The Physiology of Flowering Plants - KHAM PHA MOI

THE RATE AND DIRECTION OF TRANSLOCATION 139along with fluorescently labelled dextrans, i.e. high molecular masscarbohydrates (Balachandaran et al. 1997). These dextrans showed nocell-to-cell movement if injected by themselves. But in the presenceof the phloem sap proteins, the spread of fluorescence indicated thatdextrans with masses of at least 20 000 Da moved through up to 20cells from the site of injection within 1–2 minutes. The accompanyingphloem proteins, with masses of up to 100 000 Da, must havemoved, too. The exclusion limit for numerous ‘ordinary’ plant tissuesis 800–1000 Da. Phloem proteins from Cucurbita and castor bean(Ricinus communis) also increased the exclusion limits of plasmodesmatain the leaf mesophyll of other species, e.g. Nicotiana tabacum. Thepresence of such proteins in the SE–CC complex presumably maintainsthe high exclusion limits of the plasmodesmata. Additionally,some proteins in the SE–CC complex may act as chaperonins andunfold large proteins for passage.Some of the proteins in phloem sap have been identified asenzymes, or as components of the P-protein filaments, and togetherwith others as yet unidentified are presumably necessary for themaintenance of the life functions of the sieve tube cells. The questionthen arises how such macromolecules avoid being swept with thetranslocation stream into the sinks: maybe by adsorption on theP-protein fibrils, or to the peripheral ER (endoplasmic reticulum)?But proteins do move in the phloem, even through graft unions.Evidence is mounting that some sieve tube sap proteins and RNAare information molecules destined for transport to the sinks,where they are unloaded (Oparka & Santa Cruz 2000). Theseinformation molecules include, for instance, signals (believed tobe small RNA molecules) which can suppress the activity ofspecific genes in the sinks. The phloem is emerging as an importantcarrier of macromolecular information as well as hormonalsignals.5.3 The rate and direction of translocation5.3.1 The rate of translocation: velocity and mass transferOne measure of the rate of translocation is velocity, the distancemoved by the translocated material per unit time, expressed in, say,cm h –1 . This seems a simple value, but it is difficult to measure. Mostestimates to date have been made by introducing a marker into thephloem at a specified point and noting the time of its arrival atanother specified point. Fluorescent dyes have been timed, butthere is a time delay of unknown length between the external applicationof the dye and its entry into the sieve tubes, and the dye, as aforeign substance, might conceivably adversely affect the phloemand change the velocity of translocation. The timing of the movementof radioactivity from 14 C-labelled photosynthate avoids theintroduction of foreign material, but is not straightforward; amongother problems, 14 C is a weak b-emitter and to detect its presence,