Figure 6.

Phloem loading occurs via the ^abBS in maize. A, Arrangement and relative numbers of major veins, rank-1 intermediate veins, and rank-2 intermediate veins in a mature maize leaf. Note that rank-1 intermediate veins are distinguished from rank-2 by the presence of hypodermal sclerenchyma. B, A rank-2 intermediate vein surrounded by bundle sheath (blue outline) and mesophyll (gray) cells. Sucrose movement down its concentration gradient is indicated by blue arrows. C, The inset shows details of sucrose movement either from bundle sheath cells into the apoplasm via SWEET13 transporters or to PP (teal) via plasmodesmata, where sucrose is then exported to the apoplasmic space by SWEETs. Sucrose in the apoplasm is taken up by SUT1 into the sieve element (orange, pink) complex of the companion cells for long distance transport.