Changes in microsomal components accompanying cell differentiation of pea-seedling roots

Abstract

Microsomes were extracted from small serial segments of seedling pea-root tips by homogenization and centrifuging. A "light" microsome pellet consisting of a mixture of vesicles and ribosomes could be slid off the microsomal pellet, leaving a small firm pellet of "heavy" microsomes consisting largely of ribosomes. The protein and ribo-nucleic acid contents of the 2 fractions were determined and expressed as the yield per cell. The meristematic tip segment contained largely """"heavy" microsomes, and the yield of these fell with the age and maturation of the tissue. The yield of "light" microsomes, however, increased rapidly at the earliest stages of differentiation. The ribonucleic acid to protein ratio in the "heavy" microsomes was above 1[center dot]0 and increased with the age of the tissue, whereas the ratio in the "light" microsomes was below 0[center dot]8 and decreased with age. Electrophoresis of the 2 fractions after disruption with ribonuclease gave more nucleic acid fragments and proteins in the "light" microsomes than in the "heavy". Electron micrographs of the 2 fractions are presented. It is suggested that the synthesis of "light" microsomes is associated with the onset of differentiation.