Structure-function relationships during secondary phloem development in anangiosperm tree, Aesculus hippocastanum: microtubules and cell walls

We studied the dynamics of the cortical microtubule (CMT) cytoskeleton duri ng differentiation of axial secondary phloem elements in taproots and epico tyls of Aesculus hippocastanum L. (horse-chestnut) saplings. Indirect immun ofluorescence microscopy of cc-tubulin and transmission electron microscopy revealed that fusiform cambial cells possessed a reticulum of CMTs in whic h individual microtubules were randomly arranged. During differentiation of these cambial cell derivatives into secondary phloem cells, the CMTs were rearranged to become helically oriented, regardless of phloem cell type. Al though helical CMTs were a persistent feature of all axial elements of the secondary phloem (sieve elements, companion cells, phloem parenchyma, and f iber-sclereids), some modifications of this arrangement occurred as cells d ifferentiated. Thus, at late stages of cell differentiation, sieve elements possessed nearly transverse CMTs, pronounced bundling of CMTs was seen in phloem parenchyma, and the density of CMTs in the helical arrays of fibers increased markedly. Additionally, phloem parenchyma possessed rings of CMTs in association with developing pit areas. Aspects of the development and c hemistry of cell walls were also examined during phloem cytodifferentiation .