Shoot branching is a major determinant of plant architecture and is highly regulated by endogenous and environmental cues. Two classes of hormones, auxin and cytokinin, have long been known to have an important involvement in controlling shoot branching. Previous studies using a series of mutants with enhanced shoot branching suggested the existence of a third class of hormone(s) that is derived from carotenoids, but its chemical identity has been unknown. Here we show that levels of strigolactones, a group of terpenoid lactones, are significantly reduced in some of the branching mutants. Furthermore, application of strigolactones inhibits shoot branching in these mutants. Strigolactones were previously found in root exudates acting as communication chemicals with parasitic weeds and symbiotic arbuscular mycorrhizal fungi. Thus, we propose that strigolactones act as a new hormone class-or their biosynthetic precursors-in regulating above-ground plant architecture, and also have a function in underground communication with other neighbouring organisms.

Most higher plant species can enter a root symbiosis with arbuscular mycorrhizal fungi, in which plant carbon is traded for fungal phosphate. This is an ancient symbiosis, which has been detected in fossils of early land plants. In contrast, the nitrogen-fixing root nodule symbioses of plants with bacteria evolved more recently, and are phylogenetically restricted to the rosid I clade of plants. Both symbioses rely on partially overlapping genetic programmes. We have identified the molecular basis for this convergence by cloning orthologous SYMRK ('symbiosis receptor-like kinase') genes from Lotus and pea, which are required for both fungal and bacterial recognition. SYMRK is predicted to have a signal peptide, an extracellular domain comprising leucine-rich repeats, a transmembrane and an intracellular protein kinase domain. Lotus SYMRK is required for a symbiotic signal transduction pathway leading from the perception of microbial signal molecules to rapid symbiosis-related gene activation. The perception of symbiotic fungi and bacteria is mediated by at least one common signalling component, which could have been recruited during the evolution of root nodule symbioses from the already existing arbuscular mycorrhiza symbiosis.

The initiation of intracellular infection of legume roots by symbiotic rhizobia bacteria and arbuscular mycorrhiza (AM) fungi is preceded by the induction of calcium signatures in and around the nucleus of root epidermal cells. Although a calcium and calmodulin-dependent kinase (CCaMK) is a key mediator of symbiotic root responses, the decoding of the calcium signal and the molecular events downstream are only poorly understood. Here, we characterize Lotus japonicus cyclops mutants on which microbial infection was severely inhibited. In contrast, nodule organogenesis was initiated in response to rhizobia, but arrested prematurely. This arrest was overcome when a deregulated CCaMK mutant version was introduced into cyclops mutants, conferring the development of full-sized, spontaneous nodules. Because cyclops mutants block symbiotic infection but are competent for nodule development, they reveal a bifurcation of signal transduction downstream of CCaMK. We identified CYCLOPS by positional cloning. CYCLOPS carries a functional nuclear localization signal and a predicted coiled-coil domain. We observed colocalization and physical interaction between CCaMK and CYCLOPS in plant and yeast cell nuclei in the absence of symbiotic stimulation. Importantly, CYCLOPS is a phosphorylation substrate of CCaMK in vitro. Cyclops mutants of rice were impaired in AM, and rice CYCLOPS could restore symbiosis in Lotus cyclops mutants, indicating a functional conservation across angiosperms. Our results suggest that CYCLOPS forms an ancient, preassembled signal transduction complex with CCaMK that is specifically required for infection, whereas organogenesis likely requires additional yet-to-be identified CCaMK interactors or substrates.BiFC ͉ map-based cloning ͉ plant-microbe symbiosis ͉ protein phosphorylation ͉ protein-protein interaction L egume plants can establish endosymbiotic interactions with nitrogen-fixing rhizobia and phosphate-delivering arbuscular mycorrhiza (AM) fungi. Plant root hairs form a tight curl in which rhizobia are entrapped. From this closed infection pocket, the bacteria are guided by plant membrane-delimited infection threads (ITs) into the root nodule, a specialized organ developed by the plant to provide an optimized environment for nitrogen fixation (1). AM fungal hyphae are guided through epidermal and cortical cells toward the inner cortex (2), where arbuscules, highly branched intracellular symbiotic structures, are formed (3). Intracellular infection by rhizobia and AM fungi is preceded by an exchange of specific signaling molecules. Rhizobia produce lipochito-oligosaccharides (Nod factors) that activate host plant responses including root hair deformation, and preinfection thread formation, which are structures that determine the path of IT growth through the root (4), and initiation of cortical cell division (1). One of the earliest plant responses to stimulation by Nod factors is Ca 2ϩ -spiking, which consists of perinuclear oscillations of calcium concentration in root cells (5). In the legume