whole_TeggRobertSteven2006_thesis.pdf (11.89 MB)
Thaxtomin A toxicity in plant cells : studies associated with common scab disease of potato
thesisposted on 2023-05-27, 12:59 authored by Tegg, Robert Steven
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.
Rights statementCopyright 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). Thesis (PhD)--University of Tasmania, 2006. Includes bibliographical references