A systems study of the effect of osmotic stress on hormone crosstalk and growth in Arabidopsis thaliana roots

Abstract

Understanding the mechanisms regulating root development under drought conditions is an important question for plant biology and world agriculture. This thesis examines the effect of osmotic stress on the plant hormones abscisic acid (ABA), cytokinin and ethylene responses and how they mediate auxin transport, distribution and root growth via PIN proteins
Root growth is reduced under osmotic stress, and ABA responses increase. Root growth can be rescued by inhibiting ABA biosynthesis, indicating its critical role in the regulation of growth under stress. There was also a reduction in cytokinin signalling under stress.
The inhibition of root growth under osmotic stress does not require ethylene signalling, however auxin can rescue growth. Osmotic stress also modulates auxin transporter levels, particularly PIN1, which regulates auxin transport to the root tip.
As PIN1 levels are reduced under stress in an ABA-dependent manner, overriding the ethylene effect on PIN1 levels, and auxin responses decrease under stress, I present the hypothesis that ABA is limiting auxin transport to the root under stress to reduce growth.
However, the interplay between ABA, ethylene, cytokinin and auxin is tissue-specific, with the result that PIN1 and PIN2 differentially respond to osmotic stress.
Combining experimental analysis with extensive literature searches allowed the systematic construction of interaction networks, incorporating the known interactions between the hormones and stress. This network analysis reveals that ABA regulates root growth under osmotic stress conditions via interactions with cytokinin, ethylene and auxin demonstrating complicated non-linear relationships and providing a framework for further kinetic modelling. Kinetic modelling (using differential equations to simulate these interactions) of ethylene and ABA effects on PIN1 levels reveals that the hormones most likely act on the same pathway to regulate PIN1 levels.
The work presented here provides novel insights into how root growth is regulated by hormones under drought and osmotic stress conditions.