The evolution of minor vein phloem and phloem loading

Citation
R. Turgeon et al., The evolution of minor vein phloem and phloem loading, AM J BOTANY, 88(8), 2001, pp. 1331-1339
Citations number
61
Language
INGLESE
art.tipo
Article
Categorie Soggetti
Plant Sciences
Journal title
AMERICAN JOURNAL OF BOTANY
ISSN journal
0002-9122 → ACNP
Volume
88
Issue
8
Year of publication
2001
Pages
1331 - 1339
Database
ISI
SICI code
0002-9122(200108)88:8<1331:TEOMVP>2.0.ZU;2-K
Abstract
Phylogenetic analysis provides a rational basis for comparative studies of phloem structure and phloem loading. Although several types of minor vein c ompanion cell have been identified, and progress has been made in correlati ng structural features of these cells with loading mechanisms, little is kn own about the phylogenetic relationships of the different types. To add to the available data on companion cells, we analyzed the ultrastructure of mi nor veins in Euonymus fortunei and Celastrus orbiculatis (Celastraceae) lea ves and determined that in these species they are specialized as intermedia ry cells. This cell type has been implicated in symplastic phloem loading. The data were added to published data sets on minor vein phloem characteris tics, which were then mapped to a well-supported molecular tree. The analys is indicates that extensive plasmodesmatal continuity between minor vein ph loem and surrounding cells is ancestral in the angiosperms. Reduction in pl asmodesmatal frequency at this interface is a general evolutionary trend, p unctuated by instances of the reverse. This is especially true in the case of intermediary cells that have many plasmodesmata, but other distinguishin g characteristics as well, and have arisen independently at least four, and probably six, times in derived lineages. The character of highly reduced p lasmodesmatal frequency in minor vein phloem, common in crop plants, has se veral points of origin in the tree. Thus, caution should be exercised in ge neralizing results on apoplastic phloem loading obtained from model species . Transfer cells have many independent points of origin, not always from li neages with reduced plasmodesmatal frequency.