The gynoecium is probably the most complex floral organ, composed of many different tissues and cell types, and, after fertilization of the ovules, provides the major structural component of the fruit. In addition to its huge economic importance, the fruit represents a key innovation of the Angiosperms, greatly responsible of their evolutionary success, since it provides protection for the developing seeds and ensures their dispersal. For these reasons, there is an incredible wealth of studies directed to unravel the genetic basis for fruit patterning and its morphological diversity.
	Many of the genes required for fruit morphogenesis and seed dispersal have been identified in Arabidopsis, although, in most cases, the precise architecture of the genetic pathways involved is still far from understood.  To get further insights on these questions, it is essential to expand the genetic and molecular studies directed to unravel the functional interactions among these genes, as well as to identify new factors with key roles in gynoecium morphogenesis still uncharacterized.
	In this thesis, the functional characterization of the small NGATHA (NGA) subfamily of transcription factors of Arabidopsis has been carried out. Moreover, the position and role of the NGA factors in the genetic routes directing gynoecium morphogenesis has been studied. The results obtained have allowed to increase our knowledge on the genetic and molecular mechanisms of gynoecium patterning in Arabidopsis.
	Functional characterization of the NGA genes has been performed by identifying and characterizing loss-of-function mutants in each loci, as well as through the generation of multiple combinations of these mutations. This work has revealed the essential role of the NGA genes in the morphogenesis of the apical tissues of the Arabidopsis gynoecium, in particular directing the specification of the style and the stigma. Spatial and temporal patterns of expression of the NGA genes have been determined. In general, NGA genes appear to be expressed in domains undergoing active proliferation, and showing, in general, an overlap with domains that accumulate auxin.
	To obtain further insights on the relationship of the NGA genes and the auxin signaling pathways, different experiments were performed, such as the study of classic auxin responses in the nga mutants, the characterization of the phenotypic effect of auxin or auxin transport inhibitors in these mutants, or the expression analysis of genes involved in auxin biosynthesis, namely the YUCCA (YUC) genes, in nga mutants. Our results indicate that the gynoecium phenotypes of nga mutants are likely produced by the reduced synthesis of auxin in the apical region of the gynoecium, as a consequence of a failure to upregulate YUC genes in this domain. Therefore, NGA factors appear to be involved in auxin biosynthetic routes through YUC regulation. In addition to auxin, NGA genes may regulate brassinosteroid responses in the root, since physical interaction of NGA and BRX proteins has been revealed.
	Genetic analyses of the role of NGA genes in the genetic pathways directing gynoecium morphogeneis has been carried our extensively. Multiple combinations of loss and gain-of-function mutants, as well as expression studies have allowed to better understand the role and position of the NGA genes in these routes. The unrelated STY/SHI family of transcription factors have similar roles to NGA genes as revealed by the phenotypes of the corresponding mutants and the similarity of their expression patterns. This work has shown that both families likely do not regulate each other, but act on common targets cooperatively. In this thesis, it has been shown that STY1 and NGA3 are not able to interact directly at the protein level unless a third factor, CRABS CLAW (CRC), belonging to the YABBY family, is present. Our results indicate that a putative NGA/STY1/CRC trimer is specifically responsible of style formation.
	NGA genes have also been shown to participate in the regulation of genes directing dehiscence zone formation and fruit shattering, which also appear to have a role in apical gynoecium patterning. Genetic analyses indicate that NGA factors could be mediating the upregulation of dehiscence zone genes, such as SHATTERPROOF or INDEHISCENT, while interfering with the activation of the valve identity gene FRUITFULL. This differential effect on the regulation of SHP/IND and FUL could be mediated by the formation of protein complexes of NGA factors with YABBY3 and possibly other YABBY factors.