NODULATION Touchy DMI2 Plants

The roots of certain legumes can respond to the presence of nitrogen-fixing bacteria (rhizobia), by engulfing the microorganisms and forming a symbiotic relation between the plant and the bacteria. Plants carrying the Does Not Make Infections 2 (DMI2) mutations are incapable of forming a symbiotic relationship with rhizobia. Esseling et al. show that three species of plants with DMI2 phenotypes carrying mutations in the genes encoding the nodulation receptor-like kinase (NORK) are hyperresponsive to mechanical stimuli. Even routinely replacing the medium caused growing root hairs to develop a growth-terminating architecture. Application of nodulation factor to the roots of the mutant plants caused changes to cytoarchitecture and root reorientation, and this morphological response was enhanced compared with that in wild-type plants. When mutant plants were inoculated with their symbiotic partner bacteria, the roots curled around the bacteria, forming a simpler curled structure than that formed by a wild-type root. However, the mutant roots could not internalize the bacteria and did not show increased expression of a reporter coupled to a known nodulation-responsive promoter [Early Nodulin 11 (ENOD11)]. The authors suggest that the nodulation pathway bifurcates and that one arm of the pathway controls cytoarchitecture reorganization and the other arm, which includes NORK, governs the responses needed for bacterial internalization and regulation of nodulation-responsive genes, such as ENOD11.

J. J. Esseling, F. G. P. Lhuissier, A. M. C. Emons, A nonsymbiotic root hair tip growth phenotype in NORK-mutated legumes: Implications for nodulation factor-induced signaling and formation of a multifaceted root hair pocket for bacteria. Plant Cell 16, 933-944 (2004). [Abstract] [Full Text]