Calmodulin interacts with MLO protein to regulate defence against mildew in barley

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Abstract
In plants, defence against specific isolates of a pathogen can be triggered by the presence of a corresponding race-specific resistance gene1, whereas resistance of a more broad-spectrum nature can result from recessive, presumably loss-of-regulatory-function, mutations2. An example of the latter are mlo mutations in barley, which have been successful in agriculture for the control of powdery mildew fungus (Blumeria graminis f. sp. hordei; Bgh)3. MLO protein resides in the plasma membrane, has seven transmembrane domains, and is the prototype of a sequence-diversified family unique to plants4,5, reminiscent of the seven-transmembrane receptors in fungi and animals5. In animals, these are known as G-protein-coupled receptors and exist in three main families, lacking sequence similarity, that are thought to be an example of molecular convergence6. MLO seems to function independently of heterotrimeric G proteins. We have identified a domain in MLO that mediates a Ca2+-dependent interaction with calmodulin in vitro. Loss of calmodulin binding halves the ability of MLO to negatively regulate defence against powdery mildew in vivo. We propose a sensor role for MLO in the modulation of defence reactions.