Carbohydrate Processing in the Mesophyll Trajectory in Symplasmic and Apoplasmic Phloem Loading

Abstract

According to a recent proposal phloem loading is the transfer of photosynthates from the site of production via the mesophyll symplast to the sieve tubes. The definition implies that phloem loading comprised a range of sequential processes of which carbohydrate metabolism in the mesophyll and sieve element loading ahve received ample attention. To date, reviews on phloem loading have dealt chiefly with the transfer of photosynthates from the mesophyll domain to the sieve tubes. Crossing the border between mesophyll domain and seive element/ companion cell complex (SE/CC complex) in the minor vein has generally been regarded as crucial for phloem loading. This vision largely ignores the transport events prior to the entrance into the SE/CC-complexes, and neglects the fact that, in many species, photoassimilates are not loaded from the apoplast by the SE/CC-complexes. At the other end of the mesophyll trajectory, metabolism of photosynthesis products in the mesophyll has been highlighted over the past decade. The interplay between chloroplastic, vacuolar, and cytosolic compartmetation of photosynthate ahs been studied to a lesser extent. A drawback to the studies mentioned is that mesophyll cells are treated as uniform and isolated units. Virtually no attention has been devoted to differences between the cell types and to intercellular compartmentation and C-processing. The intra- and inter-cellular carbohydrate processing along the mesophyll trajectory is discussed here in an attempt to inventarize the known and unknown between photosynthate exit from the chloroplast and arrival in the SE/CC-complexes.