There are over 18,000 legume species, most of which nodulate with Rhizobium bacteria, but only one non-legume. What is special about legumes? Is there some developmental or physiological characteristic of legume roots that distinguishes them from the roots of other plants? Current models predict multiple origins of nodulation within the Legume family. What traits did legumes acquire that predisposed them to interact with Rhizobium? Were there any changes to legume root development in order for them to accommodate their rhizobial symbiont?
Root architecture in response to ABA: We have identified a physiological trait that distinguishes legume roots from those of non-legumes. We have found that both nodulating and non-nodulating legumes respond to the plant hormone, Abscisic acid (ABA) by increasing lateral root density. Roots of non-legumes, however, respond to ABA by decreasing lateral root density. Our analysis of this trait suggests that its acquisition in the legume family coincided with the acquisition of a predisposition for nodulation. In addition, we found that Casuarina glauca, a non-legume that forms a root symbiosis with a different nitrogen-fixing bacteria, Frankia, also acquired a legume-like response of root architecture to ABA. Thus, acquisition of an altered root response to ABA may have been an essential step to allow the formation of a symbiotic root structure regulated differently from lateral roots and thus to allow roots to accommodate a bacterial partner.
Why do legume roots respond differently to ABA? Do legume roots have different downstream targets for ABA signaling? Or have inputs that regulate the ABA signaling pathway changed? Have components of the ABA signaling pathway evolved to create novel branchpoints? We are interested in the signaling network that regulates ABA signals in legume roots and nodules and how it has changed during the legume evolution. We are currently trying to characterize various inputs that regulate the ABA signaling pathway during nodule and root development and plan to soon begin to search for downstream transcriptional targets.
An altered response of root architecture to ABA was acquired coincidently with a predisposition for nodulation in the Fabaceae (Legume family) and in an actinorhizal lineage.
rbcL sequences for our sample taxa were used when possible, or alternately for their nearest relatives for which rbcL sequences are available. These sequences were subjected to phylogenetic analysis using PAUP version 3.1 using the Eschscholzia californica rbcL sequence to root the tree. Bootstrap support is indicated above each branch. The change in lateral root density (LRD) in response to ABA is indicated by arrows to the right of each taxon. Upward arrows indicate an increase in LRD in response to ABA, downward arrows indicate a decrease and hyphens represent no LRD change in response to ABA. Red bars indicate the three independent origins of nodulation in legumes as suggested by Doyle as the most parsimonious optimization of this trait (Doyle, 1998) or by Swenson for Actinorhizal nodulation (Swensen, 1996). The pink bar represents the acquisition of a predisposition for nodulation. Filled stars indicate the most parsimonious position for the acquisition of the altered LRD ABA response. Open stars indicate loss of the altered LRD ABA response.