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The defenses of higher plants against a variety of biotic and abiotic stress agents can be grouped into two categories: Preformed and Induced. Induced defensive responses by the plant, or "alarms" -- the subject of the present review -- can be localized or systemic. Certain alarms, especially those which are induced by necrotizing pathogens, are protective against a wide variety of biotic stress agents. Other responses appear protective but the degree of host plant involvement is unclear. Finally, there are a few induced plant responses which, although protective, do not easily fit our criteria of an "alarm." Oligosaccharins may be involved as signals in both anti-herbivore and anti-pathogen alarm systems. Other specific components of plant alarms appear to be induced by only one type of stress agent. The specificity of protection, mechanisms of various alarms and comparisons between them are presented.

Populations of feral donkeys, Equus asinus, in northern Australia are limited by density-dependent mortality during the first 6 mo of individuals' lives. A female's ability to raise her offspring successfully increases with age and is dependent on her maintaining high levels of stored mineral nutrients (calcium, phosphorus, and sodium). A low proportion of offspring was successfully raised in a population at carrying capacity where females had low levels of stored mineral nutrients. Females in a growing population that had been reduced to 45% of carrying capacity had high levels of stored minerals and successfully raised a higher proportion of offspring. Females in the population at carrying capacity ingested a species-poor diet (predominantly a grass, Sehima nervosa) containing low levels of nitrogen and mineral nutrients, and high levels of crude fibre. Females in the growing population ingested a more species-rich diet with higher levels of nitrogen and minerals, and less crude fibre. Levels of calcium, phosphorus, and potassium in the faeces of females in the population at carrying capacity were higher than those in the food ingested. The reverse was true for females in the growing population. At the time of sampling (early dry season), 17% of females in the population at carrying capacity and 0% of females in the growing population were eating or had recently eaten clay. Increased salivary secretion and gastrointestinal irritation caused by high-fibre diets (even when lubricated by clay) may be responsible for mineral depletion of females at carrying capacity, and hence population limitation through the inability of females to raise offspring.

The escape hypothesis predicts that isolated individuals of a given plant species will escape their specialized herbivores and so survive better than individuals growing in clumps. This phenomenon could promote plant coexistence. I examined assumptions underlying the hypothesis by considering insect herbivores associated with neotropical Passiflora vines. I found that the hypothesis was probably of only limited value because plants were attacked by a range of herbivore species, most of which were not really specialized. Furthermore, some oligophagous herbivore species colonized isolated P. pittieri more readily than they colonized plants that were near to other P. pittieri plants: isolated plants did not escape herbivory. Because of the activities of herbivores that feed on > 1 plant species, [greater than or equal to] 2 plant species that share herbivores may not show independent patterns of herbivory or independent local distributions.

The disease resistance observed in plants was attributed to the two complementary genes called avirulence (Avr) gene and resistance gene in the host. The biochemical explanation for the hypothesis was a receptor-ligand model wherein plants stimulated defense mechanisms upon R-protein-mediated recognition of pathogen-derived Avr elements. It was determined that the nucleotide-binding leucine-rich-repeat proteins of plants specified the gene-for-gene resistance to animals, bacterial, viral and fungal pathogens and comprised a comprehensive pathogen-detection system.

Compost and compost teas can activate disease resistance genes and induce systemic resistance to diseases in plants. This had been established by Ohio State University researchers. Findings indicated that when plants were treated with plant teas, root rot protection was induced. When plants were germinated in compost, root rot protection was even greater than when the plants were treated with compost teas alone.

Bananas and plantains are found in abundance in tropical regions. Triploid bananas and plantains are made from hybridization between the diploid Musa species M. acuminata with the A genome and M. balbisiana with the B genome. Epicuticular wax is found on the leaf surface of some bananas and plantains and is the best defense against black sigatoka leaf spot disease. An attempt to breed for the epicuticular wax determined that phenotypic recurrent selection was the best hybridization method.

