Early spring defoliation,secondary leaf flush,and leafminer outbreaks on American holly

Summary Adults of Phytomyza ilicicola (Diptera: Agromyzidae), a univoltine specialist leafminer, emerge in close synchrony with leaf flush of American holly and feed on and oviposit in soft, partially expanded leaves. Early spring defoliation, such as commonly results from freezing injury to young shoots, is followed several weeks later by a second flush of young leaves from lateral buds. We simulated this phenomenon by manually defoliating whole small trees and individual shoots of large trees to test the hypothesis that freezing injury can encourage leafminer outbreaks by inducing an abundance of soft, protein rich young leaves late in the adult activity period, when availability of vulnerable leaves becomes limited. Defoliation of small trees one or two weeks after bud break resulted in six- to 13-fold increases in the incidence of feeding punctures and larval mines on second flush leaves as compared with densities on original young leaves of control trees. Similarly, we induced significant increases in feeding punctures and larval mines on second flush leaves of individual defoliated shoots, although leaves that did not open until after the flight period escaped this injury. These observations underscore the capability of adult female P. ilicicola to locate and exploit a small number of phenologically available leaves among many hundreds of older leaves on the same tree. By altering the phenology of leaf flush, certain kinds of environmental stress may predispose perennial plants to outbreaks of early season folivores that restrict their feeding or oviposition to very young leaves.     

Do holly leaf spines really deter herbivory?

Summary Although spinose teeth of holly leaves have been widely cited as an example of a physical defense against herbivores, this assumption is based largely on circumstantial evidence and on general misinterpretation of a single, earlier experiment. We studied the response of third and fifth instar larvae of the fall webworm, Hyphantria cunea Drury, a generalist, edge-feeding caterpillar, to intact American holly leaves and to leaves that had been modified by blunting the spines, by removing sections of leaf margin between the spines, or by removing the entire leaf margin. The results suggest that the thick glabrous cuticle and tough leaf margin of Ilex opaca are more important than the spinose teeth in deterring edge-feeding caterpillars. Microscopic examination of mature leaves revealed that the epidermis is thickened at the leaf margin, and that the leaf is cirucumscribed by a pair of fibrous veins. In simple choice tests neither domesticated rabbits nor captive whitetailed deer discriminated between spinescent holly foliage and foliage from which spines were removed. Nevertheless, we found little evidence of herbivory by mammals in the field, either on small experimental trees or in the forest understory. While it is possible that spinose teeth contribute to defense by reducing acceptibility of holly relative to other palatable plant species, we suggest that the high concentrations of saponins and poor nutritional quality of holly foliage may be more important than spines in deterring vertebrate herbivores. The degree of leaf spinescence and herbivory was compared at different heights with the tree canopy to test the prediction that lower leaves should be more spinescent as a deterrent to browsers. Leaves on lower branches of mature forest trees were slightly more spinescent than were upper leaves, and juvenile trees were slightly more spinescent than were mature trees. However, there was no relationship between degree of spinescence and feeding damage. The greater spinescence of holly leaves low in the canopy is probably an ontogenetic phenomenon rather than a facultative defense against browsers.The investigation reported in this paper (No. 87-7-8-77) is in connection with a project of the Kentucky Agricultural Experiment Station and is published with the approval of the Dirctor     

Interspecific variation in the defensive responses of obligate plant-ants: experimental tests and consequences for herbivory

