Résumé
Besides the development of the gene for gene resistance, one can observe a limitation of the infection by the installation of a resistance of more or less high level called partial resistance. The mechanisms of partial resistance are poorly understood despite its importance for the breeding programs. We used the rice/Magnaporthe grisea system to decipher the mechanisms involved in partial resistance. We hypothesised that some preformed defence could explain some parts of partial resistance. The expression of more than 20 regulatory and classical defence genes was measured in a set of well-characterized cultivars (japonica and indica) representative of rice diversity and partial resistance. We demonstrated that preformed defence as measured by the expression of these genes accounts for almost 50% of partial resistance, in particular in japonica type of cultivars. The indica-type of cultivars seem to use a different set of genes and regulation systems to control partial resistance. The OsMAPK6 negative regulator of defence gene expression could be a major regulator of this phenomenon. A quantitative analysis of constitutive expression of defence genes allowed the identification of eQTLs representing regions of the rice genome involved in the control of constitutive expression of several defence genes. In contrast, salicylic acid levels did not correlate with constitutive expression of defence. This work demonstrates that constitutive defence has been underscored in the past and opens new opportunities for breeding programs.