Because stalk lodging is an important problem in maize (Zea mays L.) production, improved methods of breeding for stalk lodging resistance are important. The objective of research reported in this paper was to determine the influence of six cycles of selection for rind puncture resistance, measured just prior to anthesis, on postanthesis rind puncture resistance, grain yield, grain moisture, and stalk rot reaction in two maize synthetics, RSL(MPEN) and RSSSC(MPEN), which represent two different heterotic groups. Response to selection was measured in the populations, per se, testcrosses of each cycle of each population to an appropriate inbred tester, and in crosses between cycles of the two synthetics. In all cases, selection significantly increased postanthesis rind puncture resistance. Stalk lodging was significantly reduced for all cases except RSL(MPEN) per se. Stalk rot incidence was significantly reduced in RSSSC(MPEN) per se. Few adverse effects were noted for other traits. Measuring rind puncture resistance just before pollination allowed effective improvement in stalk lodging resistance using a selection scheme which requires only one generation per cycle.

A study of the inheritance of resistance to Meloidogyne incognita in soybean for a Forrest x Bossier cross by monitoring the F1, F2 and F3 generations for gall development indicates an equal number of galls formed on the reciprocal crosses of Forrest and Bossier F1 plants that is similar to the midparent mean, suggesting that susceptibility and resistance are recessive and that the inheritance is not maternal. F2 plants with galls below the mean Forrest 95% confidence interval (CI) upper limit form the resistant group while those with gall numbers above the Bossier CI lower limit form the susceptible group. F3 families exhibit low within-family divergence.

Biochemical markers linked to pest resistance genes would be useful to plant breeders. This study was conducted to determine if biochemical markers could be found closely linked to genes conferring leaf rust resistance in wheat (Triticum aestivum L.). Near-isogenic lines have been developed in 'Thatcher' for a number of genes, Lr1 to Lr31, conferring resistance to leaf rust, caused by Puccinia recondita Rob. ex Desm. Fifteen of these Thatcher near-isogenic lines were evaluated for the isozyme endopeptidase. Two of the near-isogenic lines (Lr1 and Lr19) had endopeptidase phenotypes that differed from the Thatcher backcross parent. The endopeptidase phenotype observed in the near-isogenic line possessing Lr19 was completely different from any other phenotype normally observed in wheat. The leaf rust resistant Thatcher near-isogenic line possessing Lr19 was crossed with the leaf rust susceptible cultivar 'GA 100' (PI538257). The 59 F(sub 2) progeny were grown in the greenhouse and evaluated for leaf rust resistance and endopeptidase phenotype. All F(sub 2) progeny exhibiting the Lr19 endopeptidase phenotype were resistant to leaf rust. Maximum likelihood estimation, using the F(sub 2) phenotypic data, indicated a very tight linkage with a recombination value of 0.01 + or - 0.009 map units for the Lr19 and Ep-D1d loci. In addition, no recombinant F(sub 2) genotypes were found among the 30 F(sub 2) plants for which F(sub 3) progeny were obtained. Thus, the gene Lr19, which confers resistance to leaf rust, and the gene Ep-D1d which encodes endopeptidase phenotype EP-D1d, are linked. This linkage will enable rapid detection of seedlings possessing Lr19 in wheat improvement programs.

Irenolone and emenolone, the two phenalenone-type phytoalexins, are obtained from Musa paradisiaca varieties treated with aqueous kanamycin, tween by drop-diffusion method. Treatment with ethyl acetate results in the extraction of the phytoalexins. Irenolone that has the molecular formula C19H12O3 is a major constituent of inflected leaves. Its proton NMR spectra illustrates the presence of the aromatic spin systems AMX, AB and AA'BB'. Its structure is characterized by a phenalenone system with a hydroxyphenyl substituted ring at C-4. The proton NMR spectra of emenolone is analogous to that of irenolone.

