ARS | Publication request: Beyond the Papilionoids – What can We Learn from Chamaecrista?

Submitted to: Plant Physiology
Publication Type: Review Article
Publication Acceptance Date: September 6, 2009
Publication Date: September 15, 2009
Citation: Singer, S.R., Maki, S.L., Farmer, A.D., Ilut, D., May, G.D., Cannon, S.B., Doyle, J.J. 2009. Beyond the Papilionoids – What can We Learn from Chamaecrista? Plant Physiology. 151:1041-1047.

Interpretive Summary: The legume plant family, which contains soybeans, common beans, peas, lentils, clovers and many other crop species, also contains many under-studied species, including a group of thousands that diverged from soybeans and peas nearly 60 million years ago. That early-diverging group, called the "mimosids," shares with beans and peas the ability to create (or "fix") their own nitrogen fertilizer, through association with nitrogen-fixing soil bacteria. Learning how this association arose in these plants and bacteria and not in most others is key in understanding how to help other crop plants make better use of associations with nitrogen-fixing soil bacteria. A challenge, however, is choosing a suitable plant from the early-diverging mimosid group to use for studying the evolution of nitrogen fixation. Many mimosids are tropical trees, or have other characteristics that make them difficult to study in the laboratory. This paper reviews work on a small, annual North American prairie legume called partridge pea (Chamaecrista), which will be suitable for this purpose. The paper describes sequencing of most of the genes of the plant, as well as creation of true-breeding lines of plant variants from different ecological regions of the country. The paper describes how the plant is being used to investigate several questions important for improvement of crop legumes. When did the legumes experience a doubling of all of their genetic material? This affects how many similar genes a plant breeder or researcher needs to consider when trying to identify the genetic source of a plant trait. How did the legumes develop their characteristic flower structure? The Chamaecrista flower structure is different than a bean or pea flower; flower structure affects how a crop plant is pollinated. How do plants respond to climate changes? Chamaecrista populations from hotter, drier parts of the country and from cooler, wetter parts of the country have been studied to learn how rapidly populations of the plant are able to change to tolerate different conditions. Lastly, how did the legumes develop the ability to fix useable nitrogen fertilizer? Understanding this process is necessary in order to help other crop species obtain more of their nitrogen fertilizer from soil bacteria rather than from costly industrially-generated fertilizer.

Technical Abstract: Expanding legume research beyond the model papilionoids is necessary if we wish to capture more of the diversity of the enormous, economically important legume family. Chamaecrista fasciculata is emerging as a non-papilionoid model, belonging to the paraphyletic subfamily Caesalpinioideae within the mimosoid clade. Mimosoids diverged from the common ancestor of Glycine max, Medicago truncatula, and Lotus japonicus nearly 60 million years ago--nearly contemporaneously with the origin of legumes. There is growing interest within the legume community in C. fasciculata as a complementary legume model for a number of reasons, including phylogenetic position, nodulation within a clade of limited nodulating species, nonpapilioinid floral morphology, herbaceous growth habit, and tractability in laboratory and field settings. Whole transcriptome sequencing of C. fasciculata shoots, roots, and nodules, along with gene expression profiling and SNP profiling, provides community resources to address fundamental questions about legume evolution. A range of ecotypes, development of functional genomics tools, and an integration of research and undergraduate education leverage these genomic resources.