How CYCLOPS keeps an eye on plant symbiosis.

T he availability of phosphorus and nitrogen are major limitations to plant growth, and as such our agricultural processes apply these nutrients at high concentrations to crop plants through fertilizer. Although fertilizer application has greatly enhanced food production it comes at a significant price: the chemical fixation of nitrogen depends on high levels of fossil fuels, making fertilizers a significant cost of food production and a major cause of greenhouse gas emissions from agriculture. A number of plants have entered beneficial interactions with microorganisms that facilitate the uptake of nitrogen and phosphorus from the soil. In this issue of PNAS Yano et al. (1) provide new insights into a novel genetic component in the plant that allows the establishment of these nutrientcapturing symbioses. Plants enter a symbiosis with fungi of the taxonomic group Glomeromycota, which facilitates phosphate and mineral acquisition (2, 3). Fungal hyphae invade the plant root and form highly-branched intracellular intrusions into cortical cells, called arbuscules, where nutrient exchange occurs. A more limited set of plant species also forms interactions with rhizobial bacteria that facilitates nitrogen uptake. A novel organ, the nodule is formed on the roots of the host plants and bacteria are accommodated in plant membrane-bound compartments within the nodule cells where nitrogen fixation occurs (3). Legumes, peas and beans, are able to form both mycorrhizal and rhizobial symbioses. Genetic studies on nodulation in legumes revealed that several genes were required for both nodulation and mycorrhization, resulting in the identification of 7 distinct loci making up the so-called ‘‘common symbiosis signaling (Sym) pathway’’ (2, 4, 5). Thus far the Sym pathway consists of up to 2 putative cation channels (MtDMI1, LjCASTOR and LjPOLLUX), 1 leucine-rich repeat receptor-like kinase (MtDMI2, LjSymRK), 1 calciumand calmodulin-dependent kinase (CCaMK), and 2 members of the nuclear pore complex (NUP85, NUP133). Today, we have reached a landmark in this pioneering age with the report by Yano et al. (1) of the cloning of the seventh gene in this pathway: CYCLOPS. The Sym pathway is necessary for the recognition of the rhizobial signal Nod factor (6). Perception of Nod factor involves lysin motif-containing receptorlike kinases (LysM-RLK) that have a specific function in Nod factor recognition with no apparent role in the establishment of the mycorrhizal symbiosis (7–10). Downstream of these LysMRLKs is the Sym pathway that is involved in the activation and perception of an oscillatory calcium signal: calcium spiking (reviewed in ref. 5). It is widely believed that CCaMK is responsible for decoding and transmitting the calcium signal. Specificity is possibly encoded in the frequency of the oscillation as calcium oscillations differ be-