EVIDENCE FOR GRAFT TRANSMISSION OF STRUCTURAL PHLOEM PROTEINS OR THEIR PRECURSORS IN HETEROGRAFTS OF CUCURBITACEAE

Interspecific and intergeneric grafts of Cucurbitaceae were used to st udy the mobility of structural P-proteins in the phloem. When Cucumis sativus L. scions were grafted onto Cucurbita rootstocks, at least nin e additional proteins appeared on sodium dodecyl sulfate-polyacrylamid e electrophoresis gels of scion exudate, 9-11 d after grafting. These proteins corresponded exactly to those of the, respective Cucurbita sp . rootstock, including the filament-forming phloem protein PPI and the phloem lectin PP2, as shown by the apparent molecular weights and pep tide maps. According to probing at three sites, the additional protein s were evenly distributed within the scion. The appearance of addition al proteins was correlated with the establishment of phloem bridges ac ross the graft union. The developmental coincidence establishes that t he structural proteins or their precursors are translocated in the phl oem. This translocation was a universal phenomenon in Cucurbitaceae as shown by a comparative screening: for additional proteins in eleven g raft combinations, using Benincasa hispida (Thunb.) Cogn., Citrullus c olocynthis (L.) Schrad., Cucumis melo L, C. sativus, Cucurbita ficifol ia Bouche, Cucurbita maxima Duchesne ex Lam., and Trichosanthes cucume rina var. lobata Roxb. According to this screening, the direction of t ransmission of additional proteins depended upon the combination teste d. While some graft partners failed to show exchange, some behaved as ''donor'' for additional proteins and still others could be both ''don or'' or ''acceptor''. However, whether used as scion or stock, C. sati vus was consistently identified as an acceptor. The occurrence of addi tional proteins in heterografts is discussed with regard to the transp ort mechanism of structural P-proteins in the phloem and its relations hip to assimilate transport.