Ultrastructural Study on Companion Cells in Leaf Minor Veins of Morus alba

The transfer of photosynthates from mesophyll cells to sieve element-companion cell complex is called phloem loading. Loading mechanisms are closely related to the ultrastructural features of companion cells. To understand the ultrastructural features of companion cells in leaf blades of Morus alba, companion cells in minor veins were characterized by transmission electron microscopy. We demonstrated that (a) companion cells had smooth walls and contained a whole set of organelles such as nucleus, vacuoles, mitochondria, plastids, and ribosomes. (b) Many plasmodesmata occurred at the interfaces between companion cells and adjacent cells. Plasmodesmata usually existed in groups and were often branched. (c) Plasmodesmatal frequencies were 25.12 ± 1.83 pd/μm² at the interface between companion cells and bundle sheath cells and 20.18 ± 1.72 pd /μm² at the interface between neighboring companion cells, in contrast to 5.42 ± 0.6 pd/μm² at the interface between companion cells and vascular parenchyma cells. Based on these ultrastructural observations, it was supposed that companion cells in minor veins in leaf blades of M. alba be classified as type 1-2a, and therefore the phloem loading pathway was symplasmic.