Minor vein structure and sugar transport in Arabidopsis thaliana

Leaf and minor vein structure were studied in Arabidopsis thaliana (L.) Hey nh. to gain insight into the mechanism(s) of phloem loading. Vein density ( length of veins per unit leaf area) is extremely low. Almost all veins are intimately associated with the mesophyll and are probably involved in loadi ng. In transverse sections of veins there are, on average, two companion ce lls for each sieve element. Phloem parenchyma cells appear to be specialize d for delivery of photoassimilate from the bundle sheath to sieve element-c ompanion cell complexes: they make numerous contacts with the bundle sheath and with companion cells and they have transfer cell wall ingrowths where they are in contact with sieve elements. Plasmodesmatal frequencies are hig h at interfaces involving phloem parenchyma cells. The plasmodesmata betwee n phloem parenchyma cells and companion cells are structurally distinct in that there are several branches on the phloem parenchyma cell side of the w all and only one branch on the companion cell side. Most of the translocate d sugar in A. thaliana is sucrose, but raffinose is also transported. Based on structural evidence, the most likely route of sucrose transport is from bundle sheath to phloem parenchyma cells through plasmodesmata, followed b y efflux into the apoplasm across wall ingrowths and carrier-mediated uptak e into the sieve element-companion cell complex.