Transient expression of pepper Bs2 gene in Citrus limon as an approach to evaluate its utility for management of citrus canker disease

The pepper Bs2 gene confers resistance to Xanthomonas campestris pv. vesicatoria (Xcv) pathogenic strains containing the avrBs2 avirulence gene in susceptible pepper and tomato. The avrBs2 gene is highly conserved in the Xanthomonas genus and when bacteria lack this gene their growth in a susceptible host is diminished, indicating that the avrBs2 gene product could confer an adaptive advantage to the pathogen. The avrBs2 of Xanthomonas citri subsp. citri (Xcc), cause of citrus canker, shares 96% homology with avrBs2 of Xcv. To evaluate if Bs2 could recognize avrBs2 of Xcc in citrus plants and thereby activate plant defence mechanisms to increase resistance to canker, transient expression experiments were conducted using Agrobacterium tumefaciens in lemon plants subsequently challenged with wildtype Xcc. The results showed that transient expression of Bs2 reduced canker formation in lemon and induced plant defence mechanisms, as shown by callose deposition and PR‐1 expression. Moreover, when an avrBs2 mutant of Xcc was used, no decrease in disease symptoms was observed. This work shows that the Bs2 gene from Solanaceae is functional in lemon, a member of the Rutaceae family. Therefore, Bs2 is a potential candidate gene for stable expression in transgenic citrus plants in order to improve resistance to canker disease.     

Susceptibility of common herbaceous crops to Phytophthora cinnamomi and its influence on Quercus root rot in rangelands

The susceptibility to Phytophthora cinnamomi of four crops (wheat, oat, vetch, and yellow lupin) commonly planted in rangeland ecosystems in southern Spain was evaluated. By means of in vitro infection experiments, the presence of the pathogen into the roots of yellow lupin (symptomatic) and vetch (asymptomatic) was observed, but never into wheat and oat roots (asymptomatic). It was also demonstrated that yellow lupin stimulated the production of zoospores of P. cinnamomi. Vetch, wheat and oat did not stimulate zoospore production. Under controlled conditions, only yellow lupin induced an increase in the number of viable chlamydospores in the soil. We concluded that the culture of wheat, oat, and vetch in rangelands did not influence the epidemiology of the Quercus root disease, even when asymptomatically-infected vetch is grown, and these crops can constitute an alternative to the culture of yellow lupin in rangeland ecosystems affected by Quercus root rot.     

Airborne fungal succession in a rice field of Cuba

This study characterised the airborne mycobiota of a rice plantation in Bauta (Havana, Cuba), with a view to improving crop quality. Culturable airborne fungi were sampled between March 2007 and February 2008 using a volumetric method (SAS Super 100) with potato dextrose agar as culture medium. The total fungal count was 51,425 cfu/m³. The 39 filamentous genera identified included several major rice pathogens (Bipolaris, Curvularia, Alternaria, Pyricularia, Cercospora and Fusarium) as well as potentially mycotoxigenic species of Fusarium, Penicillium and Aspergillus. Analysis of the influence of weather-related factors on spore counts revealed that total counts and Penicillium counts correlated positively with daily relative humidity, mean temperature and dew point.     

Geminivirus C2 protein represses genes involved in sulphur assimilation and this effect can be counteracted by jasmonate treatment

Geminiviruses are plant viruses that infect a broad range of crops and cause extensive losses worldwide, having an important economic impact. C2, a multifunctional pathogenicity factor encoded by geminiviruses, has been recently shown to suppress the responses to jasmonates in the host plant, which might at least partially explain its well-established role in pathogenicity. Sulphur is one of the essential macro-elements for plant life, and is considered to have a role in plant defence, in a phenomenon named sulphur-induced resistance (SIR) or sulphur-enhanced defence (SED). In this work, we show that geminivirus C2 protein represses the expression of genes involved in the sulphur assimilation pathway in Arabidopsis, but, interestingly, this effect can be neutralized by exogenous jasmonate treatment. These preliminary results may raise the idea that geminiviruses might be affecting sulphur metabolism, and maybe counteracting SIR/SED, through the manipulation of the jasmonate signalling pathway, which would define a novel strategy in plant-virus interactions and may unveil SIR/SED as an important player in the plant defence against viruses.     

