Manipulating plant defences for improved control of diseases caused by plasmodiophorids

The potato disease powdery scab (caused by Spongospora subterranea) damages quality of potato tubers. The pathogen also harms host growth and yield through infection of roots, and transmits Potato mop top virus, and is economically important throughout the world. Chemically induced resistance to plant pathogens is a growing area of research, where exogenously applied compounds activate plant defences against pathogen attack. In this study, the efficacy of β-aminobutyric acid (BABA) was assessed for inducing defence in potato against S. subterranea, and transcription of genes that may be involved in the mechanism of defence development was analysed. The first part of the study assessed the use of qPCR to quantify S. subterranea infection in potato root tissue. A glasshouse experiment assessed development of the pathogen in roots of two potato cultivars, ‘Iwa’ (susceptible to tuber powdery scab) and ‘Gladiator’ (resistant). Intensity of root infection by the pathogen was also assessed using microscopy and counting of root galls. The resistant cultivar had half of the amount of S. subterranea in roots compared with the susceptible cultivar. qPCR provided very sensitive, rapid and consistent detection of S. subterranea in potato roots. The second part of the study focused on BABA-induced resistance to S. subterranea. Two experiments were carried out. In the first , potato plants of susceptible ‘Iwa’ and resistant ‘Gladiator’ were grown for 2 weeks, and then three different concentrations of BABA (single applications of 1, 2 or 4mM) were tested for effects on S. subterranea infection, assessed using qPCR. Results were inconsistent, showing high infection at 6 weeks, and plants also presented phytotoxicity effects of the chemical. In the second experiment, S. subterranea-susceptible plants were grown for 4 weeks before applying BABA treatments (1, 2, or 4 mM), and the plants were then inoculated with the pathogen. Infection was assessed (using qPCR and gall counting) at 2, 4 and 6 weeks post inoculation. BABA was also re-applied after each of these time periods. Infection was decreased with greater concentrations of BABA compared to non-treated plants. Multiple treatments (with 2 or 4mM BABA) of mature plants (4 weeks post planting and older) almost completely prevented infection. The final part of the study analysed transcription of genes suspected to be involved in the development of BABA-induced resistance in potato. Eighteen genes were selected from potato genome data and primers were designed for each. Plants were grown for 4 weeks and then treated with three rates of BABA (1, 2 or 4 mM). Half of the plants were then harvested at 24, 48 and 72 h post treatment, and the remaining plants were inoculated with S. subterranea (with uninoculated experimental controls included) and harvested 10 days post inoculation. Some genes increased transcription in the first 72 h, and this increase was dependent on BABA concentration. No major changes were observed with pathogen infection. Transcription of genes suggested that BABA induced systemic acquired resistance, as most genes previously related to this pathway showed greatly increased transcription after BABA application. The class I chitinase (1), PR1b and PR4 genes were good candidates for markers of BABA–induced resistance in potato. This study has demonstrated that BABA has potential as an alternative for control of S. subterranea infection of potato. This potential should be verified in field evaluations