Influence of rhizobial symbiosis on chickpea root system architecture

Since most soil resources are heterogeneously distributed, root distribution, termed root system architecture (RSA), is critical for plant nutrition. Rhizobia are bacteria that establish symbioses with legume roots, where they provide N in return for C, thus satisfying most of their hosts’ N requirements. To better understand their impact on legume RSA, this was investigated in chickpea. A large rhizobox (RB) system allows almost unrestricted root growth and to capture up to 90% of the root system against one transparent side. Local and global traits that inform on soil resources acquisition strategies can be measured. The system can differentiate between varieties with somewhat contrasting root systems, important if rhizobia modify chickpea RSA. Nodulation was optimised in RBs to obtain a high number of nodules of big dimensions with high N-fixation capacity. Experiments in RBs, and other systems, evidenced that rhizobia that significantly improve plant N nutrition increase every organ biomass and result in a higher proportion of shoot allocated biomass since the plant has less need to forage for N. Superior strains provided more N and used the C invested in the symbiosis more effectively. Changes in roots traits due to rhizobia depend on demand for more-limiting soil resources once adequate N levels have been provided by the symbiont: When water and non-N nutrients are abundant, shallow and dense root systems will be observed; while deeper and extensive roots will develop when they are scarcer. Rhizobia also improve P nutrition, which might impact plant development and root distribution. Thus, rhizobia modulate RSA through improved N nutrition, and other benefits to the host, which decreases the plant´s need to forage for N