Genomic amplification in the cotyledon parenchyma of common bean

The cotyledon parenchyma cells of common bean (Phaseolus vulgaris L.) produce large quantities of storage proteins during embryo maturation; throughout this period, these cells also accumulate nuclear DNA (nDNA). To investigate the basis of this nDNA accumulation, we have measured storage protein mRNA pools, nDNA mass, and gene copy number at specific stages of cotyledon development. RNA blotting and hybridization show that transcripts encoding the major embryo-specific storage proteins are present very early in cotyledon development, accumulate in coordinate fashion to peak during mid-maturation, and fall in abundance prior to the onset of dormancy. During this same period, nDNA mass per parenchyma cell nucleus, as measured by Feulgen microspectrophotometry, increases from 2C-4C to about 64C (C being the haploid germ cell genomic complement). The nDNA values do not cluster around integral multiples of the diploid 2C amount. DNA blotting and hybridization are used to evaluate the relative representations of different classes of the bean genome in DNA samples isolated from vegetative tissues, from cotyledons beginning to accumulate storage proteins, and from cotyledons of late maturation embryos entering dormancy. The results demonstrate that the observed DNA accumulation in the cotyledon parenchyma is due to overlapping rounds of replication of the complete genome and not to disproportionate amplification of specific sequences nor to random DNA synthesis.