Search

Deux situations dans la lutte contre le charançon rouge du palmier sont à prendre en considération : i) la situation de la zone dite ' centre-atlantique ' pour laquelle une éradication du CRP est possible et ii) la situation de la zone dite ' méditerranéenne ' pour laquelle la recherche d'une stabilisation des populations de CRP est envisageable mais demeure ambitieuse et coûteuse. Pour la zone ' méditerranéenne ', l'alternative se présente de la manière suivante : i) stabiliser si possible la population de CRP en sachant que le coût sera élevé, et chercher à limiter son aire d'extension géographique ou ii) envisager de limiter la protection à certains palmiers notamment pour leur importance patrimoniale et de proposer des espèces végétales de remplacement pour les zones non protégées.

The genera Agrobacterium, Allorhizobium, and Rhizobium belong to the family Rhizobiaceae. However, theplacement of a phytopathogenic group of bacteria, the genus Agrobacterium, among the nitrogen-fixingbacteria and the unclear position of Rhizobium galegae have caused controversy in previous taxonomicstudies. To resolve uncertainties in the taxonomy and nomenclature within this family, the phylogeneticrelationships of generic members of Rhizobiaceae were studied, but with particular emphasis on the taxaincluded in Agrobacterium and the “R. galegae complex” (R. galegae and related taxa), using multilocussequence analysis (MLSA) of six protein-coding housekeeping genes among 114 rhizobial and agrobac-terial taxa. The results showed that R. galegae, R. vignae, R. huautlense, and R. alkalisoli formed a separateclade that clearly represented a new genus, for which the name Neorhizobium is proposed. Agrobacteriumwas shown to represent a separate cluster of mainly pathogenic taxa of the family Rhizobiaceae. A. vitisgrouped with Allorhizobium, distinct from Agrobacterium, and should be reclassified as Allorhizobium vitis,whereas Rhizobium rhizogenes was considered to be the proper name for former Agrobacterium rhizogenes.This phylogenetic study further indicated that the taxonomic status of several taxa could be resolved bythe creation of more novel genera.

International audience; Agrobacteria is the name usually given to bacteria inducing the crown gall disease to numerous crops. Agrobacteria belong to different species that appeared phylogenetically intermingled with species of rhizobia, the symbiotic bacteria that induce nitrogen fixing nodules to Fabaceae, leading authors to propose the inclusion of agrobacteria within the genus Rhizobium. This proposal was contested by other authors who suggested to keep the early division in Agrobacterium and Rhizobium up to a clarification of the phylogeny. In the present work, we applied the multi-locus sequence analysis method to construct a robust phylogeny of agrobacteria and close relatives in order to clarify their taxonomic positions. Results allowed the clear assignation of bacteria to clades that fit to bona fide species previously defined by genomic methods (i.e. genomospecies). As a consequence MLSA could be used as a proxy to delineate novel species that virtually fit with the genomic definition of a species. Plant pathogenic agrobacteria were found to belong to different genomospecies distributed into three clades. Two clades encompasses species that are either bona fide Rhizobium species such as R. tropici for R. rhizogenes or more different taxa such as R. taibainensis and Allorhizobium undicola for R. vitis. A third clade encompasses all other plant pathogenic agrobacteria as well as benign plant commensals, plant growth promoting rhizobacteria, industrial strains and human opportunists. We propose to consider this large clade for a novel delineation of the genus Agrobacterium. As such, Agrobacterium spp. would include A. rubi, A. larrymooreii, A. skierniewicense (formerly R. skierniewicense), A. viscosum, A. radiobacter (i.e. the genomospecies G4 of Agrobacterium), A. fabrum, A. nepotum (formerly R. nepotum), A. pusense (formerly R. pusense) as well as bona fide genomospecies that have not yet received Latin binominals. An unifying trait of Agrobacterium members is their unique genome architecture characterized by the presence of a linear chromid (e.g. secondary chromosome) and the consequential occurrence in their genome of telA, the gene that encodes the protelomerase required to process the replication of the linear chromid. These particular traits strongly support the present proposal for a renewed definition of the genus Agrobacterium.

In the Sidi M’djahed nursery (Algeria), over 60,000 eucalyptus (Eucalyptus occidentalis) plantlets exhibited tumour-like growths localized at the crown of the plants that resembled crown galls caused by Agrobacterium tumefaciens. Bacteria colonizing the galls were isolated and purified. Most (22 out of 24) of the isolates had cultural and biochemical characteristics similar to those of strains of the biovar 1 of A. tumefaciens. Twenty out of 22 Agrobacterium isolates induced tumour formation on various test plants. In PCR experiments, DNA extracted from these virulent strains yielded an amplification signal corresponding to a 247-bp fragment located within the virulence region of nopaline type Ti plasmid. Consistent with this, the opine nopaline was detected in the tumours induced on test plants – but not on eucalyptus plants. Nopaline was degraded by the 20 pathogenic isolates that were also sensitive to agrocin 84, indicating the presence of a nopaline-type pTi in these strains. The chromosomal region encoding the 16S rRNA was analyzed in a sub-population of the pathogenic agrobacterial isolates. The analyzed strains were found to belong to the ribogroup of the reference strain B6. Interestingly, Eucalyptus camaldulensis and Eucalyptus cladocalyx grown in the same nursery and in the same soil substrate developed no galls.

