Arabidopsis conserves energy during flooding and low oxygen stress by suppressing translation of non-essential mRNA transcripts leading to the accumulation of membrane-less stress granule (SG) aggregates in the cytosol. SGs are mRNA-protein particles that represent sites of “mRNA triage” that mediate the partitioning of transcripts to various fates: storage, degradation, or return to translation after the alleviation of stress. Arabidopsis thaliana CALMODULIN-LIKE 38 (CML38) is a calcium sensor protein that is selectively induced during low oxygen stress, and which associates with SGs as well as the related mRNP granules known as siRNA bodies. It is shown here that SG and siRNA bodies accumulate during low oxygen stress, albeit with different kinetics and numbers, and are subject to selective autophagic degradation during sustained hypoxia. Autophagy of SG require the AAA+-ATPase CDC48. Previous work show that in yeast and mammalian systems, CDC48 and related proteins remodel SG in a ubiquitin-dependent manner to facilitate selective autophagy, a process referred to as granulophagy. In this study, it is shown that CML38 is necessary for this process during hypoxia, and that cml38 mutants show altered SG morphology, reduced turnover of granule associated proteins, and altered recovery during reoxygenation. Based on CML38 homology to the Regulator-of-Gene Silencing calmodulin from tobacco, a model for this class of calcium sensors as a regulator of granule dynamics and autophagic degradation during biotic and abiotic stress is presented.
Coauthors: W. Conner – University of Tennessee;Dan Roberts – University of Tennessee