NON-CELL-AUTONOMOUS FUNCTION OF THE ANTIRRHINUM FLORAL HOMEOTIC PROTEINS DEFICIENS AND GLOBOSA IS EXERTED BY THEIR POLAR CELL-TO-CELL TRAFFICKING

In Antirrhinum majus, petal and stamen organ identity is controlled by two MADS-box transcription factors, DEFICIENS and GLOBOSA. Mutations in either of these genes result in the replacement of petals by sepalo id organs and stamens by carpelloid organs. Somatically stable def and glo periclinal chimeras, generated by transposon excision events, wer e used to study the non-cell-autonomous functions of these two MADS-bo x proteins, Two morphologically distinct types of chimeras were analys ed using genetic, morphological and molecular techniques, Restoration of DEF expression in the L1 cell layer results in the reestablishment of DEF and GLO functions in L1-derived cells only; inner layer cells r etain their mutant sepaloid features, Nevertheless, this activity is s ufficient to allow the expansion of petal lobes, highlighting the role of DEF in the stimulation of cell proliferation and/or cell shape and elongation when expressed in the L1 layer, Establishment of DEF or GL O expression in L2 and L3 cell layers is accompanied by the recovery o f petaloid identity of the epidermal cells but it is insufficient to a llow petal lobe expansion. We show by in situ immunolocalisation that the non-cell-autonomy is due to direct trafficking of DEF and GLO prot eins from the inner layer to the epidermal cells, At least for DEF, th is movement appears to be polar since DEF acts cell-autonomously when expressed in the L1 cell layer. Furthermore, the petaloid revertant se ctors observed on second whorl mutant organs and the mutant margins of petals of L2L3 chimeras suggest that DEF and GLO intradermal movement is limited. This restriction may reflect the difference in the regula tion of primary plasmodesmata connecting cells from the same layer and secondary plasmodesmata connecting cells from different layers. We pr opose that control of intradermal trafficking of DEF and GLO could pla y a role in maintaining of the boundaries of their expression domains.