MORF9-dependent specific plastid RNA editing inhibits root growth under sugar starvation in Arabidopsis.

Multiple organellar RNA editing factor (MORF) complex was shown to be highly associated with C-to-U RNA editing of vascular plant editosome. However, mechanisms by which MORF9-dependent plastid RNA editing controls plant development and responses to environmental alteration remain obscure. In this study, we found that loss of MORF9 function impaired PSII efficiency, NDH activity, and carbohydrate production, rapidly promoted nuclear gene expression including sucrose transporter and sugar/energy responsive genes, and attenuated root growth under sugar starvation conditions. Sugar repletion increased MORF9 and MORF2 expression in wild-type seedlings and reduced RNA editing of matK-706, accD-794, ndhD-383 and ndhF-290 in the morf9 mutant. RNA editing efficiency of ndhD-383 and ndhF-290 sites was diminished in the gin2/morf9 double mutants, and that of matK-706, accD-794, ndhD-383 and ndhF-290 sites were significantly diminished in the snrk1/morf9 double mutants. In contrast, overexpressing HXK1 or SnRK1 promoted RNA editing rate of matK-706, accD-794, ndhD-383 and ndhF-290 in leaves of morf9 mutants, suggesting that HXK1 partially impacts MORF9 mediated ndhD-383 and ndhF-290 editing, while SnRK1 may only affect MORF9-mediated ndhF-290 site editing. Collectively, these findings suggest that sugar and/or its intermediary metabolites impair MORF9-dependent plastid RNA editing resulting in derangements of plant root development.

Translational repression of FZP mediated by CU-rich element/OsPTB interactions modulates panicle development in rice.

Panicle development is an important determinant of the grain number in rice. A thorough characterization of the molecular mechanism underlying panicle development will lead to improved breeding of high-yielding rice varieties. Frizzy Panicle (FZP), a critical gene for panicle development, is regulated by OsBZR1 and OsARFs at the transcriptional stage. However, the translational modulation of FZP has not been reported. We reveal that the CU-rich elements (CUREs) in the 3' UTR of the FZP mRNA are crucial for efficient FZP translation. The knockout of CUREs in the FZP 3' UTR or the over-expression of the FZP 3' UTR fragment containing CUREs resulted in an increase in FZP mRNA translation efficiency. Moreover, the number of secondary branches (NSB) and the grain number per panicle (GNP) decreased in the transformed rice plants. The CUREs in the 3' UTR of FZP mRNA were verified as the targets of the polypyrimidine tract-binding proteins OsPTB1 and OsPTB2 in rice. Both OsPTB1 and OsPTB2 were highly expressed in young panicles. The knockout of OsPTB1/2 resulted in an increase in the FZP translational efficiency and a decrease in the NSB and GNP. Furthermore, the over-expression of OsPTB1/2 decreased the translation of the reporter gene fused to FZP 3' UTR in vivo and in vitro. These results suggest that OsPTB1/2 can mediate FZP translational repression by interacting with CUREs in the 3' UTR of FZP mRNA, leading to changes in the NSB and GNP. Accordingly, in addition to transcriptional regulation, FZP expression is also fine-tuned at the translational stage during rice panicle development.

MORF9 Functions in Plastid RNA Editing with Tissue Specificity.

RNA editing in plant mitochondria and plastids converts specific nucleotides from cytidine (C) to uridine (U). These editing events differ among plant species and are relevant to developmental stages or are impacted by environmental conditions. Proteins of the MORF family are essential components of plant editosomes. One of the members, MORF9, is considered the core protein of the editing complex and is involved in the editing of most sites in chloroplasts. In this study, the phenotypes of a T-DNA insertion line with loss of MORF9 and of the genetic complementation line of Arabidopsis were analyzed, and the editing efficiencies of plastid RNAs in roots, rosette leaves, and flowers from the morf9 mutant and the wild-type (WT) control were compared by bulk-cDNA sequencing. The results showed that most of the known MORF9-associated plastid RNA editing events in rosette leaves and flowers were similarly reduced by morf9 mutation, with the exception that the editing rate of the sites ndhB-872 and psbF-65 declined in the leaves and that of ndhB-586 decreased only in the flowers. In the roots, however, the loss of MORF9 had a much lower effect on overall plastid RNA editing, with nine sites showing no significant editing efficiency change, including accD-794, ndhD-383, psbZ-50, ndhF-290, ndhD-878, matK-706, clpP1-559, rpoA-200, and ndhD-674, which were reduced in the other tissues. Furthermore, we found that during plant aging, MORF9 mRNA level, but not the protein level, was downregulated in senescent leaves. On the basis of these observations, we suggest that MORF9-mediated RNA editing is tissue-dependent and the resultant organelle proteomes are pertinent to the specific tissue functions.