Indirect Defense Responses to Herbivory in Grasses

The leaf trichome of tobacco (Nicotiana tabacum) represents a unique secretory structure in which the basal trichome cell is connected to the epidermis by numerous plasmodesmata (PD). Small fluorescent probes microinjected into the basal trichome cell moved apically into distal trichome cells but not into the subtending epidermal cell. In marked contrast, the same probes moved apically into trichome cells when injected into the epidermal cell. Noninvasive methods of dye loading, including ester loading into the apical secretory cell by trichome “capping” and by infiltration of caged fluorescein, produced the same result. In transgenic tobacco plants constitutively expressing photoactivatable green fluorescent protein (PAGFP), activation of PAGFP above the epidermal/trichome (e/t) boundary resulted in movement of protein apically into the distal trichome cells but not across the e/t boundary, while PAGFP activated in the epidermal cell moved apically across the e/t boundary. Experiments with apoplastic tracers also revealed the presence of a distinct apoplastic barrier to solute movement at the e/t interface. These data point to unidirectional transport of solutes through PD. PAGFP activated in individual cells equidistant between the basal cell and the apical cell moved bidirectionally from these cells, suggesting that mass flow was not the driving force for unidirectional transport. We found that unidirectional transport across the e/t boundary was not affected by virus infection or by addition of the actin inhibitor latrunculin but could be dissipated completely by addition of sodium azide. Collectively, our data suggest that active, unidirectional transport of molecules may occur through PD located at unique interfaces in the plant.Plasmodesmata (PD) are the functional units of the symplasm and occur at multiple interfaces between plant cells (Roberts, 2005; Maule, 2008). The basic structure of the PD pore reveals a plasma membrane (PM)-lined pore containing the desmotubule, an axial endoplasmic reticulum-derived structure that provides endomembrane continuity between cells (for review, see Maule, 2008). The space between the desmotubule and the PM is known as the cytoplasmic sleeve and is thought to be the main conduit for the movement of small solutes between cells (Erwee and Goodwin, 1983; Terry and Robards, 1987; Liarzi and Epel, 2005). PD have received considerable attention in recent years for their ability to traffic large macromolecules, including proteins and nucleic acids (Lucas and Lee, 2004; Maule, 2008). For many macromolecules, transport through the PD pore appears to occur by a selective mechanism that permits an exchange of macromolecules between specific cell types (Wu et al., 2002; Lucas and Lee, 2004; Zambryski, 2004).For small solutes, movement through the PD pore is thought to occur by diffusion and is strongly dependent on the hydrodynamic, or Stokes, radius of the permeant molecule in question (Liarzi and Epel, 2005). The effective channel for solute transport through PD is in the region of 3 nm (Erwee and Goodwin, 1983; Terry and Robards, 1987; Tucker, 1988), and molecules of less than 1 kD are thought to pass freely between cells (Erwee and Goodwin, 1983; Terry and Robards, 1987). However, a recalculation of the data presented by Terry and Robards (1987) by Fisher (1999) revealed that the effective diameter for transport through the cytoplasmic sleeve may be closer to 4 nm, potentially allowing significantly larger molecules to pass through the pore. Indeed, GFP (27 kD, Stokes radius of 2.8; Liarzi and Epel, 2005) may move diffusively between PD of some cell types, depending on tissue location and age (Oparka et al., 1999; Crawford and Zambryski, 2001; Wymer et al., 2001; Liarzi and Epel, 2005). Although the size exclusion limit of the PD pore was originally thought to be relatively constant (Erwee and Goodwin, 1983; Terry and Robards, 1987), it is now clear that “not all PD are equal,” in the sense that the size exclusion limit (SEL) of the pore may differ substantially during cell development (Duckett et al., 1994; Oparka et al., 1999; Crawford and Zambryski, 2000; Ruan et al., 2001) in response to environmental conditions (Epel and Erlanger, 1991; Cleland et al., 1994; Schulz, 1995) and the location of a given cell-cell interface in the plant (Erwee and Goodwin, 1983, 1985; Duckett et al., 1994; Ruan