Advances in the study of the nature of phloem transport: The activity of conducting elements

This review deals with the forms of carbon employed by plants for long-dist ance transport. The composition of sugars translocated via the phloem depen ds on the external conditions. In plant species that translocate sucrose an d its one-, two-, and three-galactosyl derivatives (raffinose, stachyose, a nd verbascose), stress factors, such as salinity, drought, and low temperat ure, induce the synthesis of oligogalactosides that contain six to eight ga lactosyl residues and move along the phloem. The ability to transport oligo galactosides comes about in leaves as they develop into a source of assimil ates, and the synthesis of specific galactosyltransferases is induced, The synthesis of oligogalactosides with variable: galactosyl composition provid es the plants affected by stress agents with the means to maintain the opti mum osmotic pressure in sieve elements and simultaneously to increase the a mount of transported carbon. Similar data were reported concerning the phlo em transport of oligofructosides (two and more fructosyl residues) and mann itol and sorbitol complexes with boron. Considered are the synthesis and ph loem transport of signaling molecules, myo-inositol and its methylated deri vatives, and systemin, a signal peptide that is also translocated via the p hloem. The functional activity of the sieve element-companion cell complex is under discussion.