Plants have developed a complex defense system against pathogens. Generally speaking, two major signaling pathways can be distinguished: the one related to biotrophs, and another one related to necrotrophs. The synthesis and perception of salicylic acid (SA) are of capital importance to plant defense, since it constitutes the major response against biotrophs. However, several important steps in the SA pathway are still unknown, mostly in relation to its perception. As a matter of fact, only one mutant has been described to be insensitive to SA (npr1) and it has not proved to be its receptor. This Dissertation takes as point of departure the hypothesis that other genetic components exist and play some role in the perception of this hormone. A biological model is proposed as an alternative to the ones previously used in the literature to study SA perception in Arabidopsis thaliana. This model is based on the decrease that the plant's fresh weight experiences as a result of the resistance being induced. Due to the phytotoxicity of SA when it is applied at high doses, benzotiadiazole (BTH) is used instead, which is one of its chemical analogues. BTH increases the plant's resistance to the biotrophic pathogen Pseudomonas syringae and produces a considerable loss of biomass. This model is applied in a high-throughput screening. The major complementation group obtained is npr1. The 43 new alleles found are insensitive to both BTH and SA, thus confirming the suitability of this model to detect mutants in SA perception. There is a bias in the distribution of the mutants present in npr1 alleles. Such bias does not correspond to any of the dominions of the protein already described, but to a zone conserved in NPR1 paralogs. Besides, no nul npr1 allele has been obtained from the screening, due to its intermediate phenotype (confirmed by getting the lines from seed banks). Such phenotype would be explained by the existence of a partial redundancy in SA perception, although with a marked preference for NPR1. The five NPR1 paralogs (NPR2, NPR3, NPR4, BOP1 and BOP2) present a secondary function in SA perception that can be detected in a null genetic background for NPR1. NPR1 has also been identified to be involved in resistance induction after applying methyl jasmonate (MeJA), even though this hormone primarily controls resistance to necrotrophs. However, such studies had been based on the few npr1 alleles described until then. The analysis of the 43 alleles from the screening and the two null alleles allows us to conclude that NPR1 is not required for resistance induced by MeJA. Furthermore, the paralogs BOP1 and BOP2 are relevant in this defensive process. TGAs, a multigenic family of transcription factors whose role in the SA signaling pathway has been reported, also participate in resistance induced by MeJA. The other complementation group characterized is NRB4 (NON- RECOGNITION-OF-BTH-4), with three alleles which present point mutations. NRB4 is the second gene required for SA perception. The three nrb4 alleles of the screening share with npr1 all the phenotypes related to defense that have been studied. On the other hand, nrb4 null alleles show a total loss of their defensive SA-mediated response, these being even more extreme than npr1 in some phenotypes. Besides, their growing habit is altered and they are sterile. No interaction between both proteins has been detected, and it has been determined that NRB4 acts in the pathway after NPR1. As a matter of fact, NRB4 is part of the Mediator complex and it is the MED15 ortholog in Arabidopsis. This complex is required for the general regulation of transcription. In spite of its function as a whole, specific functions have been described for several of its subunits in some concrete physiological processes, like the one related to SA in NRB4/MED15. The results obtained show that NRB4 is essential for the plant. The simplest explanation is that this subunit may have other functions in addition to the ones related to SA, which when suppressed make the plan unviable. However, the analysis of the transcriptomes does not point out to the suppression of any other signaling or function. Furthermore, the null alleles show phenotypes which are more severe in defense but no other relevant phenotype unless the one which is obvious in development. These results together with the fact that SA has also been described as relevant in plant growth and development suggest that NRB4 being relevant is exclusively due to its role in SA perception, and this would imply that SA is essential for a correct plant development.