Cytokinin‐induced promotion of root meristem size in the fern Azolla supports a shoot‐like origin of euphyllophyte roots

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.

Development and genetics in the evolution of land plant body plans

“…Trends in root architecture evolution somewhat mirror trends in the evolution of shoot architecture, and euphyllophyte rooting systems are hypothesized to have a shoot-like origin [7,149,150]. Although there is a good understanding of the molecular mechanisms regulating root architecture in flowering plant models [151], knowledge transfer to identify the mechanisms underpinning root evolution is limited.…”

Jasmonic and salicylic acid response in the fern Azolla filiculoides and its cyanobiont

“…In addition to emerging as a novel system for evo‐devo studies (cf. Chang, Bowman, & Meyerowitz, ; de Vries, Fischer, et al, ; F. ‐W. Li & Pryer, ; Sessa et al, ), Azolla is best‐known for its unique symbiosis (Carrapiço, ; Rai et al, ; Ran et al, ).…”

“…Azolla filiculoides is on the brink of becoming a model system for fern biology. It is fast growing (Wagner, ), has been the subject of multiple transcriptomic studies (Brouwer et al, ; Brouwer et al, ; de Vries, Fischer, et al, ), and is one of the few ferns whose genome is under investigation (F. ‐W. Li & Pryer, ; Sessa et al, ; Sessa & Der, ).…”

“…Azolla filiculoides is on the brink of becoming a model system for fern biology. It is fast growing (Wagner, 1997), has been the subject of multiple transcriptomic studies (Brouwer et al, 2014;Brouwer et al, 2017;de Vries, Fischer, et al, 2016), and is one of the few ferns whose genome is under investigation (F. -W. Li & Pryer, 2014;Sessa et al, 2014;Sessa & Der, 2016). What is more, Azolla engages in a (among the entirety of land plants) unique nitrogen fixing symbiosis with a cyanobiont that we will henceforth refer to as Nostoc azollae.…”

Azolla, a little fern with massive green potential

“…Approximately 627 million (A. filiculoides) and 259 million (S. cucullata) cleaned paired reads went into the assemblies. Libraries from experimental replicates were combined and assembled de novo by Trinity 85 (v2.0.6) and in a reference-guided manner using HISAT2 71 59 for which only a reference-guided approach was used. All programs used default parameters, and Trinity was run with the additional --trimmomatic option.…”