On the vacuolar system in maize roots.

Abstract

Root-cap cells of Zea mays L proliferate by division in the cap meristem, and subsequently differentiate and mature as they move towards the periphery of the cap, where ' they undergo autolysis and are sloughed. Vacuolar ontogeny has been shown to be complex, several different mechanisms operating not only within the root cap tissue, but within the single cells. Vacuolar initials (provacuoles) are formed in the meristem by the pinching off of single- or doublemembrane bound vesiculations of the E.R. In some instances large vacuoles appear to be formed in the mature region of the cap through the sequestering of large organelle-free regions of cytoplasm by vesicles and small cisternae, thought to be of E.R. origin. Further development of provacuoles comprises their expansion and extensive fusion, this process culminating in the formation, in a mature cell, of just one large vacuole. The vacuoles of the mature region are autophagically active, engulfing all types of cytoplasmic organelle which are subsequently lysed; these vacuoles show a positive cytochemical reaction for acid-phosphatase, further indicating that they are lysosomal in nature. The dictyosomes of the late mature cells are hypersecretory and autoradiographic and cytochemical evidence indicates that the vesicles contain an accumulation of polysaccharide. These vesicles appear to follow two secretory pathways; firstly fusion with the plasmalemma, with secretion of their content into the extra-protoplasmic space where it accumulates, finally penetrating the cell wall and middle-lamella and forming viscous polysaccharide slime on the exterior of the cap. Secondly, these vesicles appear to be engulfed by and broken down within the vacuoles. At this stage the vacuole expands considerably, and it has been postulated (Berjak and Villiers, pers. corom.) that hydrolysis of the dictyosomally-derived polysaccharide within the vacuole to monosaccharide units results in osmotic. changes leading to an influx of water into the vacuole, and its consequent expansion. Autoradiographic, cytochemical and chromatographic evidence is not inconsistent with an accumulation of monosaccharide units being at least partially responsible for the osmotic uptake of water into the swelling vacuole. Finally, the vacuolar membrane becomes discontinuous, allowing hydrolytic enzymes p~esumably contained within the vacuole to come into contact with the cytoplasm, which consequently undergoes autolysis. At this stage the cell is sloughed from the cap.