Sieve tube members have no cell nucleus, ribosomes, or leaves, companion cells help move the sugar that is produced by photosynthesis from the mesophyll tissue into the sieve tube elements.
12.
Mutant plants carrying insertions in AtSWEET11 and-12 are defective in phloem loading, thus revealing a two-step mechanism of SWEET-mediated export from parenchyma cells feeding H +-coupled import into the sieve element-companion cell complex.
