Abiotic release of nitrous acid (HONO) in equilibrium with soil nitrite (NO 2 - ) was suggested as an important contributor to the missing source of atmospheric HONO and hydroxyl radicals (OH). The ...role of total soil-derived HONO in the biogeochemical and atmospheric nitrogen cycles, however, has remained unknown. In laboratory experiments, we found that for nonacidic soils from arid and arable areas, reactive nitrogen emitted as HONO is comparable with emissions of nitric oxide (NO). We show that ammonia-oxidizing bacteria can directly release HONO in quantities larger than expected from the acid-base and Henry's law equilibria of the aqueous phase in soil. This component of the nitrogen cycle constitutes an additional loss term for fixed nitrogen in soils and a source for reactive nitrogen in the atmosphere.
Nitrous acid (HONO) is an important precursor of the hydroxyl radical (OH), the atmosphere´s primary oxidant. An unknown strong daytime source of HONO is required to explain measurements in ambient ...air. Emissions from soils are one of the potential sources. Ammonia-oxidizing bacteria (AOB) have been identified as possible producers of these HONO soil emissions. However, the mechanisms for production and release of HONO in soils are not fully understood. In this study, we used a dynamic soil-chamber system to provide direct evidence that gaseous emissions from nitrifying pure cultures contain hydroxylamine (NH
OH), which is subsequently converted to HONO in a heterogeneous reaction with water vapor on glass bead surfaces. In addition to different AOB species, we found release of HONO also in ammonia-oxidizing archaea (AOA), suggesting that these globally abundant microbes may also contribute to the formation of atmospheric HONO and consequently OH. Since biogenic NH
OH is formed by diverse organisms, such as AOB, AOA, methane-oxidizing bacteria, heterotrophic nitrifiers, and fungi, we argue that HONO emission from soil is not restricted to the nitrifying bacteria, but is also promoted by nitrifying members of the domains Archaea and Eukarya.
Atmospheric concentrations of nitrous acid (HONO), one of the major precursors of the hydroxyl radical (OH) in the troposphere, significantly exceed the values predicted by the assumption of a ...photostationary state (PSS) during daytime. Therefore, additional sources of HONO were intensively investigated in the last decades. This study presents budget calculations of HONO based on simultaneous measurements of all relevant species, including HONO and OH at two different measurement heights, i.e. 1 m above the ground and about 2 to 3 m above the canopy (24 m above the ground), conducted in a boreal forest environment. We observed mean HONO concentrations of about 6.5 × 108 molecules cm−3 (26 ppt) during daytime, more than 20 times higher than expected from the PSS of 0.2 × 108 molecules cm−3 (1 ppt). To close the budgets at both heights, a strong additional source term during daytime is required. This unidentified source is at its maximum at noon (up to 1.1 × 106 molecules cm−3 s−1, 160 ppt h−1) and in general up to 2.3 times stronger above the canopy than close to the ground. The insignificance of known gas phase reactions and other processes like dry deposition or advection compared to the photolytic decomposition of HONO at this measurement site was an ideal prerequisite to study possible correlations of this unknown term to proposed HONO sources. But neither the proposed emissions from soils nor the proposed photolysis of adsorbed HNO3 contributed substantially to the unknown source. However, the unknown source was found to be perfectly correlated to the unbalanced photolytic loss of HONO.
Because epidemics of successive cropping seasons are not independent, epidemiological studies need to encompass the processes occurring during the transmission of epidemics from one season to the ...next. With Leptosphaeria maculans, infected stubble allows carry‐over of the fungus. Generation experiments using recurrent selection on field plots are a useful means of comparing the effects of selection pressures. However, the full life cycle of the fungus, from plant infection to the next generation of ascospores, has not yet been achieved under controlled conditions. Studies were undertaken to achieve an experimental set‐up with sexual reproduction under controlled conditions. Cankered oilseed rape stems were produced under controlled conditions, after inoculation with a mixture of 12 isolates across both mating types. Stems were cut longitudinally and attached to styropore plates. Stem halves were incubated outside or in climate chambers regularly soaked in tap water to ensure maturation. Incubation was stopped when mature pseudothecia were observed. In all three independent experiments, more stem halves had pseudothecia when incubated under controlled conditions (30–100%) than incubated outside (0–80%). To the authors’ knowledge, this is the first study achieving the full life cycle of the fungus under controlled conditions, from infection of the plant to mature pseudothecia. This opens up the prospect of running experiments year‐round to better understand inoculum production, to compare fungal fitness, or to run generation experiments with exotic pathogen populations.
