Only xylem-borne factors can account for systemic wound signalling in the tomato plant

Rapid and systemic defence responses occur in various higher plants, including the tomato (Lycopersicon esculentum L.). The long-distance signalling mechanisms which permit these responses are not clear, but three models are currently considered in the literature: phloem transport, hydraulic dispersal in the xylem, and electrical transmission. Experiments presented here are designed to discriminate between these three models on the basis of some key predictions. It is demonstrated that wound signalling can be prevented by enclosure of the shoots in polythene bags, to generate high humidity. This effect can be reversed by addition of mannitol solution to the roots, showing that it depends on saturation of the plant's water status rather than on changes in the gaseous environment of the shoot. In addition, wound signals are shown to pass freely across heat-killed tissue. These results are predicted by the hydraulic-dispersal model of signalling, but they are not compatible with the other two models. We therefore conclude in favour of hydraulic dispersal.Abbreviations PI proteinase inhibitor - PIIF proteinase-inhibitor-inducing factor This work was supported by the Agriculture and Food Research Council (UK). J.-J.A. is grateful for a Fellowship from the Ministerio de Education y Ciencia (Spain).     

High overexpression of CERES,a plant regulator of translation,induces different phenotypical defence responses during TuMV infection

Mutations in the eukaryotic translation initiation factors eIF4E and eIF(iso)4E confer potyvirus resistance in a range of plant hosts. This supports the notion that, in addition to their role in translation of cellular mRNAs, eIF4E isoforms are also essential for the potyvirus cycle. CERES is a plant eIF4E- and eIF(iso)4E-binding protein that, through its binding to the eIF4Es, modulates translation initiation; however, its possible role in potyvirus resistance is unknown. In this article, we analyse if the ectopic expression of AtCERES is able to interfere with turnip mosaic virus replication in plants. Our results demonstrate that, during infection, the ectopic expression of CERES in Nicotiana benthamiana promotes the development of a mosaic phenotype when it is accumulated to moderate levels, but induces veinal necrosis when it is accumulated to higher levels. This necrotic process resembles a hypersensitive response (HR)-like response that occurs with different HR hallmarks. Remarkably, Arabidopsis plants inoculated with a virus clone that promotes high expression of CERES do not show signs of infection. These final phenotypical outcomes are independent of the capacity of CERES to bind to eIF4E. All these data suggest that CERES, most likely due to its leucine-rich repeat nature, could act as a resistance protein, able to promote a range of different defence responses when it is highly overexpressed from viral constructs.     

Rhizobacterial bioformulation for the effective management of Macrophomina root rot in mungbean

Abstract

Fluorescent pseudomonads based bioformulation was evaluated for their ability to control Macrophomina root rot disease in mungbean (Vigna mungo). P. fluorescens isolate Pf1 showed the maximum inhibition in mycelial growth of Macrophomina phaseolina under in vitro conditions. Bioformulation of Pf1 with chitin was effective in reducing the root rot incidence in green gram both under glasshouse and field conditions. The rhizosphere colonization of P. fluorescens was observed appreciable with the green gram plants. However, Pf1 amended with chitin colonized effectively. Furthermore, the induction of defence-related enzymes and chemicals in plants by Pf1 amended with or without chitin and neem were tested. Increased accumulation of defence enzymes viz., phenylalanine ammonia lyase (PAL), peroxidase (PO), polyphenol oxidase (PPO), chitinase, β-1,3-glucanse and phenolics were observed in Pf1 bioformulation amended with chitin, pre-treated plants challenge inoculated with M. phaseolina under glasshouse conditions. The present study reveals that in addition to direct antagonism and plant-growth promotion, PGPR strains amended with chitin bioformulation induced defence-related enzymes and pathogenesis related (PR) proteins which collectively enhance the resistance in green gram against the infection of M. phaseolina.     

Effects of nickel hyperaccumulation in Alyssum pintodasilvae on model arthropods representatives of two trophic levels

An experimental assessment of the defence hypothesis of nickel (Ni) hyperaccumulation in Alyssum was lacking. Also, to date no study had investigated the effects of hyperaccumulator litter on a detritivore species. We performed several experiments with model arthropods representatives of two trophic levels: Tribolium castaneum (herbivore) and Porcellio dilatatus (detritivore). In no-choice trials using artificial food disks with different Ni concentrations, T. castaneum fed significantly less as Ni concentration increased and totally rejected disks with the highest Ni concentration. In choice tests, insects preferred disks without Ni. In the no-choice experiment, mortality was low and did not differ significantly among treatments. Hence, this suggested a deterrent effect of high Ni diet. Experiments with P. dilatatus showed that isopods fed A. pintodasilvae litter showed significantly greater mortality (83%) than isopods fed litter from the non-hyperaccumulator species Iberis procumbens (8%), Micromeria juliana (no mortality) or Alnus glutinosa (no mortality). Also, isopods consumed significantly greater amounts of litter from the non-hyperaccumulator plant species. The behaviour of isopods fed A. pintodasilvae litter suggested an antifeedant effect of Ni, possibly due to post-ingestive toxic effects. Our results support the view that Ni defends the Portuguese serpentine hyperaccumulator A. pintodasilvae against herbivores, indicating that Ni can account both for feeding deterrence and toxic effects. The effects of hyperaccumulator litter on the detritivore P. dilatatus suggest that the activity of these important organisms may be significantly impaired with potential consequences on the decomposition processes.     

