Search

Phytophthora infestans is the oomycete pathogen responsible for the devastating late blight disease on potato and tomato. There is presently an intense research focus on the role(s) of effectors in promoting late blight disease development. However, little is known about how they are regulated, or how diversity in their expression may be generated among different isolates. Here we present data from investigation of RNA silencing processes, characterized by non-coding small RNA molecules (sRNA) of 19-40 nt. From deep sequencing of sRNAs we have identified sRNAs matching numerous RxLR and Crinkler (CRN) effector protein genes in two isolates differing in pathogenicity. Effector gene-derived sRNAs were present in both isolates, but exhibited marked differences in abundance, especially for CRN effectors. Small RNAs in P. infestans grouped into three clear size classes of 21, 25/26 and 32 nt. Small RNAs from all size classes mapped to RxLR effector genes, but notably 21 nt sRNAs were the predominant size class mapping to CRN effector genes. Some effector genes, such as PiAvr3a, to which sRNAs were found, also exhibited differences in transcript accumulation between the two isolates. The P. infestans genome is rich in transposable elements, and the majority of sRNAs of all size classes mapped to these sequences, predominantly to long terminal repeat (LTR) retrotransposons. RNA silencing of Dicer and Argonaute genes provided evidence that generation of 21 nt sRNAs is Dicer-dependent, while accumulation of longer sRNAs was impacted by silencing of Argonaute genes. Additionally, we identified six microRNA (miRNA) candidates from our sequencing data, their precursor sequences from the genome sequence, and target mRNAs. These miRNA candidates have features characteristic of both plant and metazoan miRNAs.

The potato late blight pathogen Phytophthora infestans has both an asexual and a sexual mode of reproduction. In Scandinavia, the pathogen is reproducing sexually on a regular basis, whereas clonal lineages dominate in other geographical regions. This study aimed at elucidating events or key genes underlying this difference in sexual behavior. First, the transcriptomes of eight strains, known as either clonal or sexual, were compared during early stages of mating. Principal component analysis (PCA) divided the samples in two clusters A and B and a clear grouping of the mating samples together with the A1 mating type parents was observed. Induction of genes encoding DNA adenine N6-methylation (6mA) methyl-transferases clearly showed a bias toward the cluster A. In contrast, the Avrblb2 effector gene family was highly induced in most of the mating samples and was associated with cluster B in the PCA, similarly to genes coding for acetyl-transferases, which play an important role in RXLR modification prior to secretion. Avrblb2 knock-down strains displayed a reduction in virulence and oospore formation, suggesting a role during the mating process. In conclusion, a number of gene candidates important for the reproductive processes were revealed. The results suggest a possible epigenetic influence and involvement of specific RXLR effectors in mating-related processes.