Abstract
Mounting evidence from genomic and transcriptomic studies suggests that most genetic networks regulating the morphogenesis of land plant sporophytes were co-opted and modified from those already present in streptophyte algae and gametophytes of bryophytes sensu lato. However, thus far, no candidate genes have been identified that could be responsible for “planation”, a conversion from a three-dimensional to a two-dimensional growth pattern. According to the telome theory, “planation” was required for the genesis of the leaf blade in the course of leaf evolution. The key transcription factors responsible for leaf blade development in angiosperms are YABBY proteins, which until recently were thought to be unique for seed plants. Yet, identification of a YABBY homologue in a green alga and the recent findings of YABBY homologues in lycophytes and hornworts suggest that YABBY proteins were already present in the last common ancestor of land plants. Thus, these transcriptional factors could have been involved in “planation”, which fosters our understanding of the origin of leaves. Here, we summarise the current data on functions of YABBY proteins in the vegetative and reproductive development of diverse angiosperms and gymnosperms as well as in the development of lycophytes. Furthermore, we discuss a putative role of YABBY proteins in the genesis of multicellular shoot apical meristems and in the evolution of leaves in early divergent terrestrial plants.
Highlights
Leaves are dorsoventral organs characterised in having determinate growth and being specialised for photosynthesis
In the protein encoded by CRABS CLAW (CRC), the first identified YABBY gene from Arabidopsis thaliana, the zinc finger is formed by two α-helices surrounded by two β-folded layers [40] and represents a zinc chelating sequence
Given that YABBY transcription factors (TFs) must have been present in the common ancestor of land plants [36] and have a function in the suppression of the expression of class I KNOX genes that encode non-cell-autonomous TFs [12,13,55], we hypothesize that they were preserved in the ancestral simplex shoot apical meristem (SAM) of the first land plants, while the modifications associated with the independent reversal of the simplex SAM to the single apical cell (AC) SAM type in some lycophytes and most ferns included the loss of YABBY-controlled regulation of leaf development
Summary
Leaves are dorsoventral organs characterised in having determinate growth and being specialised for photosynthesis. The hypotheses based on few studies that report the expression patterns of the above genes support different scenarios of leaf origin [7,25,26,28,34,35] The presence of both KNOX and ARP homologues in the shoot tips of both lycophytes with microphylls and ferns with megaphylls has been interpreted as an indication of similar molecular regulation of organogenesis in these plant taxa and of a common evolutionary origin of leaves in all land plants [28]. We set about to review the current data, available for diverse plant taxa, on phylogeny and putative functions of YABBY homologues and to speculate on their putative ancestral function and its evolutionary modifications
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