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The mode of action of phosphite: An investigation of the biochemical mechanisms involved using a Lupinus augustifolies - Phytophthora cinnamomi model system

Groves, Emma (2002) The mode of action of phosphite: An investigation of the biochemical mechanisms involved using a Lupinus augustifolies - Phytophthora cinnamomi model system. Honours thesis, Murdoch University.


The soil-borne pathogen Phytophthora cinnamomi is a serious disease affecting plant species in native plant communities of Western Australia. The fungicide phosphite effectively controls P. cinnamomi. Phosphite has a complex mode of action, acting both directly on the pathogen and indirectly stimulating host defence responses. The biochemical mechanisms involved are poorly understood and it is not known how phosphite stimulates host defences. This thesis investigated the efficiency of phosphite in controlling P. cinnamoni infection and root development, and its ability to stimulate host defence enzymes, using the Lupinus augustifolius- P. cinnamomi model system. Furthermore, the mechanism and pathways involved in phosphite-induced defence stimulation was investigated by examining reactive oxygen species generation (ROS) and salicylic acid (SA) accumulation.

Phosphite (0.25%) was applied as a foliar spray to aeroponically grown lupin plants. Phosphite was quickly translocated to the roots and levels maintained for the duration of the experiment (10 days). This was associated with a reduction in P. cinnamomi infection and lesion development. However, phosphite was found to have a negative effect on root development as the rate of root extension fell and incidence of damaged roots increased. Histological examination revealed that phosphite altered root morphology producing stunted roots with reduced root cap.

It was thought the damage induced at the root tip by phosphite might elicit ROS, important signalling molecules in plant responses to pathogen attack. Stimulation of ROS was not determined directly but rather indirectly by measurement of oxidative metabolic enzymes. This study found that each of the oxidative enzymes followed different transient patterns of stimulation following phosphite application. The lack of 111 coordinated enzyme stimulation in conjunction with the fact that ROS was not measured directly in planta made it difficult to draw any conclusions as to if ROS was generated by phosphite. However, foliar application of phosphite was found to stimulate host defence enzymes and products such as peroxidase and phenolics in both the absence and presence of P. cinnamomi.

Salicylic acid (SA) is a key molecule in signal transduction pathway involved in systemic acquired resistance (SAR). The fungicide BABA stimulates SA accumulation, thus it was hypothesised that SA may also accumulate after phosphite application. Lupin seedlings were sprayed with 0.25% phosphite and HPLC analysis used to determine the concentration of SA and its glucoside conjugate (SAG) at the root apical region over a 10 day period post application. Phosphite stimulated SA accumulation at the root tip. Therefore, it is thought that control afforded by phosphite may be mediated by SA.

The involvement of SA in phosphite mediated resistance was further investigated. SA, P-SA (a phosphite and SA combination spray) and phosphite were applied to lupin plants as a foliar spray and the stimulation of host defence enzymes, ROS generation, SA accumulation and control of P. cinnamomi examined. SA and P-SA was as effective as phosphite in controlling P. cinnamomi infection development and accumulating SA. Moreover, similar trends in enzyme stimulation in non-inoculated roots was found in root tips of SA and phosphite treated plants. Together this supports the notion that phosphite induced resistance may be SA -dependent.

Item Type: Thesis (Honours)
Murdoch Affiliation(s): School of Biological Sciences and Biotechnology
Notes: A digital copy of this thesis is not available. Your library can request a copy from Murdoch University Library via Document Delivery. A fee applies to this service.
Supervisor(s): Hardy, Giles and Burgess, Treena
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