Ruolo di HAT3 e ATHB4 nella polarità e nell’identità degli organi fiorali in Arabidopsis thaliana

Flower development is a sophisticated and critical process in angiosperms, and it's known to be regulated by hundreds of genes. In Arabidopsis, the most accepted model of flower development is the ABC model, in which three gene classes , A, B, and C, work alone or closely together to define each flower organ arranged in whorls: sepals (A), petals (A+B), stamens (B+C), gynoecium (C), from the outer to the inner. The absence of one or more ABC genes results in the loss of specific flower organs. Furthermore, the expression of A and C genes is mutually exclusive, therefore the loss of one or the other leads to the ectopic expression of these genes. It is known that the A function gene APETALA2 (AP2) recruits the corepressor TOPLESS and the histone deacetylase HDA19 with a domain called EAR and this complex ultimately leads to the negative regulation of the C function gene, namely AGAMOUS (AG), from the two outer whorls. The laboratory in which this thesis was developed studies Arabidopsis transcription factors known as homeodomain-leucine zipper class II (HD-Zip II) proteins.. Among the HD-zip II protein, HOMEOBOX ARABIDOPSIS THALIANA 3 (HAT3) and ARABIDOPSIS THALIANA HOMEOBOX 4 (ATHB4) contain the same EAR motif and have been previously shown to be involved in gynoecium development. My thesis focuses on the function of HAT3 and ATHB4 in flower organ identity and the role of the EAR motif in this this function. First, we describe the hat3 athb4 double mutant phenotype in flower organs. The expressiveness of the phenotypes is highly variable, both among different plants,different inflorescences of the same plant,and also different organs within the same flower.. Among these variable phenotypes, we find some recurring ones, including sepals with irregular edges, , radialization of sepals, petals, and staments, and gynoecium split. Strickingly, we also observed the formation of ectopic carpelloid structures in sepals and the formation of staminoid petals, suggesting that HAT3 and ATHB4 are required to prevent the ectopic expression of the C function gene in the outer whorls. With the use of marker lines we were able to show that the C function gene AGAMOUS and the SHATTERPROOF (SHP) genes are indeed overexpressed in the whole flower and also ectopically expressed in sepals and petals. Based on this evidences, we postulated that the EAR motif of the HAT3 and ATHB4 is be important for the correct expression of AG and SHP genes. To test this hypothesis, we reintroduced in the double mutant lines either the wild type HAT3 protein or a version mutated in its EAR domain in order to to evaluate the contribution of the EAR domain in rescuing the WT phenotype and the proper expression of AG and SHP., The experiments performed in my thesis uncover a previously unknown role of HAT3 and ATHB4 in the identity of floral organs, and suggest a molecular mechanism for this function.