Secondary metabolites of fleshy vertebrate-dispersed fruits: adaptive hypotheses and implications for seed dispersal

We discuss seven hypotheses to explain the adaptive significance of secondary metabolites in ripe fleshy fruits and their implications for seed dispersal. These hypotheses are the attraction/association, seed germination inhibition, attraction/repulsion, protein assimilation, gut retention time, directed toxicity, and defense trade-off hypotheses. We examine evidence that supports or refutes these hypotheses and suggest further tests of each. In addition, we summarize recent work with Solanum fruit pulp glycoalkaloids that bears directly on three of these hypotheses (directed toxicity, gut retention time, and defense trade-off). We conclude that evidence addressing many of these hypotheses is either observational or indirect, but most hypotheses find at least some level of support. Because many of the hypotheses are not mutually exclusive, we also conclude that synergistic interactions and multifunctionality in secondary metabolites may provide economical evolutionary solutions for plants facing disparate and temporally variable selective pressures that impinge on fruits and seeds.     

Determination of the phytotoxicity of barium in leach-field disposal of gas well brines

Summary A study was conducted to determine the effect of barium on yield and elemental composition of plants growing near natural gas wells. Brines containing potentially phytotoxic concentrations of barium are periodically released into leach-fields at these wells. Data on plant yield, soil and plant Ba, Ca, Mg, Na, and K concentrations, and soil pH were collected from high-barium well sites, and from greenhouse experiments using simulated brines. Barium had no detectable effect upon plant yield, and contributed only to variations in its own concentration in plants studied. Barium also exhibited biodiminution, in contrast to all other elements studied. Sodium was implicated at the major cause of phytotoxicity at gas well leach-fields. Due to interference of potassium uptake by soidum, the soil sodium/potassium ratio was found to be closely correlated with plant yield fluctuations.     

Effect of light and simulated herbivory on growth of endangered northeastern bulrush,shape Scirpus ancistrochaetus Schuyler

The independent and interactive effects of light and simulated herbivory on growth and biomass allocation in the endangered emergent sedge, Scirpus ancistrochaetus Schuyler, were examined in a 15-week common garden experiment. Light level was manipulated through the use of shade cloth to attain shading levels of 0%, 30%, 63% and 90%, while herbivory was simulated by reducing plant height by 50% six weeks into the experiment. Multivariate Analysis of Variance (MANOVA) of final total (shoot + root) plant mass, leaf width and root to shoot ratio (R:S) indicated overall significant effects of both light and the interaction of light and clipping. Means of these variables decreased with decreasing light level. Furthermore, means of these variables were generally higher for clipped plants than unclipped plants in 0% shade, while they tended to be lower in clipped plants in all other levels of shade. Maximum leaf height was also significantly affected by light. Leaf height was similar in all treatments for the first 11 weeks. For the last four weeks of the experiment, however, leaf height was lowest in plants grown in 0% shade, highest in plants grown in 63 and 90% shade, and intermediate in plants grown in 30% shade. Clipped plants tended to be taller than unclipped plants at lower levels of shade, while the opposite was true at higher levels of shade, although effects were marginally significant. Both light and the interactive effect of light and clipping were important determinants of growth of S. ancistrochaetus in this experiment and therefore may be important influences on its distribution within and among pond habitats.     

How novel are the chemical weapons of garlic mustard in North American forest understories?

The Novel Weapons Hypothesis predicts that invasive plants excel in their new ranges because they produce novel metabolites to which native species possess little resistance. We examined the novelty of the phytochemistry of the Eurasian invader, Alliaria petiolata, in North America by comparing its phytochemical profile with those of closely related Brassicaceae native to North America. We examined the profile and/or concentrations of glucosinolates, alliarinoside, flavonoids, cyanide, and trypsin inhibitors in cauline leaves of field-collected A. petiolata, Arabis laevigata, Cardamine concatenata, C. bulbosa, and C. douglassii. Cyanide and the glucosinolates and flavonoids produced by A. petiolata were detected only in A. petiolata. Trypsin inhibitor activity was highest in A. laevigata, intermediate in the Cardamine species, and lowest in A. petiolata. The phytochemical profile of A. petiolata was distinct from those of four closely related and/or abundant Brassicaceaeous species native to North America, providing support for the Novel Weapons Hypothesis.     

Leaf and root extracts of the invasive shrub, Lonicera maackii, inhibit seed germination of three herbs with no autotoxic effects

In addition to effects mediated by resource competition, some invasive plants may impact surrounding vegetation by secreting compounds that are directly inhibitory to growth. Lonicera maackii, an invasive Asian shrub of forests and open areas in eastern and midwestern North America, has devastating effects on understory vegetation, some of which persist even after this shrub is removed. In this study, we explored the potential of aqueous extracts of the leaves and roots of this plant to inhibit seed germination of Impatiens capensis, Alliaria petiolata, Arabidopsis thaliana, and L. maackii in Petri dish bioassays. Both L. maackii root and leaf extracts significantly decreased germination in the three herb species. This inhibitory effect generally increased with increasing extract concentration and was more pronounced with application of leaf extract than root extract. However, when the same extracts were applied to seeds of L. maackii itself, germination was delayed in some cases, but was not significantly reduced by the end of the experiment. Germination of L. maackii seeds even reached significantly higher levels in some extract treatments than in no-extract controls. This implies that L. maackii can successfully inhibit the germination of other plants with few autotoxic effects and may even promote the germination of its own seeds.     

