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The accumulation of soluble and cell wall-bound phenolics in the sugarcane stems of young plants from highly resistant cv. My 5514 and susceptible cv. B 42231, inoculated or not inoculated with smut sporidia, was studied. The ratio of inoculated to uninoculated plants of some cell wall-bound phenolics, such as ferulic, caffeic, and syringic acids increased for the resistant cv. My 5514, whereas it was maintained more or less constantly for the susceptible cv. B 42231. The highest increase of this ratio in the resistant cv. My 5514 corresponded to both caffeic and syringic acids. This could result in a better capacity to cv. My 5514 for an increase in the frequency of bridges between lignin fragments through ester-ether linkages for reinforcing the cell wall and major resistance to the disease. This reinforcement of the cell wall could provide an effective barrier to pathogen entry and spread. Soluble sub-fractions of all phenolics detected showed non-stable patterns. Caffeic acid, that regulates phenylalanine ammonia-lyase activity in sugarcane, showed a significant decrease in its titre at 24 h in the resistant cultivar, principally in the free soluble fraction, whilst the susceptible cultivar enhanced it. We hypothesise that the pathway of hydroxybenzoic acids is only activated once the level of p-coumaric acid justifies the accumulation of hydroxycinnamic acids required for reinforcing the cell wall after inoculation., Agencia Española de Cooperación con Iberoamérica, Ministerio de Educación y Ciencia (España), Depto. de Biodiversidad, Ecología y Evolución, Fac. de Ciencias Biológicas, TRUE, pub

Le charbon est une maladie présente dans toutes les zones de culture de la canne à sucre, à l'exception de la Papouasie-Nouvelle-Guinée, qui est pourtant le berceau de l'espèce ancestrale domestiquée, Saccharum officinarum. Des études permettent désormais de mieux cerner la diversité génétique de ce champignon et le déterminisme génétique de la résistance de la canne.

Ustilago scitaminea Sydow, which causes sugarcane smut disease, has been spreading throughout Africa and America since the 1950s. The objectives of the present study were (1) to confirm and further describe the global population structure of U. scitaminea on the basis of a larger number of isolates and (2) to infer its reproduction system. Microsatellite markers, that present the advantage of being highly polymorphic, PCR-based, reproducible and codominant, were therefore developed for U. scitaminea and used to analyse a sample of single-teliospore isolates from various sugarcane-producing countries around the world. We surveyed 142 single-spore isolates of Ustilago scitaminea for genetic diversity. The fungal samples were teliospores from 77 single whips (sori) collected on various cultivars and at different locations in 15 sugarcane-growing countries throughout the world. The overall genetic structure of this fungus was investigated using 17 polymorphic microsattelite loci. All isolates but one were homozygous for all loci, indicating that selfing could be the highly preferential predominant reproductive mode of U. scitaminea. In America and Africa, genetic diversity was found to be extremely low and all isolates belonged to a single inbred lineage. This inbred lineage was also found in some parts of the Asian continent where most U. scitaminea genetic diversity was detected. These observations support the hypothesis that the fungus originated in Asia. The strong founder effect observed in the global genetic structure of U. scitaminea suggests that the fungus migrated from Asia to other continents on rare occasions. (Texte intégral)

Ustilago scitaminea Sydow, which causes sugarcane smut disease, has been spreading throughout Africa and America since the 1950s. The objectives of the present study were (1) to confirm and further describe the global population structure of U. scitaminea on the basis of a larger number of isolates and (2) to infer its reproduction system. Microsatellite markers, that present the advantage of being highly polymorphic, PCR-based, reproducible and codominant, were therefore developed for U. scitaminea and used to analyse a sample of single-teliospore isolates from various sugarcane-producing countries around the world. We surveyed 142 single-spore isolates of Ustilago scitaminea for genetic diversity. The fungal samples were teliospores from 77 single whips (sori) collected on various cultivars and at different locations in 15 sugarcane-growing countries throughout the world. The overall genetic structure of this fungus was investigated using 17 polymorphic microsattelite loci. All isolates but one were homozygous for all loci, indicating that selfing could be the highly preferential predominant reproductive mode of U. scitaminea. In America and Africa, genetic diversity was found to be extremely low and all isolates belonged to a single inbred lineage. This inbred lineage was also found in some parts of the Asian continent where most U. scitaminea genetic diversity was detected. These observations support the hypothesis that the fungus originated in Asia. The strong founder effect observed in the global genetic structure of U. scitaminea suggests that the fungus migrated from Asia to other continents on rare occasions. (Texte intégral)

