Protoplasmic Changes in Cambial Cells Induced by a Tracheid-Differentiation Factor from Pine Needles

Direct differentiation of ‘dormant’ fusiform cambial cells (fees) into tracheids in stem cuttings of Pinus contorta Dougl. was investigated by light and transmission electron microscopy. In cuttings from which all but one pair of needles had been removed, differentiation occurred close to the needle trace without prior cell division or expansion. The highly osmiophillic, granular cytoplasm of differentiating fees exhibited numerous mitochondria, dictyosomes and endoplasmic reticulum (ER) cisternae and was enriched with vesicles and polyribosomes. The protoplasm did not become highly vacuolated, as it normally does, until differentiating tracheids were approaching maturation. During differentiation, secondary walls were deposited in fees as ribs of annular and spiralled thickenings and also as borderedpit-like structures devoid of margo and torus and lacking the typical lamellar structure of secondary-xylem tracheids. The fee plasma membrane at sites of secondary-wall growth was in dynamic flux, dilated vesicles derived from both ER and dictyosomes apparently fusing with it at these locations. In addition, electron-dense blebs were invariably plasma membrane-associated and appeared to be exocytotic; these blebs became rare during autolysis and they were never seen in non-differentiating cells. Deposition of oriented nascent cellulose microfibrils appeared to occur in the absence of associated cortical microtubules. As secondary-wall deposition neared completion, vacuoles derived from rough ER grew and fused; concomitantly, the protoplasm disappeared resulting in fully autolysed tracheids. Auxin (indol-3-ylacetic acid, IAA) promoted the gel-like protoplasm of dormant fees to became highly vacuolated, and cell division and expansion followed; however, tracheid differentiation did not occur. Polyribosomes, rough ER, and dictysomes were less abundant than in differentiating fees and the cytoplasm of auxin-treated fees was fine grained and less densely stained. The electron-dense exocytotic blebs found at the plasmalemma in differentiating fees were not induced by IAA treatment. Fees of control cuttings enlarged somewhat and became more vacuolated but otherwise remained dormant.