Monday July 29 // Mechanisms of ribosomal RNA turnover and their role in cellular homeostasis in Arabidopsis

Gustavo MacIntosh // Roy J. Carver Department of Biochemistry, Biophysics and Molecular Biology; Iowa State University, Ames, IA, USA
​
Ribosomes are essential cellular components, and a large proportion of cellular resources are dedicated to their synthesis. Yet, the pathways involved in the turnover of normal ribosomes remain poorly studied. We have shown that the Arabidopsis ribonuclease RNS2 functions in the vacuolar degradation of rRNA. Mutants lacking this RNase activity have rRNA with a longer half-life, which accumulates in the vacuole. rns2 mutants also have constitutive activation of the autophagy pathway, possibly as an attempt to compensate for the loss of rRNA degradation. A functional autophagy pathway is also necessary to maintain normal RNA levels in Arabidopsis, suggesting that plants use an autophagy-dependent mechanism to transport ribosomes to the vacuole for recycling. However, differential rRNA accumulation in vacuoles of specific atg mutants suggest that rRNA or ribosome transport to the organelle may normally occur through a selective mechanism that utilizes some, but not all, the autophagy core components. In addition to dissecting the rRNA decay pathway and the mechanisms of rRNA transport to the vacuole, we are interested in understanding why rRNA is recycled. Metabolome and transcriptome analyses indicated that carbon flux through the pentose phosphate pathways is altered in mutants that cannot recycle rRNA properly. Our results suggest that rRNA turnover is necessary to maintain cellular homeostasis, likely as part of the nucleoside salvage pathway. When this salvage pathway is blocked, the PPP is rerouted to produce ribose-5-P for de novo nucleoside synthesis. This change in carbon flux, in turn, causes growth phenotypes and the production of reactive oxygen species that are responsible for activation of the general autophagy pathway in rns2 mutants.