Lactoferrin expression in human breast cancer

We analyzed lactoferrin expression in 78 samples from patients with sporadic breast cancer and found 31/78 negative for mRNA expression. Similar results were obtained by immuno-histochemical localization of the lactoferrin protein. We did not find relationship between lactoferrin expression and clinical parameters. We investigated for the absent lactoferrin expression in some cases of breast cancer. In 68 of the samples analyzed, we found an inverse correlation between estrogen receptor expression and lactoferrin expression (P < 0,0001), thus indicating that regulation by the estrogen receptor is not the main element responsible for the expression of lactoferrin in breast cancer. Analysis of methylation of the lactoferrin genomic DNA extracted from the same patients revealed that the degree of methylation does not explain the observed absence of lactoferrin. The 937 bp lactoferrin promoter was investigated for possible mutations. By single-strand conformation polymorphism analysis one polymorphic site was found and characterized.     

Negative cross-talk between salicylate- and jasmonate-mediated pathways in the Wassilewskija ecotype of Arabidopsis thaliana

Plants often respond to attack by insect herbivores and necrotrophic pathogens with induction of jasmonate-dependent resistance traits, but respond to attack by biotrophic pathogens with induction of salicylate-dependent resistance traits. To assess the degree to which the jasmonate- and salicylate-dependent pathways interact, we compared pathogenesis-related protein activity and bacterial performance in four mutant Arabidopsis thaliana lines relative to their wild-type backgrounds. We found that two salicylate-dependent pathway mutants (cep1, nim1-1) exhibited strong effects on the growth of the generalist biotrophic pathogen, Pseudomonas syringae pv. tomato, whereas two jasmonate-dependent pathway mutants (fad3-2fad7-2fad8, jar1-1) did not. Leaf peroxidase and exochitinase activity were negatively correlated with bacterial growth, whereas leaf polyphenol oxidase activity and trypsin inhibitor concentration were not. Interestingly, leaf total glucosinolate concentration was positively correlated with bacterial growth. In the same experiment, we also found that application of jasmonic acid generally increased leaf peroxidase activity and trypsin inhibitor concentration in the mutant lines. However, the cep1 mutant, shown previously to overexpress salicylic acid, exhibited no detectable biological or chemical responses to jasmonic acid, suggesting that high levels of salicylic acid may have inhibited a plant response. In a second experiment, we compared the effect of jasmonic acid and/or salicylic acid on two ecotypes of A. thaliana. Application of salicylic acid to the Wassilewskija ecotype decreased bacterial growth. However, this effect was not observed when both salicylic acid and jasmonic acid were applied, suggesting that jasmonic acid negated the beneficial effect of salicylic acid. Collectively, our results confirm that the salicylate-dependent pathway is more important than the jasmonate-dependent pathway in determining growth of P. syringae pv. tomato in A. thaliana, and suggest important negative interactions between these two major defensive pathways in the Wassilewskija ecotype. In contrast, the Columbia ecotype exhibited little evidence of negative interactions between the two pathways, suggesting intraspecific variability in how these pathways interact in A. thaliana.     

Sexual dimorphism and cost of reproduction in the dioecious shrub Lindera benzoin (Lauraceae)>

We examined sexual dimorphism in reproductive allocation and its effects on growth and subsequent reproduction in a natural population of the dioecious woody shrub Lindera benzoin (L.) Blume. In addition to comparing natural patterns of growth and reproduction in a marked population of 251 females and 87 males, we experimentally examined the effect of reduced reproduction on future growth and reproduction in female plants and examined sexual dimorphism in carbon/nutrient balance. Our results suggest that females of L. benzoin bear greater reproductive costs in terms of both biomass and minerals. These costs were measurable in terms of current biomass and allocation of nitrogen to reproduction, as well as subsequent growth, reproduction, and tissue carbon/nutrient balance. Based upon the results of a fruit-thinning experiment and path analysis, fruit production in 1991 appeared to have direct negative effects on fruit production in 1992, an effect that was not necessarily mediated through effects on plant growth. We discuss our results in the context of other intrinsic and extrinsic factors that can influence growth, reproductive costs, and mortality in this species.     

Variation in the expression of chemical defenses in Alliaria petiolata (Brassicaceae) in the field and common garden

I examined glucosinolates, trypsin inhibitors (TI), and peroxidase (POD) activity in garlic mustard (Alliaria petiolata) plants growing naturally in Wright State University's Forest Preserve and in a common garden experiment in plants from the same populations conducted in the greenhouse. In the field, first-year plants expressed each defense, but defense levels varied significantly in plants from different sites in the forest. Patterns in site variation were consistent for glucosinolate and POD, but not for TI. The TI and POD levels were increased by mechanical wounding, but glucosinolate levels were unaffected. In the greenhouse, plants expressed each defense at higher levels than in the field, but defense levels did not vary among plants collected from each site in the field. The POD activity was increased by wounding, but glucosinolate and TI levels where unaffected. Plants from each site varied in height and leaf length when measured shortly after transplantation, but site differences substantially diminished after 4 wk. Site-based variation in defense expression in the field, which disappeared in the greenhouse, was presumably related to differences in environmental quality among the sites. Sites were shown to vary in soil moisture content, soil pH, nutrient levels, and presumably light quantity or quality. Despite an apparent lack of genetic variation in defense across sites in the field, the constitutive expression of these three chemical defenses, increases due to wounding, and phenotypic variation across sites could reduce herbivore success on garlic mustard individuals and slow the rate of herbivore adaptation to garlic mustard populations.