Ecdysteroid receptor docking suggests that dibenzoylhydrazine-based insecticides are devoid of any deleterious effect on the parasitic wasp Psyttalia concolor (Hym. Braconidae)

BACKGROUND: The moulting accelerating compounds (MACs) or ecdysteroid agonists represent a selective group of insecticides acting upon binding to the ecdysteroid receptor (EcR) and leading to lethal premature moulting in larval stages and aborted reproduction in adults. Psyttalia concolor Szèpl. is a useful parasitic wasp attacking important tephritid pests such as the medfly and olive fruit fly. RESULTS: Contact and oral exposure in the laboratory of female parasitic wasps to the dibenzoylhydrazine‐based methoxyfenozide, tebufenozide and RH‐5849 did not provoke negative effects. No mortality and no reduction in beneficial capacity were observed. The ligand‐binding domain (LBD) of the EcR of P. concolor was sequenced, and a homology protein model was constructed which confirmed a cavity structure with 12 α‐helices, harbouring the natural insect moulting hormone 20‐hydroxyecdysone. However, a steric clash occurred for the MAC insecticides owing to a restricted extent of the ligand‐binding cavity of the PcLBD‐EcR, while they did dock well in that of susceptible insects. CONCLUSIONS: The insect toxicity assays demonstrated that MACs are selective for P. concolor. The modelling/docking experiments are indications that these insecticides do not bind with the LBD‐EcR of P. concolor and support the theory that they show no biological effects in the parasitic wasp. These data may help in explaining the compatible use of MACs together with parasitic wasps in IPM programmes. Copyright © 2012 Society of Chemical Industry     

Participative site-specific agriculture analysis for smallholders

Site-specific agriculture has been adopted in a high-tech context using, for instance, in situ sensors, satellite images for remote sensing analysis, and some other technological devices. However, farmers and smallholders without the economic resources and required knowledge to use and to access the latest technology seem to find an impediment to precision agricultural practices. This article discusses the possibility of adopting precision agriculture (PA) principles for site-specific management but in a low technology context for such farmers. The proposed methodology to support PA combines low technology dependency and a participatory approach by involving smallholders, farmers and experts. The case studies demonstrate how the interplay of low technology and a participative approach may be suitable for smallholders for site-specific agriculture analysis.     

Probing the ultimate limits of plasmonic enhancement

Metals support surface plasmons at optical wavelengths and have the ability to localize light to subwavelength regions. The field enhancements that occur in these regions set the ultimate limitations on a wide range of nonlinear and quantum optical phenomena. We found that the dominant limiting factor is not the resistive loss of the metal, but rather the intrinsic nonlocality of its dielectric response. A semiclassical model of the electronic response of a metal places strict bounds on the ultimate field enhancement. To demonstrate the accuracy of this model, we studied optical scattering from gold nanoparticles spaced a few angstroms from a gold film. The bounds derived from the models and experiments impose limitations on all nanophotonic systems.     

Collapse of Classic Maya civilization related to modest reduction in precipitation

The disintegration of the Classic Maya civilization in the Yucatán Peninsula and Central America was a complex process that occurred over an approximately 200-year interval and involved a catastrophic depopulation of the region. Although it is well established that the civilization collapse coincided with widespread episodes of drought, their nature and severity remain enigmatic. We present a quantitative analysis that offers a coherent interpretation of four of the most detailed paleoclimate records of the event. We conclude that the droughts occurring during the disintegration of the Maya civilization represented up to a 40% reduction in annual precipitation, probably due to a reduction in summer season tropical storm frequency and intensity.