Two parasitic weeds, Striga gesnerioides (Willd.) Vatke and Alectra vogelii Benth., cause heavy yield losses in cowpea (Vigna unguiculata (L.) Walp.) throughout the semiarid regions of sub-Saharan Africa. Several sources of resistance have been identified, including B 301, a landrace from Botswana that has shown complete resistance to both striga and alectra. Striga resistance in B 301 is conditioned by a single dominant gene. Our objective was to determine the inheritance of alectra resistance. B 301 was crossed to three susceptible cultivars (IT84S-2246-4, SUVITA-2, and IT82D-849) and F1, F2, and backcross seeds were obtained. The reaction of parental, F1, F2, and backcross plants to alectra infestation was evaluated. Test populations were planted in plastic pots of 13 cm diam. filled with a mixture of sand and soil and approximately equal to 800 alectra seeds. The roots of each plant were examined for alectra attachment 10 wk after planting, and the number of resistant and susceptible plants in each population were recorded. All plants of B 301 and backcrosses to B 301 were completely free from alectra. All plants of IT84S-2246-4, SUVITA-2, and IT82D-849 were susceptible. All F2 populations segregated 15 resistant : 1 susceptible, and backcrosses to the susceptible parents segregated 3 resistant : 1 susceptible indicating duplicate dominant gene inheritance. We propose the gene symbols Rav(sub 1) and Rav(sub 2) (resistance to Alectra vogelii) for this trait.

Synthesis of Phytoalexins in Sorghum as a Site-Specific Response to Fungal Ingress PLANTS RESIST INFECTION BY PATHOGENS in various ways. One is the production of antimicrobial compounds, called phytoalexins, that [...]

In tomato, Ve is implicated in race-specific resistance to infection by Verticillium species causing crop disease. Characterization of the Ve locus involved positional cloning and isolation of two closely linked inverted genes. Expression of individual Ve genes in susceptible potato plants conferred resistance to an aggressive race 1 isolate of Verticillium albo-atrum. The deduced primary structure of Ve1 and Ve2 included a hydrophobic N-terminal signal peptide, leucine-rich repeats containing 28 or 35 potential glycosylation sites, a hydrophobic membrane-spanning domain, and a C-terminal domain with the mammalian E/DXXXL[Phi] or YXX[Phi] endocytosis signals ([Phi] is an amino acid with a hydrophobic side chain). A leucine zipper-like sequence occurs in the hydrophobic N-terminal signal peptide of Ve1 and a Pro-Glu-Ser-Thr (PEST)-like sequence resides in the C-terminal domain of Ve2. These structures suggest that the Ve genes encode a class of cell-surface glycoproteins with receptor-mediated endocytosis-like signals and leucine zipper or PEST sequences.

Flock house virus (FHV), a single-stranded RNA insect virus, has previously been reported to cross the kingdom barrier and replicate in barley protoplasts and in inoculated leaves of several plant species [Selling, B. H., Allison, R. F. & Kaesberg, P. (1990) Proc. Natl. Acad. Sci. USA 87, 434-438]. There was no systemic movement of FHV in plants. We tested the ability of movement proteins (MPs) of plant viruses to provide movement functions and cause systemic spread of FHV in plants. We compared the growth of FHV in leaves of nontransgenic and transgenic plants expressing the MP of tobacco mosaic virus or red clover necrotic mosaic virus (RCNMV). Both MPs mobilized cell-to-cell and systemic movement of FHV in Nicotiana benthamiana plants. The yield of FHV was more than 100-fold higher in the inoculated leaves of transgenic plants than in the inoculated leaves of nontransgenic plants. In addition, FHV accumulated in the noninoculated upper leaves of both MP-transgenic plants. RCNMV MP was more efficient in mobilizing FHV to noninoculated upper leaves. We also report here that FHV replicates in inoculated leaves of six additional plant species: alfalfa, Arabidopsis, Brassica, cucumber, maize, and rice. Our results demonstrate that plant viral MPs cause cell-to-cell and long-distance movement of an animal virus in plants and offer approaches to the study of the evolution of viruses and mechanisms governing mRNA trafficking in plants as well as to the development of promising vectors for transient expression of foreign genes in plants.