et al., 2001). It is also established that the SEL of the pore may respond to intracellular cues such as water relations (Oparka and Prior, 1992; Schulz, 1995), metabolic status (Tucker, 1993; Cleland et al., 1994; Wright and Oparka, 1997), cytosolic Ca²⁺ levels (Tucker, 1988; Tucker and Boss, 1996; Holdaway-Clarke et al., 2000), and hormones (Ormenese et al., 2006). The picture emerging is one in which the PD pore functions as a dynamic structure that alters its SEL in response to changing local conditions (Maule, 2008). Despite this, simple diffusion remains the most likely mechanism determining solute exchange between symplasmically coupled cells (Liarzi and Epel, 2005).A common feature in plant tissues is the presence of symplasmic “domains” within which PD share a common SEL (for review, see Roberts and Oparka, 2003). Examples of symplasmic domains occur at interfaces where cells of differing functions are connected by PD. In extreme cases, such as guard cells, PD are lost during development from the guard cell walls that connect the stomatal complex to the epidermis, effectively isolating the guard cells from the epidermal symplasm (Palevitz and Hepler, 1985). Commonly, however, cells of differing functions remain connected by PD, although these interfaces appear to be tightly regulated with respect to solute flow (Roberts and Oparka, 2003). An example of strict control occurs in the PD that connect the sieve element-companion cell complex with mesophyll cells (van Bel and van Rijen, 1994; Knoblauch and van Bel, 1998; Oparka and Turgeon, 1999). These PD appear to be closed during normal long-distance phloem functions (van Bel and van Rijen, 1994; Knoblauch and van Bel, 1998; Oparka and Turgeon, 1999) but may open in response to stress conditions (Schulz, 1995; Wright and Oparka, 1997). For those differing cell types that remain connected by PD, a major challenge is to retain cell autonomous functions in the presence of symplasmic connections.Recently, we made an extensive study of the interface that connects the basal leaf trichome cell of tobacco (Nicotiana tabacum) with the underlying epidermal cell (Faulkner et al., 2008). A simple freeze-fracture protocol allowed PD to be viewed in surface fractures of the cell wall and PM, and we were able to image all of the PD across the cell-cell interface of the epidermis/trichome (e/t) boundary. Our data showed that during wall development, simple primary PD were gradually replaced with secondary PD by a multiple twinning process that involved the de novo formation of PD pores. During trichome development, the number of PD at the e/t boundary increased 5-fold due to the insertion of new secondary PD pores. In mature trichomes, in excess of 2,000 PD were found in the cell wall spanning the e/t boundary. The ability to image large numbers of PD at this interface prompted us to explore more closely the function of PD at the e/t interface. Previously, trichomes have proved a useful experimental system for studying cell-cell transport through PD (Waigmann and Zambryski, 2000). Interestingly, trichome cells of Nicotiana clevelandii have been shown to be connected by PD with a large basal SEL of about 7 kD (Waigmann and Zambryski, 1995) and appear to differ in structure from mesophyll PD (Waigmann et al., 1997). However, the specific interface between the trichome basal cell and the epidermis has not been examined.To examine the function of PD at the e/t interface in more detail, we developed a number of different approaches for introducing fluorescent probes into the symplasm of the basal trichome cell and its supporting epidermal cell. These included microinjection, cell-specific dye loading, the use of caged fluorescein, and the generation of transgenic tobacco plants constitutively expressing photoactivatable GFP (PAGFP) in the cytosol. Our data show consistently that fluorescent probes introduced into the basal trichome cell are unable to cross the e/t boundary while probes introduced below this boundary are able to enter the trichome freely, providing strong evidence for unidirectional transport through PD at a specific cell-cell interface. In addition, we show that sodium azide, a metabolic inhibitor that alters PD SEL (Tucker, 1993), dissipates unidirectional solute flow into the trichome and allows molecules to exit the trichome. Our data are consistent with unidirectional solute movement into the basal trichome cell.     

