SUMMARY

       The B-oxidation is an essential metabolic process for eukaryotic cells. Traditionally, this catabolic pathway has been considered in plants as the main route for the degradation of fatty acids in cells. However, the B-oxidation in plants has lured a great scientific interest because its involvement in to the regulation of development and defence. Although previous biochemical studies had suggested the involvement of B-oxidation reactions in the biosynthesis of salicylic acid (SA) and jasmonic acid (JA), which are key signalling molecules in the defense responses of plants to pathogens and wounding, the involvement of any enzyme of B-oxidation has not been described to this work. The family of 3-ketoacyl-CoA thiolases (KAT) are the least studied enzymes and genes of all steps involved in this pathway. This work has focused on the functional and molecular characterization of the different genes that encode KAT proteins, and specifically on KAT2 gene that encodes the main protein with KAT function of B-oxidation in Arabidopsis. To assess the possible functions of KAT2 gene in processes related to development and defense in plants, we generated transgenic plants with loss or gain of function as well as others where the KAT2 promoter sequence was fused to reporter genes. These tools allowed us to deeply characterize the expression pattern of this gene in Arabidopsis. The expression analysis showed a specific pattern of wound-activated expression of B-oxidation genes. Specifically, the KAT2 expression increased through a JA-independent signalling pathway and possibly dependent on abscisic acid. In addition, transgenic plants with reduced expression of KAT2 gene showed a severe decrease in the accumulation of JA in response to wounding, indicating that KAT2 is involved in the wound-activated biosynthesis of JA and possibly other JA-dependent responses. This result is the first evidence of the involvement of a B-oxidation enzyme in the JA biosynthesis in plants. In contrast, the response to biotrophic pathogen such as Pseudomonas syringae or ultraviolet C light, which depend largely on the synthesis and signaling of SA, suggested that KAT2 is not involved in the synthesis of this hormone. Consequently, the B-oxidation-dependent biosynthesis of SA, which had been previously described in Solanaceae, is not active in response to stress in Arabidopsis. Furthermore, we analyzed the function of KAT2 gene in developmental processes such as germination, postgerminative establishment and senescence. These analyses showed that KAT2 is necessary for seedling development and it is an essential regulator for the timely activation of leaf senescence program. The transgenic plants with reduced KAT2 expression showed serious problems of postgerminative growth in the absence of sucrose and a delayed natural and dark-induced senescence in leaves. In addition, KAT2 expression was high throughout development and maturation of floral organs, which could be related to its role in the biosynthesis of JA and other hormones such as auxins, which synthesis also proceeds through B-oxidation reactions. On the other hand, since B-oxidation is located in peroxisomes, and these organelles have been linked repeatedly with responses to stress in plants, we examined the role of peroxisomes in response to wounding, analyzing both the number and the size of these organelles and the possible components of the signalling pathways involved in these processes. Wounding did not change either the number or the size of these organelles, contrasting with the peroxisome proliferator activity of the well characterized hypolipidemic drug clofibrate (CFB). JA acts as a negative regulator of the proliferation of these organelles since it decreased the number of peroxisomes and increased their size. Moreover, the analysis of signalling pathways activated in response to CFB, unknown in plants to date, showed that share common elements with wound-activated pathways such as COI1. Nevertheless, both processes are uncoupled and differ in the capacity to promote proliferation of peroxisomes. In addition, the transcriptional activation of B-oxidation and wound-related genes in response to treatment with JA, CFB or wounding is not linked to the integrity of peroxisomes, as demonstrated by the results of transgenic plants with loss of function of PEX14 gene, which contained altered peroxisomes and serious defects in the transport of peroxisomal proteins. Finally, the genome-wide analysis of the CFB-responsive transcriptome suggested that CFB is perceived primarily as a xenobiotic compound in plants, mostly by activating the expression of genes coding for components of the typical detoxification responses.