Genes controlling legume nodule numbers affect phenotypic plasticity responses to nitrogen in the presence and absence of rhizobia

We investigated the role of three autoregulation of nodulation (AON) genes in regulating of root and shoot phenotypes when responding to changing nitrogen availability in the model legume, Medicago truncatula. These genes, RDN1‐1 (ROOT DETERMINED NODULATION1‐1), SUNN (SUPER NUMERIC NODULES), and LSS (LIKE SUNN SUPERNODULAOR), act in a systemic signalling pathway that limits nodule numbers. This pathway is also influenced by nitrogen availability, but it is not well known if AON genes control root and shoot phenotypes other than nodule numbers in response to nitrogen. We conducted a controlled glasshouse experiment to compare root and shoot phenotypes of mutants and wild type plants treated with four nitrate concentrations. All AON mutants showed altered rhizobia‐independent phenotypes, including biomass allocation, lateral root length, lateral root density, and root length ratio. In response to nitrogen, uninoculated AON mutants were less plastic than the wild type in controlling root mass ratio, root length ratio, and lateral root length. This suggests that AON genes control nodulation‐independent root architecture phenotypes in response to nitrogen. The phenotypic differences between wild type and AON mutants were exacerbated by the presence of nodules, pointing to resource competition as an additional mechanism affecting root and shoot responses to nitrogen.     

Structure of root nodules and nitrogen fixation in Egyptian wild herb legumes

Five wild herb legumes (Trifolium resupinatum, Melilotus indica, Medicago intertexta, Trigonella hamosa, and Alhagi murarum) were collected from cultivated lands of the Nile Valley, and compared with clover (Trifolium alexandrinum), a cultivated forage legume. The wild herb legumes exhibited great variation in nodulation percentage, nodule number, nodule mass and acetylene reduction activity with regard to locality. Nodulation of T. resupinatum and M. indica ranged between 50 - 100% and 33 - 100%, respectively, compared to 50 - 100% for T. alexandrinum. The number of nodules formed on T. resupinatum was 9 - 128 and that of M. indica 6 - 39, compared to 13 - 122 nodule per plant for T. alexandrinum. Nodule mass was correlated with nodule number. In M. indica, a small number of nodules was compensated with high specific nitrogenase activity. The herb legumes formed nodules of small size, varying shape (globose, cylindrical, branched, etc.), and of different types (crotalaroid and astragaloid). Microscopic examination of root-nodules from T. resupinatum, M. indica and M. intertexta, showed that these legumes formed indeterminate and effective nodules, containing apical meristems, central symbiotic tissue with characteristic zonation and peripheral vascular bundles. The nodules harboured bacteroids with pleiomorphic morphology. This revised version was published online in July 2006 with corrections to the Cover Date.     

The ecological significance of plasticity in root weight ratio in response to nitrogen: Opinion

We analyzed data on root weight ratio from a range of experimental studies documenting plant allocation changes in response to altered nitrogen availability. Our goal was to determine the degree to which plasticity in allocation between roots and shoots exists and to search for patterns in such plasticity among species. Our survey included 77 studies representing 206 cases and 129 species. As expected, we found that root weight ratio decreased with increased nitrogen availability in the majority of cases examined, and this response was most consistent when plants were grown individually or in intraspecific competition (versus interspecific competition). Surprisingly, however, we found no evidence to support existing hypotheses that fast-growing species adapted to high soil fertilities exhibit the highest levels of morphological plasticity, or that plasticity is positively associated with competitive ability. Rather, we found that average amounts of plasticity in root weight ratio in response to nitrogen availability were similar among species grouped by maximum relative growth rate and habitat fertility. Similar results were obtained for species categorized by life form, life history or root weight ratio itself, and plasticity in root weight ratio also had no consistent relationship with competitive ability. Numerous difficulties are associated with the attempt to search for pattern using independent studies, however our results lead to the conclusion that strong patterns in plasticity of root weight ratio in response to nitrogen availability among species do not exist. We discuss two reasons for this: (1) the costs of plasticity relative to its benefits are lower than previously predicted and (2) plasticity in traits other than root weight ratio is more important to plant foraging ability.     

Effects of drought on nitrogen fixation in soybean root nodules

Soybean plants [Glycine max (L.) Merr.] were grown in silica sand and were drought stressed for a 4 week period during reproductive development and without any mineral N supply in order to maximize demand for fixed nitrogen. A strain of Bradyrhizobium japonicum that forms large quantities of polysaccharide in nodules was used to determine whether or not the supply of reduced carbon to bacteroids limits nitrogenase activity. A depression of 30–40% in nitrogen content in leaves and pods of stressed plants indicated a marked decline in nitrogen fixation activity during the drought period. A 50% increase in the accumulation of bacterial polysaccharide in nodules accompanied this major decrease in nitrogen fixation activity and this result indicates that the negative impact of drought on nodules was not due to a depression of carbon supply to bacteroids. The drought treatment resulted in a statistically significant increase in N concentration in leaves and pods. Because N concentration and chlorophyll concentration in leaves were not depressed, there was no evidence of nitrogen deficiency in drought‐stressed plants, and this result indicates that the negative impact of drought on nodule function was not the cause of the depression of shoot growth. At the end of the drought period, the concentration of carbohydrates, amino nitrogen, and ureides was significantly increased in nodules on drought‐stressed plants. The overall results support the view that, under drought conditions, nitrogen fixation activity in nodules was depressed because demand for fixed N to support growth was lower.