Nodulation and growth phenotypes of M. truncatula plants after inoculation with S. meliloti strain Rm41. A, F83005.5 and A20 lines after inoculation by S. meliloti strain Rm41. A20 (right) grew normally under nitrogen-free conditions, whereas F83005.5 (left) can hardly survive under the same conditions. B, Nodulation phenotypes of the same plants in panel A. A20 roots formed nitrogen-fixing root nodules (right), while F83005.5 cannot nodulate with Rm41. C and D, a closer look at the nodulation phenotypes of A20 and F83005.5, showing fully developed functional nodules on the roots of A20 (D) and the nodule primordial (arrows) formed on the roots of F83005.5 (C).