Role of petunia pMADS3 in determination of floral organ and meristem identity, as revealed by its loss of function

Abstract

SummarypMADS3, a petunia class C gene, is the candidate homologue of Arabidopsis AGAMOUS (AG), which is involved in the specification of stamens and carpels. We report the characterization of loss-of-function phenotype of pMADS3 that resulted from silencing of this gene. Silencing of pMADS3 resulted in homeotic conversion of stamens into petaloid structures, whereas the carpels were only weakly affected. Ectopic secondary inflorescences emerged from the interstamenal region in the third whorl, which is unique and has not been reported for any class C gene of other plant species. Third-order inflorescences emerged at corresponding positions in the third whorl of inner flowers of secondary inflorescences, indicating reiterative conversion of parts of the floral meristem into inflorescence meristem. On the basis of phenotypic analysis of the pMADS3-silenced plants, we propose that pMADS3 is involved in determination of floral organ and floral meristem identity in petunia. Two hybrid studies in yeast showed that PMADS3 protein interacted specifically with FBP2, a candidate homologue of Arabidopsis SEPALLATA3 (SEP3). The evidence presented here suggest that a complex involving PMADS3 and FBP2 is responsible for specification of organ identity in the third whorl.

Redefining C and D in the Petunia ABC

“…A similar observation was made in snapdragon, where PLE expression is enhanced in the far mutant (Davies et al, 1999). The PMADS3-RNAi phenotype reported here is similar but not identical to the PMADS3 loss-offunction lines that have been described earlier (Kapoor et al, 2002). Most notably, we never observed secondary inflorescences arising from within the third whorl of PMADS3-RNAi flowers.…”

“…Expression of all four AG genes is mostly restricted to the inner two whorls of developing flowers, as described previously Kater et al, 1998;Kapoor et al, 2002) The individual expression patterns show some differences, which correlate to a large extent with their functions (as presented below). While PMADS3 is expressed at similar levels within the stamens, the pistil, and ovules, the expression of FBP6 is markedly lower in the stamens and ovary compared with the pistil, in which the levels of PMADS3 and FBP6 are relatively equal.…”

“…PMADS3 is specifically expressed in the vascular bundles of the stamens and the pistil, in developing ovules, the epidermis of the pistil, nonsporogenic anther tissue, and the nectaries. FBP6 is more highly expressed in the transmitting tissue of the style and in the stigma (Kater et al, 1998;Kapoor et al, 2002). Overexpression and silencing experiments have already shed some light on the function of these genes Kater et al, 1998;Kapoor et al, 2002).…”

“…FBP6 is more highly expressed in the transmitting tissue of the style and in the stigma (Kater et al, 1998;Kapoor et al, 2002). Overexpression and silencing experiments have already shed some light on the function of these genes Kater et al, 1998;Kapoor et al, 2002). Silencing of PMADS3 leads to a partial loss of stamen identity and slightly altered carpel morphology.…”

Integration of reproductive meristem fates by a SEPALLATA -like MADS-box gene

“…Downregulation of TM29, a SEP-like gene, causes reversion of the floral meristem into a shoot meristem (26). In Petunia, downregulation of FBP2 (SEP homologue) causes floral reversion (6,27), but down-regulation of the AG homologue pMADS3 also does, at least in whorl 3 (28). No simple loss of floral determinacy is known in Petunia.…”