Small RNAs in Phytophthora infestans and cross-talk with potato

Small RNAs (sRNAs) are small non-coding RNAs usually ranging in size 20-30 nt. They are playing important roles in plant-pathogen interactions. This thesis aimed at studying the sRNA populations in potato and Phytophthora infestans and their role in the potato-P. infestans interaction. An attempt was also made to implement such knowledge to improve resistance in potato against P. infestans. P. infestans is an oomycete that causes severe damage to potato and tomato, and is well known for its ability to evolve rapidly to overcome resistance. It possesses active RNA silencing pathways and sRNAs are playing important roles to control the large numbers of transposable elements (TE) present in its genome and effector genes. Through deep sequencing of sRNAs from two isolates of P. infestans differing in pathogenicity, three clear size classes (21, 25/26 and 32 nt) of sRNAs were identified. RxLR and Crinkler (CRN) effector gene-derived sRNAs were present in both isolates, but exhibited marked differences in abundance. Some effector genes, such as PiAvr3a and PiAvrblb2, to which sRNAs were found, also exhibited differences in transcript accumulation between the two isolates. Majority of sRNAs also mapped to TEs. An additional group of sRNAs, the tRNA-derived RNA fragments (tRFs) ranging in size from 19-40 nt was identified in P. infestans as well. Some tRFs accumulated differentially during infection. A host-induced gene-silencing (HIGS) approach was proven to be successful in the potato-P. infestans pathosystem. Four different endogenous genes in P. infestans were targeted in my study by HIGS constructs and choice of target gene was shown crucial for a successful outcome. HIGS has the potential to be adopted in new resistance breeding to improve resistance in potato against P. infestans. Single late blight resistant potato cultivar was sequenced upon infection with compatible isolate of P. infestans. In order to decipher the molecular events underlying its resistance breakdown, analysis of transcript change and the possible role of sRNAs on transcript regulation during infection are ongoing. In infected potato, resistance genes are targeted by its own miRNAs leading to suppression. Additional components are most likely involved in this process and near future analysis will help to enhance our understanding of how P. infestans uses sRNAs to evade plant immune responses.