Functional characterization of AP3, SOC1 and WUS homologues from citrus (Citrus sinensis)

Flowering and flower formation are defining features of angiosperms and the control of these developmental processes involves a common repertoire of genes which are shared among different species of flowering plants. These genes were first identified using various homeotic and flowering time mutants of Arabidopsis and snapdragon, and homologous genes have subsequently been isolated from a wide range of different plant species based on the conservation of protein sequence and function. Using degenerate reverse-transcriptase polymerase chain reaction, we have isolated one APETALA3-like (CitMADS8) and two SOC1 (SUPPRESSOR OF OVEREXPRESSION OF CONSTANS 1)-like (CsSL1 and CsSL2) homologues from sweet orange (Citrus sinensis L.). Although the translated amino acid sequence of CitMADS8 shares many similarities with other higher plant APETALA3 proteins, CitMADS8 fails to complement the floral organ identity defects of the Arabidopsis ap3-3 mutant. By contrast, the two citrus SOC1-like genes, particularly CsSL1, are able to shorten the time taken to flower in the Arabidopsis wild-type ecotypes Columbia and C24, and functionally complement the late flowering phenotype of the soc1 mutant, essentially performing the endogenous function of Arabidopsis SOC1. Once flowering has commenced, interactions between specific flowering genes and a gene required for meristem maintenance, WUSCHEL, ensure that the Arabidopsis flower is a determinate structure with four whorls. We have isolated a citrus WUSCHEL homologue (CsWUS) that is capable of restoring most of the meristem function to the shoots and flowers of the Arabidopsis wus-1 mutant, implying that CsWUS is the functional equivalent of Arabidopsis WUSCHEL.