Turnover of ribosomal RNA is essential to maintain cellular homeostasis. Arabidopsis thaliana plants missing the ribonuclease RNS2 are deficient in this process and have constitutive autophagy. Using transcriptome and metabolite analyses, Morriss et al. found that defects in rRNA recycling cause changes in carbon flux through the pentose phosphate pathway in order to produce metabolites needed to synthesize nucleosides.
Metabolic changes also increase reactive oxygen species that signal the activation of autophagy, likely to compensate for the lack of rRNA degradation. Lack of rRNA recycling in rns2-2 cells triggers a change in carbon flux, which is redirected through the PPP to produce ribose-5-phosphate for de novo nucleoside synthesis. rRNA or ribosome turnover is thus essential for cellular homeostasis, probably through maintenance of nucleoside levels as part of the salvage pathway.