Major biocontrol of plant tumors targets tRNA synthetase.
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Crops can be devastated by pathogenic strains of Agrobacterium tumefaciens that cause crown gall tumors. This devastation can be prevented by the nonpathogenic biocontrol agent A. radiobacter K84, which prevents disease by production of the "Trojan horse" toxin agrocin 84, which is specifically imported into tumorgenic A. tumefaciens strains to cause cell death. We demonstrate that this biocontrol agent targets A. tumefaciens leucyl-tRNA synthetase (LeuRS), an essential enzyme for cell viability, while the agent itself survives by having a second, self-protective copy of the synthetase. In principle, this strategy from nature could be applied to other crop diseases by direct intervention. (+info)
The effect of cellulose overproduction on binding and biofilm formation on roots by Agrobacterium tumefaciens.
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Agrobacterium tumefaciens growing in liquid attaches to the surface of tomato and Arabidopsis thaliana roots, forming a biofilm. The bacteria also colonize roots grown in sterile quartz sand. Attachment, root colonization, and biofilm formation all were markedly reduced in celA and chvB mutants, deficient in production of cellulose and cyclic beta-(1,2)-D-glucans, respectively. We have identified two genes (celG and cell) in which mutations result in the overproduction of cellulose as judged by chemical fractionation and methylation analysis. Wild-type and chvB mutant strains carrying a cDNA clone of a cellulose synthase gene from the marine urochordate Ciona savignyi also overproduced cellulose. The overproduction in a wild-type strain resulted in increased biofilm formation on roots, as evaluated by light microscopy, and levels of root colonization intermediate between those of cellulose-minus mutants and the wild type. Overproduction of cellulose by a nonattaching chvB mutant restored biofilm formation and bacterial attachment in microscopic and viable cell count assays and partially restored root colonization. Although attachment to plant surfaces was restored, overproduction of cellulose did not restore virulence in the chvB mutant strain, suggesting that simple bacterial binding to plant surfaces is not sufficient for pathogenesis. (+info)
Both hypersensitive and non-hypersensitive responses are associated with resistance in Salix viminalis against the gall midge Dasineura marginemtorquens.
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Hypersensitivity responses (HR) play a major role in plant resistance to pathogens. It is often claimed that HR is also important in plant resistance to insects, although there is little unambiguous documentation. Large genotypic variation in resistance against the gall midge Dasineura marginemtorquens is found in Salix viminalis. Variation in larval performance and induced responses within a full-sib S. viminalis family is reported here; 36 sibling plants were completely resistant (larvae died within 48 h after egg hatch, no gall induction), 11 plants were totally susceptible, 25 plants were variable (living and dead larvae present on the same plant). Resistance was associated with HR, but to different degrees; 21 totally resistant genotypes showed typical HR symptoms (many distinct necrotic spots) whereas the remaining 15 genotypes showed no, or very few, such symptoms. Hydrogen peroxide, used as a marker for HR, was induced in genotypes expressing HR symptoms but not in resistant genotypes without symptoms, or in susceptible genotypes. These data suggest that production of hydrogen peroxide, and accompanying cell death, cannot explain larval mortality in the symptomless reaction. Another, as yet unknown, mechanism of resistance may be present. If so, then it is possible that this unknown mechanism also contributes to resistance in plants displaying HR. The apparent complexity observed in this interaction, with both visible and invisible plant responses associated with resistance against an adapted insect species, may have implications for the study of resistance factors in other plant-insect interactions. (+info)
The Agrobacterium tumefaciens virD3 gene is not essential for tumorigenicity on plants.
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Genetic studies indicate that three of the four polypeptides encoded within the virD operon of the Agrobacterium tumefaciens Ti plasmid are essential for virulence. In order to determine whether the fourth polypeptide, VirD3, has any role in virulence, complementation analysis was used. An A. tumefaciens strain, A348 delta D, which lacked the entire virD operon in the Ti plasmid pTiA6, was constructed. Plasmids containing defined regions of the virD operon were introduced into this strain, and virulence was tested by the strains' abilities to form tumors on Kalanchoe leaves, tomato stems, and potato tubers. As expected, deletion of the virD operon led to an avirulent phenotype. The virulence of this strain could be restored by providing virD1, virD2, and virD4 in trans. No requirement for virD3 in tumor formation was observed in these assays. (+info)
Inbreeding ancestors: the role of sibmating in the social evolution of gall thrips.
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We used microsatellite data to estimate levels of inbreeding in four species of solitary gall thrips that are in the same clade as the six species with soldier castes. Three of the four species were highly inbred (Fis 0.54-0.68), and the other apparently mated randomly (Fis near zero). These estimates, combined with previous data from species with soldiers, suggest that inbreeding is a pervasive life-history feature of the gall-inducing thrips on Australian Acacia. Mapping of inbreeding estimates onto the phylogeny of the gall inducers showed that the ancestral lineage that gave rise to soldiers was apparently highly inbred, and therefore, inbreeding could have played a role in the origin of sociality within this group. Moreover, there was a trend from high levels of inbreeding at the origin of soldiers to low levels in the most derived species with soldiers, which exhibits the highest levels of reproductive division of labor and soldier altruism. These patterns are consistent with considerations from population genetics, which show that the likelihood of the origin of soldier altruism is higher in inbreeding populations but that, once soldiers have evolved, a reduction in inbreeding levels may facilitate the evolution of enhanced division of labor and reproductive skew. (+info)
Morphogenesis of galls induced by Baccharopelma dracunculifoliae (Hemiptera: Psyllidae) on Baccharis dracunculifolia (Asteraceae) leaves.