The susceptibility of different fruit rootstocks to crown gall disease was investigated in greenhouse and field experiments with numerous strains of Agrobacterium tumefaciens over three years. Plants were inoculated in the roots and shoots for pot experiments. Field experiments were performed in a naturally contaminated nursery plot. The genotypes Prunus dulcis and P. persica showed a high level of susceptibility to A. tumefaciens. Among the stone rootstocks, bitter almond was highly susceptible in all experiments. Apricot and Cadaman rootstocks displayed low susceptibility but larger galls, showing that there was no relation between rootstock susceptibility and gall size. Among pome rootstocks, quince BA29 was resistant to the disease, while MM106 was susceptible in potted trials; however, in the field essays, pome rootstocks did not become galled, possibly because the strains had selected for and adapted to stone rootstocks.

International audience

Une comparaison entre plants sains et plants spontanément atteints de crown gall a été effectuée dans un dispositif expérimental de comparaison de clones de peupliers grisards (obtenus par croisements contrôlés entre Populus tremula et P alba). Les arbres présentant des tumeurs au collet sont plus petits que les arbres sains (- 18%) alors que la présence de tumeurs limitées aux racines n’a pas d’effet significatif. La fréquence des plants atteints de crown gall a été notée chez 31 clones de grisards lors de plusieurs campagnes de production dans la même pépinière. En dépit d’importantes variations annuelles et clonales, certaines familles d’hybrides sont toujours moins atteintes que d’autres, suggérant l’existence d’une base génétique de résistance au crown gall exploitable en sélection. En outre, une étude épidémiologique a montré la présence de la même population caractéristique d’Agrobacterium tumefaciens pathogène dans plusieurs pépinières de grisards. Ceci indique que les agrobactéries à l’origine de la contagion du crown gall chez les peupliers Leuce sont probablement transportées de pépinière en pépinière par les plantes elles-mêmes. La diffusion de grisards porteurs de bactéries pathogènes latentes fait courir un grave risque épidémiologique aux pépinières forestières car les plasmides Ti en cause peuvent potentiellement infecter de nombreuses essences forestières. La sélection de clones résistant au crown gall doit, de ce fait, être accompagnée de mesures prophylactiques sévères visant à limiter la diffusion des plants porteurs sains., Crowngalled and crown-gall free plants of Leuce poplar (Populus tremula x P alba) were observed to be spontaneously contaminated by Agrobacterium tumefaciens in experimental plots primarly designed to compare agronomic properties of poplar clones. This offered a rare possibility for the survey of clonal responses to spontaneous attacks of crown gall in the field, with the aim of serving as basic reference for breeding purposes. This study took into account the vigour and aspect of the plant, the crown gall frequency of the clone and an estimation of the epidemiological risks in nurseries. Regarding the effect of crown gall on shoot size, 2-yr-old poplars harbouring crown galls were statistically shorter than apparently healthy plants growing in the same plot (table I). Regarding the effect of size and location of galls on shoot growth inhibition, even small galls (< 2 cm in diameter) reduced relative shoot lengths (- 8%), but the biggest growth inhibitions were recorded with the largest galls (- 19% with galls > 5 cm). The location of the galls at the crown instead of on the roots had a significant effect : trees with crown-located galls were 18% shorter than healthy trees, while root-located galls had no significant effect (table II). Regarding clonal and family effect of spontaneous crown gall frequencies, the frequencies of crown-galled plants of 31 clones belonging to 7 breeding families were recorded over a period of several years in a single nursery (table III). In spite of significant annual and clonal variations, some hybrid families were constantly less crown-galled than others, indicating a possible genetic basis for selection of resistant poplars. Regarding the epidemiological survey, the pathogenic population of A tumefaciens, already described in a single poplar tree nursery (Nesme et al, 1987) was looked for in galls originating from several other nurseries. This showed the constant occurrence of the same pathogenic population of agrobacteria in every Leuce poplar nursery. This indicates that agrobacteria, which mediates crown-gall contagion in Leuce poplars were mainly carried from nursery to nursery by poplar plants themselves. Therefore, selection of crown gall-resistant poplar clones must be combined with severe prophylactic measures to limit the diffusion of contaminated plants carrying latent pathogenic agrobacteria.