Microtubules and root protophloem ontogeny in wheat.

Protophloem ontogeny in roots of Triticum aestivum has been investigated ultrastructurally. Each protophloem pole consists of three cells, a protophloem sieve element and two companion cells, all originating from a single precursor cell usually having a pentahedral shape. This protophloem mother cell (PMC) undergoes two successive asymmetrical divisions: the first one gives rise to a smaller cell that will differentiate into a companion cell, and a larger one that divides again asymmetrically yielding another companion cell and a protophloem sieve element. The latter divides once more, but now symmetrically, increasing the number of cells. Both asymmetrical and symmetrical divisions are preceded by preprophase microtubule bands (PMBs), well demarcated by a great number (more than 100 profiles in a single band section) of microtubules (MTs). The plane of a PMB coincides with that of the succeeding cell plate, which fuses with parent walls at sites previously occupied by the PMB. The strict correspondence between PMB and cell plate suggests that a cytokinesis the latter bisects the PMB cortical zone. The possible role of PMB cortical zone in positioning the cell plate and guiding its expanding edges towards predetermined sites is discussed in relation to recent discoveries in other anatomical situations. The plane of PMBs (and hence of divisions) changes from one division to the next, so that the three successive divisions occur in three spatial planes transversely to each other. This change is probably influenced by cell polarity. Prior to each asymmetrical division peri-nuclear MTs were observed besides the MTs of the PMB. They appear before the PMB organization and persist throughout preprophase, but they change their position and orientation in response to the transition from PMB to the spindle organization.