The rust caused by Puccinia psidii is a major disease of Eucalyptus in Brazil, causing losses in nurseries and in the field. Attacks occur in young plants with less than two years old, always in new branches and new leaves. The genetic resistance is recognized how the best option for disease control. Nevertheless the susceptible genotypes is possible to activate the resistance mechanisms through treatment with biotic and abiotic agents. In order to study the biochemical and molecular mechanisms involved in the process of Eucalyptus defense against P. psidii four assays were carried out with two seedlings of hybrid genotypes (Eucalyptus grandis x E. urophylla) called VR and C0. In the first assay, seedlings were treated with Acibenzolar-S-methyl (ASM), Agro-Mos, Saccharomyces cerevisiae, Dipel, Ecolife40 and Crop-Set, five days before the inoculation with the pathogen. The severity of the disease was estimated 15 days after inoculation by comparison with a scale of notes. In the most effective treatments analyzed the activity of enzymes chitinase and peroxidase, 48 hours after inoculation (h.a.i). In the second assay seedlings were treated with ASM five days before the inoculation and peroxidase and chitinase activity was determined 0, 24, 72 and 96 h.a.i. in leaves of different stages of development (1st, 2nd and 3rd pairs of leaves). In the third assay the action of ASM and S. cerevisiae, applied five days before the inoculation, was correlated with possible biochemical changes in plants. The chitinase, peroxidase, phenylalanine ammonia-lyase (PAL) and polyphenoloxidase activity in leaves of 1st and 2nd pairs collected in 5 periods 0, 24, 48, 72 and 96 h.a.i was determined. Finally, in the fourth test was analyzed the pattern of gene expression over time in seedlings of both genotypes after treatment with ASM and, after 5 days, inoculation with P. psidii in five periods (0, 24, 48, 72 and 96 h.a.i), through of macroarray technique. In the first assay it was found that the treatments ASM, S. cerevisiae and Ecolife were more efficient in controlling the rust. There was also an increase in activity of chitinase and peroxidase, in both genotypes, when treated with ASM and S. cerevisiae. In the second test results showed that the effect of ASM or the action of P. psidii contributed to changes of enzymes levels only in the development leaves (1st and 2nd pairs), with larger increases observed in VR plants 72 h.a.i., treated with ASM. The third test was found differences in chitinase, peroxidase and PAL activity in both clones, either by the action of the pathogen, the effect of inducers of resistance or by the interaction between pathogen- inducers. In the fourth assay, on plants of the VR genotype, several genes related to the strengthening of the cell wall, cell polarization, PR-proteins, enzymes involved in the route of phenylpropanoides and peroxidation of lipids, accumulation of phenolic compounds, among others, were differentially expressed in the plants were challenged by the pathogen, when only were treated with the resistance inducers or treated with inducer followed by inoculation. However, plants in the genotype C0 differential expression was only when the plants were treated with ASM.
