Quantitative expression analysis of the ABC genes in Sophora tetraptera, a woody legume with an unusual sequence of floral organ development

Abstract

Sophora is a woody genus of the Leguminosae in which an unusual order and process of floral organ development is often observed. The SEM results for Sophora tetraptera revealed precocious initiation of the carpel, delayed development of petals, and floral organ development interrupted by an unusual prolonged summer-autumn dormant period which occurred between organ initiation and organ differentiation. These observations provided an opportunity to track key floral identity genes over an extended developmental period. Homologues of LEAFY, APETALA1, PISTILLATA, and AGAMOUS were isolated from S. tetraptera. Real-time PCR enabled a simultaneous and quantitative analysis of both the temporal and spatial expression patterns of these four genes. Expression differences in the range of three to five orders of magnitude were detected between different genes and between different stages of flower development for the same gene. Although not functionally tested, the spatial expression patterns of the genes were consistent with expectations based on the ABC model of floral development. Their temporal expression patterns were consistent with the timing of flower initiation and the unusual order of organ development. Quantitatively, while the expression levels of the LFY homologue and the A-class gene were high during the periods of organ initiation and organ differentiation and low during the summer-autumn dormant period, high expression levels of the B- and C-class genes were detected only during the rapid, albeit delayed, phase of organ differentiation. Additionally, the sustained expression of the floral organ identity genes after differentiation reflects on-going roles for these genes during subsequent organ development.