Interactions between plants and soil affect plant-plant interactions and community composition. Festuca arundinacea (tall fescue) is often infected with the mutualistic fungal endophyte Neotyphodium coenophialum. Infected (E+) F. arundinacea has been reported to inhibit soil pathogens, nematodes, and mycorrhizal fungi, and to have greater mineral uptake from the soil relative to uninfected (E-) F. arundinacea. In addition, endophyte presence may indirectly influence the soil by altering the plant-plant interactions and the composition of communities. Recent studies have shown that local plant species diversity was reduced in experimental field plots with E+ F. arundinacea compared to plots with E- plants. Endophyte-induced changes, direct or indirect, in biotic or abiotic soil properties could therefore affect subsequent plant growth. We investigated soil-mediated effects of E+ F. arundinacea on E+ and E- F. arundinacea, Trifolium repens, and Plantago lanceolata by growing each in soil collected from the aforementioned field plots seeded with either E+ or E- F. arundinacea. Infection status of F. arundinacea plots had no direct effect on any measured characteristic of plants grown in soil from those plots. However, responses of experimental plants suggest that, by inducing changes in plant community composition, E+ F. arundinacea may indirectly affect the soil. Most notable was an increase in E+ F. arundinacea total biomass when grown in soil previously dominated by Poa pratensis and other grass species compared to biomass when grown in soil previously dominated by E+ F. arundinacea, indicative of an indirect negative feedback on E+ F. arundinacea growth. In addition, Plantago and Trifolium root biomass was significantly higher when grown in soil that previously supported a high proportion of P. pratensis relative to other plant species. Such plant-soil dynamics may have important community-level implications in areas dominated by F. arundinacea. Key words: community structure; endophyte; Festuca arundinacea; negative feedback; Neotyphodium coenophialum; Plantago lanceolata; plant-soil interaction; Poa pratensis; soil community; Trifolium repens.

Production of reactive oxygen intermediates (ROI) and a form of programmed cell death called hypersensitive response (HR) are often associated with disease resistance of plants. We have previously shown that the Rac homolog of rice, OsRac1, is a regulator of ROI production and cell death in rice. Here we show that the constitutively active OsRac1 (i) causes HR-like responses and greatly reduces disease lesions against a virulent race of the rice blast fungus; (ii) causes resistance against a virulent race of bacterial blight; and (iii) causes enhanced production of a phytoalexin and alters expression of defense-related genes. The dominant-negative OsRac1 suppresses elicitor-induced ROI production in transgenic cell cultures, and in plants suppresses the HR induced by the avirulent race of the fungus. Taken together, our findings strongly suggest that OsRac1 has a general role in disease resistance of rice.

To facilitate plant colonization, some pathogenic fungi produce phytotoxic metabolites that damage tissues; plants may be resistant to a particular pathogen if they produce an enzyme(s) that catalyzes detoxification of this metabolite(s). Alternaria blackspot is one of the most damaging and significant fungal diseases of brassica crops, with no source of resistance known within the Brassica species. Destruxin B is the major phytotoxin produced by the blackspot-causing fungus, Alternaria brassicae (Berkley) Saccardo. We have established that a blackspot-resistant species (Sinapis alba) metabolized [sup.14]C-labeled destruxin B to a less toxic product substantially faster than any of the susceptible species. The first metabolite, hydroxydestruxin B ([sup.14]C-labeled), was further biotransformed to the [Beta]-D-glucosyl derivative at a slower rate. The structures of hydroxydestruxin B and [Beta]-D-glucosyl hydroxydestruxin B were deduced from their spectroscopic data [NMR, high resolution (HR)-MS, Fourier transform infrared (FTIR)] and confirmed by total chemical synthesis. Although these hydroxylation and glucosylation reactions occurred in both resistant (S. alba) and susceptible (Brassica napus, Brassica juncea, and Brassica rapa) species, hydroxylation was the rate limiting step in the susceptible species, whereas glucosylation was the rate limiting step in the resistant species. Remarkably, it was observed that the hydroxydestruxin B induced the biosynthesis of phytoalexins in blackspot-resistant species but not in susceptible species. This appears to be a unique example of phytotoxin detoxification and simultaneous phytoalexin elicitation by the detoxification product. Our studies suggest that S. alba can overcome the fungal invader through detoxification of destruxin B coupled with production of phytoalexins.

