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Abstract

Common scab, a bacterial disease of potato causes significant losses to the Australian
potato industry through rejected seed and/or increased processing costs. Disease
symptoms caused by plant pathogenic Streptomyces sp., are attributable to a key
phytotoxin it produces, namely thaxtomin A. The development of resistance to this
disease, thrqugh selection of somaclonal varieties tolerant to thaxtomin A is a current
key industry supported program. However, mechanisms relating to resistance and
patterns of toxicity produced by thaxtomin A are not well understood and represent
the key objectives of this project.
The purpose of this study was to initially assess the impact of thaxtomin A on
various plants and plant cell systems from an electrophysiological, morphological
and pathological perspective. The effect of thaxtomin A in combination with various
auxin sources and analogues was also examined. Further work aimed to quantify
resistance to thaxtomin A within mutant strains of potato plants and calli, and
whether any specific change to a known thaxtomin A susceptible gene may be
responsible for altered levels of resistance to thaxtomin A.
Electrophysiological data obtained using ion-selective microelectrode ion flux
estimation (the MIFE) technique showed that interaction between plant and toxin
was characterised by a rapid and short-lived Ca2
+ influx and activation of the plasma
membrane proton pump. Thaxtomin A was more effective in young, physiologically
active tissues (root elongation zone or pollen tube apex), suggesting higher density of
thaxtomin A-binding sites in these regions. This provided the first evidence that
thaxtomin A triggers an early signalling cascade, which may be crucial in plantpathogen
interactions.
Glasshouse trials showed that foliar application of sublethal concentrations of 2,4-D
sprays on potato foliage reduced severity and occurrence of common scab. Lenticel
numbers, lenticel external dimensions and periderm structure (key features critical to
S. scabiei entry and penetration into a tuber) were generally not changed by 2,4-D sprays, suggesting no direct effect of 2,4-D on these morphological structures. In
contrast, tubers harvested from 2,4-D treated plants had a decreased sensitivity to
th~xtomin A compared with the controls, which may explain enhanced resistance. This suggests an indirect effect of2,4-D impacting on the toxin, thaxtomin A, rather
than morphological changes to the developing tuber.
Further experimental evidence confirmed an interaction between thaxtomin A and
auxin. fuhibition of tomato pollen tube growth by thaxtomin A was ameliorated by
addition of NAA. Also, auxin/auxin transport inhibitor sensitive 'ucu2-2/gi2 'A.
thaliana mutant showed significantly greater sensitivity to thaxtomin A, observed as
root growth suppression, increased rates of necrosis ( chlorosis ), plant death, and
more severely altered W flux profiles ( electrophysiological data) in the mutant
compared to the wild-type. Moreover, inhibition root growth assays with the
thaxtomin A-resistant 'txr I' A. thaliana mutant showed a 3 fold increase in
resistance to the polar auxin transport inhibitor, 1-NPA, suggesting an interaction
between thaxtomin A and the auxin efflux carrier associated with the NP A binding
'protein. Cross-resistances to 1-NPA and isoxaben of 'txrl' and the isoxaben
resistant 'ixr 1 'A. thaliana mutant suggests a similarity of function between isoxaben
and thaxtomin A, and also 'txrl' and 'ixrl '.
The 'txrl' gene homolog from potato has been successfully cloned and sequenced
from a series of mutant potato lines, selected for resistance to thaxtomin A.
Compared with the parent (control) there were no mutations within the 'txr 1 ' gene
examined suggesting the resistance phenotype is due to some other genetic change.
These studies have contributed to a better understanding of mechanisms of toxicity
of thaxtomin A in plant cells and advanced our knowledge of pathogen: host
interactions within the common scab disease pathosystem.

Item Type:	Thesis - PhD
Authors/Creators:	Tegg, Robert Steven
Keywords:	Potatoes
Copyright Holders:	The Author
Copyright Information:	Copyright 2006 the Author - The University is continuing to endeavour to trace the copyright owner(s) and in the meantime this item has been reproduced here in good faith. We would be pleased to hear from the copyright owner(s).
Additional Information:	Thesis (PhD)--University of Tasmania, 2006. Includes bibliographical references
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