The aggressive behavior of ants that protect plants from herbivores in exchange for rewards such as shelter or food is thought to be an important form of biotic defense against herbivory, particularly in tropical systems. To date, however, no one has compared the defensive responses of different ant taxa associated with the same plant species, and attempted to relate these differences to longer-term efficacy of ant defense. We used experimental cues associated with herbivory—physical damage and extracts of chemical volatiles from leaf tissue—to compare the aggressive responses of two ant species obligately associated with the Amazonian myrmecophyte Tococa bullifera (Melastomataceae). We also conducted a colony removal experiment to quantify the level of resistance from herbivores provided to plants by each ant species. Our experiments demonstrate that some cues eliciting a strong response from one ant species elicited no response by the other. For cues that do elicit responses, the magnitude of these responses can vary interspecifically. These patterns were consistent with the level of resistance provided from herbivores to plants. The colony removal experiment showed that both ant species defend plants from herbivores: however, herbivory was higher on plants colonized by the less aggressive ant species. Our results add to the growing body of literature indicating defensive ant responses are stimulated by cues associated with herbivory. However, they also suggest the local and regional variation in the composition of potential partner taxa could influence the ecology and evolution of defensive mutualisms in ways that have previously remained unexplored.     

Secretory avocado idioblast oil cells: evidence of their defensive role against a non-adapted insect herbivore

We tested the hypothesis that avocado idioblast oil cells play a defensive role against herbivorous insects. Toxicities of the intact avocado idioblast oil cells and the extracted idioblast oil were compared for three insect herbivores. Spodoptera exigua (Hübner) larvae are generalists that do not feed on avocados. By contrast, Sabulodes aegrotata (Guenée) and Pseudoplusia includens (Walker) larvae are generalist herbivores that readily feed on avocados. All bioassays were performed at a naturally occurring concentration of idioblast oil cells (2% w/w). Choice experiments showed that S. exigua larvae avoided diet treated with avocado idioblast oil cells and consume more control than treated diet. In contrast, idioblast oil cells had no significant antifeedant effects on the adapted S. aegrotata and P. includens larvae. Subsequent experiments designed to assess resistance mechanisms separated pre-ingestive (behavioral) and post-ingestive (physiological) effects of the avocado idioblast oil cells, and the extracted idioblast oil, on the two adapted herbivores. Post-ingestive adaptation was the mechanism that allows feeding. Because the impact of the avocado idioblast oil cells was greatest on the performance of non-adapted S. exigua, additional experiments determined that larvae fed diet containing the oil cells had higher mortality and reduced larval growth compared to controls. Developmental times were significantly prolonged for the survivors. Thus, increased mortality, reduced developmental rates, and antifeedant activity in the non-adapted insect indicate that defense against non-adapted herbivores may be an important function of idioblast cells in avocados.     

Aphid-ant interaction reduces chrysomelid herbivory in a cottonwood hybrid zone

In a cottonwood (Populus) hybrid zone, Chaitophorus aphids attract aphid-tending ants which subsequently reduce herbivory by the leaf-feeding beetle, Chrysomela confluens. Observations and experimental manipulations of aphids and beetle larvae on immature cottonwood trees demonstrated that: 1) via their recruitment of ants, aphids reduced numbers of beetle eggs and larvae on the host; 2) these interactions occurred within a few days of the host being colonized by aphids; and 3) although aphid colonies were ephemeral, their presence resulted in a 2-fold reduction in beetle herbivory. The aphid-ant interaction is most important in the hybrid zone where 93% of the beetle population is concentrated (for reasons unrelated to aphids and ants). Because beetle defoliation of immature trees is high (ca. 25%), the indirect effect of aphids in reducing herbivory is likely more beneficial to trees in the hybrid zone than in adjacent pure zones where beetle herbivory is virtually absent. Tree genotype likely affects the impact of the aphid-ant interaction on trees within the hybrid zone, since levels of herbivory differ between sympatric Fremont and hybrid cottonwoods.     

Arbuscular mycorrhizal fungal species suppress inducible plant responses and alter defensive strategies following herbivory

In a greenhouse experiment using Plantago lanceolata, plants grown with different arbuscular mycorrhizal (AM) fungal species differed in constitutive levels of chemical defense depending on the species of AM fungi with which they were associated. AM fungal inoculation also modified the induced chemical response following herbivory by the specialist lepidopoteran herbivore Junonia coenia, and fungal species varied in how they affected induced responses. On average, inoculation with AM fungi substantially reduced the induced chemical response as compared with sterile controls, and inoculation with a mixture of AM fungi suppressed the induced response of P. lanceolata to herbivory. These results suggest that AM fungi can exert controlling influence over plant defensive phenotypes, and a portion of the substantial variation among experimental tests of induced chemical responses may be attributable to AM fungi.     

