Resurrecting Root Hairs in Duckweed - a trait lost for millions of years

Lead Research Organisation: University of Nottingham
Department Name: Sch of Biosciences

Abstract

Context: In both plants and animals, organ loss has occurred frequently during evolution. There are several examples in animal systems, such as the loss of limbs in snakes and vision in cavefish, where the molecular mechanisms underpinning organ loss are well understood. Despite the process being common in plants, we lack a molecular understanding of the underpinning processes.
Duckweeds are the ideal plant system to investigate the molecular mechanisms underpinning organ loss and uncover how downstream gene regulatory networks atrophy. Duckweed is a monocot that has returned to the aquatic environment. In doing so it has undergone phenomenal anatomical simplification in both shoot and root systems. Duckweeds are reduced to a single frond with individual genera being either rooted or rootless. Within the root-bearing genera, all species have lost pericycle cells meaning that roots cannot branch and all duckweed species have lost the ability to form root hairs. Quite simply, we do not know of a single family of plants or animals that has undergone a comparable degree of structural reduction. Advances in genomics and duckweed transformation place duckweeds as ideal models with which to study organ loss and templates in which gene regulatory networks that have been dormant for millions of years can be revived.
Understanding the molecular mechanisms governing the evolution and loss of organs informs us of the rules controlling radical changes in body plans and reverse engineer how complex networks arise. In this project, the student will investigate the loss of one trait, root hairs. Why root hairs? The molecular network controlling root hair formation is highly conserved. The same group of transcription factors (RSLs) that regulate root hair formation in flowering plants regulate rhizoid formation in liverworts. Liverwort RSL genes can even complement root hair mutants in Arabidopsis.
RSL genes are present in every multicellular plant from liverworts to flowering plants except one key group (the Class II RSLs) are missing from duckweeds. In this project, the student will re-introduce these genes to duckweed to test whether this is sufficient to restore a feature that has been absent from duckweeds for millions of years.
Work Plan: The student will firstly investigate components upstream of the Class II RSLs in the greater duckweed Spirodela polyrhiza. This will be done by creating reporters in Spirodela to examine expression patterns and expressing the Spirodela genes in Arabidopsis to test whether they can trans-complement root hair mutants. The student will then create transgenic Spirodela lines in which Class II RSL genes will be expressed. We predict that this will activate a suite of downstream genes that were previously dormant, as many Spirodela genes still contain root hair-specific cis elements within there promoters. The student will combine RNASeq and promoter motif analysis to determine how much of the root hair network is activated, and use confocal/light sheet microscopy to visualise epidermal cells and test for root hairs.
Impact: Beyond duckweed, resurrecting ancestral traits has importance in opening new synthetic biology projects and can be used to restore features present in wild relatives but missing from crop plants, such as secondary metabolites with benefits to health.

Publications

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Studentship Projects

Project Reference Relationship Related To Start End Student Name
BB/T008369/1 30/09/2020 29/09/2028
2886062 Studentship BB/T008369/1 30/09/2023 29/09/2027