CoPI: Rajeev K. Varshney (International Crops Research Institute for the Semi-Arid Tropics [ICRISAT], Patancheru, India)
Collaborator: Eric von Wettberg (Florida International University)
Senior Personnel: R. Varma Penmetsa (University of California - Davis)
Legumes are the third largest family of flowering plants, and second only to the grasses in agricultural importance. On a global scale, legumes contribute 1/3 of humankind's protein intake, a fact that is directly related to their unusual capacity to access atmospheric nitrogen through symbiosis with nitrogen-fixing bacteria. Provision of reduced ("fixed") nitrogen represents a key challenge in all modern agriculture. In the developed world, intensive agriculture depends on the input of industrially-produced nitrogen fertilizers, derived from the energy of fossil fuels. In resource poor areas of the world, however, the cost of nitrogen fertilizers is prohibitive to their use, and crop yields and soil fertility suffer proportionally. Despite the important implications, little is known about the mechanisms that underlie efficient symbiotic nitrogen fixation in legumes, or how and to what extent domestication and breeding has impacted the ancestral capacity for symbiotic nitrogen fixation. This project will characterize the genetic mechanisms that underlie efficient symbiotic nitrogen fixation in the agricultural context, and contribute knowledge and resources to a new round of knowledge-driven legume crop improvement strategies that will benefit developing-world and developed-world agriculture alike.
In agricultural systems crop rotation with legume species is an important means to maintain soil fertility and crop productivity. Increasing the efficiency of symbiotic nitrogen fixation in legume crops - overcoming the genetic bottleneck of domestication - has great potential to improve the livelihoods of resource poor farmers. The goal of this project is to simultaneously satisfy the curiosity that drives basic science, while providing understanding that can lead to new tools for applied agriculture. This project will enhance quality and visibility of international agricultural research, and provide for the training of young scientists as undergraduate, graduate and postdoctoral students. Information about the project will be available through a project website (www.icrisat.org/gt-bt/ICGGC/homepage.htm). All project data will be available at www.comparative-legumes.org and the Legume Information System (LIS; http://www.lis.org) long-term. All sequence data will also be available at the relevant National Center for Biotechnology Information (NCBI) databases. Seed and bacterial strains will be available through ICRISAT and the University of California - Davis, respectively, upon request.
Legumes provide an estimated 30% of humankind’s nutritional nitrogen, while in agricultural systems crop rotations with legume species provide an important means to maintain soil fertility. Despite legumes’ comparative advantage of N-fixation, nitrogen fertilizers are often added during legume cultivation; moreover, agricultural soils are typically richer in nutrient content than are the soils in which the species originally evolved. These practices may relax selection on nitrogen fixation traits and potentially create legume crops that are less efficient for nitrogen fixation than their wild progenitor species. Improved understanding of the processes that regulate N-fixation in natural and agricultural environments will ultimately enable rationale strategies to mitigate these situations. In this project, we investigated the domestication of chickpea, the world’s second most important grain legume. Research from this project suggests that shifts in the genetics of domesticated chickpea have created legume crops that are more reliant on soil nitrogen and potentially less able to harness symbiotic nitrogen fixation. Impacted phenotypes include an increased sensitivity to soil nitrogen in the domesticated crop, as well as increases in the number of symbiotic organs formed on roots. The former is likely to be an advantage in intensively managed agricultural systems where soil fertility tends to be high, while the later phenotype may reflect a lower per-unit function in symbiotic organs. Under this project, we also have contributed to the whole genome sequence of chickpea. In addition to deep sequencing of a single reference genotype, re-sequencing of numerous cultivated accessions reveals the impact of human selection (breeding) on genetic diversity within the species. In related molecular genetic studies, we have identified candidate genes for a range of domestication-related traits. These include flowering time control (a homolog of FT), plant architecture (a homolog of TFL), flower color and seed coat tannins (a transcription factor controlling both phenotypes, and a rare allele of anthocyanidin synthase controlling flower color only), nodule number and disease resistance (NBS-LRR genes).
