymptotic tree height and maximum diameter showed the opposite trend. Such patterns suggest a trade-off between leaf traits and tree size which may be constrained by the same environmental factors along a dry-cold to humid-warm latitudinal gradient.     

Faster returns on ‘leaf economics’ and different biogeochemical niche in invasive compared with native plant species

Plant‐invasive success is one of the most important current global changes in the biosphere. To understand which factors explain such success, we compared the foliar traits of 41 native and 47 alien‐invasive plant species in Oahu Island (Hawaii), a location with a highly endemic flora that has evolved in isolation and is currently vulnerable to invasions by exotic plant species. Foliar traits, which in most cases presented significant phylogenetic signal, i.e. closely related species tended to resemble each other due to shared ancestry, separated invasive from native species. Invasive species had lower leaf mass per area and enhanced capacities in terms of productivity (photosynthetic capacity) and nutrient capture both of macro‐ (N, P, K) and microelements (Fe, Ni, Cu and Zn). All these differences remain highly significant after removing the effects of phylogenetic history. Alien‐invasive species did not show higher efficiency at using limiting nutrient resources, but they got faster leaf economics returns and occupied a different biogeochemical niche, which helps to explain the success of invasive plants and suggests that potential increases in soil nutrient availability might favor further invasive plant success.