Herbivory and Stoichiometric Feedbacks to Primary Production

Established theory addresses the idea that herbivory can have positive feedbacks on nutrient flow to plants. Positive feedbacks likely emerge from a greater availability of organic carbon that primes the soil by supporting nutrient turnover through consumer and especially microbially-mediated metabolism in the detrital pool. We developed an entirely novel stoichiometric model that demonstrates the mechanism of a positive feedback. In particular, we show that sloppy or partial feeding by herbivores increases detrital carbon and nitrogen allowing for greater nitrogen mineralization and nutritive feedback to plants. The model consists of differential equations coupling flows among pools of: plants, herbivores, detrital carbon and nitrogen, and inorganic nitrogen. We test the effects of different levels of herbivore grazing completion and of the stoichiometric quality (carbon to nitrogen ratio, C:N) of the host plant. Our model analyses show that partial feeding and plant C:N interact because when herbivores are sloppy and plant biomass is diverted to the detrital pool, more mineral nitrogen is available to plants because of the stoichiometric difference between the organisms in the detrital pool and the herbivore. This model helps to identify how herbivory may feedback positively on primary production, and it mechanistically connects direct and indirect feedbacks from soil to plant production.     

Specificity in Mesograzer-Induced Defences in Seagrasses

Grazing-induced plant defences that reduce palatability to herbivores are widespread in terrestrial plants and seaweeds, but they have not yet been reported in seagrasses. We investigated the ability of two seagrass species to induce defences in response to direct grazing by three associated mesograzers. Specifically, we conducted feeding-assayed induction experiments to examine how mesograzer-specific grazing impact affects seagrass induction of defences within the context of the optimal defence theory. We found that the amphipod Gammarus insensibilis and the isopod Idotea chelipes exerted a low-intensity grazing on older blades of the seagrass Cymodocea nodosa, which reflects a weak grazing impact that may explain the lack of inducible defences. The isopod Synischia hectica exerted the strongest grazing impact on C. nodosa via high-intensity feeding on young blades with a higher fitness value. This isopod grazing induced defences in C. nodosa as indicated by a consistently lower consumption of blades previously grazed for 5, 12 and 16 days. The lower consumption was maintained when offered tissues with no plant structure (agar-reconstituted food), but showing a reduced size of the previous grazing effect. This indicates that structural traits act in combination with chemical traits to reduce seagrass palatability to the isopod. Increase in total phenolics but not in C:N ratio and total nitrogen of grazed C. nodosa suggests chemical defences rather than a modified nutritional quality as primarily induced chemical traits. We detected no induction of defences in Zostera noltei, which showed the ability to replace moderate losses of young biomass to mesograzers via compensatory growth. Our study provides the first experimental evidence of induction of defences against meso-herbivory that reduce further consumption in seagrasses. It also emphasizes the relevance of grazer identity in determining the level of grazing impact triggering resistance and compensatory responses of different seagrass species.     

Herbivory on coral reefs: algal susceptibility to herbivorous fishes

Summary The susceptibility of several tropical algal species to fish grazing was studied on the Belizean barrier reef off the Caribbean coast of Central America. Short-term transplant experiments indicate that plant species vary markedly in their rates of biomass loss to grazing by a shallow-water guild of herbivorous fishes. Algal species transplanted from habitats with low grazing pressure are highly susceptible to grazing, while species occurring in habitats with high herbivore densities are highly resistant to grazing. Algal species show differential susceptibility to grazing by two major components of the tropical herbivore guild, Acanthurus (surgeonfishes) and Sparisoma (parrotfishes).Variability in plant susceptibility to grazing by herbivorous fishes was not clearly correlated with morphological or chemical characteristics that have been previously suggested as plant defenses against herbivory. Plants found to be highly resistant to fish grazing, such as Halimeda, exhibit both morphological characteristics and secondary chemical compounds which do appear to reduce herbivory. In contrast, species of Caulerpa, Sargassum, Turbinaria, and Padina, which also possess alleged morphological and/or chemical defenses, are nevertheless highly susceptible to fish grazing.     