Sugarcane smut, caused by Ustilago scitaminea Syd., is present in all sugarcane growing areas with the exception o f Papua New Guinea, Fiji and the eastern side of Australia. Breeding for smut resistance is efficient because this trait is fairly heritable but it requires complicated screenings. Moreover, the genetic control o f smut resistance is still unknown. With the objective to identify the mendelian factors involved in sugarcane resistance to smut, two strategies have been implemented (1) QTL mapping in a bi-parental progeny derived from a cross between a resistant cultivar 'R 570' and a highly susceptible clone 'MQ 76/53', evaluated in Reunion island for resistance to smut (2) Association study in a population of cultivars, evaluated in Burkina Faso for resistance to smut. First, the worldwide genetic structure of U. scitaminea was investigated. A total of 142 singleteliospore isolates from 15 countries worldwide were analysed using 17 polymorphic microsatellite loci. All isolates but one were homozygous for all loci, indicating that selfing is the highly preferential reproductive mode of U. scitaminea. In America and Africa, genetic diversity was found to be extremely low and all isolates belonged to a single inbred lineage. This inbred lineage was also found in some parts of the Asian continent where most U. scitaminea genetic diversity was detected. These observations support the hypothesis that the fungus originated in Asia. Thus, the bi-parental population and the population of cultivars used to study the genetic determinism of smut resistance have been evaluated towards isolates from the worldwide lineage. The genetic maps of the two parents o f the bi-parental progeny, 'R 570' and 'MQ 76/53', were constructed using a population o f 198 progeny. A total o f 1666 polymorphic markers were produced using 37 AFLP primer pairs combinations, 46 SSRs and 9 RFLP probes. Linkage analysis allowed the construction of 86 cosegregation groups for 'R 570' and 105 cos

Sugarcane smut, caused by Ustilago scitaminea Syd., is present in all sugarcane growing areas with the exception o f Papua New Guinea, Fiji and the eastern side of Australia. Breeding for smut resistance is efficient because this trait is fairly heritable but it requires complicated screenings. Moreover, the genetic control o f smut resistance is still unknown. With the objective to identify the mendelian factors involved in sugarcane resistance to smut, two strategies have been implemented (1) QTL mapping in a bi-parental progeny derived from a cross between a resistant cultivar 'R 570' and a highly susceptible clone 'MQ 76/53', evaluated in Reunion island for resistance to smut (2) Association study in a population of cultivars, evaluated in Burkina Faso for resistance to smut. First, the worldwide genetic structure of U. scitaminea was investigated. A total of 142 singleteliospore isolates from 15 countries worldwide were analysed using 17 polymorphic microsatellite loci. All isolates but one were homozygous for all loci, indicating that selfing is the highly preferential reproductive mode of U. scitaminea. In America and Africa, genetic diversity was found to be extremely low and all isolates belonged to a single inbred lineage. This inbred lineage was also found in some parts of the Asian continent where most U. scitaminea genetic diversity was detected. These observations support the hypothesis that the fungus originated in Asia. Thus, the bi-parental population and the population of cultivars used to study the genetic determinism of smut resistance have been evaluated towards isolates from the worldwide lineage. The genetic maps of the two parents o f the bi-parental progeny, 'R 570' and 'MQ 76/53', were constructed using a population o f 198 progeny. A total o f 1666 polymorphic markers were produced using 37 AFLP primer pairs combinations, 46 SSRs and 9 RFLP probes. Linkage analysis allowed the construction of 86 cosegregation groups for 'R 570' and 105 cos

A Quantitative Trait Locus (QTL) mapping study is under way to analyse the genetic determinism underlying sugarcane smut resistance. A total of 1295 polymorphic AFLP markers have been generated on a population of 200 individuals derived from a cross between R 570 (resistant) and MQ 76/53 (highly susceptible). This population is under evaluation for its resistance to smut in the field over three successive crop cycles and in two different locations on Reunion island. Inoculation was performed by dipping three-bud cuttings in a spore suspension, and susceptible clones were regularly interspersed in the field to trigger the epidemy. Different inoculation methods were also compared in greenhouse trials. The bud puncture method appeared the most efficient and will be used for further greenhouse evaluations of the progeny. Detection of marker-trait associations has been performed and so far has indicated a complex determinism for smut resistance. Many markers are involved with little effects. Nevertheless, among other traits that were looked at, two putative major genes originating from MQ 76/53 have been tagged: a gene involved in the red color of the internode and a rust resistance gene. Locus specific markers, SSR markers from sugarcane and maize and sequences differentially expressed in response to challenge by smut (provided by the South African Sugar Association Experiment Station), will be mapped. This should help explain the genetic variation in smut resistance observed in the trials.