LOCALIZATION OF REPETITIVE PROLINE-RICH PROTEINS IN THE EXTRACELLULAR-MATRIX OF PEA ROOT-NODULES

Early responses of legume roots to Rhizobium inoculation include new c ell wall synthesis and induction of some putative wall protein genes. Although the predicted amino acid sequences of several early nodulins indicate that they encode proline-rich proteins (PRPs), the proteins h ave been neither isolated nor has their presence been demonstrated in cell walls. We have used polyclonal antibodies against PRP2 from soybe an to identify and localize proline-rich proteins in pea nodules. On i mmunoblots, several PRPs were detected, ranging from less than 20 kDa to 110 kDa. Immunocytochemistry revealed that tissues of the Vascular cylinder contained abundant PRPs, particularly in the secondary cell w alls of xylem elements and phloem fibers. PRPs were also found within the primary wall of the nodule endodermis and within Casparian strips of the vascular endodermis. Of symbiotic importance, PRPs were a promi nent component of the infection thread matrix in newly infected root c ells and in nodules. PRPs were also secreted by cells in the uninfecte d nodule parenchyma, where they were found occluding intercellular spa ces outside the middle lamella. Despite structural conservation among members of this class of cell wall proteins, PRPs were targeted to dis tinct layers of the extracellular matrix dependent upon cell type, and may thus play separate roles in the biology of plant cells. The putat ive functions and the potential for interactions between PRPs and othe r wall polymers are discussed.