Abstract
The ovules and seeds of most angiosperm groups are enclosed by two integuments, whose evolutionary origins are considerably separated in time, as the inner integument arose over 300 million years ago (MYA) in an ancestor of all living seed plants, while the outer integument arose, perhaps as recently as 164 MYA, in an ancestor of all living angiosperms. Studies of the model angiosperm Arabidopsis thaliana indicate that the mechanisms of development of the inner and outer integuments depend on largely different sets of molecular players. However, it was not known, in most cases, whether these differences were already present in early flowering plants, or arose later in the Arabidopsis lineage. Here, we analyze the expression patterns of integument regulators in Amborella trichopoda, the likely sister to all other living angiosperms. The data obtained indicate that regulators of the YABBY, KANADI, and homeodomain-leucine zipper class III transcription factor families have largely conserved their integument-specific expression profiles in the Amborella and Arabidopsis lineages since the most recent common ancestor (MRCA) of living angiosperms. We identified only one case, involving the paralogous genes ETTIN and AUXIN RESPONSE FACTOR4, in which integument-specific expression patterns had clearly diverged between Amborella and Arabidopsis. We use the data obtained to partially reconstruct molecular mechanisms of integument development in the MRCA of living angiosperms and discuss our findings in the context of alternative hypotheses for the origin of the angiosperm outer integument.
Highlights
The ovules and seeds of most seed plants are covered by one or two integuments
Phylogenetic reconstruction of the KANADI family (Supplementary Figure S3), generated from the protein alignment shown in Supplementary Figure S4, succeeded in identifying a clear Amborella ortholog of the Arabidopsis inner integument regulator ATS (KAN4), which grouped in a small basal clade of ATS-like sequences
The data presented here strongly suggest the extensive conservation of integument-specific developmental roles of genes of the YABBY, KANADI, and HD-ZIP III families in both the Amborella and Arabidopsis lineages since the most recent common ancestor (MRCA) of living flowering plants
Summary
The ovules and seeds of most seed plants are covered by one or two integuments. These maternal tissues function to (1) protect the internal tissues of the ovule and later the seed, (2) define a route, via the micropyle, for pollen or pollen-tube entry, and (3) contribute in many cases to the regulation of seed hydration and dormancy (Linkies et al, 2010). Conservation of Integument Development Mechanisms in Angiosperms integument, whereas the majority of angiosperm groups, including the most basally diverging of these, possess two integuments, of which the inner integument is considered homologous to the single integument of gymnosperms. Much of what is known of the molecular mechanisms of integument development comes from the study of the model angiosperm Arabidopsis thaliana (hereafter referred to as Arabidopsis), which possesses anatropous, bitegmic ovules. In the case of the bitegmic ovule, the lateral organs produced in this way are the inner and outer integuments, which arise from the chalazal tissue in the central region of the elongating ovule primordium. The inner integument arises first, followed closely by the outer integument (Endress, 2011)
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