Abstract
Summary The phytohormones cytokinin and auxin orchestrate the root meristem development in angiosperms by determining embryonic bipolarity. Ferns, having the most basal euphyllophyte root, form neither bipolar embryos nor permanent embryonic primary roots but rather an adventitious root system. This raises the questions of how auxin and cytokinin govern fern root system architecture and whether this can tell us something about the origin of that root.Using Azolla filiculoides, we characterized the influence of IAA and zeatin on adventitious fern root meristems and vasculature by Nomarski microscopy. Simultaneously, RNAseq analyses, yielding 36 091 contigs, were used to uncover how the phytohormones affect root tip gene expression.We show that auxin restricts Azolla root meristem development, while cytokinin promotes it; it is the opposite effect of what is observed in Arabidopsis. Global gene expression profiling uncovered 145 genes significantly regulated by cytokinin or auxin, including cell wall modulators, cell division regulators and lateral root formation coordinators.Our data illuminate both evolution and development of fern roots. Promotion of meristem size through cytokinin supports the idea that root meristems of euphyllophytes evolved from shoot meristems. The foundation of these roots was laid in a postembryonically branching shoot system.
Highlights
Roots are a key innovation of vascular plants
Ferns, having the most basal euphyllophyte root, form neither bipolar embryos nor permanent embryonic primary roots but rather an adventitious root system. This raises the questions of how auxin and cytokinin govern fern root system architecture and whether this can tell us something about the origin of that root
Using Azolla filiculoides, we characterized the influence of IAA and zeatin on adventitious fern root meristems and vasculature by Nomarski microscopy
Summary
Roots are a key innovation of vascular plants They probably arose twice, once in the fern lineage and once in lycopods (Pires & Dolan, 2012). Regardles of whether they are uni- or multicellular (Jones & Dolan, 2012), rhizoids are elementary structures that grow through tip growth in the same manner as root hairs (Menand et al, 2007). Gametophytes of ferns form rhizoids, too (Banks, 1999), but in contrast to the bryophytes, their life cycle is dominated by the sporophytic stage (as in angiosperms), in which they build proper roots (Gunning et al, 1978; Banks, 1999). Gametophyte and sporophyte development in basal land plants share control mechanisms (Landberg et al, 2013; Bennett et al, 2014; Viaene et al, 2014), but we know little about complex roots of basal vascular plants beyond that
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