FRACTIONATION AND CHARACTERIZATION OF CELLULAR MEMBRANES FROM ROOT-TIPS OF GARDEN CRESS (LEPIDIUM-SATIVUM L)

The first step in the gravitropic reaction chain, i.e., perception, occurs in the statenchyma of the root cap. Because of the importance of the root tip in graviperception, a procedure has been developed to isolate root tips from garden cress (L. sativum L.). The root tip fraction contains the tissues of the root cap plus the lower half of the meristem zone, but is clearly separated from the tissues of the elongation zone, the zone of gravitropic response. Membranes from the root tip and root base fractions were centrifuged on sucrose density gradients and the marker enzyme profiles analyzed. These results show that the marker enzyme profiles for vacuoles, dictyosomes, mitochondria, and plasma membranes are similar in the root tip or root base fractions. The endoplasmic reticulum (ER) has a shoulder of cytochromme c reductase activity at a density of 1.16 g cm-3 which is distinct from the other enzyme activities and is only observed in root tip preparations. The specific enzyme activity for ER, cytochrome c reductase, was enriched in root tip membranes 1.7 fold. This latter increase is interpreted as at least in part an increased RE content in the root tip.