Quantitative Resistance to Verticillium Wilt in Medicago truncatula Involves Eradication of the Fungus from Roots and Is Associated with Transcriptional Responses Related to Innate Immunity

Resistance mechanisms to Verticillium wilt are well studied in tomato, cotton and Arabidopsis, but much less in legume plants. Because legume plants establish nitrogen-fixing symbioses in their roots, resistance to root-attacking pathogens merits particular attention. The interaction between the soil-borne pathogen Verticillium alfalfae and the model legume Medicago truncatula was investigated using a resistant (A17) and a susceptible (F83005.5) line. As shown by histological analyses, colonization by the pathogen was initiated similarly in both lines. Later on, the resistant line A17 eliminated the fungus, whereas the susceptible F83005.5 became heavily colonized. Resistance in line A17 does not involve homologs of the well-characterized tomato Ve1 and V. dahliae Ave1 genes. A transcriptomic study of early root responses during initial colonization (i.e. until 24 h post-inoculation) similarly was performed. Compared to the susceptible line, line A17 displayed already a significantly higher basal expression of defense-related genes prior to inoculation, and responded to infection with up-regulation of only a small number of genes. Although fungal colonization was still low at this stage, the susceptible line F83005.5 exhibited a disorganized response involving a large number of genes from different functional classes. The involvement of distinct phytohormone signaling pathways in resistance as suggested by gene expression patterns was supported by experiments with plant hormone pretreatment before fungal inoculation.Gene co-expression network analysis highlighted five main modules in the resistant line, whereas no structured gene expression was found in the susceptible line. One module was particularly associated to the inoculation response in A17. It contains the majority of differentially expressed genes, genes associated with PAMP perception and hormone signaling, and transcription factors. An in silico analysis showed that a high number of these genes also respond to other soil-borne pathogens in M. truncatula, suggesting a core of transcriptional response to root pathogens. Taken together, the results suggest that resistance in M. truncatula line A17 might be due to innate immunity combining preformed defense and PAMP-triggered defense mechanisms, and putative involvement of abscisic acid

Plusieurs niveaux de contrôle sont mis en jeu lors de flétrissement bactérien chez la légumineuse modèle Medicago truncatula

Nous présentons l étude de l interaction entre la bactérie pathogène racinaire Ralstonia solanacearum et la légumineuse modèle Medicago truncatula. Un pathosystème avec les lignées A17 et F83005.5, respectivement sensible et résistante à la souche GMI1000, a été mis en place avec une procédure d inoculation sur racines intactes. Ce dispositif expérimental nous a permis de suivre le processus infectieux, de la pénétration de la bactérie par l extrémité racinaire au développement des symptômes foliaires. L analyse des étapes précoces de l interaction a permis de décrire l apparition de symptômes racinaires qui se mettent en place rapidement après l infection, que les lignées soient résistantes ou sensibles à la bactérie. Un arrêt de croissance de la racine s'observe dès 24 heures post-inoculation, ainsi qu une mortalité de l épiderme de l extrémité racinaire. Ces phénotypes sont notés suite à des inoculations avec de faibles concentrations bactériennes, et ce sur plusieurs espèces hôtes ou non-hôtes testées. La mise en place des symptômes racinaires est dépendante de l appareil de sécrétion de type III. Un crible de mutants d effecteurs de type III de la souche GMI1000, basé sur l apparition des symptômes racinaires, a permis de montrer que des pools différents d effecteurs interviennent chez A17 et F83005.5. Chez la lignée sensible A17, deux effecteurs sont principalement impliqués, Gala7 et AvrA. L étude de la colonisation de cette lignée a montré que le mutant gala7 ne pénètre pas la plante et n induit pas de symptômes de flétrissement. Le mutant avrA s est révélé capable d induire la maladie chez la lignée A17 mais de manière nettement réduite par rapport à la souche sauvage. L analyse des extrémités racinaires des lignées sensible et résistante infectées par la souche GMI1000 a révélé qu au niveau des parois de l endoderme, la présence de lignine est induite de manière plus précoce chez la lignée résistante. Des phénomènes de division cellulaire ont été identifiés autour du cylindre central et semblent également liés à une restriction de la propagation bactérienne. Au niveau du contenu cellulaire, une autofluorescence et une production de ROS semblent liés à une phase nécrotrophe de la bactérie lors de sa propagation dans la zone corticale de l extrémité racinaire. L étude de la colonisation bactérienne en s affranchissant de l étape de pénétration a révélé que des mécanismes de résistances peuvent intervenir au niveau de collet chez la lignée F83005.5 et lors de la colonisation racinaire des vaisseaux conducteurs suite à une inoculation avec le mutant gala7ManquantTOULOUSE-ENSAT-Documentation (315552324) / SudocSudocFranceF

