Quantification of induced resistance against <i>Phytophthora</i> species expressing GFP as a vital marker : β-aminobutyric acid but not BTH protects potato and <i>Arabidopsis</i> from infection

Induced resistance was studied in the model pathosystem <i>Arabidopsis-Phytophthora brassicae</i> (formerly <i>P. porri</i>) in comparison with the agronomically important late blight disease of potato caused by <i>Phytophthora infestans</i>. For the quantification of disease progress, both <i>Phytophthora</i> species were transformed with the vector p34GFN carrying the selectable marker gene neomycine phosphotransferase (<i>nptII</i>) and the reporter gene green fluorescent protein (<i>gfp</i>). Eighty five per cent of the transformants of <i>P. brassicae</i> and <i>P. infestans</i> constitutively expressed GFP at high levels at all developmental stages both <i>in vitro</i> and <i>in planta</i>. Transformants with high GFP expression and normal <i>in vitro</i> growth and virulence were selected to quantify pathogen growth by measuring the <i>in planta</i> emitted GFP fluorescence. This non-destructive monitoring of the infection process was applied to analyse the efficacy of two chemical inducers of disease resistance, a functional SA-analogue, benzothiadiazole (BTH), and β-aminobutyric acid (BABA) which is involved in priming mechanisms of unknown nature. BABA pre-treatment (300 µm) via soil drench applied 24 h before inoculation completely protected the susceptible <i>Arabidopsis</i> accession Landsberg erecta (Ler) from infection with <i>P. brassicae</i>. A similar treatment with BTH (330 µm) did not induce resistance. Spraying the susceptible potato cultivar Bintje with BABA (1 mm) 2 days before inoculation resulted in a phenocopy of the incompatible interaction shown by the resistant potato cultivar Matilda while BTH (1.5 mm) did not protect Bintje from severe infection. Thus, in both pathosystems, the mechanisms of induced resistance appeared to be similar, suggesting that the <i>Arabidopsis-P. brassicae</i> pathosystem is a promising model for the molecular analysis of induced resistance mechanisms of potato against the late blight disease.