A study into the regulation of root development under osmotic stress

Abstract

The mechanisms behind the regulation of root development under a lack of water (osmotic stress) is a critical subject for plant biology and global agriculture. Previously osmotic stress has been shown to inhibit root growth via an abscisic acid (ABA)-mediated reduction in auxin transport, independent of ethylene signalling.
This thesis examines the impact of osmotic stress on critical developmental signals: auxin, ABA, and reactive oxygen species (ROS) minutes after encountering osmotic stress and following a longer stress period of 24 hours. The roles of these signals in the root were analysed via loss-of-function mutants, gene expression analysis, and bioimaging in concert with exogenous chemical treatment.
Auxin, ABA, and ROS (represented by level of oxidation) levels were all found to rapidly increase within 10 minutes of osmotic stress leading to downstream responses. ROS and ABA appear to have a strong positive feedback relationship that can develop with minutes. Under the longer stress period of 24 hours, auxin responses were found to decrease while both ROS and ABA responses were shown to increase.
PIN-mediated auxin transport was shown to play a key role in the reduction of auxin in the root tip following 24-hour osmotic stress treatment. PIN3 and PIN7 gene expression and protein distribution were altered under osmotic stress, associated with a reduction of the auxin maximum at the root tip.
ROP2 was found to play a central role in root development under osmotic stress root, along with the decrease of auxin and increase in ABA signalling after 24 hours. It was also shown that several respiratory burst oxidase homologs (RBOHs) play a role in root development under standard conditions as well as under osmotic stress. PERK4 and RBOHC likely play a key role in ROS production under ABA-mediated osmotic stress response, with the loss-of-function of PERK4 significantly improving root growth under osmotic stress.