Quantitative trait loci associated with isolate specific and isolate nonspecific partial resistance to Phoma macdonaldii in sunflower

Black stem, caused by Phoma macdonaldii, is one of the most important diseases of sunflower in the world. Quantitative trait loci (QTLs) implicated in partial resistance to two single pycnidiospore isolates of P. macdonaldii (MP8 and MP10) were investigated using 99 recombinant inbred lines (RILs) from the cross between sunflower parental lines PAC2 and RHA266. The experimental design was a randomized complete block with three replications. High genetic variability and transgressive segregation were observed among RILs for partial resistance to P. macdonaldii isolates. QTL‐mapping was performed using a recently developed high‐density SSR/AFLP sunflower linkage map. A total of 10 QTLs were detected for black stem resistance. The phenotypic variance explained by each QTL (R2) was moderate, ranging from 6 to 20%. Four QTLs were common between two isolates on linkage group 5 and 15 whereas the others were specific for each isolate. Regarding isolate‐specific and isolate‐nonspecific QTLs detected for partial resistance, it is evident that both genetic effects control partial resistance to the disease isolates. This confirms the need to consider different isolates in the black stem resistance breeding programmes. The four SSR markers HA3700, SSU25, ORS1097 and ORS523_1 encompassing the QTLs for partial resistance to black stem isolates could be good candidates for marker assisted selection

QTL mapping of partial resistance to Phoma basal stem and root necrosis in sunflower (Helianthus annuus L.)

Phoma macdonaldii infects different tissues of sunflower and causes reduction in yield and oil content. The aim of present research was to identify genomic regions involved in partial resistance of sunflower to four Phoma macdonaldii basal stem and root necrosis isolates using our improved map constructed with 191 SSR and 304 AFLP markers. The experiment was conducted using F9 recombinant inbred lines (RILs) from a cross between ‘PAC2’ and ‘RHA266’. Results showed that ‘PAC2’ was more resistant than ‘RHA266’ to basal stem necrosis isolate ‘TA6’ and root necrosis isolate ‘TA4’. By contrast ‘RHA266’ was more resistant than ‘PAC2’ to basal stem necrosis isolate ‘TA9’ and root necrosis isolate ‘TA2’. Transgressive segregation was observed for partial resistance to all four isolates. Some recombinant lines presented partial resistance or susceptibility to all isolates. Twenty-seven QTL with phenotypic variance ranging from 7 to 29% were detected. Among them 13 were ‘isolate-specific’ and others were common for partial resistance to different isolates (isolate-non-specific). Most of the QTLs in common have major effects for resistance to each isolate. The ‘isolate-non-specific’ QTLs were located on linkage groups (LG) 5, 6, 8, 12, 13 and 15. The markers ‘HA3555’ on LG12 and ‘E33M48_26’ on LG6 as well as ‘E33M48_20’ on LG13, which are each linked to QTLs of different basal stem and root necrosis isolates, could be used in marker-assisted selection to introduce tolerance to four Phoma macdonaldii isolates into elite sunflower breeding lines

A QTL analysis of sunflower partial resistance to downy mildew (Plasmopara halstedii) and black stem (Phoma macdonaldii) by the use of recombinant inbred lines (RILs)

