Identification of small non-coding RNAs responsive to GUN1 and GUN5 related retrograde signals in Arabidopsis thaliana.

Chloroplast perturbations activate retrograde signalling pathways, causing dynamic changes of gene expression. Besides transcriptional control of gene expression, different classes of small non-coding RNAs (sRNAs) act in gene expression control, but comprehensive analyses regarding their role in retrograde signalling are lacking. We performed sRNA profiling in response to norflurazon (NF), which provokes retrograde signals, in Arabidopsis thaliana wild type (WT) and the two retrograde signalling mutants gun1 and gun5. The RNA samples were also used for mRNA and long non-coding RNA profiling to link altered sRNA levels to changes in the expression of their cognate target RNAs. We identified 122 sRNAs from all known sRNA classes that were responsive to NF in the WT. Strikingly, 142 and 213 sRNAs were found to be differentially regulated in both mutants, indicating a retrograde control of these sRNAs. Concomitant with the changes in sRNA expression, we detected about 1500 differentially expressed mRNAs in the NF-treated WT and around 900 and 1400 mRNAs that were differentially regulated in the gun1 and gun5 mutants, with a high proportion (~30%) of genes encoding plastid proteins. Furthermore, around 20% of predicted miRNA targets code for plastid-localised proteins. Among the sRNA-target pairs, we identified pairs with an anticorrelated expression as well pairs showing other expressional relations, pointing to a role of sRNAs in balancing transcriptional changes upon retrograde signals. Based on the comprehensive changes in sRNA expression, we assume a considerable impact of sRNAs in retrograde-dependent transcriptional changes to adjust plastidic and nuclear gene expression.