Katabolism of plant cytoplasmic ribosomes: RNA breakdown in senescent cotyledons of germinating broad-bean seedlings.

Broad-bean cotyledons lost about 80% of their RNA during the first 30 days of seed germination. Both high-molecular-weight ribosomal (r) RNA and low-molecular-weight RNA were degraded at about the same rate. Only small quantities of breakdown products of rRNA were detected within senescent tissue. This finding contrasts strongly with the very rapid cleavage of rRNA and the accumulation of breakdown products when the same cotyledons were homogenised and then incubated at 25°. It would appear, therefore, that there was a barrier between ribonuclease and most ribosomes in vivo which was disrupted by homogenisation. The results are discussed in relation to the mechanism of ribosome katabolism during senescence.