A NEW PROLINE-RICH EARLY NODULIN FROM MEDICAGO-TRUNCATULA IS HIGHLY EXPRESSED IN NODULE MERISTEMATIC CELLS

We cloned and characterized MtPRP4, a new member of the repetitive pro line-rich protein gene family in Medicago trun catula. The sequence of MtPRP4 predicts a 62-kD protein consisting of a 22-amino acid N-termi nal signal peptide and a 527-amino acid repetitive proline-rich domain composed of three repetitive pentapeptide motifs arranged into two de capeptide repeats: PPVEKPPVHK and PPVEKPPVYK. MtPRP4 is the largest PR P described to date and contains repeated motifs that have not previou sly been found together in a single polypeptide. RNA gel blot experime nts detected MtPRP4 transcripts in symbiotic root nodules, but not in roots, hypocotyls, or leaves. Accumulation of MtPRP4 transcript was an early response to Rhizobium inoculation and did not depend on nodule infection. In situ hybridization experiments demonstrated that MtPRP4 was expressed early in the development of the nodule meristem and that expression was highest in the meristematic cells of mature indetermin ate nodules. These data support the proposition that an important earl y response of legume host roots to Rhizobium involves remodeling the h ost extracellular matrix and that proline-rich wall proteins play an i mportant role in this architectural modification.