Studies: on the nucleic acids of developing, dormant and germinating seeds of vicia faba (L)

Abstract

Seeds of Vicia faba (L)take about 130 days from the fertilisation of the ovule to become fully mature. After an initial phase of cell division, storage protein is synthesised in the cotyledon cells and accumulates in organelles termed protein bodies. During the latter phase, the RNA, contents of cotyledon cells increase sixfold. Methylated albumin-kieselguhr chromatography revealed that the newly synthesised RNA, consists of 13% low molecular weight RNA and 87% ribosomal RNA, suggesting that storage protein synthesis may be mediated by the mRNA/tRNA/ ribosome mechanism. Isolated protein bodies contain only traces of RNA, indicating that storage protein is not synthesised in these organelles. Biochemical methods show that, during the phase of cell division, there are many free ribosomes and relatively few membrane bound ribosomes. Correlated with the onset of storage protein synthesis, there is a large increase in the numbers of membrane bound ribosomes, but the numbers of free3ribosomes remain approximately constant. Experiments with ( H) - uridine show that there is no interconversion of these two classes of ribosomes, and both were shown to incorporate amino-acids "in vivo". It is suggested, therefore, that each class of ribosomes may be synthesising different groups of proteins, the membrane bound ribosomes being responsible for the synthesis of storage protein.At the cessation of nutrient reserve accumulation, seeds dehydrate and there is a gradual loss of membrane boundribosomes with a subsequent rise in the numbers of free ribosomes.Upon germination, endoplasmic reticulum with associated32ribosomes is formed once more. Experiments with ( P) - ortho-phosphate indicate that these ribosomes are synthesised "de novo", rather than being formed by the attachment of pre-existing, free ribosomes to membranes. The breakdown of various types of nucleic acids was studied during the senescence of testas in seed development, and cotyledons in seed germination.