Quantitative resistance mediated by multiple genetic factors has been shown to increase the potential for durability of major resistance genes. This was demonstrated in the Leptosphaeria ...maculans/Brassica napus pathosystem in a 5year recurrent selection field experiment on lines harboring the qualitative resistance gene Rlm6 combined or not with quantitative resistance. The quantitative resistance limited the size of the virulent isolate population. In this study we continued this recurrent selection experiment in the same way to examine whether the pathogen population could adapt and render the major gene ineffective in the longer term. The cultivars Eurol, with a susceptible background, and Darmor, with quantitative resistance, were used. We confirmed that the combination of qualitative and quantitative resistance is an effective approach for controlling the pathogen epidemics over time. This combination did not prevent isolates virulent against the major gene from amplifying in the long term but the quantitative resistance significantly delayed for 5years the loss of effectiveness of the qualitative resistance and disease severity was maintained at a low level on the genotype with both types of resistance after the fungus population had adapted to the major gene. We also showed that diversity of AvrLm6 virulence alleles was comparable in isolates recovered after the recurrent selection on lines carrying either the major gene alone or in combination with quantitative resistance: a single repeat-induced point mutation and deletion events were observed in both situations. Breeding varieties which combine qualitative and quantitative resistance can effectively contribute to disease control by increasing the potential for durability of major resistance genes.
Three avirulence genes, AvrLm1, AvrLm6, and AvrLm4-7, were recently identified in Leptosphaeria maculans and found to be localized as solo genes within large noncoding, heterochromatin-like regions ...mainly composed of retrotransposons, truncated and degenerated by repeat-induced point mutation (RIP). The Rlm6 resistance gene has been overcome within 3 years in outdoor experiments in France and, here, we investigate the molecular basis of evolution toward virulence at the AvrLm6 locus. A region of 235 kb was sequenced in a virulent isolate and showed the deletion of AvrLm6 and three divergent mosaics of retrotransposons. AvrLm6 was found to be absent from 66% of 70 virulent isolates, with multiple events of deletion. The sequencing of virulent alleles in 24 isolates revealed a few cases of point mutations that had created stop codons in the sequence. The most frequent mutation events, however, were RIP, leading to the modification of 4 to 9% of the bases compared with the avirulent allele and generating 2 to 4 stop codons. Thus, RIP is described for the first time as an efficient mechanism leading to virulence and the multiple patterns of mutation observed suggest that multiple RIP events could occur independently in a single field population during 1 year.
Fast ozone (O3) measurements (1–50 Hz) in the atmosphere are required for airborne studies and for the measurement of ground-based O3 fluxes by the eddy covariance technique. Fast response analyzers, ...based on heterogeneous chemiluminescence, need dye coated sensor discs on which the chemiluminescence is generated. In this study, we present three new preparation methods for those sensor discs. Currently available sensor discs exhibit a fast temporal decay of sensitivity, resulting in short duty times which is troublesome for many field applications. To produce sensor discs that provide more stable signals over time, three dyes and nine energy transfer reagents were tested (as well as different stoichiometric mixtures). The resulting optimal method saves 80% of the solid chemicals and shows a duty ozone dose that is prolonged by a factor of 3.5, revealing the same average sensitivity as currently available discs. In addition, we observed a strong effect of the adsorption matrix on the O3 sensitivity, although silica discs from the same manufacturer were used. Application of the new sensor discs during field measurements showed that the results are consistent with the laboratory data.
