SPECIFIC PROTEINS IN THE SIEVE-TUBE EXUDATE OF RICINUS-COMMUNIS L SEEDLINGS - SEPARATION, CHARACTERIZATION AND IN-VIVO LABELING

Ricinus communis L. seedlings exuded pure phloem sap from the cut hypo cotyl for several hours. Throughout the entire exudation period protei ns were present in the phloem exudate at a constant concentration rang ing from 0.11 to 0.41 mg.ml-1 depending on the culture conditions and the age of the seedlings. Manipulation of the nutrient supply at the c otyledons after removal of the endosperm did not change the protein co ncentration in the exudate. Comparison of sieve-tube exudate proteins (STEPs) with soluble proteins extracted from the hypocotyl and the cot yledons showed a unique abundance of small proteins in the exudate, wi th molecular weights ranging from 10 to 25 kDa. Bands at 18, 19 and 20 kDa were especially dominant. The proteins found transiently in the x ylem exudate, which might represent proteins secreted at the wound sur face, were different in pattern. Two-dimensional separation of STEPs r evealed that more than 100 distinct polypeptides occurred in the sieve -tube exudate, most of them slightly acidic with isoelectric points ra nging from 4 to 6 and a few basic ones around 8. [S-35]Methionine fed to the cotyledons led to labelling of STEPs, demonstrating their rapid synthesis. It is concluded that there is a continuous synthesis and t ranslocation of specific sieve-tube proteins, whose function is unknow n.