Molecular evidence for hybridization ofIlex x wandoensis (Aquifoliaceae) by RAPD analysis

Based on the presence of intermediate morphological characters, such as serrated leaf margins and flower structures,Ilex x wandoensis was initially described as a putative natural hybrid betweenI. cornuta andI. Integra, and was formally described as a new hybrid species,I. x wandoensis C. F. Mill., and M. Kim. However, using molecular markers generated via random amplified polymorphic DNA (RAPD), we have now discovered hybridization in populations of theI. x wandoensis complex collected from Wando and Jeju Islands, Korea. Marker bands of the putative parent taxa also were found in some populations ofI. x wandoensis, confirming its hybrid origin. Morphological variability within and among those populations was confirmed by model-based clustering methods, using multilocus genotype data. Phenograms generated from RAPD bands indicated that some accessions ofI. x wandoensis clustered with one of the parental species. This implied the occurrence of hybridization and recurring backcrosses of the hybrid to both parents, resulting in various hybrid derivatives because of the segregation and recombination of traits.Ilex x wandoensis was more closely related toI. cornuta than toI. integra suggesting that it backcrossed more with the former than with the latter.     

Optimal central-place foraging by beavers: Tree-size selection in relation to defensive chemicals of quaking aspen

Summary At a newly occupied pond, beavers preferentially felled aspen smaller than 7.5 cm in diameter and selected against larger size classes. After one year of cutting, 10% of the aspen had been cut and 14% of the living aspen exhibited the juvenile growth form. A phenolic compound which may act as a deterrent to beavers was found in low concentrations in aspen bark, and there was no significant regression of relative concentration of this compound on tree diameter. At a pond which had been intermittently occupied by beavers for over 20 years, beavers selected against aspen smaller than 4.5 cm in diameter, and selected in favor of aspen larger than 19.5 cm in diameter. After more than 28 years of cutting at this site, 51% of the aspen had been cut and 49% of the living aspen were juvenileform. The phenolic compound was found in significantly higher concentrations in aspen bark than at the newly occupied site, and there was a significant negative regression of relative concentration on tree diameter. The results of this study show that responses to browsing by trees place constraints on the predictive value of standard energy-based optimal foraging models, and limitations on the use of such models. Future models should attempt to account for inducible responses of plants to damage and increases in concentrations of secondary metabolites through time.     

Members only: induced systemic resistance to herbivory in a clonal plant network

The stoloniferous herb Trifolium repens was used to study the expression of induced systemic resistance (ISR) to the generalist caterpillar Spodoptera exigua in interconnected ramets of clonal fragments. The ISR was assessed as caterpillar preference in dual choice tests between control and systemically induced plants. The ISR was detected in young ramets, after inducing older sibling ramets on the same stolon by a controlled herbivore attack. However, older ramets did not receive a defense induction signal from younger ramets unless the predominant phloem flow was reversed by means of basal shading. This provides evidence for the notion that in T. repens the clone-internal expression of ISR is coupled to phloem transport and follows source–sink gradients. The inducibility of the genotypes was not linked to their constitutive ability to produce cyanide, implying the absence of a trade-off between these two defense traits. To our knowledge, this is the first study that explores ISR to herbivory in the context of physiological integration in potentially extensive clonal plant networks.     