Blackleg (stem canker) caused by the fungus Leptosphaeria maculans is one of the most damaging diseases of oilseed rape (Brassica napus). Crop relatives represent a valuable source of “new” ...resistance genes that could be used to diversify cultivar resistance. B. rapa, one of the progenitors of B. napus, is a potential source of new resistance genes. However, most of the accessions are heterozygous so it is impossible to directly detect the plant genes conferring specific resistance due to the complex patterns of avirulence genes in L. maculans isolates. We developed a strategy to simultaneously characterize and introgress resistance genes from B. rapa, by homologous recombination, into B. napus. One B. rapa plant resistant to one L. maculans isolate was used to produce B. rapa backcross progeny and a resynthesized B. napus plant from which a population of doubled haploid lines was derived after crossing with natural B. napus. We then used molecular analyses and resistance tests on these populations to identify and map the resistance genes and to characterize their introgression from B. rapa into B. napus. Three specific genes conferring resistance to L. maculans (Rlm1, Rlm2 and Rlm7) were identified in B. rapa. Comparisons of genetic maps showed that two of these genes were located on the R7 linkage group, in a region homologous to the region on linkage group N7 in B. napus, where these genes have been reported previously. The results of our study offer new perspectives for gene introgression and cloning in Brassicas.
The value of Katanning Early Maturing (KEM) breeding lines from Western Australia, derived from Brassica napus x B. juncea crosses, was assessed as a source of germplasm for resistance to blackleg ...disease (caused by Leptosphaeria maculans) in spring-type oilseed rape cultivars. The stability of blackleg resistance in these KEM lines was related to key cytological characteristics to determine why there are poor levels of introgression of this resistance into progeny. Promising recombinant KEM lines were crossed with the spring-type B. napus cv. Dunkeld, which has useful polygenic resistance to blackleg, and screened for resistance. The lines were analyzed cytologically for pairing of bivalents in each generation to aid in the selection of stable recombinant lines. KEM recombinant lines showing regular meiotic behavior and a high level of blackleg resistance were obtained for the first time. We also showed that the stable introgression of the B. juncea resistance from the KEM lines into a 'Dunkeld' background was possible. Inoculation of selfing and backcross populations with isolates of L. maculans having different AvrLm genes indicated that the B. juncea resistance gene, Rlm6, had been introgressed into a B. napus spring-type cultivar carrying polygenic resistance. The combination of both resistances would enhance the overall effectiveness of resistance against L. maculans. This is clearly needed in Australia and France where cultivars relying upon single dominant gene-based resistance for their effectiveness have proved not durable.
Breeding varieties for increased disease resistance is a major means to control epidemics. However, the deployment of resistance genes through space and time drives the genetic composition of the ...pathogen population, directionally altering pathotype frequencies. In France,
Leptosphaeria maculans
causes stem canker on
Brassica napus
oilseed rape crops but not on
B. juncea
condiment mustard. Prior to the deployment of winter
B. napus
varieties with
Rlm6
resistance gene introduced from
B. juncea
, the aim of our study was to investigate if this deployment could impact disease control in condiment mustard. We assessed the presence of resistance genes against the disease in a set of current French
B. juncea
varieties and breeding lines with a set of five differential
L. maculans
isolates carrying know Avr/avr genes.
Rlm6
was detected in all 12 condiment mustard varieties.
Rlm5
was also detected in eight varieties. No additional resistance genes were detected with the set of isolates used. Because the frequency of isolates virulent on
Rlm6
is very low in France, these results indicate that
Rlm6
gene is a major component of disease control in the French
B. juncea
mustards tested. Using
Rlm6
in oilseed rape varieties will very likely induce an increase in frequency of
Rlm6
virulent isolates. This raises the acute concern of a wise deployment of oilseed rape around the condiment mustard growing area. Scientific knowledge on adaptation dynamics, spatial segregation of crops and cooperation between actors is currently available in order to mitigate the risk and advert negative consequences of the introduction of
Rlm6
resistance gene in oilseed rape varieties.
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