Root responses to mechanical impedance and the role of ethylene signalling.

Abstract

Plant roots encounter a number of physical stresses in the soil and must be able to respond their growth appropriately. One such stress is mechanical impedance, which becomes an increasing problem in drying soils as soil strength increases with decreasing water content. In addition, the use of larger, heavier farming machinery leads to soil compaction, further increasing soil strength. Mechanical impedance has previously been shown to reduce root elongation and may have a negative impact on crop yields. It is therefore important to understand how root development is affected and growth regulated in response to mechanical impedance.
This thesis investigates the effect of mechanical impedance on root growth of Arabidopsis thalina and focuses on the role of the plant hormone ethylene in mediating this response. In particular the role of ethylene signalling in mediating root growth via crosstalk with auxin is examined. In addition the involvement of other plant hormones such as ABA, cytokinin and gibberellin is also briefly investigated. Experiments were carried out using a previously developed method whereby seedlings grown on horizontally orientated, dialysis membrane covered agar experience sufficient mechanical impedance to induce a response.
Mechanically impeded roots exhibited a characteristic ethylene response, with decreased primary root growth, increased diameter and root hair growth occurring closer to the tip. Analysis of mutants with altered responses to ethylene and auxin, and the effect of inhibitors of ethylene signalling and auxin demonstrated that both correct ethylene signalling and auxin transport are required for a mechanical impedance response. Confocal microscopy demonstrated that under mechanical impedance, auxin is redistributed at the root tip with increases in the expression of the transporters PIN1 and PIN2. ABA signalling is not required for a response to mechanical impedance and cytokinin responses appear to be reduced.