Conserved differential expression of paralogous DEFICIENS ‐ and GLOBOSA ‐like MADS‐box genes in the flowers of Orchidaceae: refining the ‘orchid code’

Summary In flowering plants, class‐B floral homeotic genes encode MADS‐domain transcription factors, which are key in the specification of petal and stamen identity, and have two ancient clades: DEF ‐like and GLO ‐like genes. Many species have one gene of each clade, but orchids have typically four DEF ‐like genes, representing ancient gene clades 1, 2, 3 and 4. We tested the ‘orchid code’, a combinatorial genetic model suggesting that differences between the organs of the orchid perianth (outer tepals, inner lateral tepals and labellum) are generated by the combinatorial differential expression of four DEF ‐like genes. Our experimental test involves highly sensitive and specific measurements, with qRT‐PCR of the expression of DEF ‐ and GLO ‐like genes from the distantly related Vanilla planifolia and Phragmipedium longifolium , as well as from wild‐type and peloric Phalaenopsis hybrid flowers. Our findings support the first ‘orchid code’ hypothesis, in that absence of clade‐3 and ‐4 gene expression distinguishes the outer tepals from the inner tepals. In contrast to the original hypothesis, however, mRNA of both clade‐3 and ‐4 genes accumulates in wild‐type inner lateral tepals and the labellum, and in labellum‐like inner lateral tepals of peloric flowers, albeit in different quantities. Our data suggest a revised hypothesis where high levels of clade‐1 and ‐2, and low levels of clade‐3 and ‐4, gene expression specify inner lateral tepals, whereas labellum development requires low levels of clade‐1 and ‐2 expression and high levels of clade‐3 and ‐4 expression.