Comparative behavior of Ralstonia solanacearum biovar 2 in diverse plant species

Ralstonia solanacearum causes bacterial wilt in numerous plant species worldwide. Although biovar 2 mostly affects solanaceous crops, identification of new hosts remains a matter of concern since there is still no clear-cut distinction between host and nonhost plants. In this work we provide data based on histological studies on the status of 20 plant species, most of them of potential interest in crop rotation. Plants were watered with a P-glucuronidase-expressing derivative of R. solanacearum biovar 2, and after a month of incubation, sections of roots and stems were analyzed to localize the pathogen on surface, in cortex and/or xylem. Depending on whether the xylem was colonized or not, plants were classified as hosts or nonhosts, respectively. Hosts generally affected in a few xylem vessels or occasionally in all xylem bundles were classified as tolerant. These included some cabbage, kidney bean, and rutabaga cultivars, and the weed bittersweet nightshade (Solanum dulcamara). Nonhosts were the cultivars tested of alfalfa, barley, black radish, carrot, celery, colocynth, fennel, fiber flax, field bean, field pea, horseradish, maize, and zucchini. However, barley and maize, though nonhosts, may act as reservoirs for the pathogen. The present work constitutes a basis for further studies on cropping systems in fields where R. solanacearum has been detected

Bacteriocin Typing of Burkholderia (Pseudomonas) solanacearum Race 1 of the French West Indies and Correlation with Genomic Variation of the Pathogen

Burkholderia solanacearum race 1 isolates indigenous to the French West Indies were characterized by bacteriocin typing and two genomic fingerprinting methods: pulsed-field gel electrophoresis of genomic DNA digested by rare-cutting restriction endonucleases (RC-PFGE) and PCR with primers corresponding to repetitive extragenic palindromic (REP), enterobacterial repetitive intergenic consensus (ERIC), and BOX elements (collectively known as rep-PCR). The survey comprised 24 reference strains and 65 isolates obtained from a field trial in Guadeloupe in 1993. Comparison of the data identified RC-PFGE as the most discriminatory method, delineating 17 pulsed-field gel profile types. rep-PCR and bacteriocin typing identified nine rep-PCR profile types and nine bacteriocin groups. Independent determination of similarity coefficients and clustering of RC-PFGE and rep-PCR data identified six groups common to both sets of data that correlated to biovar and bacteriocin groups. Further study of bacteriocin production in planta gave results consistent with in vitro bacteriocin typing. It was observed that spontaneous bacteriocin-resistant mutants exhibited a cross-resistance to other bacteriocins as identified by the typing scheme and that such mutants possessed a selective advantage for growth over isogenic nonmutants in the presence of a bacteriocin. The results are significant in the search for biological control of disease by nonpathogenic mutants of the wild-type organism

Effect of land abandonment on soil organic carbon fractions along a Mediterranean precipitation gradient

Land abandonment has been the main land use change in rural Mediterranean areas over the last decades. The secondary succession following land abandonment is strongly affected by precipitation, which in turn determines the change of soil organic carbon (SOC) and other soil properties. However, SOC consists of different fractions with contrasting resistance to decomposition that are differently affected by land abandonment. The aim of this study is assessing the evolution of different carbon fractions after land abandonment along a West– East precipitation gradient in southern Spain (Gaucín (GAU): 1080.5 mm y−1–Almogía: 650 mm y−1–Gérgal (GER): 350mmy−1). In each site, samples from a chronosequence of land abandonment were size fractionated, yielding three fractions with increasing degrees of carbon stability (coarse, intermediate, fine). Following land abandonment, there was a transitional grassland state in GAU and ALM that promoted a quick SOC gain of 0.87 and 0.42 kg m−2, respectively. The majority of this gain, i.e. 79.2% in GAU and 71.4% in ALM, occurred in the most stable fractions. SOC was rapidly stabilised due to the presence of litter with a low C:N ratio. The colonisation of grasslands, in GAU and ALM, or croplands, in GER, by woody vegetation triggered SOC accumulation of 0.07, 0.78 and 0.74 kgm−2. However, in GAU there was at the same time a SOC loss in the most stable fractions (0.23 kgm−2). In ALMand GER a reverse trendwas observed: SOC accumulated in the intermediate and fine fractions. SOC accumulation in the coarse fraction was related to the lowquality of the litter fromwoody vegetation, that had a higher C:N ratio than litter from grasslands. The largest effect of precipitation was found in the fine fraction, whereas SOC in the coarse fraction was mostly controlled by land abandonment and related factors, such as vegetation, litter quality and soil conditions

Changes in soil organic carbon pools along a chronosequence of land abandonment in southern Spain

Land abandonment is the dominant land use change in theMediterranean and the resulting vegetation recovery has a strong influence on soil organic carbon (SOC). Yet, the gradual changes in SOC pools during secondary succession remain poorly studied. As SOC is a mixture of pools with distinct functional properties, isolating these pools may provide a better understanding of the decadal SOC dynamics. Topsoil samples were collected along a chronosequence of cropland abandonment in the region north of Málaga (Spain). Five fractions were isolated: particulate organic matter (POM), dissolved organic carbon (DOC), SOC bound to silt and clay (s + c), SOC attached to sand particles or occluded in aggregates (S+A) and a chemically resistant fraction (rSOC). The significant increase in the S+A mass over time indicated aggregation processes along the vegetation recovery stages. Carbon concentrations within S + A or rSOC fractions did not change over time. In contrast, carbon in the s + c fraction significantly increased after three decades from 7.9 g C·kg−1 to 20.4 g C·kg−1 at the final stage of the chronosequence. Hence, in addition to the frequently reported increases in POM, carbon was also allocated to the more stable SOC fractions. Taking active carbon (DOC + POM; high turnover rate) and intermediate carbon (s+c, S+A; lowturnover rate) as indicators for carbon turnover, the proportion of active carbon increased from 17% to 39% along the chronosequence. Consequently, the proportion of slow cycling carbon decreased from 72% to 54%. These results showthat the proportion of labile carbon increases along the secondary succession while, at the same time, the slow cycling carbon fractions sequester carbon (from 0.6 to 1.12 kg C·m−2)

Impact of three selected biotechnological strategies for potato pathogen control on the indigenous soil microbiot

In this project, we investigate possibilities to control plant diseases caused by bacteria according to three different approaches, i.e. genetic modification of plants (two approaches) and application of avirulent competitive bacterial strains. For that purpose we selected Ralstonia solanacearum and Erwinia carotovora as model organisms for controlling diseases, caused by both agents, in genetically modified potato lines and constructed avirulent mutants of R. solanacearum for suppression of brownrot in unmodified potato lines. Genetically modified potato lines are obtained from MPB Cologne (Germany) and from the Austrian Research Centre , Seibersdorf and these lines possesses insertions of genes encoding for bacterial lytic proteins. The lines obtained from MPB Cologne, denoted DL11, DL 12 and DL13 containT4 Lysozym gene from bacterial fage T4, whereas lines mas2C4-mas1 C4 and mas2C4-mas1 Att posses genes cecropin and both cecropin and attacin, respectively, from the giant silk moth Hyalophora cecropia. Further, mutants of R. solanacearum corrupted in expression of virulence genes via transposon insertion into the hrpB gene are used to control brownrot disease in potato. Using transgenic potato lines and non-pathogenic R. solanacearum strains, greenhouse and field studies are performed to investigate the efficacy of both approaches. Next to efficacy screening, the impact of heterologous gene expression and the presence of non-pathogenic R. solanacearum mutants on the microbial phytosphere community of potato will be investigated