The evidence of light, electronic, and confocal microscopy collected within the 30-year period is reviewed to revise the concept of assimilate loading in phloem. It is the starting point located in mesophyll cells, which determines the route of assimilate export from mesophyll to phloem, rather than its final segment located in the terminal phloem. Plastids, photosynthesis, and the primary pool of photosynthates are localized in the vacuome of mesophyll cells. All chemicals applied to leaf surface are loaded to phloem via apoplast, even in the symplastic plants. It follows that photoassimilates are not loaded via apoplast because they cannot leave mesophyll and not due to the lack of pumps and transporters in the terminal phloem cells. Of two membranes separating vacuome and apoplast, the tonoplast confers the barrier function. The impossibility to overcome this barrier raises the hydrostatic pressure in the vacuome to the level that induces plasmodesma development between the cells. With the loss of tonoplast barrier function for assimilates, the latter leave for apoplast, this process is incompatible with building the vacuolar loading route. Two alternative mechanisms of phloem loading diverge initially because of different barrier functions of tonoplast. The radical change in these functions makes up the crucial advantage of the young group of apoplastic dicot plants (about 20 000 species), whose evolution is associated with expansion of meadow-steppe vegetation 5–7 million years ago. Such change would evolve due to the climate differentiation in the late myocene period, when heat and moisture were lacking at vast territories. A large group of temperate herbs evolved and expanded because of these changes in the assimilate compartmentalization.
Russian Journal of Plant Physiology – Springer Journals
Published: Jan 6, 2007
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