Abstract
The rhizobial-derived signaling molecule Nod factor is essential for the establishment of the Medicago truncatula/Sinorhizobium meliloti symbiosis. Nod factor perception and signal transduction in the plant involve calcium spiking and lead to the induction of nodulation gene expression. It has previously been shown that the heterotrimeric G-protein agonist mastoparan can activate nodulation gene expression in a manner analogous to Nod factor activation of these genes and this requires DOESN'T MAKE INFECTIONS3 (DMI3), a calcium- and calmodulin-dependent protein kinase (CCaMK) that is required for Nod factor signaling. Here we show that mastoparan activates oscillations in cytosolic calcium similar but not identical to Nod factor-induced calcium spiking. Mastoparan-induced calcium changes occur throughout the cell, whereas Nod factor-induced changes are restricted to the region associated with the nucleus. Mastoparan-induced calcium spiking occurs in plants mutated in the receptor-like kinases NOD FACTOR PERCEPTION and DMI2 and in the putative cation channel DMI1, which are all required for Nod factor induction of calcium spiking, indicating either that mastoparan functions downstream of these components or that it uses an alternative mechanism to Nod factor for activation of calcium spiking. However, both mastoparan and Nod factor-induced calcium spiking are inhibited by cyclopiazonic acid and n-butanol, suggesting some common mechanisms underpinning these two calcium agonists. The fact that mastoparan and Nod factor both activate calcium spiking and can induce nodulation gene expression in a DMI3-dependent manner strongly implicates CCaMK in the perception and transduction of the calcium signal.