Lack of active defence responses revealed in a soil-free Arabidopsis/Peronospora sterile co-cultivation system

Abstract

The molecular basis of organ specificity in plant diseases is little characterised. Downy mildew of Arabidopsis caused by the oomycete Peronospora parasitica is characteristically a leaf disease. Resistant host genotypes recognise the pathogen in a gene-for-gene dependent manner and respond with the production of H<sub>2</sub>O<sub>2</sub> and the execution of a genetically programmed hypersensitive cell death (HR). We inoculated the roots of Arabidopsis genotypes Col-0, Ws-0 and Wei-0 with the NOCO and WELA races of the pathogen and compared the responses with those observed in infected leaves. Combinations of incompatible genotypes of host and pathogen showed the expected responses of an oxidative burst and the HR in leaves but, surprisingly, roots showed no signs of active defence and appeared completely susceptible to all the P. parasitica isolates tested. RT-PCR showed that the R genes RPP1 and RPP13, which mediate resistance in leaves to P. parasitica isolates NOCO and WELA, respectively, were expressed in leaves as well as in roots. Similarly, NDR1 and EDS1, two components of RPP1-mediated gene signalling pathways, are also expressed in both tissues. Thus, we show for the first time that expression of R genes and at least some of the known downstream components of the signalling cascade are not sufficient for the induction of avirulence gene-mediated defence mechanisms.