HsYABBY expression in H. selago has also showed a significant difference with angiosperms. While in angiosperms YABBY genes are never expressed in the SAM, and their ectopic expression leads to its disappearance, transcription of HsYABBY marked the SAM (except for the apical initials), leaf and sporangia primordia. On later stages of leaf development HsYABBY transcription was confined to the surface cells of its abaxial and adaxial sides (the protoderm), while in seed plants YABBY expres- sion is confined to the abaxial domain of the leaf, controlling its histogenesis.
Thus, both H. selago and P. abies are characterized by expression of the ho- mologs of “leaf” TFs YABBY (in H. selago) and ARP (in P. abies) in the SAM, which is different from angiosperms. This data suggest that the regulation of mi- crophyll and megaphyll origin in simplex SAM have more common features as compared to regulation of megaphyll origin in the simplex (spruce) and duplex (angiosperms) SAMs.
These data confirm the viewpoint that despite common partition into CZ and PZ, different SAM types do have functional differences (Romanova et al., 2010). In the monoplex SAM both microphylls of lycophytes and megaphylls of monilo- phytes initiate through origin of the single LAC (leaf apical cell) resulting from formative division of one of the surface cells, that is in the CZ of the SAM, not PZ as in two other structural types. In simplex SAM both lycophyte microphylls and gymnosperm megaphylls originate through proliferation of a group of PZ cells. Thus both cellular and molecular aspects of leaf origin seem to be correlated with the SAM structural type, but not with the leaf type (mirophyll or megaphyll).