The nuclear-encoded plastid ribosomal protein bL31c of Arabidopsis thaliana functions in chloroplast biogenesis and leaf development.

Plastid ribosomes are composed of large (50S) and small (30S) subunits, each made up of ribosomal RNAs (rRNA) and plastid ribosomal proteins (PRPs). While numerous PRPs have been characterized, the functions of several PRPs remain elusive. We followed a reverse genetics approach in Arabidopsis to functionally characterize nuclear genes encoding PRPs for which no viable mutant alleles had been described. We report here the characterization of 50S subunit PRP bL31c. Its gene, RPL31, was previously identified as essential for embryo development (EMB2184), but viable alleles were unavailable. RPL31 is a single-copy gene and we confirmed that rpl31-1 and emb2184 (renamed rpl31-2) are allelic. rpl31 mutants display reduced growth, paleness, pointed rosette leaves with toothed margins, loosely packed and small mesophyll cells with abnormal chloroplasts, and disrupted cotyledon venation. Chloroplast targeting of the bL31c protein was confirmed using an RPL31pro:RPL31-GFP construct. RPL31 loss of function affects transcript and protein accumulation from nuclear and plastid genes and modifies the abundance of rRNAs, suggesting defects in plastid ribosome biogenesis and/or translation. Remarkably, rpl31 mutants show altered expression of the auxin activity markers DR5pro:GUS and DR5pro:GFP, as well as auxin-related genes, and exhibit reduced sensitivity to the synthetic auxin 2,4-D. These findings and the abnormalities in venation and leaf morphology suggest compromised auxin homeostasis in rpl31 mutants. RPL31-mediated leaf margin development depends on CUP-SHAPED COTYLEDON2 and the rpl31-1 mutation synergistically interacts with others affecting regulators of plastid gene expression. Overall, our results demonstrate that besides embryo development, RPL31 plays a key role in leaf morphogenesis, highlighting its broader developmental significance.