Targeting and release of phytohemagglutinin from the roots of bean seedlings.

Phytohemagglutinin (PHA), an abundant vacuolar seed protein of the common bean (Phaseolus vulgaris), is a tetramer of two homologous polypeptides, PHA-E and PHA-L. The roots of bean seedlings release into the culture medium a cross-reacting lectin that is most closely related to PHA-E. Reverse-transcriptase polymerase chain reaction with root mRNA as template was used to identify PHA transcripts in the roots of bean seedlings. Roots were found to contain mRNA for PHA-E but not for PHA-L. Indirect immunocytochemical detection with colloidal gold and antibodies to deglycosylated PHA showed that in the meristem of the primary root, PHA accumulates in vacuoles. However, in elongated root cells PHA was found only in the cell walls, indicating targeting to an alternate location. These results are discussed in relation to the various mechanisms that may account for the release of a normally vacuolar protein by roots.