Defects in secondary cell-wall synthesis mitigate the effects of lincomycin on early chloroplast development

Lincomycin (LIN)-mediated inhibition of protein synthesis in chloroplasts prevents greening of seedlings, represses the activity of photosynthesis-related genes in the nucleus including LHCB1.2, and induces the phenylpropanoid pathway, resulting in production of anthocyanins. In genomes uncoupled (gun) mutants, LHCB1.2 expression is maintained in the presence of LIN or other inhibitors of early chloroplast development. In a screen using concentrations of LIN lower than those employed to isolate gun mutants, we have identified happy on lincomycin (holi) mutants. Several holi mutants show increased tolerance to LIN, exhibiting de-repressed LHCB1.2 expression and chlorophyll synthesis in seedlings. The mutations responsible were identified by whole-genome SNP mapping, and most were found to affect the phenylpropanoid pathway. However, LHCB1.2 expression appears not to be directly regulated by phenylpropanoids, as indicated by metabolic profiling of mutants. The most potent holi mutant is defective in a subunit of cellulose synthase encoded by IRREGULAR XYLEM3, and comparative analysis of this and other cell-wall mutants establishes a link between secondary cell-wall integrity and tolerance to LIN, possibly involving altered ABA metabolism or sensing. Overall design: Examination of differential gene expression in the Arabdidopsis thaliana irx3-2 mutant compared to wild type grown on MS or on MS supplemented with Lincomcycin