A collection of 200 bacterial isolates recovered from citrus plants (Citrus limon, Citrus sinensis, and Citrus reticulata), Medicago truncatula and Laurus nobilis, was established. In vitro screening indicated that 28 isolates exhibited an inhibitory activity against the vascular pathogens Phoma tracheiphila and Verticillium albo-atrum. Isolates were screened according to their hydrolytic activities, plant growth-promoting bacteria (PGPB) abilities, as well as for the presence of nonribosomal peptide synthetase (NRPS) genes responsible of the lipopeptide biosynthesis. The results were positive for 16 isolates which exhibited at least two PGPB activities and a single NRPS gene. Genetic diversity of the selected isolates was studied using random amplified polymorphic DNA (RAPD) and repetitive element PCR (REP) tools that showed clustering of strains into three major groups (I, II, and III) (i, ii, and iii), respectively. Clustering was further confirmed by the 16S rDNA sequencing that assigned nine isolates to Bacillus velezensis, four isolates to Bacillus methyltrophicus, one isolate to Bacillus amyloliquefaciens, and two isolates to Bacillus mojavensis. Organ-bacterial genotype interaction as well as positive correlation with NRPS genes are discussed.
Surveys of yellowing viruses under nonheated and geothermal heated plastic tunnels and in open field crops of melon (Cucumis melo), cucumber (C. sativus), zucchini (Cucurbita pepo), squash (C. maxima), watermelon (Citrullus lanatus), and ware cucurbit (Ecballium elaterium) were carried out year-round during 2000–2001, 2003, and 2004 in the major cucurbit-growing areas in Tunisia. Severe yellowing symptoms on older leaves of cucurbits were observed in open fields and under plastic-tunnel production systems. These yellowing symptoms and large populations of aphids (Aphis gossypii) on a diversity of cucurbit crops in Tunisia support the hypothesis of a viral cause of the disease. Virus identification using double-antibody sandwich enzyme-linked immunosorbent assay (DAS-ELISA), followed by reverse transcription–polymerase chain reaction (RT-PCR) and immunocapture (IC)-RT-PCR showed that Cucurbit aphid-borne yellows virus (CABYV) was largely distributed in melon, cucumber, zucchini, squash, and watermelon crops. Ware cucurbit (E. elaterium) and lettuce (Lactuca sativa) crops were identified as potential CABYV reservoirs. The RT-PCR-amplified partial coat protein (CP) and P4 genes were cloned and sequenced from nine Tunisian CABYV isolates. CP and P4 gene nucleotide and amino acid sequence comparisons as well as phylogenetic reconstructions showed that the Tunisian isolates clustered into two major subgroups. Comparisons with CABYV sequences retrieved from GenBank showed high nucleotide and CP amino acid identities, and close relationships of the Tunisian isolates with Italian and French isolates.
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