Pathogen-activated induced resistance of cucumber: response of arthropod herbivores to systemically protected leaves

Summary Restricted (non-systemic) inoculation of cucurbits, green bean, tobacco, and other plants with certain viruses, bacteria, or fungi has been shown to induce persistent, systemic resistance to a wide range of diseases caused by diverse pathogens. The non-specificity of this response has fueled speculation that it may also affect plant suitability for arthropod herbivores, and there is limited evidence, mainly from work with tobacco, which suggests that this may indeed occur. Young cucumber plants were immunized by restricted infection of a lower leaf with tobacco necrosis virus (TNV), and upper leaves were later challenged with anthracnose fungus, Colletotrichum lagenarium, to confirm induction of systemic resistance to a different pathogen. The response of arthropod herbivores was simultaneously measured on non-infected, systemically protected leaves of the same plants. As has been reported before, immunization with TNV gave a high degree of protection from C. lagenarium, reducing the number of lesions and the area of fungal necrosis by 65–93%. However, there was no systemic effect on population growth of twospotted spider mites, Tetranychus urticae Koch, on upper leaves, nor did restricted TNV infection of leaf tissue on one side of the mid-vein systemically affect mite performance on the opposite, virus-free side of the leaf. Similarly, there were no effects on growth rate, pupal weight, or survival when fall armyworm larvae were reared on systemically protected leaves from induced plants. In free-choice tests, greenhouse whiteflies oviposited indiscriminately on induced and control plants. Feeding preference of fall armyworms was variable, but striped cucumber beetles consistently fed more on induced than on control plants. There was no increase in levels of cucurbitacins, however, in systemically-protected foliage of induced plants. These findings indicate that pathogen-activated induced resistance of cucumber is unlikely to provide significant protection from herbivory. The mechanisms and specificity of induced resistance in cucurbits apparently differ in response to induction by pathogens or herbivores.     

Constraints on an induced defense: the role of leaf area

Folivory results in both leaf damage and the loss of photosynthetic capacity. Leaf damage activates the production of induced defenses, but diminished photosynthetic capacity resulting from lost leaf area may impair a plant's ability to respond defensively. Because damage-induced nicotine production in Nicotiana sylvestris (Solanaceae) is an energy-demanding, active process, we predicted that the loss of leaf area would constrain this plant's ability to produce an induced nicotine pool. We examined our prediction in an experiment which combined leaf puncture and removal protocols, quantified induced nicotine pools on a whole-plant basis, and accounted for losses in the nicotine pool due to removed leaves and lost growth potential. In contrast to our prediction, leaf removal did little to diminish the growth-corrected estimates of the induced nicotine pool in plants with sufficient damage cue; only when plants had lost 88% or more of their leaf area did the induced nicotine pool decline significantly. These results demonstrate that the induced defense is relatively insensitive to current photosynthetic capacity. In contrast to the size of the induced nicotine pool, the concentration of nicotine in the remaining shoot tissues continued to rise as puncture damage increased over all defoliation levels tested. The mechanisms responsible for inducible nicotine production may have evolved as a means of providing shoot tissues with protection that is proportional to the amount of damage incurred while keeping production costs constant for the remaining plant parts.     

Bigger ant colonies reduce herbivory and herbivore residence time on leaves of an ant-plant: Azteca muelleri vs. Coelomera ruficornis on Cecropia pachystachya

Summary The effect of defence force size in colonies of the ant Azteca muelleri on the time spent to localize, attack and expel the specialized herbivorous beetle Coelomera ruficornis from Cecropia pachystachya bushes was studied in an area of Atlantic forest in northeastern Brazil. Our results show that Azteca muelleri expel Coelomera ruficornis from Cecropia pachystachya and that the number of ants leaving a colony (defence force size) is negatively correlated with the residence time of an adult beetle on the plant. Colonies with larger defence forces recruited larger numbers of ants, resulting in faster herbivore discovery (r ²=0.80; n=17; P<0.001) and reduced herbivore residence time on a leaf (r ²=0.79 n=23; P<0.001) before being driven off by the ants. We also found a negative and significant relationship between herbivore damage on leaves and ant colony size (r ²=0.28; n=17; P<0.05). We conclude that larger colonies have more individuals available to patrol a plant and recruit defenders toward herbivores. This reduces the time spent to locate and expel susceptible herbivores from the plant. Since the plant probably benefits from reduced herbivory and the plant provides food for the ants, the association between Azteca muelleri and Cecropia pachystachya appears mutualistic.     

