The evolution of minor vein phloem and phloem loading

Phylogenetic analysis provides a rational basis for comparative studies of phloem structure and phloem loading. Although several types of minor vein companion cell have been identified, and progress has been made in correlating structural features of these cells with loading mechanisms, little is known about the phylogenetic relationships of the different types. To add to the available data on companion cells, we analyzed the ultrastructure of minor veins in Euonymus fortunei and Celastrus orbiculatis (Celastraceae) leaves and determined that in these species they are specialized as intermediary cells. This cell type has been implicated in symplastic phloem loading. The data were added to published data sets on minor vein phloem characteristics, which were then mapped to a well-supported molecular tree. The analysis indicates that extensive plasmodesmatal continuity between minor vein phloem and surrounding cells is ancestral in the angiosperms. Reduction in plasmodesmatal frequency at this interface is a general evolutionary trend, punctuated by instances of the reverse. This is especially true in the case of intermediary cells that have many plasmodesmata, but other distinguishing characteristics as well, and have arisen independently at least four, and probably six, times in derived lineages. The character of highly reduced plasmodesmatal frequency in minor vein phloem, common in crop plants, has several points of origin in the tree. Thus, caution should be exercised in generalizing results on apoplastic phloem loading obtained from model species. Transfer cells have many independent points of origin, not always from lineages with reduced plasmodesmatal frequency.