Sublethal cadmium intoxication in Arabidopsis thaliana impacts translation at multiple levels.

To study the impact of translational regulation during heavy metal poisoning, Arabidopsis thaliana cell cultures were submitted to sublethal cadmium stress. At the concentration used, cadmium had a minimal impact on the growth of the culture but induced an accumulation of high molecular weight polysomes without de novo production of new ribosomes together with a reduction of protein synthesis. In addition, cadmium stress induces phosphorylation of eukaryotic initiation factor 2α by GCN2 and, in planta, gcn2 mutants are more sensitive to cadmium stress, suggesting a role for this translational regulation mechanism in the response to cadmium stress. Microarray analysis of total and polysomal RNAs in control and cadmium-treated cells reveals a large class of genes for which a variation in total RNA abundance is not linked to a variation in polysomal loading, suggesting that transcription and translation are uncoupled and that these genes are not recruited at the initiation step of translation.

Factors affecting distribution and mobility of trace elements (Cu, Pb, Zn) in a perennial grapevine (Vitis vinifera L.) in the Champagne region of France.

Soil and Vitis vinifera L. (coarse and fine roots, leaves, berries) concentration and geochemical partitioning of Cu, Pb and Zn were determined in a contaminated calcareous Champagne plot to assess their mobility and transfer. Accumulation ratios in roots remained low (0.1-0.4 for Cu and Zn, <0.05 for Pb). Differences between elements resulted from vegetation uptake strategy and soil partitioning. Copper, significantly associated with the oxidisable fraction (27.8%), and Zn with the acid soluble fraction (33.3%), could be mobilised by rhizosphere acidification and oxidisation, unlike Pb, essentially contained in the reducible fraction (72.4%). Roots should not be considered as a whole since the more reactive fine roots showed higher accumulation ratios than coarse ones. More sensitive response of fine roots, lack of correlation between chemical extraction results and vegetation concentrations, and very limited translocation to aerial parts showed that fine root concentrations should be used when assessing bioavailability.

Pentatricopeptide repeat (PPR) proteins as sequence-specificity factors in post-transcriptional processes in organelles.

PPR (pentatricopeptide repeat) genes form a large family particularly prevalent in higher plants and targeted to organelles. They are involved in many post-transcriptional processes such as splicing, editing, processing and translation. Current data suggest that PPR proteins are involved in targeting effectors to the correct sites on the correct transcripts but the molecular mechanisms for RNA binding and effector recruitment by PPR proteins are not understood yet.

Resistance of cotton towards Xanthomonas campestris pv. malvacearum.

Interactions between Gossypium spp. and the bacterial pathogen Xanthomonas campestris pv. malvacearum are understood in the context of the gene-for-gene concept. Reviewed here are the genetic basis for cotton resistance, with reference to resistance genes, resistance gene analogs, and bacterial avirulence genes, together with the physiological mechanisms involved in the hypersensitive response to the pathogen, including production of signaling hormones, synthesis of antimicrobial molecules and alteration of host cell structures. This host-pathogen interaction represents the most complex resistance gene/avr gene system yet known and is one of the few in which phytoalexins are known to be specifically localized in HR cells at anti-microbial concentrations.

Activity of class III peroxidases in the defense of cotton to bacterial blight.

Cotton cotyledons displayed a hypersensitive reaction (HR) in the cultivar Réba B50 after infiltration with the avirulent race 18 from Xanthomonas campestris pv. malvacearum. Two sets of peroxidases were associated with the HR time course. Early but transient accumulation of peroxidase in material encapsulating the bacteria in intercellular areas was observed by immunocytochemistry at 3 h postinfection and coincided with the oxidative burst. Total guaiacol-peroxidase activity was highly increased in cells undergoing HR, from 12 h after treatment. Molecular characterization of seven cloned peroxidase genes revealed highly conserved B, D, and F domains, with similarities to plant class III peroxidases. Analysis of gene expression showed variation in transcript accumulation during both compatible (race 20) and incompatible interactions for four of these genes: pod2, pod3, pod4, and pod6. Pod4 and pod6 were more intensely up-regulated during resistance than during disease and in the control, while pod3 was specifically down-regulated during the HR after the oxidative burst. Pod2 was induced by pathogen infection and weakly stimulated in the control. These data suggest that cotton peroxidases may have various functions in the defense response to Xanthomonas infections.

Lipid peroxidation in cotton: Xanthomonas interactions and the role of lipoxygenases during the hypersensitive reaction.

Lipid peroxidation, often associated with hypersensitive cell death, may be initiated either by active oxygen species (AOS) or lipoxygenases (LOX). Here we report a detailed analysis of this oxidative process in both incompatible and compatible interactions between the cotton cultivar Reba B50 and Xanthomonas campestris pv. malvacearum (Xcm). The hypersensitive reaction (HR) was characterized by a massive production of polyunsaturated fatty acid (PUFA) hydroperoxides together with typical tissue dehydration. Among these, isomers peroxidized on carbon 9, largely predominant, were chiral, showing an excess in the S enantiomer. The HR process was accompanied by an increase in 9S-LOX activity and preceded by transcription of a LOX gene (GhKLox1). These results showed that: (i) AOS produced during the oxidative burst were not involved in PUFA peroxidation during HR; and (ii) as previously described in elicited leaves of tobacco, the massive enzymatic lipid peroxidation was closely associated with hypersensitive cell death. During disease development in this cotton cultivar, the 9-lipoxygenation of PUFAs was late, weak, preceded by a faint accumulation of GhKLox1 transcripts, and associated with chlorosis but not with necrosis. Consequently, the main difference between incompatible and compatible interactions was in the precocity and intensity of the oxidative process, rather than in its nature. These data provide the evidence for a correlation between lipid peroxidation and hypersensitive cell death induced by pathogens.