Phloem proteins of the Cucurbitaceae : ultrastructure and properties
The ultrastructure of sieve elements and phloem exudates from a number of species of the Cucurbitaceae, but especially from stems of C. maxima, were examined by a variety of electron microscopic methods. The methods included conventional chemical fixation and ultrathin sectioning, negative staining and freeze-etching. The rates of release of exudate and P-proteins from cut stems of C. maxima and C. melo, and the ultrastructure of exuding sieve tubes were also investigated. Results of these investigations suggest that: The range and types of P-protein filaments in mature sieve elements are ultrastructurally similar for all species of the Cucurbitaceae examined; they also resemble P-protein filaments found in plants of other families. The different forms of P-protein are built from filaments no more than 4 rim diameter; 2 of these fine filaments intertwine to form narrow tubular types 6 to 10 am diameter, and 3 of these narrow tubular types intertwine to form each wide super-helical tubular filament. The super-helices are filaments 14 to 24 run diameter. Fine elementary filaments may be composed mainly of spherical protein subunits 3 nm diameter, with other non-spherical protein subunits. P-protein filaments exist in helical and super-helical forms within intact sieve tubes. Changes in the internal cellular environment of Cucurbit sieve tubes cause the helical and super-helical tubules to break down to form masses of fine elementary filaments. When cut, Cucurbit sieve tubes release P-protein filaments from anchored positions into the stream of exudate. P-protein filaments are converted into a gel at the cut surface of Cucurbit stems by some potent factor which causes the proteins to crosslink. Flow of exudate is stopped by the formation of a layer of gelled protein at the cut end. P-proteins from several species of the Cucurbitaceae also form gels when simply exposed to oxygen. The possibility is discussed that the main function of P-protein in all plants is to seal, at the point of damage, cut or punctured sieve tubes.