Mechanisms that determine the cellular volume allocated to organelles are largely unknown. We demonstrate that in the plant Arabidopsis thaliana, a small gene family that encodes proteins of unknown function contributes to a mechanism that establishes the proportion of cellular volume devoted to chloroplasts. We show that this mechanism resides outside of the chloroplast by demonstrating that the protein that makes the greatest contribution to this mechanism resides in the cytoplasm and nucleus and that the trafficking of this protein between the cytoplasm and nucleus may regulate this mechanism. A deeper understanding of this mechanism may lead to the rational manipulation of chloroplast compartment size, which may lead to more efficient photosynthesis and increased yields from important crop plants.

Abstract

Eukaryotic cells require mechanisms to establish the proportion of cellular volume devoted to particular organelles. These mechanisms are poorly understood. From a screen for plastid-to-nucleus signaling mutants in Arabidopsis thaliana, we cloned a mutant allele of a gene that encodes a protein of unknown function that is homologous to two other Arabidopsis genes of unknown function and to FRIENDLY, which was previously shown to promote the normal distribution of mitochondria in Arabidopsis. In contrast to FRIENDLY, these three homologs of FRIENDLY are found only in photosynthetic organisms. Based on these data, we proposed that FRIENDLY expanded into a small gene family to help regulate the energy metabolism of cells that contain both mitochondria and chloroplasts. Indeed, we found that knocking out these genes caused a number of chloroplast phenotypes, including a reduction in the proportion of cellular volume devoted to chloroplasts to 50% of wild type. Thus, we refer to these genes as REDUCED CHLOROPLAST COVERAGE (REC). The size of the chloroplast compartment was reduced most in rec1 mutants. The REC1 protein accumulated in the cytosol and the nucleus. REC1 was excluded from the nucleus when plants were treated with amitrole, which inhibits cell expansion and chloroplast function. We conclude that REC1 is an extraplastidic protein that helps to establish the size of the chloroplast compartment, and that signals derived from cell expansion or chloroplasts may regulate REC1.