Citrus sinensis L. Osbeck orthologs of FRUITFULL and SHATTERPROOF are differentially expressed during fruit development

Several regulatory steps and genes involved in fruit development were identified and characterized in Arabidopsis thaliana. FRUITFULL (FUL) and SHATTERPROOF (SHP), which belong to the MADS-box family of transcription factors, act together to promote the differentiation of the dehiscence zone and thus control the process of pod shattering in Arabidopsis. Homologs to these genes have been described in fleshy fruit species, but the specific nature of the regulatory hierarchy and interactions among these key regulators remains elusive in most plant species. Here, Citrus sinensis putative orthologs to FUL and SHP, named CsFUL and CsSHP respectively, were characterized. Phylogenetic comparisons indicated that CsFUL belongs to FRUITFUL sub-clade within the AP1/SQUA major clade while CsSHP falls into PLENA sub-clade from the AG/PLE clade. CsFUL and CsSHP protein sequences possess all of the characteristic conserved domains commonly found in A- and C-lineages of MIKC MADS-box proteins, respectively. Semi-quantitative RT-PCR showed preferential expression of both genes in developing fruits. In situ hybridization and a detailed analysis of Citrus fruit development using scanning electron microscopy allowed further characterization of these genes during C. sinensis fruit development. CsFUL and CsSHP are differentially expressed in exocarp, mesocarp and endocarp tissues in early stages of fruit development but their expression diminishes with fruit maturation. Moreover, the co-localization of CsFUL and CsSHP mRNA during oil glands and juice vesicle formation suggests a potential role in the development of such structures. Altogether, there results might contribute to a better understanding of the molecular mechanisms involved in Citrus fruit development.