Sunday, February 24, 2013

Keeping the Defenses Quiet


Plants and animals alike fall sick when infected by something pathogenic: viruses, bacteria, oomycetes, and more. Thankfully, higher-order eukaryotes have evolved multiple ways to stop many of these bugs from “taking over”. However, these mini critters don’t give up quite so easily, and have a few clever tricks of their own to combat the host immune system. This paper investigates the strategy of RNA silencing suppressors, a counter defense utilized by the Phytophthora microbial plant pathogens.
            Before discussing the paper, however, why bother to study this topic? First of all, humans rely on plants for everyday life. If one understands the ways this group of notorious crop pathogens causes plant sickness, we will have an edge in curbing the economical and physical damage caused by these microbes to the supply of food worldwide. For example, in Ireland, the potato famine was caused by
 Phytophthora infes­tans; since 1845, the pest causes endemic loss of crop of up to 50% seven out of ten years (1). More importantly, however, we will have a model that may be extrapolated to other critical plant species and even animals when trying to tailor specific cures for diseases in the future. As with all science, understanding a little piece of the puzzle has multifaceted usefulness, because often this piece can help us solve the puzzles in other plants and animals. This paper successfully characterizes factors in Phytophthora plant pathogens that contribute to its successful virulence in the hopes of demonstrating that further characterizations of factors in relevant organisms will help in finding effective cures.
            One method by which a host may prevent, or at least lessen, the infection of pathogens, especially against RNA viruses, is to encode for miRNAs or siRNAs; small-RNAs that work to degrade the genetic material of the invading species (2). But what happens when Phytophthora creates something called an effector, a factor boosting virus fitness (3) where the small-RNA defense mechanism is actually disabled? You get successful infection of the invading species, or at least enhanced succeptibility. In a series of clever experiments, Qiao et al. characterized PSR1 and PSR2, miRNA/siRNA interfering proteins of pathogens belonging to the Phytophthora genus.

            As can be seen in Figure 1, the authors’ first experiment demonstrated that the RNA silencing defense system is interrupted in the leaves of N. benthamiana when infected with a PSR-containing version of Agrobacterium tumefaciens, which was designed to express GFP. Agrobacterium tumefaciens is a soil bacterium that causes tumors in plants when introduced into a plant (4). Normally, the plant’s small-RNA defense system would destroy the invader, disallowing it to express GFP. However, when coexpressed with the effectors, high levels of fluorescence were observed, suggesting that these effectors were disabling the plant’s small-RNA immune response. They also showed that PSR1 effectors might have an amplified immune-system suppression effect (systemic silencing); this places an even greater stress on trying to characterize these elements, as they may be key factors in the pathogenesis of particular invaders.
            The authors also demonstrated in Figure 2 that PSR1 is affecting the biogenesis of siRNAs and miRNAs, not their activity. Since bugs expressing the PSR1 (versus the control, which was not) caused levels of pre-miRNA to drop but not pri-miRNA levels, and because pre-miRNAs are derived by processing of pri-miRNAs via DCL1 (which is also responsible in part for the biogenesis of siRNAs and miRNAs), the differential abundance suggests that the PSR1 is interacting with DCL1 and preventing pre-miRNA synthesis which is also interrupting small-RNA biogenesis. This finding is valuable: it provides information on the type of mechanism for getting around the host defenses. Many VSRs (viral suppressors of RNA silencing) actually operate by binding to directly to small-RNAs (CITATION), but by knowing where in the pathway host defenses are failing, it will make a targeted cure far more effective. Further, the authors showed that PSR2 does not affect small-RNA biogenesis, but rather, plays a role in the block of other host-defense genes. This shows that the two effectors are likely working in tandem to promote infection. Figures 3, 4, and 5 were experiments separate from 1 and 2, designed to further support the hypothesis that the PSR1 and PSR2 are important for the promotion of Phytophthora pathogenesis.
            What is important to understand from this paper is not necessarily what PSR1 and PSR2 effectors do, but rather understand the importance of characterizing such effectors. By having a better grasp on understanding what is responsible for thwarting our bodies’ defenses, steps may be taken to research potential ways to disable these invaders’ virulence strategies. This paper is a fundamental step in moving towards clinical solutions for ailed mammals as well as in plants ravaged by these pathogens.

Primary article:

Supporting articles:

1) Dowley L, Grant J, Griffin D. (2008) Yield losses caused by late blight (Phytophthora infestans (Mont.) de Bary) in potato crops in Ireland.
 Irish Journal of Agricultural and Food Research 47: 69–78

2) McManus, M. (2004) Small RNAs and Immunity. Immuntiy. Volume 21, Issue 6, Pages 747–756

3) Jones J, Dangl J. (2006) The plant immune system. Nature. doi:10.1038/nature05286

4) Hoekema A, Hirsch P, Hooykaas P, Schilperoort R. (1983) A binary plant vector strategy based on separation of vir- and T-region of the Agrobacterium tumefaciens Ti-plasmid. Nature. doi:10.1038/303179a0

5) Burgyán J, Havelda Z. Viral suppressors of RNA silencing. Cell. Volume 16, Issue 5, May 2011, Pages 265–272

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