The enhanced disease resistance 1 (edr1) mutation of Arabidopsis confers resistance to powdery mildew disease caused by the fungus Erysiphe cichoracearum. Resistance mediated by the edr1 mutation is correlated with induction of several defense responses, including host cell death. Double mutant analysis revealed that all edr1-associated phenotypes are suppressed by mutations that block salicylic acid (SA) perception (nim1) or reduce SA production (pad4 and eds1). The NahG transgene, which lowers endogenous SA levels, also suppressed edr1. In contrast, the ein2 mutation did not suppress edr1-mediated resistance and associated phenotypes, indicating that ethylene and jasmonic acid-induced responses are not required for edr1 resistance. The EDR1 gene was isolated by positional cloning and was found to encode a putative MAP kinase kinase kinase similar to CTR1, a negative regulator of ethylene responses in Arabidopsis. Taken together, these data suggest that EDR1 functions at the top of a MAP kinase cascade that negatively regulates SA-inducible defense responses. Putative orthologs of EDR1 are present in monocots such as rice and barley, indicating that EDR1 may regulate defense responses in a wide range of crop species.

Plants can defend themselves from herbivorous insects by emitting volatile chemical signals that attract natural enemies of the herbivore. For example, maize seedlings attacked by beet armyworm larvae (Spodoptera exigua) produce a mixture of terpenoid and indole volatiles that serve to attract parasitic wasps. A key step in terpenoid biosynthesis is the conversion of acyclic prenyl diphosphates to terpenoid compounds by specific terpenoid synthases (cyclases). We have cloned a maize sesquiterpene cyclase gene, stc1, by transposon tagging and have identified two deletion mutations of the gene. The stc1 gene is located on chromosome 95 and does not seem to have a closely related ortholog in the maize genome. It is induced 15- to 30-fold in maize leaves by beet armyworm larvae feeding or by application of purified volicitin, the insect-derived elicitor, at a mechanically wounded site. stc1 induction is systemic, because undamaged leaves of the same plant show a similar increase in stc1 transcription. Analysis of volatiles from volicitin-treated seedlings revealed that a major naphthalene-based sesquiterpene was present in wild-type seedlings but absent in the Ac-insertion and x-ray-deletion mutants. Therefore, we have identified a maize gene that responds to caterpillar herbivory by producing a chemical defense signal that most likely serves to attract natural enemies of the herbivore.

Maize and a variety of other plant species release volatile compounds in response to herbivore attack that serve as chemical cues to signal natural enemies of the feeding herbivore. N-(17-hydroxy-linolenoyl)-L-glutamine is an elicitor component that has been isolated and chemically characterized from the regurgitant of the herbivore-pest beet armyworm. This fatty acid derivative, referred to as volicitin, triggers the synthesis and release of volatile components, including terpenoids and indole in maize. Here we report on a previously unidentified enzyme, indole-3-glycerol phosphate lyase (IGL), that catalyzes the formation of free indole and is selectively activated by volicitin. IGL's enzymatic properties are similar to BX1, a maize enzyme that serves as the entry point to the secondary defense metabolites DIBOA and DIMBOA. Genesequence analysis indicates that Igl and Bx1 are evolutionarily related to the tryptophan synthase alpha subunit.

The plant-intracellular interaction of the avirulence protein AvrPto of Pseudomonas syringae pathovar tomato, the agent of bacterial speck disease, and the corresponding tomato resistance protein Pto triggers responses leading to disease resistance. Pto, a serine/threonine protein kinase, also interacts with a putative downstream kinase, Pto-interactor 1, as well as with members of a family of transcription factors Pto-interactors 4, 5, and 6. These proteins are likely involved, respectively, in a phosphorylation cascade resulting in hypersensitive cell death, and in defense gene activation. The mechanism by which the interaction of AvrPto and Pto initiates defense response signaling is not known. To pursue the hypothesis that tertiary interactions are involved, we modified the yeast two-hybrid protein interaction trap and conducted a search for tomato proteins that interact with Pto only in the presence of AvrPto. Five classes of AvrPto-dependent Pto interactors were isolated, and their interaction specificity confirmed. Also, to shed light on a recently demonstrated virulence activity of AvrPto, we conducted a standard two-hybrid screen for tomato proteins in addition to Pto that interact with AvrPto: i.e., potential virulence targets or modifiers of AvrPto. By constructing an N-terminal rather than a C-terminal fusion of AvrPto to the LexA DNA binding domain, we were able to overcome autoactivation by AvrPto and identify four classes of specific AvrPto-interacting proteins.

