Patterned cell development in the secondary phloem of dicotyledonous trees: a review and a hypothesis

The secondary phloem of dicotyledonous trees and shrubs is constructed of sieve tube cells (S) and their companion cells, as well as parenchyma (P) and fibre (F) cells. Different species have characteristic sequences of these S, P and F cells within the radial files of their phloem. The sequences are recurrent, and are evidence of rhythmic cell determination and differentiation. A model was devised to account for the sequences found in various dicot tree species. It is based on the pattern of radial displacement of cells through a gradient of morphogen which supports secondary phloem development. According to this model, each tree species shows a particular pattern of post-mitotic cellular displacement along each radial file as a result of a corresponding sequence of periclinal division in the cambial initial and its descendents. The divisions and displacements ensure that at each timestep (equivalent to an interdivisional interval) each cell resides in a specific location within the morphogenic gradient. Cells then emerge from the post-mitotic zone of cell determination, having acquired different final positional values. These values lie above a series of thresholds that permit the respective determination and subsequent differentiation of one or other of the three cell types S, P and F. The recurrent nature of the sequences of the three cell types within each radial cell file, as well as their tangential banding, are a consequence of a shared rhythmic spatio-temporal pattern of periclinal cambial divisions. With a single set of morphogen parameters required for cell determination, and using three positions for cambial cell divisions, all the cellular sequences of secondary phloem illustrated in the literature can be accounted for.