Soldier bug predation on swallowtail caterpillars (Lepidoptera: Papilionidae): Circumvention of defensive chemistry

All larvae in the family Papilionidae possess osmeteria, eversible glands that release defensive chemicals upon disturbance. These chemicals have been shown to be repellent to ants, mantids, and other predaceous arthropods with chewing mouthparts. In this study, we demonstrate that the pentatomid Podisus maculiventris,an important predator of the black swallowtail Papilio polyxenes,is capable of piercing the body wall of the caterpillar and consuming body fluids without eliciting osmeterial eversion. Tenebrio molitorlarvae coated with osmeterial secretions of Papilio polyxeneswere universally rejected by Podisus maculiventris,suggesting that the predator is in fact repelled by the swallowtail defensive chemistry. By circumventing the defensive behavior of the caterpillar, the pentatomid is able to utilize an otherwise chemically unsuitable prey species.     

Perfect is best: low leaf fluctuating asymmetry reduces herbivory by leaf miners

Fluctuating asymmetry (FA) represents small, random variation from symmetry and can be used as an indicator of plant susceptibility to herbivory. We investigated the effects of FA of two oak species, Quercus laevis and Q. geminata, and the responses of three herbivore guilds: leaf miners, gallers, and chewers. To examine differences in FA and herbivory between individuals, 40 leaves from each tree were collected, and FA indices were calculated. To examine differences in FA and herbivory within-individuals, we sampled pairs of mined and unmined leaves for asymmetry measurements. Differences in growth of leaf miners between leaf types were determined by tracing 50 mines of each species on symmetric leaves and asymmetric leaves. Asymmetric leaves contained significantly lower concentrations of tannins and higher concentrations of nitrogen than symmetric leaves for both plant species. Both frequency of asymmetric leaves on plants and levels of asymmetry positively influenced the abundance of Brachys, Stilbosis and other leaf miners, but no significant relationship between asymmetry and herbivory was observed for Acrocercops. Brachys and Stilbosis mines were smaller on asymmetric leaves, but differences in mine survivorship between symmetric and asymmetric leaves were observed only for Stilbosis mines. This study indicated that leaf miners might use leaf FA as a cue to plant quality, although differential survivorship among leaf types was not observed for all species studied. Reasons for the different results between guilds are discussed.     

The defensive role of trichomes in black medick (Medicago lupulina,Fabaceae)

Simple and gland-tipped trichomes occur in varying proportions in the cosmopolitan weed black medick (Medicago lupulina L.). Densities of both simple and glandtipped trichomes were separately negatively correlated with the amount of damage to leaves in herbarium collections from throughout the world. This indicates that both types of trichomes are protective against herbivores, but suggests that the two types of hair differ in effectiveness against different species. Since an average of less than 1% herbivore consumption of the foliage of black medick was observed, it is remarkable that adaptive value of the trichomes was demonstrable. In a greenhouse experiment, both gland-tipped and simple hairs provided considerable resistance against whitefly oviposition. For both herbarium collections and the greenhouse experiment, conspicuously fewer gland-tipped hairs than simple hairs were required to achieve a given level of resistance to insects. Low trichome density considerably reduced damage in comparison to the absence of trichomes, while increasing density was progressively less efficient, suggesting that moderate pubescence is the most desirable strategy.     