Herbivory simulations in ecological research

Because of the experimental advantages that they offer, mechanical simulations of grazing are more commonly used than true herbivory in ecological studies of the impact of herbivory on plants. However, few studies have explicitly compared plant responses to herbivory and to mechanical simulations. Most such comparisons report differences in plant responses to mechanical versus true herbivory, even though the amounts and types of tissue removed were similar. Moreover, studies that also attempted to mimic the timing of leaf damage report differences in plant responses to the different damage modes. Because a plant's response to herbivory is complex and is activated by more than merely the removal of tissue, exact mechanical simulations may prove difficult.     

Pathogen-Triggered Ethylene Signaling Mediates Systemic-Induced Susceptibility to Herbivory in Arabidopsis

Multicellular eukaryotic organisms are attacked by numerous parasites from diverse phyla, often simultaneously or sequentially. An outstanding question in these interactions is how hosts integrate signals induced by the attack of different parasites. We used a model system comprised of the plant host Arabidopsis thaliana, the hemibiotrophic bacterial phytopathogen Pseudomonas syringae, and herbivorous larvae of the moth Trichoplusia ni (cabbage looper) to characterize mechanisms involved in systemic-induced susceptibility (SIS) to T. ni herbivory caused by prior infection by virulent P. syringae. We uncovered a complex multilayered induction mechanism for SIS to herbivory. In this mechanism, antiherbivore defenses that depend on signaling via (1) the jasmonic acid–isoleucine conjugate (JA-Ile) and (2) other octadecanoids are suppressed by microbe-associated molecular pattern–triggered salicylic acid (SA) signaling and infection-triggered ethylene signaling, respectively. SIS to herbivory is, in turn, counteracted by a combination of the bacterial JA-Ile mimic coronatine and type III virulence-associated effectors. Our results show that SIS to herbivory involves more than antagonistic signaling between SA and JA-Ile and provide insight into the unexpectedly complex mechanisms behind a seemingly simple trade-off in plant defense against multiple enemies.     

Herbivory in an acid stream

• 1 Spatial and temporal variation in the distribution and feeding of non‐predatory macroinvertebrates was investigated in a first‐order, acid stream in the Ashdown Forest, southern England.
• 2 Stonefly (Nemouridae) and chironomid (Orthocladiinae) larvae were abundant on the upper surfaces of mineral substrata of three sizes (small stones, large stones, bedrock). The density of larvae in each taxonomic group did not vary among substrata of different sizes, although strong seasonal variation existed.
• 3 Nemourids and chironomids (H. marcidus) collected from the upper surfaces of substrata exhibited generalist feeding habits, consuming algae (diatoms, coccoid and filamentous green algae), detritus (biofilm matrix material and fine particulate organic matter (FPOM)) and inorganic debris.
• 4 There was spatial variation in the gut contents of nemourids. The proportion of algae in the guts of larvae often increased with the size of the substratum from which they were collected. Strong temporal variation in the composition of the diet also existed. Nemourids ingested a large quantity of attached algae and biofilm matrix from the biofilm in spring and winter, but consumed loose FPOM and associated microflora in summer and autumn.
• 5 We conclude that, in this acid stream, the trophic linkage between algae and grazers is maintained by ‘detritivorous’ stonefly and chironomid species. The relationship between the feeding habits of these larvae and other life‐history attributes, such as mouthpart morphology and mobility, is discussed.

     

Herbivory in sun and shade

Observations of several plant species suggest that individuals incur greater herbivore damage in shaded than in nearby sunny areas. Two hypotheses are presented to explain this pattern of herbivory; a preliminary test of one suggests that plants growing in the sun are usually tastier, although eaten less, than those in the shade. The phenomenon has several implications for the nature of plant-herbivore interactions in terrestrial communities.     