Salicylic acid (SA) plays a critical signaling role in the activation of plant defense responses after pathogen attack. We have identified several potential components of the SA signaling pathway, including (i) the [H.sub.2][O.sub.2]-scavenging enzymes catalase and ascorbate peroxidase, (ii) a high affinity SA-binding protein (SABP2), (iii) a SA-inducible protein kinase (SIPK), (iv) NPR1, an ankyrin repeat-containing protein that exhibits limited homology to I[Kappa]B[Alpha] and is required for SA signaling, and (v) members of the TGA/OBF family of bZIP transcription factors. These bZIP factors physically interact with NPR1 and bind the SA-responsive element in promoters of several defense genes, such as the pathogenesis-related 1 gene (PR-l). Recent studies have demonstrated that nitric oxide (NO) is another signal that activates defense responses after pathogen attack. NO has been shown to play a critical role in the activation of innate immune and inflammatory responses in animals. Increases in NO synthase (NOS)-like activity occurred in resistant but not susceptible tobacco after infection with tobacco mosaic virus. Here we demonstrate that this increase in activity participates in PR-1 gene induction. Two signaling molecules, cGMP and cyclic ADP ribose (cADPR), which function downstream of NO in animals, also appear to mediate plant defense gene activation (e.g., PR-l). Additionally, NO may activate PR-1 expression via an NO-dependent, cADPR-independent pathway. Several targets of NO in animals, including guanylate cyclase, aconitase, and mitogen-activated protein kinases (e.g., SIPK), are also modulated by NO in plants. Thus, at least portions of NO signaling pathways appear to be shared between plants and animals. MAP kinases | [Ca.sup.+2] | cADP ribose | cGMP | disease resistance

Genetic analysis of plant-pathogen interactions has demonstrated that resistance to infection is often determined by the interaction of dominant plant resistance (R) genes and dominant pathogen-encoded avirulence (Avr) genes. It was postulated that R genes encode receptors for Avr determinants. A large number of R genes and their cognate Avr genes have now been analyzed at the molecular level. R gene loci are extremely polymorphic, particularly in sequences encoding amino acids of the leucine-rich repeat motif. A major challenge is to determine how Avr perception by R proteins triggers the plant defense response. Mutational analysis has identified several genes required for the function of specific R proteins. Here we report the identification of Rcr3, a tomato gene required specifically for Cf-2-mediated resistance. We propose that Avr products interact with host proteins to promote disease, and that R proteins 'guard' these host components and initiate Avr-dependent plant defense responses.

When a plant is attacked by another organism, such as a fungus or a herbivorous insect, it cannot very well flee; instead it produces noxious chemical compounds. Engelberth et al. [...]

The Douglas-fir Needle Midge (Contarinia pseudotsugae) is a serious pest of Douglas-fir Christmas trees in the Pacific Northwest. In this study, levels of monoterpenes were measured in needles of current (1999) and previous year's (1998) growth from infested and uninfested branches of Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) trees. Samples were taken at approximately bi-monthly intervals from May through October. Monoterpenes were identified and quantified using gas chromatography/mass spectroscopy. Nine monoterpenes occurred regularly in all trees. Beta-pinene was the most prevalent compound in all samples. Total terpenes were higher in 1998 age-class needles than the 1999 age-class needles in the early season. Camphene and bornyl acetate were higher in uninfested needles than in infested 1999 age-class needles. These two compounds are known to decrease budworm larval growth and may be important in influencing needle midge host choice.

Genomic positions of phenotypically defined disease resistance genes (R genes) and R gene homologues were analyzed in three solanaceous crop genera, Lycopersicon (tomato), Solanum (potato), and Capsicum (pepper). R genes occurred at corresponding positions in two or more genomes more frequently than expected by chance; however, in only two cases, both involving Phytophthora spp., did genes at corresponding positions have specificity for closely related pathogen taxa. In contrast, resistances to Globodera spp., potato virus Y, tobacco mosaic virus, and tomato spotted will virus were mapped in two or more genera and did not occur in corresponding positions. Without exception, pepper homologues of the cloned R genes Sw-5, N, Pto, Prf, and 12 were found in syntenous positions in other solanaceous genomes and in some cases also mapped to additional positions near phenotypically defined solanaceous R genes. This detailed analysis and synthesis of all available data for solanaceous R genes suggests a working hypothesis regarding the evolution of R genes. Specifically, while the taxonomic specificity of host R genes may be evolving rapidly, general functions of R alleles (e.g., initiation of resistance response) may be conserved at homologous loci in related plant genera.