Indirect versus direct effects of grasses on growth of a cactus (Opuntia fragilis): insect herbivory versus competition

Variation in plant performance between microhabitats is usually attributed to direct mechanisms, such as plant physiological tolerances or competitive interactions. However, indirect mechanisms, such as differences in herbivore pressure mediated by microhabitat differences, could create the same pattern of variation. In this study, we investigated the effect of insect herbivore pressure on the growth of the grassland cactus Opuntia fragilis under different regimes of grassland canopy cover. Our purpose was to establish the extent to which canopy cover plays a direct, competitive role versus an indirect, mediatory role in cactus growth. We manipulated aboveground microhabitat, specifically the cover of adjacent grasses. The three treatments were: (1) open canopy, with grass pinned down away from the cactus; (2) shaded canopy, with a partial mesh cage staked over the cactus; and (3) ambient grass canopy. We measured seasonal plant growth and recorded changes in insect herbivore occurrence and damage in relation to cover. Cactus growth, defined as the change in number of live cladodes, was higher in the open than under either treatment where the plant was more shaded (P<0.05). However, allocation to new growth, measured as the proportion of new segments (cladodes) in a patch, did not differ among cover treatments. Thus, the hypothesis that physiological constraints, or competition for light, limited cactus performance in grass is rejected. Instead, we found that both cladode mortality, caused by the larvae of a cactus moth borer (Melitara dentata), and occurrence of the moth were lower in the open microhabitat than in either shaded microhabitat. Thus, higher net growth in the open, unshaded treatment, rather than representing a release from competition for light with grasses, was better explained as an indirect effect of grass cover on the activity and impact of the cactus moth. These results show that indirect effects can lead to a misinterpretation of experimental data on direct effects. These data also contribute to an improved understanding of mixed results in the biological control of weedy cacti. Clearly, future evaluations of the relative importance of physiology, competition, and insect herbivory in plant performance must be environmentally explicit.     

Plant growth inhibitory activity of Ophiopogon japonicus Ker-Gawler and role of phenolic acids and their analogues: a comparative study

Allelopathic potential of Ophiopogon japonicus was investigated. The methanolic extract of O. japonicus roots strongly inhibited root and hypocotyls growth of lettuce. Sequential partitioning of the methanol extract with organic solvents showed that the diethyl ether and n-butanol extract possess strong plant growth inhibitory activities. The allelopathic constituents of the diethyl ether extract were isolated and identified as salicylic acid and p-hydroxybenzoic acid by NMR spectroscopy. Both of these phenolic acids were found in the aqueous extracts of leaves as well. The concentration of salicylic acid in roots and leaves were estimated as 0.011 and 0.02%, respectively, and it inhibited the root and shoot of tested plants by 50% even at less than 3 ppm. The p-hydroxybenzoic acid on the other hand was in less abundance (0.005%) and inhibited the plant growth to a lesser extent. The biological activity of commercially available O-methyl derivatives of these phenolic acids was also determined to establish structure–activity relationship. Among these, salicylic acid was found to be the most active one. These results suggest that Ophiopogon japonicus produces plant growth inhibitors, which are responsible for its potential allelopathic activity.     

Agroinfiltration of grapevine leaves for fast transient assays of gene expression and for long-term production of stable transformed cells

Agrobacterium-mediated transient assays for the analysis of gene function are used as alternatives to genetic complementation and stable plant transformation. Although such assays are routinely performed in several plant species, they have not yet been successfully applied to grapevines. We explored genetic background diversity of grapevine cultivars and performed agroinfiltration into in vitro cultured plants. By combining different genotypes and physiological conditions, we developed a protocol for efficient transient transformations of selected grapevine cultivars. Among the four cultivars analyzed, Sugraone and Aleatico exhibited high levels of transient transformation. Transient expression occurred in the majority of cells within the infiltrated tissue several days after agroinfiltration and, in a few cases, it later spread to a larger portion of the leaf. Three laboratory strains of Agrobacterium tumefaciens with different virulence levels were used for agroinfiltration assays on grapevine plants. This method promises to be a powerful tool to perform subcellular localization analyses. Grapevine leaf tissues were transformed with fluorescent markers targeted to cytoplasm (free GFP and mRFP1), endoplasmatic reticulum (GFP::HDEL), chloroplast (GAPA1::YFP) and mitochondria (β::GFP). Confocal microscope analyses demonstrated that these subcellular compartments could be easily visualized in grapevine leaf cells. In addition, from leaves of the Sugraone cultivar agroinfiltrated with endoplasmic reticulum-targeted GFP-construct, stable transformed cells were obtained that show the opportunity to convert a transiently transformed leaf tissue into a stably transformed cell line. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Effects of light and nutrient availability on the growth,allocation, carbon/nitrogen balance,phenolic chemistry,and resistance to herbivory of two freshwater macrophytes