Dynamic Precision Phenotyping Reveals Mechanism of Crop Tolerance to Root Herbivory

The western corn rootworm (WCR; Diabrotica virgifera virgifera LeConte) is a major pest of maize (Zea mays) that is well adapted to most crop management strategies. Breeding for tolerance is a promising alternative to combat WCR but is currently constrained by a lack of physiological understanding and phenotyping tools. We developed dynamic precision phenotyping approaches using ¹¹C with positron emission tomography, root autoradiography, and radiometabolite flux analysis to understand maize tolerance to WCR. Our results reveal that WCR attack induces specific patterns of lateral root growth that are associated with a shift in auxin biosynthesis from indole-3-pyruvic acid to indole-3-acetonitrile. WCR attack also increases transport of newly synthesized amino acids to the roots, including the accumulation of Gln. Finally, the regrowth zones of WCR-attacked roots show an increase in Gln turnover, which strongly correlates with the induction of indole-3-acetonitrile-dependent auxin biosynthesis. In summary, our findings identify local changes in the auxin biosynthesis flux network as a promising marker for induced WCR tolerance.The western corn rootworm (WCR; Diabrotica virgifera virgifera LeConte; Supplemental Fig. S1) is a voracious pest of maize (Zea mays). Larvae hatch in the soil during late spring and immediately begin feeding on the crop’s root system. Over time, active feeding can result in substantial root damage with significant loss of water and/or nutrient uptake, thus weakening plants (Flint-Garcia et al., 2009). Plants also become highly susceptible to lodging when major damage is inflicted upon the anchoring root system. Taken together, these effects can result in significant corn yield losses and management costs totaling between $650 million to $1 billion in the U.S. annually (Flint-Garcia et al., 2009; Gray et al., 2009).History reveals the enormous resilience and adaptability of this pest and just how quickly it can evolve to overcome management strategies. For example, resistance to application of chemical pesticides, including cyclodienes (benzene hexachloride, aldrin) and organophosphates (methyl parathion), was seen over just a 10-year period of their use in Nebraska’s cornfields during the 1950s and 1990s, respectively (Ball and Weekman, 1963; Meinke et al., 1998). Alternate management practices, including rotation of corn with other crops on a seasonal basis, was generally considered the best choice for management since 1909 (Levine et al., 2002). In east/central Illinois, 95% to 98% of cropland had adopted a management strategy using only soybean as the rotation crop. Unfortunately, the enthusiastic adoption of this strategy over a broad area combined with the efficacy of the technique created a strong selection that favored a less common D. v. virgifera phenotype with reduced egg laying fidelity to cornfields. Over time, natural selection afforded a strong reproductive advantage to females laying their eggs in soybean fields. Since the late 1990s, a strain of the western corn rootworm with resistance to crop rotation can be found in parts of Illinois, Indiana, and parts of bordering states (Gray et al., 2009; Levine et al., 2002).More recently, D. v. virgifera resistance to deployed genetically modified organisms has been reported. First introduced into the market to target this pest back in 2003, genetically altered Bt-maize expressing one or more proteins from the soil bacteria Bacillus thuringiensis provided enhanced plant defenses to larval feeding. When a vulnerable insect ate the Bt-containing plant, the protein became activated in its gut, forming a toxin that paralyzed the digestive system and caused it to stop feeding. Unfortunately, resistance began to show within three generations of selection (Meihls et al., 2008).An alternative strategy to reduce the negative impact of D. v. virgifera attack without triggering counter adaptations in the pest is plant tolerance, which relies on a plant’s capacity to maintain growth and yield even in the presence of substantial damage. While D. v. virgifera-tolerant maize germplasms exhibiting slight to moderate tolerances to D. v. virgifera have been reported (Flint-Garcia et al., 2009), more effective lines are needed. Unfortunately, we know very little about the underlying mechanisms for crop tolerance. Over the years, one resounding message has been that the physiological processes affected by herbivory should be better characterized before breeding tools can be leveraged in a rational way to generate improved varieties that maintain high yields under herbivore pressure (Riedell, 1990). Rational decision making in the breeding selection process requires rigorous phenotyping; however, present phenotyping tools tell us little about the plasticity of root systems, especially when it comes to understanding mechanisms for crop tolerance to attack belowground. It was recently suggested that the timing for allocation of newly fixed carbon resources as soluble sugars between leaves, stalks, and root systems, and their coordination with mobilization of other resources including amino acids, may play significant roles in determining the ability of maize plants to survive an attack by D. v. virgifera (Orians et al., 2011; Robert et al., 2014).In this work, our systematic evaluation of the physiological, metabolic, and genetic basis for root regrowth as a tolerance trait sheds new light on the regulation of the growth hormone auxin (indole-3-acetic acid [IAA]) and its role in this process. Radioactive decay of ¹¹C (β⁺ emitter, t_1/2 = 20.4 min), dynamic whole-plant positron emission tomography, root autoradiography, and radiometabolite flux analyses allowed us to map the transport, allocation and metabolism of carbon and nitrogen resources against genetic and radiolabeled biochemical markers including [¹¹C]IAA, [¹¹C]indole, [¹¹C]indole-3-acetonitrile ([¹¹C]IAN), [¹¹C]indole-3-acetamide ([¹¹C]IAM), and l-[5-¹¹C]Gln (Supplemental Fig. S2). Taken together, these tools enabled us for the first time, to our knowledge, to rigorously map out the auxin biosynthesis flux network at regional tissue levels and in turn provide new insights on auxin regulation and its coordination with the availability of a key amino acid, l-Gln. The developed phenotyping tools can now be employed for the rapid identification and selection of D. v. virgifera-tolerant maize germplasm.     

