Molecular and biochemical mechanisms of potato defense suppression by P. infestans

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Late blight, caused by the oomycete Phytophthora infestans in both potato and tomato, has re-emerged as the most important disease on potatoes worldwide. Concurrently, the world populations of this pathogen have drastically changed in the last two decades. The new populations, mainly composed of the US-8 genotype, are much more aggressive and successful than the old US-1 genotype. To date, however, very few studies have addressed potato differential interactions with the new versus the old strains. Recent studies in my lab suggested the suppression of plant defense genes as one strategy used by the US-8 strain to counteract potato defense mechanisms at the site of infection, while the US-1 strain did not. Interestingly, such suppression applied to some genes, but not to others. Defense suppression in this pathosystem has been suspected for many years, and suppressors of glucan nature have been identified in P. infestans. However, the mechanisms of such suppression and its role are not understood. Through my research program, I seek to understand the molecular and biochemical mechanisms governing the plant defense suppression that allows the currently predominant strain of P. infestans to overcome potato defenses. I will investigate whether P. infestans ability to inhibit the expression of defense genes is only constitutive or if it is also induced in this pathogen upon its interaction with the host tissues. Then, I will study the relative involvement of P. infestans glucan versus peptidic suppressors in this defense gene downregulation. On the plant side, I will investigate whether other isoforms accumulate at the expense of the suppressed gene in response to P. infestans suppressors. Finally, I will assess the effects of salicylic acid (SA), methyl jasmonate (MJ), abscissic acid (ABA) and ethylene (ET) on the suppression/induction of these isoforms. This research is important in order to understand how the most aggressive strains of P. infestans defeat potato defense reactions, and hopefully will lay the groundwork for more efficient knowledge-based disease control strategies.