STRUCTURAL AND REGULATORY ASPECTS OF MORPHOGENESIS IN <i>EQUISETUM</i> <i>SYLVATICUM</i> AND <i>EQUISETUM</i> <i>FLUVIATILE</i> AND THE ISSUE OF HOMOLOGY OF LEAVES OF HORSETAILS AND OTHER FERNS

Abstract

The structure of the shoot apical meristem (SAM), ultrastructure of its cells and the origin of primordia of organs in the SAM of horsetails Equisetum sylvaticum L. and E. fluviatile L. were studied. A relatively low degree of vacuolation and structural homogeneity of the cells that compose the surface initials zone (SI), the presence of single starch grains in their plastids and single lipid droplets in the cytoplasm of the SI cells were revealed. These features are more similar to those in the monoplex SAM of lycophytes than to those in leptosporangiate ferns. It is shown that initiation of leaves in E. sylvaticum and E. fluviatile via emergence of the leaf apical initials (LAIs) in the surface initials zone is similar to that of other plants with monoplex SAM; both ferns and lycophytes. The main peculiarity of organogenesis in horsetails is the origin of the leaf whorl as a single structure; each whorl develops from the triplet of the shoot apical initial merophytes. The inner cells of the emerging leaf whorl that belong to the single triplet of merophytes differentiate into the intercalary meristem. Each phytomer of E. sylvaticum and E. fluviatile is composed of the whorl of congenitally fused leaves and a whorl of buds that emerge in the axils of leaf bases located between the blades and thus alternate with leaves. Each bud bears a single root primordium; bud primordia develop on aerial shoots while root primordia – on the underground shoots. Termination of the leaf apical meristem functioning, that consequences from the vacuolation of the LAI, leads to the absence of the marginal meristem and the vascular tissues in the leaf blade. Proximodistal development of the leaf whorl and origin of buds results from meristematic activity of the peripheral part of the intercalary meristem. The search for the homologues of genes encoding known for angiosperms regulators of the development of adaxial (C3HDZ and ARP) and abaxial (YABBY and KANADI) leaf domains in available transcriptomes of horsetails and matching of the found homologues with that from the genomes of mosses, ferns and gymnosperms was undertaken in the context of assessment of presumptive regulatory similarities or differences of leaves of horsetails and other ferns in the context of leaf homology in the two groups. MEME analysis has shown that R2-R3 MYB proteins from transciptomes of horsetails are not ARP homologues. Thus it has been revealed that horsetail have single adaxial domain regulator (C3HDZ) and single abaxial domain regulator (KANADI), as other ferns. This finding suggests the probable loss of other regulators of the adaxial (ARP) and abaxial (YABBY) domains in the common ancestor of Polypodiophyta. Phylogenetic analysis of the identified homologues of genes that encode WOX proteins suggest that the T3 clade, that includes regulators of marginal (WOX3) and plate (WOX1) leaf meristems, originated in a common ancestor of Polypodiophyta, also indicating similarities in the molecular genetic regulation of the leaves in horsetails and other ferns.