Plastid translation is essential for lateral root stem-cell patterning in <i>Arabidopsis thaliana</i>.

The plastid evolved from a symbiotic cyanobacterial ancestor and is an essential organelle for plant life, but its developmental roles in roots have been largely overlooked. Here, we show that plastid translation is connected to the stem-cell patterning in lateral root primordia. The RFC3 gene encodes a plastid-localized protein that is a conserved bacterial ribosomal protein S6 of β/γ proteobacterial origin. The rfc3 mutant developed lateral roots with disrupted stem-cell patterning and associated with decreased leaf photosynthetic activity, reduced accumulation of plastid rRNAs in roots, altered root plastid gene expression, and changes in expression of several root stem-cell regulators. These results suggest that deficiencies in plastid function affect lateral root stem cells. Treatment with the plastid translation inhibitor spectinomycin phenocopied the defective stem-cell patterning in lateral roots and altered plastid gene expression observed in the rfc3 mutant. Additionally, when prps17 defective in a plastid ribosomal protein was treated with low concentrations of spectinomycin, it also phenocopied the lateral root phenotypes of rfc3 The spectinomycin treatment and rfc3 mutation also negatively affected symplasmic connectivity between primary root and lateral root primordia. This study highlights previously unrecognized functions of plastid translation in the stem-cell patterning in lateral roots.