Functional analysis of mTERF5 and mTERF9 contribution to salt tolerance, plastid gene expression and retrograde signalling in Arabidopsis thaliana

Abstract

We previously showed that Arabidopsis mda1 and mterf9 mutants, defective in the chloroplast-targeted mitochondrial transcription termination factors mTERF5 and mTERF9, respectively, display altered responses to abiotic stresses and abscisic acid (ABA), as well as perturbed development, likely through abnormal chloroplast biogenesis.• To advance the functional analysis of mTERF5 and mTERF9, we obtained and characterized overexpression (OE) lines. Additionally, we studied genetic interactions between sca3-2, affected in the plastid-RNA polymerase RpoTp, and the mda1-1 and mterf9 mutations. We also investigated the role of mTERF5 and mTERF9 in plastid translation and plastid-to-nucleus signalling.• We found that mTERF9 OE reduces salt and ABA tolerance, while mTERF5 or mTERF9 OE alter expression of nuclear and plastid genes. We determined that mda1-1 and mterf9 mutations genetically interact with sca3-2. Further, plastid 16S rRNA levels were reduced in mda1-1 and mterf9 mutants, and mterf9 was more sensitive to chemical inhibitors of chloroplast translation. Expression of the photosynthesis gene LHCB1, a retrograde signalling marker, was differentially affected in mda1-1 and/or mterf9 compared to wild-type Col-0, after treatments with inhibitors of carotenoid biosynthesis (norflurazon) or chloroplast translation (lincomycin). Moreover, mterf9, but not mda1-1, synergistically interacts with gun1-1, defective in GUN1, a central integrator of plastid retrograde signals.• Our results show that mTERF9, and to a lesser extent mTERF5, are negative regulators of salt tolerance and that both genes are functionally related to RpoTp, and that mTERF9 is likely required for plastid ribosomal stability and/or assembly. Furthermore, our findings support a role for mTERF9 in retrograde signalling.

Arabidopsis Plastid-RNA Polymerase RPOTp Is Involved in Abiotic Stress Tolerance

“…As such, as these results show that both RPOTp-and PEP-mediated plastid transcription are involved in plant salt tolerance, we consider it feasible that PGE alteration at the transcriptional level may account for the abiotic stress phenotype of mutants sca3, mterf and sig5-2. Consistently with this, sca3-2 synergistically interacts with mutations mterf6-5 and mterf9 [31,49].…”

“…We consider that the hypersensitivity to stress displayed by the sca3 mutants is due to impaired RPOTp function, and does not result from a delayed germination caused by the stress treatments. Consistent with this, other chloroplast-defective mutants, such as mterf9 [which also shows, like sca3 plants, stunted growth and impaired chloroplast biogenesis, and is morphologically very similar to sca3 (both mutations even interact synergistically)], display enhanced tolerance to abiotic stress [31,42]. As such, these results suggest that RPOTp promotes tolerance to environmental stress during germination and seedling establishment.…”

“…To determine the specificity of the effect of salt stress on the expression of the NEP genes, we studied the transcript levels of two nuclear genes that encode chloroplast-targeted proteins also involved in PGE: the mitochondrial transcription termination factors mTERF5 and mTERF9. These mTERFs are functionally related to RPOTp and are required for chloroplast development and response to abiotic stress [31]; furthermore, mTERF5 regulates transcription of the plastid psbEFLJ polycistron [32]. In stark contrast to the NEP genes, we found that 100 mM of NaCl significantly repressed mTERF5 expression (1.3-fold down-regulated) and did not noticeably modify mTERF9 transcript abundance (Table 1).…”

The Arabidopsis mTERF‐repeat MDA1 protein plays a dual function in transcription and stabilization of specific chloroplast transcripts within the psbE and ndhH operons

“…By contrast, the family expanded to c. 30 members in higher plants (Babiychuk et al, 2011;Kleine, 2012;Zhao et al, 2014) and recent studies have suggested that this expansion has been accompanied by a functional diversification in RNA metabolism such as intron splicing and rRNA maturation in organelles (Hammani & Barkan, 2014;Hsu et al, 2014;Romani et al, 2015;Zhang et al, 2018). In addition to these studies, transcriptomic and physiological analyses conducted in Arabidopsis and crop species have highlighted the importance of mTERF genes for plant response to a variety of abiotic stresses (Zhao et al, 2014;Zhou et al, 2016;Xu et al, 2017;Robles et al, 2018;Nunez-Delegido et al, 2019). Nevertheless, only a handful of mTERF genes have been characterized molecularly and biochemically in plants to better understand their functional diversification.…”

Arabidopsis mTERF9 protein promotes chloroplast ribosomal assembly and translation by establishing ribonucleoprotein interactionsin vivo

“…In plants, several of these genes are essential for embryo viability (11)(12)(13). Others have been linked to a variety of abiotic stress-responses (8,(14)(15)(16)(17) but how these genes trigger these responses in plants is not understood. Plant mTERFs are predicted to act in mitochondria or chloroplasts but knowledge about their roles in organelles is scarce.…”