Several plant pathogens can secrete effectors molecules that once delivered into the host could manipulate plant defences allowing the pathogen to complete its life cycle. As an obligate biotroph, the oomycete Hyaloperonospora arabidopsidis (Hpa) that colonizes A. thaliana, requires living host tissue for growth and reproduction. These effectors are predominantly small secreted proteins containing a signal peptide and the RxLR EEr motifs. Bioinformatics analysis of the Hpa Emoy2 race genome was carried out and around 140 potential effectors were identified. Selection of candidates expressed during the infection process was done by sequencing ESTs of a 3dpi cDNA library and short reads of a 7dpi sequence tag library (www.illumina.com). Together with the ERA-PG Consortium we have cloned 102 of these potential effectors and are studying them for (i) Functions in virulence or avirulence in A. thaliana (ii) Genetic variation in the host for their recognition. For this, we use the EDV system of Hpa effectors delivery through Pseudomonas syringae TTSS (Pst) (Sohn et al., Plant Cell 2007). Using different Psts we could assess if a given Hpa effector could increase/decrease bacterial growth ?in planta? (Pst-LUX), and if can suppress PAMP triggered immunity (PTI) by monitoring callose deposition suppression when delivered through Pst deltaCEL. Preliminary results indicate that around 80% of the effectors tested can enhance bacterial growth on at least one of 12 A.t. accessions tested while 18% of them can restrict bacterial growth, and some of these provoke HR-like lesions on A.t . Also, 50% of the effectors are able to suppress callose accumulation indicating PTI suppression. Mixed infections using sets of Psts carrying different effectors are also being developed and results will be presented.