Differential Responses of Herbivores and Herbivory to Management in Temperate European Beech

Forest management not only affects biodiversity but also might alter ecosystem processes mediated by the organisms, i.e. herbivory the removal of plant biomass by plant-eating insects and other arthropod groups. Aiming at revealing general relationships between forest management and herbivory we investigated aboveground arthropod herbivory in 105 plots dominated by European beech in three different regions in Germany in the sun-exposed canopy of mature beech trees and on beech saplings in the understorey. We separately assessed damage by different guilds of herbivores, i.e. chewing, sucking and scraping herbivores, gall-forming insects and mites, and leaf-mining insects. We asked whether herbivory differs among different forest management regimes (unmanaged, uneven-aged managed, even-aged managed) and among age-classes within even-aged forests. We further tested for consistency of relationships between regions, strata and herbivore guilds. On average, almost 80% of beech leaves showed herbivory damage, and about 6% of leaf area was consumed. Chewing damage was most common, whereas leaf sucking and scraping damage were very rare. Damage was generally greater in the canopy than in the understorey, in particular for chewing and scraping damage, and the occurrence of mines. There was little difference in herbivory among differently managed forests and the effects of management on damage differed among regions, strata and damage types. Covariates such as wood volume, tree density and plant diversity weakly influenced herbivory, and effects differed between herbivory types. We conclude that despite of the relatively low number of species attacking beech; arthropod herbivory on beech is generally high. We further conclude that responses of herbivory to forest management are multifaceted and environmental factors such as forest structure variables affecting in particular microclimatic conditions are more likely to explain the variability in herbivory among beech forest plots.     

