As more and more information becomes available, one marvels (and also frets) at the sophisticated strategies that pathogens have evolved in order to evade their hosts’ defense mechanisms. Many pathogens of plants and animals deliver effectors into their hosts in order to suppress immune responses. To date, the vast majority of these molecules have turned out to be proteins. However, for some two decades now, we have lived in an era when RNAs, especially small ones, have been found to play a surprisingly broad set of regulatory roles. The repertoire of these molecules includes both positive and negative effectors—those that enhance gene expression, as well as those that suppress it. Weiberg and coworkers now report that small RNAs (sRNAs) from a fungal pathogen of plants act as effectors to block the immune systems of Arabidopsis and tomato.
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