Figure 7: Suppression by sp1 of the atToc33 Knockout Mutation, ppi1

With the aim of identifying new factors involved in the chloroplast protein import mechanism, or in its regulation, we conducted a novel forward genetic screen: we screened for second-site suppressors of the well-characterized Arabidopsis chloroplast biogenesis mutation, plastid protein import 1 (ppi1). One of the new mutants identified in the screen was suppressor of ppi1 locus 1 (sp1). Panel A shows plants that have been grown on soil for 30 days, and illustrates clearly how the sp1 mutation substantially suppresses the chlorotic phenotype caused by the ppi1 mutation. Panel B shows the ultrastructure of typical cotyledon chloroplasts in 10-day-old plants grown in vitro. It is evident that the visible sp1-mediated suppression shown in panel A is associated with significant improvements in chloroplast biogenesis: the organelles are larger in the double mutant than in ppi1, and have more highly developed thylakoid membrane networks. Scale bar, 2 μm. Taken from Science, 2012, 338: 655-659.