Towards the functional characterization of the clementine Asp-rich protein encoding genes, candidates for regulating gametophytic self-incompatibility

Self-incompatibility (SI), typical of many mandarin and pummelo varieties, is a valuable trait to obtain seedless fruits. Despite the importance of SI, its genetic basis in citrus is poorly understood. Two clementine genotypes showing divergent behaviour related to self-pollen rejection (‘Comune’, self-incompatible’; and ‘Monreal’, self-compatible) were chosen as model to study the molecular events occurring during pollen-pistil interaction and self-incompatible response. In a previous work, the comparative analysis of the floral transcriptome of these two clementine genotypes led to the identification of a set of candidate genes related to SI. The identified genes showed no homology to genes involved in SI mechanism of other species and many of them lacked annotation. Functional analyses were therefore needed to unravel their function and to confirm their role in citrus progamic phase. Our attention has been focused on 3 uncharacterized genes overrepresented in the ‘Comune’ transcriptome whose expression drastically increased in concomitance with the stop of pollen tube elongation. These genes, due to the abundance in aspartic acid residues in their deduced protein sequences, have been named Citrus clementina aspartic-acid rich (CcAsp-rich) protein encoding genes. A first step of the functional characterization consisted in their over-expression in tobacco plants. Moreover, the CcASP-RICH proteins were expressed in a cell-free system and purified for a biochemical characterization. The preliminary results of the functional analyses are discussed.