Partial resistance to downy mildew (Plasmopara halstedii) and to black stem (Phoma macdonaldii) in sunflower were investigated under natural field infection and a controlled growth chamber respectively. Genetic control for resistance to the diseases was determined in recombinant inbred lines (RILs) and their two parents, ’PAC-2’ and ’RHA-266.’ The experiments were undertaken in a randomized complete block design with two replications, in a field severely infected by downy mildew and in a controlled growth chamber with plants inoculated with an agressive French isolate of P. macdonaldii. Each replication consisted of three rows, 4.6-m long, giving 48 plants per RIL or parent in the field and 15 plants in the growth chamber. Genetic variability was observed among the RILs for resistance to both diseases. When 10% of the selected RILs were compared with the mean of the two parents genetic gain was significant for partial resistance to the diseases. Four putative QTLs for resistance to downy mildew on linkage groups 1, 9 and 17 were detected using composite interval mapping. The QTLs explained 54.9% of the total phenotypic variance. Major QTLs (dmr1–1 and dmr1–2) for resistance were found on linkage group 1 with up to 31% of the phenotypic variability explained by two peaks. QTL analysis of resistance to black stem showed seven QTLs on linkage groups 3, 6, 8, 9, 11, 15 and 17. The detected QTLs together explain 92% of the phenotypic variation of the trait. Crosses between RILs contrasted for their resistance to downy mildew and black stem, and exhibiting molecular polymorphism in detected QTLs, will be made in order to focus more-precisely on the genomic region of interest

Differential regulation of ACC synthase genes in cold-dependent and -independent ripening in pear fruit

Late pear cultivars such as Passe-Crassane (PC) require a long chilling treatment before they are capable of ripening. Early cultivars such as Old-Home (OH) have no cold prerequisite. The regulation of 1-aminocyclopropane-1-carboxylic acid synthase (ACS) genes was studied in OH, PC and in OH x PC hybrids in order to determine the role of this gene family in the cold requirement. Of the seven Pc-ACS cDNAs isolated, four (Pc-ACSla/b and Pc-ACS2a/b) showed differential expression associated with the cold requirement. Pc-ACS1a transcripts accumulated throughout the cold treatment and, with Pc-ACS2a, during ripening of cold-dependent cultivars. Pc-ACS1b and Pc-ACS2b were detected only during ripening of cold-independent genotypes. Furthermore, Pc-ACS2a transcript accumulation was negatively regulated by ethylene, whereas Pc-ACS2b was positively regulated by the hormone. Pc-ACS3, 4 and 5 transcript accumulation was similar in all genotypes. Genetic analyses of OH, PC, and 22 OH x PC progenies demonstrated that late, cold-dependent cultivars were homozygous for Pc-ACS1a and 2a whereas early, cold-independent cultivars were heterozygous for Pc-ACS1(a/b) and homozygous for Pc-ACS2b. A model is presented in which differences in Pc-ACS alleles and gene expression between cold- and non-cold-requiring pears are critical in determining the ripening behaviour of the cultivars

A cDNA microarray approach to decipher sunflower (Helianthus annuus) responses to the necrotrophic fungus Phoma macdonaldii

To identify the genes involved in the partial resistance of sunflower (Helianthus annuus) to the necrotrophic fungus Phoma macdonaldii, we developed a 1000‐element cDNA microarray containing carefully chosen genes putatively involved in primary metabolic pathways, signal transduction and biotic stress responses. A two‐pass general linear model was used to normalize the data and then to detect differentially expressed genes. This method allowed us to identify 38 genes differentially expressed among genotypes, treatments and times, mainly belonging to plant defense, signaling pathways and amino acid metabolism. Based on a set of genes whose differential expression was highly significant, we propose a model in which negative regulation of a dual‐specificity MAPK phosphatase could be implicated in sunflower defense mechanisms against the pathogen. The resulting activation of the MAP kinase cascade could subsequently trigger defense responses (e.g. thaumatin biosynthesis and phenylalanine ammonia lyase activation), under the control of transcription factors belonging to MYB and WRKY families. Concurrently, the activation of protein phosphatase 2A (PP2A), which is implicated in cell death inhibition, could limit pathogen development. The results reported here provide a valuable first step towards the understanding and analysis of the P. macdonaldii–sunflower interaction

Évaluation du risque simplifiée du tomato brown rugose fruit virus pour la France métropolitaine: Avis de l'Anses. Rapport d'expertise collective

Évaluation du risque simplifiée du tomato brown rugose fruit virus pour la France métropolitaine. Avis de l'Anses. Rapport d'expertise collectiv