The Glycine max Xylem Sap and Apoplast Proteome

Molecular signaling interactions in the plant apoplast are important for defense and developmental responses. We examined the soybean proteome of the apoplastic conduit of root-to-shoot communication, the xylem stream, using gel electrophoresis combined with two types of tandem mass spectrometry. We examined soybeans for the presence of a Bradyrhizobium japonicum-induced, long distance developmental signal that controls autoregulation of nodulation (AON) to determine if xylem proteins (XPs) were involved directly or indirectly in AON. The xylem and apoplast fluids collected in hypocotyl, epicotyl, and stem tissue contained a highly similar set of secreted proteins. The XPs were different from those secreted from imbibing seed implying they play important basic roles in xylem function. The XPs of wild-type and nts1007 plants were indistinguishable irrespective of plant age, inoculation status, or time after inoculation suggesting that none was directly involved in AON. XPs were continuously loaded into the xylem stream, as they were present even 28 h after shoot decapitation. These results were consistent with semiquantitative RT-PCR studies that examined the expression of genes corresponding to the XPs under inoculated or uninoculated conditions. Monitoring the expression of XP genes by RT-PCR showed that four possessed root biased expression. This suggested that the corresponding protein products could be produced in roots and travel long distances to shoots. Of these, a species of lipid transfer protein is a candidate for a water-soluble, long-distance signal-carrier due to the presence of hydrophobic clefts that bind known plant signals in vitro. Two soybean XPs identified in this study, lipid transfer protein and Kunitz trypsin inhibitor (KTI), have known roles in plant signaling.