Reductive stress triggers ANAC017-mediated retrograde signaling to safeguard the endoplasmic reticulum by boosting mitochondrial respiratory capacity

Abstract

Redox processes are at the heart of universal life processes, such as metabolism, signaling or folding of secreted proteins. Redox landscapes differ between cell compartments and are strictly controlled to tolerate changing conditions and to avoid cell dysfunction. While a sophisticated antioxidant network counteracts oxidative stress, our understanding of reductive stress responses remains fragmentary. Here, we observed root growth impairment in Arabidopsis thaliana mutants of mitochondrial alternative oxidase 1a (aox1a) in response to the model thiol reductant dithiothreitol (DTT). Mutants of mitochondrial uncoupling protein 1 (ucp1) displayed a similar phenotype indicating that impaired respiratory flexibility led to hypersensitivity. Endoplasmic reticulum (ER) stress was enhanced in the mitochondrial mutants and limiting endoplasmic reticulum oxidoreductin (ERO) capacity in the aox1a background led to synergistic root growth impairment by DTT, indicating that mitochondrial respiration alleviates reductive ER stress. The observations that DTT triggered NAD reduction in vivo and that the presence of thiols led to electron transport chain activity in isolated mitochondria offer a biochemical framework of mitochondrion-mediated alleviation of thiol-mediated reductive stress. Ablation of transcription factor ANAC017 impaired the induction of AOX1a expression by DTT and led to DTT hypersensitivity, revealing that reductive stress tolerance is achieved by adjusting mitochondrial respiratory capacity via retrograde signaling. Our data reveal an unexpected role for mitochondrial respiratory flexibility and retrograde signaling in reductive stress tolerance involving inter-organelle redox crosstalk.

Proteolytic Activation of Plant Membrane-Bound Transcription Factors

“…Six atMB-TFs (NTL6, NTL7, NTL13, bZIP17, bZIP28, and bZIP60) play a role during ER stress by regulating the expression of unfolded protein-responsive genes ( Liu et al, 2008 ; Tajima et al, 2008 ; Nagashima et al, 2011 ; Yang et al, 2014a , b ; Fuchs et al, 2022 ; Table 2 ). Also in rice ( Oryza sativa ) and maize ( Zea mays ), orthologs of bZIP17/28/60 were discovered in ER stress regulation ( Hayashi et al, 2012 ; Li et al, 2012 ; Takahashi et al, 2012 ; Yang et al, 2013 , 2022 ; Table 3 ).…”

“…Interestingly, NTL13 also controls programmed cell death in response to DTT overreduction of the chloroplasts by repressing stromal ascorbate peroxidase-encoding genes ( Klein et al, 2012 ). Moreover, NTL7, well-characterized for its function in mitochondrial stress responses, also plays a role during DTT-induced ER stress by boosting mitochondrial respiration to enable oxidization of excess reducing equivalents from the ER ( Fuchs et al, 2022 ). Whether these membrane NAC and bZIP TFs have distinct and/or overlapping functions in UPR and which are the precise mechanisms of ER stress sensing and their consequent activation are still not understood.…”

“…NTL1, NTL3, and NTL7 have mainly been studied for their function in mitochondrial retrograde signaling, in which stressed or dysfunctional mitochondrial status (for instance, by treatment with the mitochondrial complex III inhibitor antimycin A) is communicated to the nucleus to induce adaptation responses for the maintenance of the mitochondrial as well as the whole cellular homeostasis ( De Clercq et al, 2013 ; Ng et al, 2013 ; Van Aken et al, 2016 ; Broda et al, 2021 ). Since their discovery as regulators of mitochondria-to-nucleus communication pathways, NTL1, NTL3, and NTL7 were studied in the context of various cellular and environmental stress responses, including the mitochondrial and ER unfolded protein responses ( Kacprzak et al, 2020 ; Fuchs et al, 2022 ), drought ( Van Aken et al, 2016 ), and flooding stress ( Meng et al, 2020 ). NTL7 is constitutively produced and acts as a ‘master switch’ that regulates the expression of NTL1 , NTL3 and NTL4 as downstream targets ( Van Aken et al, 2016 ; Broda et al, 2021 ).…”

“…However, four atMB-TFs, Myc106, bHLH155, SCP, and SPL7, were exclusively detected in the nucleus, implying either that only a TMD-truncated isoform had been produced, or that the TF had been posttranslationally activated under the experimental conditions. Six atMB-TFs (NTL6, NTL7, NTL13, bZIP17, bZIP28, and bZIP60) play a role during ER stress by regulating the expression of unfolded protein-responsive genes (Liu et al, 2008;Tajima et al, 2008;Nagashima et al, 2011;Yang et al, 2014a,b;Fuchs et al, 2022; Table 2). Also in rice (Oryza sativa) and maize (Zea mays), orthologs of bZIP17/28/60 were discovered in ER stress regulation (Hayashi et al, 2012;Li et al, 2012;Takahashi et al, 2012;Yang et al, 2013Yang et al, , 2022; Table 3).…”

The retrograde signaling regulator ANAC017 recruits the MKK9–MPK3/6, ethylene, and auxin signaling pathways to balance mitochondrial dysfunction with growth

“…A central role of ANAC017 is also evident from its function in regulating some chloroplast retrograde responses, and it is therefore a convergence point for organellar signaling pathways ( Van Aken et al, 2016 ; Meng et al, 2019 ). Correspondingly, the mutation of ANAC017 leads to increased sensitivity to combined light/drought stress, submergence, and reductive stress ( Ng et al, 2013 ; Meng et al, 2019 ; Fuchs et al, 2022 ). ANAC017 also regulates plant growth, as overexpression and mutation leads to reduced and increased growth under nonlimiting conditions, respectively, indicating a role of ANAC017 in balancing mitochondrial function in response to stress with plant growth ( Ng et al, 2013 ; Meng et al, 2019 , 2020 ).…”