Phenotypic responses of Potamogeton amplifolius and Nuphar advena to different light (7% and 35% of surface irradiance) and nutrient environments were assessed with field manipulation experiments. Higher light and nutrient availability enhanced the growth of P. amplifolius by 154% and 255%, respectively. Additionally, biomass was allocated differently depending on the resource: high light availability resulted in a higher root/shoot ratio, whereas high nutrient availability resulted in a lower root/shoot ratio. Low light availability and high nutrient availability increased the nitrogen content of leaf tissue by 53% and 40% respectively, resulting in a 37% and 31% decrease in the C/N ratio. Root nitrogen content was also increased by low light and high nutrient availability, by 50% (P=0.0807) and 77% respectively, resulting in a 20% and 40% decrease in root C/N ratio. Leaf phenolics were significantly increased 72% by high light and 31% by high nutrient availability, but root phenolic concentrations were not altered significantly. None of these changes in tissue constituents resulted in altered palatability to crayfish. N. advena was killed by the same high nutrient treatment that stimulated growth in P. amplifolius, preventing assessment of phenotypic responses to nutrient availability. However, high light availability increased overall growth by 24%, but this was mainly due to increased growth of the rhizome (increased 100%), resulting in a higher root/shoot ratio. High light tended to increase the production of floating leaves (P=0.09) and significantly decreased the production of submersed leaves. High light availability decreased the nitrogen content by 15% and 25% and increased the phenolic concentration by 88% and 255% in floating and submersed leaves, respectively. These differences in leaf traits did not result in detectable differences in damage by herbivores.     

Pleiotropic effect of the insertion of the <Emphasis Type="Italic">Agrobacterium rhizogenes rolD</Emphasis> gene in tomato (<Emphasis Type="Italic">Lycopersicon esculentum</Emphasis> Mill.)

The Agrobacterium rhizogenes rolD gene, coding for an ornithine cyclodeaminase involved in the biosynthesis of proline from ornithine, has been inserted in Lycopersicon esculentum cv Tondino with the aim of studying its effects on plant morphological characters including pathogen defense response. The analysis of plants transgenic for rolD did not show major morphological modifications. First generation transgenic plants however were found to flower earlier, and showed an increased number of inflorescences and higher fruit yield. Transformed plants were also analysed for parameters linked to pathogen defense response, i.e. ion leakage in the presence of the toxin produced by the fungus Fusarium oxysporum f. sp. lycopersici, and expression of the pathogenesis-related PR-1 gene. All the plants harbouring the rolD gene were shown to be more tolerant to the toxin in ion leakage experiments, with respect to the untransformed regenerated controls and the cv Tondino. PR-1 gene expression was quantitated by means of real-time PCR both at the basal level and after treatment with salicylic acid, an inducer of Systemic Acquired Resistance. In both cases the amount of PR-1 mRNA was higher in the transgenic plants. It seems therefore that the transformation of tomato plants with rolD could lead to an increased competence for defense response, as shown by toxin tolerance and increased expression of the Systemic Acquired Resistance marker gene PR-1. The results are finally discussed in view of their possible economic relevance.Communicated by G. Wenzel