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Pigeonpea is an important legume food crop grown primarily by smallholder farmers in many semi-arid tropical regions of the world. We used the Illumina next-generation sequencing platform to generate 237.2 Gb of sequence, which along with Sangerbased bacterial artificial chromosome end sequences and a genetic map, we assembled into scaffolds representing 72.7% (605.78 Mb) of the 833.07 Mb pigeonpea genome. Genome analysis predicted 48,680 genes for pigeonpea and also showed the potential role that certain gene families, for example, drought tolerance-related genes, have played throughout the domestication of pigeonpea and the evolution of its ancestors. Although we found a few segmental duplication events, we did not observe the recent genome-wide duplication events observed in soybean. This reference genome sequence will facilitate the identification of the genetic basis of agronomically important traits, and accelerate the development of improved pigeonpea varieties that could improve food security in many developing countries.

The arbuscular mycorrhizal (AM) symbiosis is a mutualistic endosymbiosis formed by plant roots and AM fungi. Most vascular flowering plants have the ability to form these associations, which have a significant impact on plant health and consequently on ecosystem function. Nutrient exchange is a central feature of the AM symbiosis, and AM fungi obtain carbon from their plant host while assisting the plant with the acquisition of phosphorus (as phosphate) from the soil. In the AM symbiosis, the fungus delivers P i to the root through specialized hyphae called arbuscules. The molecular mechanisms of Pi and carbon transfer in the symbiosis are largely unknown, as are the mechanisms by which the plant regulates the symbiosis in response to its nutrient status. Plants possess many classes of P i transport proteins, including a unique clade (Pht1, subfamily I), members of which are expressed only in the AM symbiosis. Here, we show that MtPT4, a Medicago truncatula member of subfamily I, is essential for the acquisition of P i delivered by the AM fungus. However, more significantly, MtPT4 function is critical for AM symbiosis. Loss of MtPT4 function leads to premature death of the arbuscules; the fungus is unable to proliferate within the root, and symbiosis is terminated. Thus, Pi transport is not only a benefit for the plant but is also a requirement for the AM symbiosis.biotrophic ͉ membrane ͉ mineral nutrition ͉ mutualism ͉ root P lants and their arbuscular mycorrhizal (AM) fungal symbionts have coexisted for Ͼ400 million years (1), and the association is widespread in terrestrial ecosystems. AM symbiosis provides multiple benefits for the plant, not only enhanced phosphorus and nitrogen nutrition but also tolerance to pathogens and abiotic stresses (2, 3). For AM fungi, formation of a symbiosis is an obligate requirement. Their large spores contain reserves sufficient to support growth of a hyphal germ tube, but they must form an association with a plant to acquire additional carbon to complete their life cycle (4). Development of symbiosis, in particular intraradical development of the fungus in the root cortex, is reduced if P i availability is high (5-7). This suggests that the plant has a mechanism of regulating fungal growth, presumably to avoid the unnecessary allocation of carbon resources. The molecular basis is unknown but could occur by the control of carbon allocation (8).In the AM symbiosis, the fungal hyphae grow through the intercellular spaces of the root and subsequently invade the inner cortical cells, developing branched hyphae, called arbuscules, within the cells (9, 10). As each arbuscule forms, the plant cell envelops it in a membrane, the periarbuscular membrane, and the result is an extensive plant-fungal interface specialized for nutrient exchange (11)(12)(13)(14). It is predicted that P i and carbon transfer occur at the arbuscule/cortical cell interface, although direct evidence for carbon transfer at this location is lacking (14). Current data suggest that P i is translocated through the ...

Development of the Rhizobium-legume symbiosis is controlled by the host plant, although the underlying mechanisms have remained obscure. A mutant in the annual legume Medicago truncatula exhibits an increase of more than an order of magnitude in the number of persistent rhizobial infections. Physiological and genetic analyses indicate that this same mutation confers insensitivity to the plant hormone ethylene for multiple aspects of plant development, including nodulation. These data support the hypothesis that ethylene is a component of the signaling pathway controlling rhizobial infection of legumes.

Rhizobial bacteria activate the formation of nodules on the appropriate host legume plant, and this requires the bacterial signaling molecule Nod factor. Perception of Nod factor in the plant leads to the activation of a number of rhizobial-induced genes. Putative transcriptional regulators in the GRAS family are known to function in Nod factor signaling, but these proteins have not been shown to be capable of direct DNA binding. Here, we identify an ERF transcription factor, ERF Required for Nodulation (ERN), which contains a highly conserved AP2 DNA binding domain, that is necessary for nodulation. Mutations in this gene block the initiation and development of rhizobial invasion structures, termed infection threads, and thus block nodule invasion by the bacteria. We show that ERN is necessary for Nod factor-induced gene expression and for spontaneous nodulation activated by the calcium-and calmodulin-dependent protein kinase, DMI3, which is a component of the Nod factor signaling pathway. We propose that ERN is a component of the Nod factor signal transduction pathway and functions downstream of DMI3 to activate nodulation gene expression.