Control of Organ Asymmetry in Flowers of Antirrhinum

Abstract

Organ asymmetry is thought to have evolved many times independently in plants. In Antirrhinum, asymmetry of the flower and its component organs requires cyc and dich gene activity. We show that, like cyc, the dich gene encodes a product belonging to the TCP family of DNA-binding proteins that is first expressed in the dorsal domain of early floral meristems. However, whereas cyc continues to be expressed throughout dorsal regions, expression of dich eventually becomes restricted to the most dorsal half of each dorsal petal. This correlates with the effects of dich mutations and ectopic cyc expression on petal shape, providing an indication that plant organ asymmetry can reflect subdomains of gene activity. Taken together, the results indicate that plant organ asymmetry can arise through a series of steps during which early asymmetry in the developing meristem is progressively built upon.

Genetic Control of Organ Shape and Tissue Polarity

“…The effect of each dorsoventral gene on different petal regions of the mature dorsal and lateral petals has been established through shape analysis of various mutant and over-expression lines (Figure 6B,C) [11]. In accordance with observed expression patterns [6], the CYC identity factor was activated in two adjacent petals of the starting canvas defining the dorsal domain, while DICH was expressed in the most dorsal half of these petals (Figure 6D). Subsequent interactions between the dorsoventral genes were modelled through a GRN based on known gene interactions (Figure 6E).…”

“…One remaining problem with the model was that the dorsal lobes were much more splayed out (Figure 8E) than observed in wild type, resembling the petals of dich mutants [6]. This arises because of the curvature of the tube generated by differential growth of its dorsal regions.…”

“…Floral asymmetry is established by the dorsoventral genes CYCLOIDEA ( CYC ), DICHOTOMA ( DICH ), RADIALIS ( RAD ), and DIVARICATA ( DIV ). CYC and DICH code for TCP transcription factors that are expressed in the dorsal domain of the early corolla [6]. Mutants that lack CYC and DICH have radially symmetrical flowers in which all petals resemble the ventral petal of wild type.…”

Quantitative Control of Organ Shape by Combinatorial Gene Activity

“…The distinctive shapes of the upper and lower petals depend on the activities of four dorsoventral genes: CYCLOIDEA ( CYC ), DICHOTOMA ( DICH ), RADIALIS ( RAD ), and DIVARICATA ( DIV ) [5]–[9]. CYC and DICH encode TCP transcription factors that are expressed from an early stage in the dorsal domain of the flower bud.…”

“…At later developmental stages, DIV expression becomes restricted to lateral and ventral petals through the action of the dorsally expressed genes. A cis -acting dominant mutant of CYC ( backpetals ) has been characterised in which CYC is ectopically expressed, leading to lower petals acquiring dorsal identity [9]. However, it is unclear whether the phenotype is a result of ectopic expression of CYC and/or its target gene RAD .…”