1 The effects of fungus infection on a natural population of the pseudoannual plant Trientalis europaea were followed for 4 years. 2 The incidence of the disease was low, showed little temporal variation during the period of study and was not affected by ramet size. Disease reduced flowering, fruiting, stolon length and the number and size of daughter tubers, all of which were positively correlated with ramet size. The year-disease interaction was not significant, except for flowering, suggesting little variation in the aggressiveness of the pathogen. 3 Disease reduced survival of ramets to the end of the growing season, although the effect varied with ramet size, and decreased tuber survival both by reduction of tuber size and by reduction of the overwintering ability of tubers of a given size. 4 For two of the three annual transitions the size of the offspring ramets was affected negatively by infection in the previous year. 5 Disease transmission occurred along the stolons of only 31% of the diseased ramets. The probability of disease being shown in the following year decreased with stolon length. 6 Although disease had a detrimental effect on ramet fitness, the low level of incidence and the stability of the clone dynamics in simulation models suggest only a minor role of the disease in population regulation in this species. Key-words: clonal plants, host--pathogen interaction, population, pseudoannual, temporal patterns Journal of Ecology (1999) 87, 995-1004

The Bs2 resistance gene of pepper specifically recognizes and confers resistance to strains of Xanthomonas campestris pv. vesicatoria that contain the corresponding bacterial avirulence gene, avrBs2. The involvement of avrBs2 in pathogen fitness and its prevalence in many X. campestris pathovars suggests that the Bs2 gene may be durable in the field and provide resistance when introduced into other plant species. Employing a positional cloning strategy, the Bs2 locus was isolated and the gene was identified by coexpression with avrBs2 in an Agrobacterium-mediated transient assay. A single candidate gene, predicted to encode motifs characteristic of the nucleotide binding site-leucine-rich repeat class of resistance genes, was identified. This gene specifically controlled the hypersensitive response when transiently expressed in susceptible pepper and tomato lines and in a nonhost species, Nicotiana benthamiana, and was designated as Bs2. Functional expression of Bs2 in stable transgenic tomatoes supports its use as a source of resistance in other Solanaceous plant species.

To understand better cotton (Gossypium hirsutum L.) plant compensation for early-season floral bud (square) loss due to insect damage, a field study was conducted in 1994 and 1995 at Marianna, AR. The control treatment was protected by insecticide applications, while tarnished plant bugs (Lygus lineolaris Palisot de Beauvois) and bollworms (Helicoverpa zea Boddie) were released in the plots of the other treatment three times before flowering. Square abscission at the first sympodial fruiting position was 5 and 33% for the control and infested plants, respectively, and yield was reduced 21% by insect infestation. Insect treatment resulted in 4% more light penetration through the canopy, which may have contributed to the 17% increase in photosynthesis of the eighth main-stem leaf from the terminal leaf as compared with the control plants. Canopy photosynthesis recorded 4 wk after the initiation of flowering was 21% higher in the infested plants. [CO.sub.2] labeling showed infestation also resulted in more 14C recovered in the terminal node (terminal leaf plus main stem above the terminal leaf) and less remaining in the branch at the same node as the source leaf, which corresponded to an increase in plant height, although node number was not affected. Since our insect-induced abscission treatments had similar effects as manual fruit removal treatments reported by others, future studies seem justified in using either approach. Early fruit loss in the U.S. Mid-south results in changes in carbon exchange and allocation, but poor late-season growing conditions often prevent yield compensation., Key insect pests of cotton have been shown to feed preferentially on flower buds (squares) and young bolls (Hearn and Fitt, 1992; Williams et al., 1987). Investigation of cotton plant [...]