Involvement of a maize proline-rich protein in secondary cell wall formation as deduced from its specific mRNA localization

A clone encoding a proline-rich protein (ZmPRP) has been obtained from maiz e root by differential screening of a maturing elongation root cDNA library . The amino acid sequence deduced from the full length cDNA contains a puta tive signal peptide and a highly repetitive sequence containing the PEPK mo tif, indicating that the ZmPRP mRNA may code for a cell wall protein. The P EPK repeat is also found in a previously reported wheat sequence but differ s from the repeated sequences found in hydroxyproline rich glycoproteins (H RGP) and in dicot proline-rich proteins (PRP). In the maize genome, the ZmP RP protein is encoded by a single gene that is expressed in maturing region s of the root, in the hypocotyl and in the pericarp. In these organs, the Z mPRP mRNA accumulates in the xylem and surrounding cells, and in the epider mis. No ZmPRP mRNA was found in the phloem. The pattern of mRNA accumulatio n is very similar to the one observed for genes coding for proteins involve d in lignin biosynthesis and, like most cell wall proteins, ZmPRP synthesis is also induced by wounding. These data support the hypothesis that ZmPRP is a member of a new class of fibrous proteins involved in the secondary ce ll wall formation in monocot species.