White clover genetics drive root soil Rhizobium interactions

Abstract

The legume white clover (Trifolium repens L.) is an important source of biologically fixed nitrogen (N) for temperate agronomy in mixed swards. To date, there has been little focus on identifying and exploiting variation in plant genetic factors underpinning effective symbioses with rhizobia. Combining plant and Rhizobium genetics for improved N fixation, and concomitant reductions in fertiliser use, provides new avenues towards sustainable agriculture. However, in order to breed new white clover cultivars with improved interactions with rhizobia, the genetic factors of phenotypic variation in symbiotic performance need to be examined. In this study, we quantified the variation in symbiotic performance across a large number (n=360) of individuals from 17 white clover cultivars released between 1920 and 2003, in addition to a wild ecotype and a T. repens x T. uniflorum hybrid. Symbiotic responses were assessed using an effective and partially effective Rhizobium strain. There were significant interactions between white clover populations and the two Rhizobium strains. Productivity of the white clover ecotype was low with the partially effective Rhizobium strain, and high with the effective strain. In contrast, some cultivars performed similarly well with both strains. Shoot dry matter production was positively correlated with the number of pink (effective) nodules on the roots of individuals inoculated with the partially effective strain. In contrast, shoot and root symbiotic potentials were positively correlated with the number of pink nodules when inoculated with the effective strain. Canonical discriminant analysis (CDA) separated the population response to N availability. This was reflected by an increased root:shoot ratio (RSR) under the low-nitrogen and partially effective Rhizobium strain treatments. Dry matter production and symbiotic potential increased with effective Rhizobium and N supplemented treatments. Further, the RSR of individuals inoculated with the partially effective Rhizobium strain was positively correlated with the decade of cultivar release: when inoculated with a partially effective N-fixing strain of Rhizobium, older cultivars produced more root biomass relative to shoot biomass than newer cultivars. This study showed that the N-fixing abilities of white clover cultivars play a key role in determining symbiotic performance with Rhizobium strains. The results highlight the potential for manipulating clover genetics to improve nodulation and symbiotic performance of white clover cultivars in the future.