Phloem loading and plasmodesmata

Phloem loading, the active accumulation of photosynthate in minor veins, is thought to be the motivating force for translocation. Can loading take place along an entirely symplastic route, through the plasmodesmata, from mesophyll to phloem? Transport of small molecules through the plasmodesmata is apparently passive, and the concept of symplastic phloem loading thus appears to violate thermodynamic principles. Nonetheless, evidence for such a pathway in many plants has been accumulating steadily and an answer to the thermodynamic argument has been put forward. The proposed mechanism involves synthesis and trapping of raffinose and stachyose in the phloem.