Developmentally-Regulated Virulence Factors of Trypanosoma cruzi and Their Relationship to Evasion of Host Defences1

ABSTRACT. Developmental preadaptation of virulent stages of Trypanosoma cruzi correlates with their ability to survive and establish infection in mammalian hosts. Infective trypomastigote stages must first preadapt to survival in the extracellular milieu and then to the rigors of establishing an intracellular infection. Selected phenotypic variations in evading host defences have been correlated with expression of stage-specific proteins or functions. Resistance of trypomastigotes to complement-mediated killing correlates with the presence of a stage-specific molecule that exhibits an analogous function to mammalian decay-accelerating factor, and with the presence of a neuraminidase/trans-sialidase that transfers sialic acid moieties to the parasite surface, thereby enabling it to avoid complement activation. Trypomastigotes enter cells by a mechanism that involves sorting of cell surface receptors and avoids eliciting a respiratory burst. Once within a membrane-bound vacuole, which undergoes acidification, the neuraminidase/trans-sialidase and an acid-active, transmembrane pore-forming protein are released by the parasite and are capable of acting together to accelerate rupture of the vacuolar membrane and the parasite's escape into the cytoplasm of the host cell. Escape from the parasitophorous vacuole allows virulent stages of T. cruzi to avoid compartmental, non-oxidative killing mechanisms such as degradation by lysosomal hydrolases.     

Deer Herbivory as an Ecological Constraint to Restoration of Degraded Riparian Corridors

Ungulate herbivory can impact riparian vegetation in several ways, such as by reducing vigor or reproductive output of mature plants, and through increased mortality of seedlings and saplings. Much work has focused on the effects of livestock grazing within riparian corridors, while few studies have addressed the influence of native ungulate herbivory on riparian vegetation. This study investigated the effect of deer herbivory on riparian regeneration along three streams with degraded riparian corridors in Mendocino County, California. We utilized existing stream restoration efforts by private landowners and natural resource agencies to compare six deer exclosures with six upstream control plots. Livestock were excluded from both exclosure and control plots. Three of the deer exclosures had been in place for 15 years, one for 6 years, and two for 4 years. The abundance and size distribution of woody riparian plant species such as Salix exigua, S. laevigata, S. lasiolepis, Alnus rhombifolia, and Fraxinus latifolia were quantified for each exclosure and control plot. The mean density of saplings in deer exclosures was 0.49 ± 0.15/m², while the mean density of saplings in control plots was 0.05 ± 0.02/m². Within exclosures, 35% of saplings were less than 1 m and 65% were greater than 1 m; within control plots, 97% of saplings were less than 1 m in height. The fact that little regeneration had occurred in control plots suggests that deer herbivory can substantially reduce the rate of recovery of woody riparian species within degraded riparian corridors. Exclusionary fencing has demonstrated promising results for riparian restoration in a region with intense deer herbivory.     

Defences against oxidative stress in vibrios associated with corals

Bacteria colonizing healthy coral tissue may produce enzymes capable of overcoming the toxic effects of reactive oxygen species, including superoxide dismutase (SOD) and catalase. Significant differences in the activities of these enzymes were observed in cultures of Vibrio campbellii, Vibrio coralliilyticus, Vibrio harveyi, Vibrio mediterranei, Vibrio pelagius, Vibrio rotiferanus, Vibrio tasmaniensis, and Photobacterium eurosenbergii isolated from healthy, bleached or necrotic tropical and cold water corals. Levels of SOD in exponential phase cultures of V. coralliilyticus grown at 28 degrees C were only slightly higher than those grown at 16 degrees C whereas the levels in stationary phase cultures at 28 degrees C were 7.3 x higher than those at 16 degrees C. The increase in catalase activity of V. coralliilyticus and V. harveyi upon entry to stationary phase conferred protection against killing by oxidative stress. Increased temperature affected up-regulation of enzymes in stationary phase cultures, but pretreatment of cultures with hydrogen peroxide had no significant effect on induction of catalase or SOD. The increased activities appear to be due to up-regulation of gene expression rather than induction of different forms of the enzymes. We suggest that SOD and catalase are unlikely to be major factors in the virulence of these bacteria for corals and that their main function may be to protect against endogenous superoxide.