Tomato linalool synthase is induced in trichomes by jasmonic acid

Tomato (Lycopersicon esculentum) plants emit a blend of volatile organic compounds, which mainly consists of terpenes. Upon herbivory or wounding, the emission of several terpenes increases. We have identified and characterized the first two tomato monoterpene synthases, LeMTS1 and LeMTS2. Although these proteins were highly homologous, recombinant LeMTS1 protein produced (R)-linalool from geranyl diphosphate (GPP) and (E)-nerolidol from farnesyl diphosphate (FPP), while recombinant LeMTS2 produced β-phellandrene, β-myrcene, and sabinene from GPP. In addition, these genes were expressed in different tissues: LeMTS1 was expressed in flowers, young leaves, stems, and petioles, while LeMTS2 was strongest expressed in stems and roots. LeMTS1 expression in leaves was induced by spider mite-infestation, wounding and jasmonic acid (JA)-treatment, while LeMTS2 did not respond to these stimuli. The expression of LeMTS1 in stems and petioles was predominantly detected in trichomes and could be induced by JA. Because JA treatment strongly induced emission of linalool and overexpression of LeMTS1 in tomato resulted in increased production of linalool, we propose that LeMTS1 is a genuine linalool synthase. Our results underline the importance of trichomes in JA-induced terpene emission in tomato.     

Effect of light intensity on methylxanthine contents of Ilex paraguariensis A. St. Hil.

Maté (Ilex paraguariensis A. St. Hil.) is a shade-tolerant species, frequently cultivated on agroforestry systems or even in monoculture. Secondary metabolism of maté includes the methylxanthines, caffeine and theobromine. Shading could alter the quantitative profile of these compounds in the plant, and since the secondary chemistry of maté is decisive to determine the quality of raw material, the caffeine and theobromine content under different light intensities were investigated. Artificial shading obtained by a nylon screen and natural shading produced by other trees were tested. Methylxanthines content was increased with the reduction of light, but only in the low light intensity treatment. A negative correlation was found between biomass accumulation and methylxanthines content. But the total methylxanthines yield per plant was not altered.     

Presence of sub-epithelial lymphoid tissues in the teat of ewe-lambs and adult ewes

In the first part of the study, 24 clinically healthy teats from non-lactating ewe-lambs were examined bacteriologically and histologically. No bacteria were isolated from any of these teats; lymphocytes were observed in teat cisterns of six teats (25%) from three ewes. In the second part, 87 teats from adult ewes were examined; their origin was from lactating mammary glands with no bacteria isolated (n = 23), from glands after lactation with no bacteria isolated (n = 25), from lactating glands with bacteria isolated (n = 22) or from glands after lactation with bacteria isolated (n = 17). The salient histological feature was sub-epithelial leucocytic infiltration. In teat cisterns, lymphocytes were the predominant cell type and in teat ducts, lymphocytes and neutrophils were seen in equal proportions. Sub-epithelial lymphoid nodules, some with germinal centers, were detected in 43 (49%) teats; their majority was observed at the border between teat duct and teat cistern. Presence of bacteria was significantly associated with presence of leucocytic activity (P < 0.001) and with presence of lymphoid nodules (P = 0.032). We conclude that the presence of induced sub-epithelial lymphoid tissue at the border between teat duct and teat cistern appears to be important in protecting the mammary gland during the early stages of bacterial invasion.     

Nutrient affinity,half‐saturation constants and the cost of toxin production in dinoflagellates

The two parameters of the Michaelis–Menten model, the maximum uptake rate and the half‐saturation constant, are not stochastically independent, and the half‐saturation constant is not a measure of nutrient affinity, as commonly assumed. Failure to realise their interdependence and mechanistic interpretation may lead to the emergence of false trade‐offs.     

The importance of who infects whom: the evolution of diversity in host resistance to infectious disease

Variation for resistance to infectious disease is ubiquitous and critical to host and parasite evolution and to disease impact, spread and control. However, the processes that generate and maintain this diversity are not understood. We examine how ecological feedbacks generate diversity in host defence focussing on when polymorphism can evolve without co-evolution of the parasite. Our key result is that when there is heritable variation in hosts in both their transmissibility and susceptibility along with costs to resistance, there is the possibility of the evolution of polymorphism. We argue that a wide range of behavioural or physiological mechanisms may lead to relationships between transmissibility and susceptibility that generate diversity. We illustrate this by showing that a tendency for higher contacts between related individuals leads to polymorphism. Only dimorphisms can evolve when infection is determined only by an individuals' susceptibility or when transmissibility and susceptibility are simply positively or negatively correlated.