The effect of agricultural management practices on geochemical cycles in moderate ecosystems is by far better understood than in semiarid regions, where fertilizer availability and climatic conditions are less favorable. We studied the impact of different fertilizer regimens in an agricultural long-term observatory in Burkina Faso at three different plant development stages (early leaf development, flowering, and senescence) of sorghum cultivars. Using real-time PCR, we investigated functional microbial communities involved in key processes of the nitrogen cycle (nitrogen fixation, ammonia oxidation, and denitrification) in the rhizosphere. The results indicate that fertilizer treatments and plant development stages combined with environmental factors affected the abundance of the targeted functional genes in the rhizosphere. While nitrogen-fixing populations dominated the investigated communities when organic fertilizers (manure and straw) were applied, their numbers were comparatively reduced in urea-treated plots. In contrast, ammonia-oxidizing bacteria (AOB) increased not only in absolute numbers but also in relation to the other bacterial groups investigated in the urea-amended plots. Ammonia-oxidizing archaea exhibited higher numbers compared to AOB independent of fertilizer application. Similarly, denitrifiers were also more abundant in the urea-treated plots. Our data imply as well that, more than in moderate regions, water availability might shape microbial communities in the rhizosphere, since low gene abundance data were obtained for all tested genes at the flowering stage, when water availability was very limited.

Summary: 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.

In tropical ecosystems, termite mound soils constitute an important soil compartment covering around 10% of African soils. Previous studies have shown (S. Fall, S. Nazaret, J. L. Chotte, and A. Brauman, Microb. Ecol. 28:191-199, 2004) that the bacterial genetic structure of the mounds of soil-feeding termites (Cubitermes niokoloensis) is different from that of their surrounding soil. The aim of this study was to characterize the specificity of bacterial communities within mounds with respect to the digestive and soil origins of the mound. We have compared the bacterial community structures of a termite mound, termite gut sections, and surrounding soil using PCR-denaturing gradient gel electrophoresis (DGGE) analysis and cloning and sequencing of PCR-amplified 16S rRNA gene fragments. DGGE analysis revealed a drastic difference between the genetic structures of the bacterial communities of the termite gut and the mound. Analysis of 266 clones, including 54 from excised bands, revealed a high level of diversity in each biota investigated. The soil-feeding termite mound was dominated by theActinobacteriaphylum, whereas theFirmicutesandProteobacteriaphyla dominate the gut sections of termites and the surrounding soil, respectively. Phylogenetic analyses revealed a distinct clustering ofActinobacteriaphylotypes between the mound and the surrounding soil. TheActinobacteriaclones of the termite mound were diverse, distributed among 10 distinct families, and like those in the termite gut environment lightly dominated by theNocardioidaceaefamily. Our findings confirmed that the soil-feeding termite mound (C. niokoloensis) represents a specific bacterial habitat in the tropics.

Cotton cotyledons displayed a hypersensitive reaction (HR) in the cultivar Réba B50 after infiltration with the aviru-lent 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.

We analyzed the production of reactive oxygen species, the accumulation of salicylic acid (SA), and peroxidase activity during the incompatible interaction between cotyledons of the cotton (Gossypium hirsutum) cv Reba B50/Xanthomonas campestris pvmalvacearum (Xcm) race 18. SA was detected in petioles of cotyledons 6 h after infection and 24 h post inoculation in cotyledons and untreated leaves. The first peak of SA occurred 3 h after generation of superoxide (O₂^·−), and was inhibited by infiltration of catalase. Peroxidase activity and accumulation of SA increased in petioles of cotyledons and leaves following H₂O₂ infiltration of cotyledons from 0.85 to 1 mm. Infiltration of 2 mm SA increased peroxidase activity in treated cotyledons and in the first leaves, but most of the infiltrated SA was rapidly conjugated within the cotyledons. When increasing concentrations of SA were infiltrated 2.5 h post inoculation at the beginning of the oxidative burst, the activity of the apoplastic cationic O₂^·−-generating peroxidase decreased in a dose-dependent manner. We have shown that during the cotton hypersensitive response to Xcm, H₂O₂ is required for local and systemic accumulation of SA, which may locally control the generation of O₂^·−. Detaching cotyledons at intervals after inoculation demonstrated that the signal leading to systemic accumulation of SA was emitted around 3 h post inoculation, and was associated with the oxidative burst. SA produced 6 h post infection at HR sites was not the primary mobile signal diffusing systemically from infected cotyledons.