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The commonest insect gall on Baccharis dracunculifolia (Asteraceae) leaves is induced by Baccharopelma dracunculifoliae (Hemiptera, Psyllidae). The gall-inducing insect attacks young leaves in both the unfolded and the fully expanded stages. Four developmental phases were observed in this type of gall: 1) A folding phase, during which the leaf lamina folded upward alongside the midrib and the edges of the upper portion of the leaf approached each other, forming a longitudinal slit. A single chamber was formed on the adaxial surface of the leaf; 2) A swelling phase, in which the folded leaf tissues thickened and the edges of the leaf drew closer together, narrowing the slit. In this phase the gall matured, turning succulent, fusiform and pale green. The single nymphal chamber was lined with white wax and was able to house from one to several nymphs; 3) A dehiscence phase, characterized by the opening of the slit to release inducers; and 4) A senescence phase, when the gall turned dark and dry. The dermal system of the mature gall was composed of a single-layered epidermis. The mesophyll was swollen, and the swelling was due mainly to hyperplasia of the parenchyma. The vascular tissues along the midrib vein were conspicuous and the perivascular fibers resembled parenchymal cells. The hypertrophied secretory cavities contained low lipophylic content. This gall does not form nutritive tissue, but salivary sheaths left by the inducers were observed near the parenchyma, vascular bundles and secretory cavities. This study complements our current knowledge of gall biology and sheds further light on the plasticity of plant tissues stimulated by biotic factors. (+info)
Galls and gall makers in plants from the Pe-de-Gigante Cerrado Reserve, Santa Rita do Passa Quatro, SP, Brazil.
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Thirty-six morphologically different types of galls were obtained in leaves, leaflets, veins, petioles, stems, tendrils and flower buds from twenty-five species of plants in the Pe-de-Gigante Reserve, municipality of Santa Rita do Passa Quatro, state of Sao Paulo, Brazil. The host plant species belong to the closely related families Anacardiaceae, Annonaceae, Asteraceae, Bignoniaceae, Caryocaraceae, Erythroxylaceae, Fabaceae, Malpighiaceae, Melastomataceae, Myrtaceae, Ochnaceae, Polygalaceae, Sapindaceae, Sapotaceae, and Smilacaceae. The most common gall makers included Cecidomyiidae (Diptera), Pteromalidae (Hymenoptera) and Diaspididae (Sternorrhyncha-Hemiptera). This is the first report of galls found in the following plant genera: Gochnatia (Asteraceae), Distictela (Bignoniaceae), Banisteriopsis (Malpighiaceae), Ouratea (Ochnaceae), and Bredemeyera (Polygalaceae). The results of this work contribute to the body of knowledge about the relationship among host plants, gall makers, and the gall morphology of Pe-de-Gigante Cerrado Reserve. (+info)
Bases of biocontrol: sequence predicts synthesis and mode of action of agrocin 84, the Trojan horse antibiotic that controls crown gall.
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Agrobacterium radiobacter K84, used worldwide to biocontrol crown gall disease caused by Agrobacterium tumefaciens, produces an antiagrobacterial compound called agrocin 84. We report the nucleotide sequence of pAgK84, a 44.42-kb plasmid coding for production of this disubstituted adenine nucleotide antibiotic. pAgK84 encodes 36 ORFs, 17 of which (agn) code for synthesis of or immunity to agrocin 84. Two genes, agnB2 and agnA, encode aminoacyl tRNA synthetase homologues. We have shown that the toxic moiety of agrocin 84 inhibits cellular leucyl-tRNA synthetases and AgnB2, which confers immunity to the antibiotic, is a resistant form of this enzyme. AgnA, a truncated homologue of asparaginyl tRNA synthetase could catalyze the phosphoramidate bond between a precursor of the methyl pentanamide side group and the nucleotide. We propose previously undescribed chemistry, catalyzed by AgnB1, to generate the precursor necessary for this phosphoramidate linkage. AgnC7 is related to ribonucleotide reductases and could generate the 3'-deoxyarabinose moiety of the nucleoside. Bioinformatics suggest that agnC3, agnC4, and agnC6 contribute to maturation of the methyl pentanamide, whereas agnC2 may produce the glucofuranose side group bound to the adenine ring. AgnG is related to bacterial exporters. An agnG mutant accumulated agrocin 84 intracellularly but did not export the antibiotic. pAgK84 is transmissible and encodes genes for conjugative DNA processing but lacks a type IV secretion system, suggesting that pAgK84 transfers by mobilization. By sequence analysis, the deletion engineered into pAgK1026 removed the oriT and essential tra genes, confirming the enhanced environmental safety of this modified form of pAgK84. (+info)